Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1509.05297
Ising Spin Network States for Loop Quantum Gravity: a Toy Model for Phase Transitions
Alexandre Feller, Etera R. Livine
(Submitted on 17 Sep 2015)
Non-perturbative approaches to quantum gravity call for a deep understanding of the emergence of geometry and locality from the quantum state of the gravitational field. Without background geometry, the notion of distance should entirely emerge from the correlations between the gravity fluctuations. In the context of loop quantum gravity, quantum states of geometry are defined as spin networks. These are graphs decorated with spin and intertwiners, which represent quantized excitations of areas and volumes of the space geometry. Here, we develop the condensed matter point of view on extracting the physical and geometrical information out of spin network states: we introduce new Ising spin network states, both in 2d on a square lattice and in 3d on a hexagonal lattice, whose correlations map onto the usual Ising model in statistical physics. We construct these states from the basic holonomy operators of loop gravity and derive a set of local Hamiltonian constraints which entirely characterize our states. We discuss their phase diagram and show how the distance can be reconstructed from the correlations in the various phases. Finally, we propose generalizations of these Ising states, which open the perspective to study the coarse graining and dynamics of spin network states using well-known condensed matter techniques and results.
17 pages

possibly of wider interest:
http://arxiv.org/abs/1509.04645
Non-Thermal Corrections to Hawking Radiation Versus the Information Paradox
Gia Dvali
(Submitted on 15 Sep 2015)
We provide a model-independent argument indicating that for a black hole of entropy N the non-thermal deviations from Hawking radiation, per each emission time, are of order 1/N, as opposed to exp(-N). This fact abolishes the standard a priori basis for the information paradox.
5 pages

http://arxiv.org/abs/1509.04282
Scalar Dark Matter: Direct vs. Indirect Detection
Michael Duerr, Pavel Fileviez Perez, Juri Smirnov
(Submitted on 14 Sep 2015)
We revisit the simplest model for dark matter. In this context the dark matter candidate is a real scalar field which interacts with the Standard Model particles through the Higgs portal. We discuss the relic density constraints as well as the predictions for direct and indirect detection. The final state radiation processes are investigated in order to understand the visibility of the gamma lines from dark matter annihilation. We find two regions where one could observe the gamma lines at gamma-ray telescopes. We point out that the region where the dark matter mass is between 100 and 300 GeV can be tested in the near future at direct and indirect detection experiments.
Comments: 27 pages, 13 figures

http://arxiv.org/abs/1509.04325
Anomalies of the Entanglement Entropy in Chiral Theories
Nabil Iqbal, Aron C. Wall
(Submitted on 14 Sep 2015)
We study entanglement entropy in theories with gravitational or mixed U(1) gauge-gravitational anomalies in two, four and six dimensions. In such theories there is an anomaly in the entanglement entropy: it depends on the choice of reference frame in which the theory is regulated. We discuss subtleties regarding regulators and entanglement entropies in anomalous theories. We then study the entanglement entropy of free chiral fermions and self-dual bosons and show that in sufficiently symmetric situations this entanglement anomaly comes from an imbalance in the flux of modes flowing through the boundary, controlled by familiar index theorems. In two and four dimensions we use anomalous Ward identities to find general expressions for the transformation of the entanglement entropy under a diffeomorphism. (In the case of a mixed anomaly there is an alternative presentation of the theory in which the entanglement entropy is not invariant under a U(1) gauge transformation. The free-field manifestation of this phenomenon involves a novel kind of fermion zero mode on a gravitational background with a twist in the normal bundle to the entangling surface.) We also study d-dimensional anomalous systems as the boundaries of d + 1 dimensional gapped Hall phases. Here the full system is non-anomalous, but the boundary anomaly manifests itself in a change in the entanglement entropy when the boundary metric is sheared relative to the bulk.
45 pages + appendices, 5 figures

http://arxiv.org/abs/1509.04497
General relativity and cosmology
Martin Bucher (APC, U. Paris 7/CNRS, Paris, France & University of KwaZulu-Natal, Durban, South Africa), Wei-Tou Ni (National Tsing Hua University, Hsinchu, Taiwan)
(Submitted on 15 Sep 2015)
15 pages, 4 figures, to appear as introductory chapter in "One Hundred Years of General Relativity: From Genesis and Empirical Foundations to Gravitational Waves, Cosmology and Quantum Gravity"

 

[marcus]

http://arxiv.org/abs/1509.05693
Detailed analysis of the predictions of loop quantum cosmology for the primordial power spectra
Ivan Agullo, Noah A. Morris
(Submitted on 18 Sep 2015)
We provide an exhaustive numerical exploration of the predictions of loop quantum cosmology (LQC) with a post-bounce phase of inflation for the primordial power spectrum of scalar and tensor perturbations. We extend previous analysis by characterizing the phenomenologically relevant parameter space and by constraining it using observations. Furthermore, we characterize the shape of LQC-corrections to observable quantities across this parameter space. Our analysis provides a framework to contrast more accurately the theory with forthcoming polarization data, and it also paves the road for the computation of other observables beyond the power spectra, such as non-Gaussianity.
24 pages, 5 figures

possible wider interest:
http://arxiv.org/abs/1509.07767
Brief reply to "Can gravity account for the emergence of classicality?"
Igor Pikovski, Magdalena Zych, Fabio Costa, Časlav Brukner
(Submitted on 24 Sep 2015)
In a series of comments, Bonder et al. criticized our work on decoherence due to time dilation [Nature Physics 11, 668-672 (2015)]. First the authors erroneously claimed that our results contradict the equivalence principle, only to "resolve" the alleged conflict in a second note. The resolution - relativity of simultaneity - was already explained in our reply [arXiv:1508.03296], which Bonder et al. now essentially reiterate. The newly raised points were also already extensively clarified in our note. The physical prediction of our work remains valid: systems with internal dynamics decohere if the superposed paths have different proper times.
1 page

http://arxiv.org/abs/1509.07666
Summary of the XXVIIth Rencontres de Blois: Particle Physics and Cosmology
Gavin P. Salam
(Submitted on 25 Sep 2015)
This writeup summarises some of the highlights from the 2015 Rencontres de Blois, with a compression ratio of about 100:1 relative to the original presentations.
22 pages, 24 figures, presented at XXVIIth Rencontres de Blois, France, 31 May - 5 June 2015

http://arxiv.org/abs/1509.07506
Density Jumps Near the Virial Radius of Galaxy Clusters
Anna Patej, Abraham Loeb
(Submitted on 24 Sep 2015)
Recent simulations have indicated that the dark matter halos of galaxy clusters should feature steep density jumps near the virial radius. Since the member galaxies are expected to follow similar collisionless dynamics as the dark matter, the galaxy density profile should show such a feature as well. We examine the potential of current datasets to test this prediction by selecting cluster members for a sample of 56 low-redshift (0.1<z<0.3) galaxy clusters, constructing their projected number density profiles, and fitting them with two profiles, one with a steep density jump and one without. Additionally, we investigate the presence of a jump using a non-parametric spline approach. We find that some of these clusters show strong evidence for a model with a density jump. We discuss avenues for further analysis of the density jump with future datasets.
11 pages, 8 figures

http://arxiv.org/abs/1509.07501
Cosmological Hints of Modified Gravity ?
Eleonora Di Valentino, Alessandro Melchiorri, Joseph Silk
(Submitted on 24 Sep 2015)
The recent measurements of Cosmic Microwave Background temperature and polarization anisotropies made by the Planck satellite have provided impressive confirmation of the ΛCDM cosmological model. However interesting hints of slight deviations from ΛCDM have been found, including a 95% c.l. preference for a "modified gravity" structure formation scenario. In this paper we confirm the preference for a modified gravity scenario from Planck 2015 data, find that modified gravity solves the so-called Alensanomaly in the CMB angular spectrum, and constrains the amplitude of matter density fluctuations to σ₈=0.815+0.032−0.048, in better agreement with weak lensing constraints. Moreover, we find a lower value for the reionization optical depth of τ=0.059±0.020 (to be compared with the value of τ=0.079±0.017obtained in the standard scenario), more consistent with recent optical and UV data. We check the stability of this result by considering possible degeneracies with other parameters, including the neutrino effective number, the running of the spectral index and the amount of primordial helium. The indication for modified gravity is still present at about 95% c.l., and could become more significant if lower values of τ were to be further confirmed by future cosmological and astrophysical data.
10 pages, 5 figures

 

[marcus]

http://arxiv.org/abs/1509.08899
Generalized effective description of loop quantum cosmology
Abhay Ashtekar, Brajesh Gupt
(Submitted on 29 Sep 2015)
The effective description of loop quantum cosmology (LQC) has proved to be a convenient platform to study phenomenological implications of the quantum bounce that resolves the classical big-bang singularity. Originally, this description was derived using Gaussian quantum states with small dispersions. In this paper we present a generalization to incorporate states with large dispersions. Specifically, we derive the generalized effective Friedmann and Raychaudhuri equations and propose a generalized effective Hamiltonian which are being used in an ongoing study of the phenomenological consequences of a broad class of quantum geometries. We also discuss an interesting interplay between the physics of states with larger dispersions in standard LQC, and of sharply peaked states in (hypothetical) LQC theories with larger area gap.
21 pages, 4 figures

http://arxiv.org/abs/1509.09182
Loop quantum cosmology and the fate of cosmological singularities
Parampreet Singh
(Submitted on 30 Sep 2015)
Singularities in general relativity such as the big bang and big crunch, and exotic singularities such as the big rip are the boundaries of the classical spacetimes. These events are marked by a divergence in the curvature invariants and the breakdown of the geodesic evolution. Recent progress on implementing techniques of loop quantum gravity to cosmological models reveals that such singularities may be generically resolved because of the quantum gravitational effects. Due to the quantum geometry, which replaces the classical differential geometry at the Planck scale, the big bang is replaced by a big bounce without any assumptions on the matter content or any fine tuning. In this manuscript, we discuss some of the main features of this approach and the results on the generic resolution of singularities for the isotropic as well as anisotropic models. Using effective spacetime description of the quantum theory, we show the way quantum gravitational effects lead to the universal bounds on the energy density, the Hubble rate and the anisotropic shear. We discuss the geodesic completeness in the effective spacetime and the resolution of all of the strong singularities. It turns out that despite the bounds on energy density and the Hubble rate, there can be divergences in the curvature invariants. However such events are geodesically extendible, with tidal forces not strong enough to cause inevitable destruction of the in-falling objects.
26 pages, 1 figure. Invited review based on M K Vainu Bappu gold medal award lecture. Published in Bulletin of Astronomical Society of India

http://arxiv.org/abs/1509.08788
Recent results in CDT quantum gravity
Jan Ambjorn, Daniel Coumbe, Jakub Gizbert-Studnicki, Jerzy Jurkiewicz
(Submitted on 29 Sep 2015)
We review some recent results from the causal dynamical triangulation (CDT) approach to quantum gravity. We review recent observations of dimensional reduction at a number of previously undetermined points in the parameter space of CDT, and discuss their possible relevance to the asymptotic safety scenario. We also present an updated phase diagram of CDT, discussing properties of a newly discovered phase and its possible relation to a signature change of the metric.
6 pages, 3 figures, 1 table. Contribution to the proceedings of MG14, Rome July 2015

http://arxiv.org/abs/1509.09122
On selfdual spin-connections and Asymptotic Safety
Ulrich Harst, Martin Reuter
(Submitted on 30 Sep 2015)
We explore Euclidean quantum gravity using the tetrad field together with a selfdual or anti-selfdual spin-connection as the basic field variables. Setting up a functional renormalization group (RG) equation of a new type which is particularly suitable for the corresponding theory space we determine the non-perturbative RG flow within a two-parameter truncation suggested by the Holst action. We find that the (anti-)selfdual theory is likely to be asymptotically safe. The existing evidence for its non-perturbative renormalizability is comparable to that of Einstein-Cartan gravity without the selfduality condition.
14 pages, 4 fgures

possibly of wider interest:
http://arxiv.org/abs/1509.08772
Bounce Inflation Cosmology with Standard Model Higgs Boson
Youping Wan, Taotao Qiu, Fa Peng Huang, Yi-Fu Cai, Hong Li, Xinmin Zhang
(Submitted on 28 Sep 2015)
It is of great interest to connect cosmology in the early universe to the Standard Model of particle physics. In this paper, we try to construct a bounce inflation model with the standard model Higgs boson, where the one loop correction is taken into account in the effective potential of Higgs field. In this model, a Galileon term has been introduced to eliminate the ghost mode when bounce happens. Moreover, due to the fact that the Fermion loop correction can make part of the Higgs potential negative, one naturally obtains a large equation of state(EoS) parameter in the contracting phase, which can eliminate the anisotropy problem. After the bounce, the model can drive the universe into the standard higgs inflation phase, which can generate nearly scale-invariant power spectrum.
13 pages, 9 figures

 

[marcus]

http://arxiv.org/abs/1510.01925
Projective Loop Quantum Gravity II. Searching for Semi-Classical States
Suzanne Lanéry, Thomas Thiemann
(Submitted on 7 Oct 2015)
In [arXiv:1411.3592] an extension of the Ashtekar-Lewandowski (AL) state space of Loop Quantum Gravity was set up with the help a projective formalism introduced by Kijowski [Kijowski 1977; see also: arXiv:1304.6330, arXiv:1411.3590]. The motivation for this work was to achieve a more balanced treatment of the position and momentum variables (aka. holonomies and fluxes). Indeed, states in the AL Hilbert spaces describe discrete quantum excitations on top of a vacuum which is an eigenstate of the flux variables (a `no-geometry' state): in such states, most holonomies are totally spread, making it difficult to approximate a smooth, classical 4-geometry. However, going beyond the AL sector does not fully resolve this difficulty: one uncovers a deeper issue hindering the construction of states semi-classical with respect to a full set of observables. In the present article, we analyze this issue in the case of real-valued holonomies (we will briefly comment on the heuristic implications for other gauge groups, eg. (2)). Specifically, we show that, in this case, there does not exist any state on the holonomy-flux algebra in which the variances of the holonomies and fluxes observables would all be finite, let alone small. It is important to note that this obstruction cannot be bypassed by further enlarging the quantum state space, for it arises from the structure of the algebra itself: as there are too many (uncountably many) non-vanishing commutators between the holonomy and flux operators, the corresponding Heisenberg inequalities force the quantum uncertainties to blow up uncontrollably. A way out would be to suitably restrict the algebra of observables. In a companion paper we take the first steps in this direction by developing a general framework to perform such a restriction without giving up the universality and diffeomorphism invariance of the theory.
51 pages, 1 figure

http://arxiv.org/abs/1510.01926
Projective Limits of State Spaces IV. Fractal Label Sets
Suzanne Lanéry, Thomas Thiemann
(Submitted on 7 Oct 2015)
Instead of formulating the state space of a quantum field theory over one big Hilbert space, it has been proposed by Kijowski [Kijowski 1977] to represent quantum states as projective families of density matrices over a collection of smaller, simpler Hilbert spaces. One can thus bypass the need to select a vacuum state for the theory, and still be provided with an explicit and constructive description of the quantum state space, at least as long as the label set indexing the projective structure is countable. Because uncountable label sets are much less practical in this context, we develop in the present article a general procedure to trim an originally uncountable label set down to countable cardinality. In particular, we investigate how to perform this tightening of the label set in a way that preserves both the physical content of the algebra of observables and its symmetries. This work is notably motivated by applications to the holonomy-flux algebra underlying Loop Quantum Gravity. Building on earlier work by Okolow [arXiv:1304.6330], a projective state space was introduced for this algebra in [arXiv:1411.3592]. However, the non-trivial structure of the holonomy-flux algebra prevents the construction of satisfactory semi-classical states. Implementing the general procedure just mentioned in the case of a one-dimensional version of this algebra, we show how a discrete subalgebra can be extracted without destroying universality nor diffeomorphism invariance. On this subalgebra, states can then be constructed whose semi-classicality is enforced step by step, starting from collective, macroscopic degrees of freedom and going down progressively toward smaller and smaller scales.
42 pages, 1 figure

For convenience here are the previous four papers in this series:
arXiv:1411.3592
Projective Loop Quantum Gravity I. State Space
Suzanne Lanéry, Thomas Thiemann
81 pages, many figures

arXiv:1411.3589
Projective Limits of State Spaces I. Classical Formalism
Suzanne Lanéry, Thomas Thiemann
51 pages, many figures

arXiv:1411.3590
Projective Limits of State Spaces II. Quantum Formalism
Suzanne Lanéry, Thomas Thiemann
56 pages, 2 figures

arXiv:1411.3591
Projective Limits of State Spaces III. Toy-Models
Suzanne Lanéry, Thomas Thiemann
40 pages

 

[marcus]

http://arxiv.org/abs/1510.02182
Entropy in the interior of a black hole and thermodynamics
Baocheng Zhang
(Submitted on 8 Oct 2015)
Based on a recent proposal for the volume inside a black hole, we calculate the entropy associated with this volume and show that such entropy is proportional to the surface area of the black hole. Together with the consideration of black hole radiation, we find that the thermodynamics associated with the entropy is likely to be caused by the vacuum polarization near the horizon.
11 pages.

briefly noted:
http://arxiv.org/abs/1510.02751
Barbero-Immirzi parameter as a solution of the simplicity constraints
Leonid Perlov, Michael Bukatin
(Submitted on 9 Oct 2015)
This paper contains three main achievements. The first one is naturally obtaining the values of the Barbero-Immirzi parameter as the solution of the simplicity constraints rather than setting it a priori. Particularly the Main theorem shows that if γ=±i then the simplicity constraints require that the corresponding Lorentz group representations be necessary finite dimensional and therefore non-unitary. The second main achievement is the ability to define the Lorentzian spin-network Hilbert space without slicing the space by the 3-dimensional ADM-like hyper-surfaces and thus not breaking the Lorentz covariance. The third achievement is the ability to use the well-defined converging inner-product directly in 4-dimensional Lorentzian Hilbert space.
13 pages.

http://arxiv.org/abs/1510.02239
Comment on decoherence by time dilation
H. Dieter Zeh
(Submitted on 8 Oct 2015)
Remarks regarding a novel decoherence mechanism [arXiv:1311.1095].
3 pages

not QG but possibly of general interest:
http://arxiv.org/abs/1510.01733
spotted by Chronos who started this discussion thread about it:
https://www.physicsforums.com/threads/first-detection-of-direct-collapse-black-holes.836599/

 

[marcus]

http://arxiv.org/abs/1510.00699
A Perfect Bounce
Steffen Gielen, Neil Turok
(Submitted on 2 Oct 2015)
We study the quantum cosmology of a universe with conformal matter comprising a perfect radiation fluid and a number of conformally coupled scalar fields. For FRW backgrounds, we are able to perform the quantum gravity path integral exactly. We find the evolution to describe a "perfect bounce," in which the universe passes smoothly through the singularity. The Feynman path integral amplitude is precisely that of a relativistic oscillator, for which the scale factor of the universe is the time and the scalar fields are the spatial coordinates. This picture provides natural, unitary quantum mechanical evolution across a bounce. We also study the quantum evolution of anisotropies and of inhomogeneous perturbations, at linear and nonlinear order. We provide evidence for a semiclassical description in which all fields pass "around" the cosmological singularity along complex classical paths.
5 pages.

http://arxiv.org/abs/1510.02985
Shocks in the Early Universe
Ue-Li Pen, Neil Turok
(Submitted on 10 Oct 2015)
We point out a surprising consequence of the usually assumed initial conditions for cosmological perturbations. Namely, a scale-invariant spectrum of Gaussian, linear, adiabatic, scalar, growing mode perturbations not only creates acoustic oscillations, of the kind observed in great detail on large scales today, it also leads to the production of shock waves in the radiation fluid of the very early universe. At very early epochs, 1 GeV<T<10⁷ GeV, assuming standard model physics, viscous damping is negligible and nonlinear effects turn acoustic waves into shocks after ∼10⁴ oscillations. The resulting scale-invariant network of shocks provides a natural mechanism for creating significant departures from local thermal equilibrium as well as primordial vorticity and gravitational waves.
5 pages, 3 figures (4 figure files).

http://arxiv.org/abs/1510.01696
Optomechanical test of the Schrödinger-Newton equation
André Großardt, James Bateman, Hendrik Ulbricht, Angelo Bassi
(Submitted on 6 Oct 2015)
The Schrödinger-Newton equation has been proposed as an experimentally testable alternative to quantum gravity, accessible at low energies. It contains self-gravitational terms, which slightly modify the quantum dynamics. Here we show that it distorts the spectrum of a harmonic system. Based on this effect, we propose an optomechanical experiment with a trapped microdisc to test the Schrödinger-Newton equation, and we show that it can be realized with existing technology.
13 pages, 4 figures, 1 table, 1 page of supplemental material

http://arxiv.org/abs/1510.03135
Loop quantum cosmology: the horizon problem and the probability of inflation
Long Chen, Jian-Yang Zhu
(Submitted on 12 Oct 2015)
Anomaly-free perturbations of loop quantum cosmology reveal a deformed space-time structure, in which the signature changes when the energy density is ρ=ρ_c/2. Furthermore, in loop quantum cosmology, one can obtain an effective causal structure only for a low density region (ρ≤ρ_c/2), which gives a natural initial condition to consider the horizon problem. Choosing the initial value at ρ(0)=ρ_c/2 in this paper, we investigate the horizon problem and the probability of inflation in the framework of loop quantum cosmology. Two models are considered: the quadratic inflation and the natural inflation. We use the Liouville measure to calculate the probability of inflation which solves the horizon problem, and find that, for the quadratic inflation model, the probability is very close to unity, while for the natural inflation model, the probability is about 35%.
7 pages, 2 figures

possible wider interest:
http://arxiv.org/abs/1510.02788
Witnessing the birth of a supermassive protostar
Muhammad A. Latif, Dominik R. G. Schleicher, Tilman Hartwig
(Submitted on 9 Oct 2015)
The detection of z>6 quasars reveals the existence of supermassive black holes of a few 10⁹M_⊙. One of the potential pathways to explain their formation in the infant universe is the so-called direct collapse model which provides massive seeds of 10⁵−10⁶ M_⊙. An isothermal direct collapse mandates that halos should be of a primordial composition and the formation of molecular hydrogen remains suppressed in the presence of a strong Lyman Werner flux. In this study, we perform high resolution cosmological simulations for two massive primordial halos employing a detailed chemical model which includes H⁻ cooling as well as realistic opacities for both the bound-free H⁻ emission and the Rayleigh scattering of hydrogen atoms. We are able to resolve the collapse up to unprecedentedly high densities of ∼10⁻³g/cm³ and to scales of about 10⁻⁴ AU. Our results show that the gas cools down to ∼ 5000 K in the presence of H⁻ cooling, and induces fragmentation at scales of about 8000 AU in one of the two simulated halos, which may lead to the formation of a binary. In addition, fragmentation also occurs on the AU scale in one of the halos but the clumps are expected to merge on short time scales. Our results confirm that H⁻ cooling does not prevent the formation of a supermassive star and the trapping of cooling radiation stabilises the collapse on small scales.
Submitted for publication in MNRAS, comments are welcome and high resolution version is available at http://www2.iap.fr/users/latif/DCBH.pdf

 

[marcus]

http://arxiv.org/abs/1510.03858
The thermodynamics of quantum spacetime histories
Lee Smolin
(Submitted on 13 Oct 2015)
We show that the simplicity constraints, which define the dynamics of spin foam models, imply, and are implied by, the first law of thermodynamics, when the latter is applied to causal diamonds in the quantum spacetime. This result reveals an intimate connection between the holographic nature of gravity, as reflected by the Bekenstein entropy, and the fact that general relativity and other gravitational theories can be understood as constrained topological field theories.
To state and derive this correspondence we describe causal diamonds in the causal structure of spin foam histories and generalize arguments given for the near horizon region of black holes by Frodden, Gosh and Perez and Bianchi. This allows us to apply a recent argument of Jacobson to show that if a spin foam history has a semiclassical limit described in terms of a smooth metric geometry, that geometry satisfies the Einstein equations.
These results suggest also a proposal for a quantum equivalence principle.
39 pages, 6 figures

http://arxiv.org/abs/1510.03855
Inflationary spectra with inverse-volume corrections in loop quantum cosmology and their observational constraints from Planck 2015 data
Tao Zhu, Anzhong Wang, Klaus Kirsten, Gerald Cleaver, Qin Sheng, Qiang Wu
(Submitted on 13 Oct 2015)
We derive the primordial power spectra, spectral indices and runnings of both cosmological scalar perturbations and gravitational waves in the framework of loop quantum cosmology with the inverse-volume quantum corrections. This represents an extension of our previous treatment for σ being integers to the case with any given value of σ. For this purpose, we adopt a new calculational strategy in the uniform asymptotic approximation, by expanding the involved integrals first in terms of the inverse-volume correction parameter to its first-order, a consistent requirement of the approximation of the inverse-volume corrections. In this way, we calculate explicitly the quantum gravitational corrections to the standard inflationary spectra and spectral indices to the second-order of the slow-roll parameters, and obtain the observational constraints on the inverse-volume corrections from Planck 2015 data for various values of σ. Using these constraints we discuss whether these quantum gravitational corrections lead to measurable signatures in the cosmological observations. We show that the scale-dependent contributions to inflationary spectra from the inverse-volume corrections could be well within the range of the detectability of the forthcoming generation of experiments.
19 pages, 4 figures, and 2 tableshttp://arxiv.org/abs/1510.03915
Kaluza-Klein Aspects of Noncommutative Geometry
J. Madore
(Submitted on 13 Oct 2015)
Using some elementary methods from noncommutative geometry a structure is given to a point of space-time which is different from and simpler than that which would come from extra dimensions. The structure is described by a supplementary factor in the algebra which in noncommutative geometry replaces the algebra of functions. Using different examples of algebras it is shown that the extra structure can be used to describe spin or isospin.
13 pages, published in Differential Geometric Methods in Theoretical Physics, A. I. Solomon, ed., pp. 243--252. World Scientific Publishing, 1989

http://arxiv.org/abs/1510.04154
The algebra of observables in Gaußian normal spacetime coordinates
Norbert Bodendorfer, Paweł Duch, Jerzy Lewandowski, Jędrzej Świeżewski
(Submitted on 14 Oct 2015)
We discuss the canonical structure of a spacetime version of the radial gauge, i.e. Gaussian normal spacetime coordinates. While it was found for the spatial version of the radial gauge that a "local" algebra of observables can be constructed, it turns out that this is not possible for the spacetime version. The technical reason for this observation is that the new gauge condition needed to upgrade the spatial to a spacetime radial gauge does not Poisson-commute with the previous gauge conditions. It follows that the involved Dirac bracket is inherently non-local in the sense that no complete set of observables can be found which is constructed locally and at the same time has local Dirac brackets. A locally constructed observable here is defined as a finite polynomial of the canonical variables at a given physical point specified by the Gaussian normal spacetime coordinates.
16 pages

http://arxiv.org/abs/1510.04243
The holographic principle and the Immirzi parameter of loop quantum gravity
Muhammad Sadiq
(Submitted on 14 Oct 2015)
The geometrical spectra in loop quantum gravity (LQG) suffer from ambiguity up to the free Immirzi parameter that is often determined by comparing results from the theory with the established dynamics at the black hole horizon. We address conceptual difficulties associated with such approaches and point out that the Immirzi parameter can be fixed naively by applying the LQG version of the equipartition rule at a holographic boundary such that the Hawking-Unruh temperature law follows. The value of the Immirzi parameter derived in this way should possesses universal validity. This approach also provides a clue that this parameter could be rooted in the holographic principle.
7 pages.

 

[marcus]

http://arxiv.org/abs/1510.04990
Gravitational waves from isolated systems: The phantom menace of a positive cosmological constant
Abhay Ashtekar, Béatrice Bonga, Aruna Kesavan
(Submitted on 16 Oct 2015)
There is a deep tension between the well-developed theory of gravitational waves from isolated systems and the presence of a positive cosmological constant Λ, however tiny. In particular, even the post-Newtonian quadrupole formula, derived by Einstein in 1918, has not been generalized to include a positive Λ. We first explain the principal difficulties and then show that it is possible to overcome them in the weak field limit. These results also provide concrete hints for constructing the Λ>0 generalization of the Bondi-Sachs framework for full, non-linear general relativity.
5 pages, 1 Figure

http://arxiv.org/abs/1409.3816
Asymptotics with a positive cosmological constant: I. Basic framework
Abhay Ashtekar, Beatrice Bonga, Aruna Kesavan
(Submitted on 12 Sep 2014)
The asymptotic structure of the gravitational field of isolated systems has been analyzed in great detail in the case when the cosmological constant Λ is zero. The resulting framework lies at the foundation of research in diverse areas in gravitational science. Examples include: i) positive energy theorems in geometric analysis; ii) the coordinate invariant characterization of gravitational waves in full, non-linear general relativity; iii) computations of the energy-momentum emission in gravitational collapse and binary mergers in numerical relativity and relativistic astrophysics; and iv) constructions of asymptotic Hilbert spaces to calculate S-matrices and analyze the issue of information loss in the quantum evaporation of black holes. However, by now observations have established that Λ is positive in our universe. In this paper we show that, unfortunately, the standard framework does not extend from the Λ=0 case to the Λ>0 case in a physically useful manner. In particular, we do not have positive energy theorems, nor an invariant notion of gravitational waves in the non-linear regime, nor asymptotic Hilbert spaces in dynamical situations of semi-classical gravity. A suitable framework to address these conceptual issues of direct physical importance is developed in subsequent papers.
42 pges, 5 figures. To appear in CQG

http://arxiv.org/abs/1506.06152
Asymptotics with a positive cosmological constant: II. Linear fields on de Sitter space-time
Abhay Ashtekar, Béatrice Bonga, Aruna Kesavan
(Submitted on 19 Jun 2015)
Linearized gravitational waves in de Sitter space-time are analyzed in detail to obtain guidance for constructing the theory of gravitational radiation in presence of a positive cosmological constant in full, nonlinear general relativity. Specifically: i) In the exact theory, the intrinsic geometry of $\scr{I}$ is often assumed to be conformally flat in order to reduce the asymptotic symmetry group from Diff to the de Sitter group. Our {results show explicitly} that this condition is physically unreasonable; ii) We obtain expressions of energy-momentum and angular momentum fluxes carried by gravitational waves in terms of fields defined at $\scr{I}^+$; iii) We argue that, although energy of linearized gravitational waves can be arbitrarily negative in general, gravitational waves emitted by physically reasonable sources carry positive energy; and, finally iv) We demonstrate that the flux formulas reduce to the familiar ones in Minkowski space-time in spite of the fact that the limit Λ→0 is discontinuous (since, in particular, $\scr{I}$ changes its space-like character to null in the limit).
22 pages, 4 figures. To appear in PRD

http://arxiv.org/abs/1510.05593
Asymptotics with a positive cosmological constant: III. The quadrupole formula
Abhay Ashtekar, Béatrice Bonga, Aruna Kesavan
(Submitted on 19 Oct 2015)
Almost a century ago, Einstein used a weak field approximation around Minkowski space-time to calculate the energy carried away by gravitational waves emitted by a time changing mass-quadrupole. However, by now there is strong observational evidence for a positive cosmological constant, Λ. To incorporate this fact, Einstein's celebrated derivation is generalized by replacing Minkowski space-time with de Sitter space-time. The investigation is motivated by the fact that, because of the significant differences between the asymptotic structures of Minkowski and de Sitter space-times, many of the standard techniques, including the standard 1/r expansions, can not be used for Λ>0. Furthermore since, e.g., the energy carried by gravitational waves is always positive in Minkowski space-time but can be arbitrarily negative in de Sitter space-time irrespective of how small Λ is, the limit Λ→0 can fail to be continuous. Therefore, a priori it is not clear that a small Λ would introduce only negligible corrections to Einstein's formula. We show that, while even a tiny cosmological constant does introduce qualitatively new features, in the end, corrections to Einstein's formula are negligible for astrophysical sources currently under consideration by gravitational wave observatories.
31 pages, 2 figures

http://arxiv.org/abs/1510.04896
Phenomenological investigation of a quantum gravity extension of inflation with the Starobinsky potential
Béatrice Bonga, Brajesh Gupt
(Submitted on 16 Oct 2015)
We investigate the pre-inflationary dynamics of inflation with the Starobinsky potential, favored by recent data from the Planck mission, using techniques developed to study cosmological perturbations on quantum spacetimes in the framework of loop quantum gravity. We find that for a large part of the initial data, inflation compatible with observations occurs. There exists a subset of this initial data that leads to quantum gravity signatures that are potentially observable. Interestingly, despite the different inflationary dynamics, these quantum gravity corrections to the powerspectra are similar to those obtained for inflation with a quadratic potential, including suppression of power at large scales. Furthermore, for super horizon modes the tensor modes show deviations from the standard inflationary paradigm that are unique to the Starobinsky potential and could be important for non-Gaussian modulation and tensor fossils.
30 pages, 11 figures

http://arxiv.org/abs/1510.04701
Timelike information broadcasting in cosmology
Ana Blasco, Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez
(Submitted on 15 Oct 2015)
We study the transmission of information and correlations through quantum fields in cosmological backgrounds. With this aim, we make use of quantum information tools to quantify the classical and quantum correlations induced by a quantum massless scalar field in two particle detectors, one located in the early universe (Alice's) and the other located at a later time (Bob's). In particular, we focus on two phenomena: a) the consequences on the transmission of information of the violations of the strong Huygens principle for quantum fields, and b) the analysis of the field vacuum correlations via correlation harvesting from Alice to Bob. We will study a standard cosmological model first and then assess whether these results also hold if we use other than the general relativistic dynamics. As a particular example, we will study the transmission of information through the Big Bounce, that replaces the Big Bang, in the effective dynamics of Loop Quantum Cosmology.
16 pages, 10 figures.

 

[marcus]

http://arxiv.org/abs/1510.06991
Entropy of isolated horizons from quantum gravity condensates
Daniele Oriti, Daniele Pranzetti, Lorenzo Sindoni
(Submitted on 23 Oct 2015)
We construct condensate states encoding the continuum spherically symmetric quantum geometry of an isolated horizon in full quantum gravity, i.e. without any classical symmetry reduction, in the group field theory formalism. Tracing over the bulk degrees of freedom, we show how the resulting reduced density matrix manifestly exhibits an holographic behavior. We derive a complete orthonormal basis of eigenstates for the reduced density matrix of the horizon and use it to compute the horizon entanglement entropy. By imposing consistency with the isolated horizon boundary conditions and semi-classical thermodynamical properties, we recover the Bekenstein--Hawking entropy formula for any value of the Immirzi parameter. Our analysis supports the equivalence between the von Neumann (entanglement) entropy interpretation and the Boltzmann (statistical) one.
4.5 pages

http://arxiv.org/abs/1510.07018
Asymptotic safety of gravity-matter systems
Jan Meibohm, Jan M. Pawlowski, Manuel Reichert
(Submitted on 23 Oct 2015)
We study the ultraviolet stability of gravity-matter systems for general numbers of minimally coupled scalars and fermions. This is done within the functional renormalisation group setup put forward in \cite{Christiansen:2015rva} for pure gravity. It includes full dynamical propagators and a genuine dynamical Newton's coupling, which is extracted from the graviton three-point function.
We find ultraviolet stability of general gravity-fermion systems. Gravity-scalar systems are also found to be ultraviolet stable within validity bounds for the chosen generic class of regulators, based on the size of the anomalous dimension. Remarkably, the ultraviolet fixed points for the dynamical couplings are found to be significantly different from those of their associated background counterparts, once matter fields are included. In summary, the asymptotic safety scenario does not put constraints on the matter content of the theory within the validity bounds for the chosen generic class of regulators.
18 pages, 10 figures

http://arxiv.org/abs/1510.08348
Beyond the Standard Model with noncommutative geometry, strolling towards quantum gravity
Pierre Martinetti
(Submitted on 28 Oct 2015)
Noncommutative geometry, in its many incarnations, appears at the crossroad of various researches in theoretical and mathematical physics: from models of quantum space-time (with or without breaking of Lorentz symmetry) to loop gravity and string theory, from early considerations on UV-divergencies in quantum field theory to recent models of gauge theories on noncommutative spacetime, from Connes description of the standard model of elementary particles to recent Pati-Salam like extensions. We list several of these applications, emphasizing also the original point of view brought by noncommutative geometry on the nature of time.
This text serves as an introduction to the volume of proceedings of the parallel session "Noncommutative geometry and quantum gravity", as a part of the conference "Conceptual and technical challenges in quantum gravity" organized at the University of Rome "La Sapienza" in September 2014.
8 pages, 2 figures.

http://arxiv.org/abs/1510.07883
Heavy neutrinos in particle physics and cosmology
Marco Drewes
(Submitted on 27 Oct 2015)
Neutrinos are the only particles in the Standard Model of particle physics that have only been observed with left handed chirality to date. If right handed neutrinos exist, they would not only explain the observed neutrino oscillations, but could also be responsible for several phenomena in cosmology, including the baryon asymmetry of the universe, dark matter and dark radiation. A crucial parameter in this context is their Majorana mass, which in principle could lie anywhere between the eV scale and GUT scale. The implications for experiments and cosmology strongly depend on the choice of the mass scale. We review recent progress in the phenomenology of right handed neutrinos with different masses, focusing on scenarios in which the mass is at least a keV. We emphasise the possibility to discover heavy neutrinos that are responsible for the baryon asymmetry of the universe via low scale leptogenesis in near future experiments, such as LHC, BELLE II, SHiP, FCC-ee or CEPC.
6 pages, one figure. Contribution to the proceedings of The European Physical Society Conference on High Energy Physics 2015 (EPS-HEP2015) in Vienna.

http://arxiv.org/abs/1510.06764
On the maximal efficiency of the collisional Penrose process
Elly Leiderschneider, Tsvi Piran
(Submitted on 22 Oct 2015)
The center of mass (CM) energy in a collisional Penrose process - a collision taking place within the ergosphere of a Kerr black hole - can diverge under suitable extreme conditions (maximal Kerr, near horizon collision and suitable impact parameters). We present an analytic expression for the CM energy, refining expressions given in the literature. Even though the CM energy diverges, we show that the maximal energy attained by a particle that escapes the black hole's gravitational pull and reaches infinity is modest. We obtain an analytic expression for the energy of an escaping particle resulting from a collisional Penrose process, and apply it to derive the maximal energy and the maximal efficiency for several physical scenarios: pair annihilation, Compton scattering, and the elastic scattering of two massive particles. In all physically reasonable cases (in which the incident particles initially fall from infinity towards the black hole) the maximal energy (and the corresponding efficiency) are only one order of magnitude larger than the rest mass energy of the incident particles. The maximal efficiency found is ≈13.92 and it is obtained for the scattering of an outgoing massless particle by a massive particle.
14 pages, 9 figures.

curious MNRAS article, not QG, might be of interest to cosmologists:
http://arxiv.org/abs/1507.00675
A giant ring-like structure at 0.78<z<0.86 displayed by GRBs
L. G. Balazs, Z. Bagoly, J. E. Hakkila, I. Horvath, J. Kobori, I. Racz, L. V.Toth
(Submitted on 2 Jul 2015)
According to the cosmological principle, Universal large-scale structure is homogeneous and isotropic. The observable Universe, however, shows complex structures even on very large scales. The recent discoveries of structures significantly exceeding the transition scale of 370 Mpc pose a challenge to the cosmological principle.
We report here the discovery of the largest regular formation in the observable Universe; a ring with a diameter of 1720 Mpc, displayed by 9 gamma ray bursts (GRBs), exceeding by a factor of five the transition scale to the homogeneous and isotropic distribution. The ring has a major diameter of 43^o and a minor diameter of 30^o at a distance of 2770 Mpc in the 0.78<z<0.86 redshift range, with a probability of 2×10⁻⁶ of being the result of a random fluctuation in the GRB count rate.
Evidence suggests that this feature is the projection of a shell onto the plane of the sky. Voids and string-like formations are common outcomes of large-scale structure. However, these structures have maximum sizes of 150 Mpc, which are an order of magnitude smaller than the observed GRB ring diameter. Evidence in support of the shell interpretation requires that temporal information of the transient GRBs be included in the analysis.
This ring-shaped feature is large enough to contradict the cosmological principle. The physical mechanism responsible for causing it is unknown.
13 pages, 8 figures and 4 tables
http://mnras.oxfordjournals.org/content/452/3/2236.abstract
My reaction: might just be apparent pattern in random distribution of GRBs

 

[marcus]

http://arxiv.org/abs/1510.08766
Observational Exclusion of a Consistent Quantum Cosmological Scenario
Boris Bolliet, Aurelien Barrau, Julien Grain, Susanne Schander
(Submitted on 29 Oct 2015)
It is often argued that inflation erases all the information about what took place before it started. Quantum gravity, relevant in the Planck era, seems therefore mostly impossible to probe with cosmological observations. In general, only very ad hoc scenarios or hyper fine-tuned initial conditions can lead to observationally testable theories. Here we consider a well-defined and well motivated candidate quantum cosmology model that predicts inflation. Using the most recent observational constraints on the cosmic microwave background B modes, we show that the model is excluded for all its parameter space, without any tuning. Some important consequences are drawn for the deformed algebra approach to loop quantum cosmology. We emphasize that neither loop quantum cosmology in general nor loop quantum gravity are disfavored by this study but their falsifiability is established.
5 pages, 1 figure

http://arxiv.org/abs/1510.08828
Non-singular rotating black hole with a time delay in the center
Tommaso De Lorenzo, Andrea Giusti, Simone Speziale
(Submitted on 29 Oct 2015)
As proposed by Bambi and Modesto, rotating non-singular black holes can be constructed via the Newman-Janis algorithm. Here we show that if one starts with a modified Hayward black hole with a time delay in the centre, the algorithm succeeds in producing a rotating metric, but curvature divergences reappear. To preserve finiteness, the time delay must be introduced directly at the level of the non-singular rotating metric. This is possible thanks to the deformation of the inner stationarity limit surface caused by the regularisation, and in more than one way. We outline three different possibilities, distinguished by the angular velocity of the event horizon. Along the way, we provide additional results on the Bambi-Modesto rotating Hayward metric, such as the structure of the regularisation occurring at the centre, the behaviour of the quantum gravity scale alike an electric charge in decreasing the angular momentum of the extremal black hole configuration, or details on the deformation of the ergosphere.
15 pages, many figures

http://arxiv.org/abs/1510.08706
Discrete quantum geometries and their effective dimension
Johannes Thürigen
(Submitted on 29 Oct 2015)
In several approaches towards a quantum theory of gravity, such as group field theory and loop quantum gravity, quantum states and histories of the geometric degrees of freedom turn out to be based on discrete spacetime. The most pressing issue is then how the smooth geometries of general relativity, expressed in terms of suitable geometric observables, arise from such discrete quantum geometries in some semiclassical and continuum limit. In this thesis I tackle the question of suitable observables focusing on the effective dimension of discrete quantum geometries. For this purpose I give a purely combinatorial description of the discrete structures which these geometries have support on. As a side topic, this allows to present an extension of group field theory to cover the combinatorially larger kinematical state space of loop quantum gravity. Moreover, I introduce a discrete calculus for fields on such fundamentally discrete geometries with a particular focus on the Laplacian. This permits to define the effective-dimension observables for quantum geometries. Analysing various classes of quantum geometries, I find as a general result that the spectral dimension is more sensitive to the underlying combinatorial structure than to the details of the additional geometric data thereon. Semiclassical states in loop quantum gravity approximate the classical geometries they are peaking on rather well and there are no indications for stronger quantum effects. On the other hand, in the context of a more general model of states which are superposition over a large number of complexes, based on analytic solutions, there is a flow of the spectral dimension from the topological dimension d on low energy scales to a real number 0<α<d on high energy scales. In the particular case of α=1 these results allow to understand the quantum geometry as effectively fractal.
PhD thesis, Humboldt-Universit Berlin this http URL

http://arxiv.org/abs/1510.08719
The effective action in four-dimensional CDT
Jakub Gizbert-Studnicki
(Submitted on 29 Oct 2015)
We present recent results concerning the measurement and analysis of the effective action in four-dimensional Causal Dynamical Triangulations. The action describes quantum fluctuations of the spatial volume of the CDT universe (or alternatively the scale factor) after integrating out other degrees of freedom. We use the covariance of volume fluctuations to measure and parametrize the effective action inside the de Sitter phase, also called the C phase. We show that the action is consistent with a simple discretization of the minisuperspace action (with a reversed overall sign). We discuss possible subleading corrections and show how to construct a more complicated effective action comprising both integer and half-integer discrete proper time layers. We introduce a new method of the effective action measurement based on the transfer matrix. We show that the results of the new method are fully consistent with the covariance matrix method inside the de Sitter phase. We use the new method to measure the effective action in the small volume range and to explain the behaviour of the stalk part of the CDT triangulations. Finally we use the transfer matrix method to measure and parametrize the effective action inside the A and B phases, and to analyze the phase transitions. The results lead to an unexpected discovery of a new "bifurcation" phase separating the "old" C and B phases. We analyze geometric properties of triangulations inside the new phase and draw a new phase diagram.
133 pages. PhD thesis, Jagiellonian University, July 2014

possible interest, I don't think Eardley's instability (1974) applies in the case of LQG BH bounce where critical density has been achieved and quantum corrections make gravity repel, but the authors apply it anyway:
http://arxiv.org/abs/1511.00633
Black holes turn white fast, otherwise stay black: no half measures
Carlos Barceló, Raúl Carballo-Rubio, Luis J. Garay
(Submitted on 2 Nov 2015)
Recently, various authors have proposed that the first ultraviolet effect on the gravitational collapse of massive stars to black holes is the transition between a black-hole geometry and a white-hole geometry, though their proposals are radically different in terms of their physical interpretation and characteristic time scales [1,2]. Several decades ago, it was shown by Eardley that white holes are highly unstable to the accretion of small amounts of matter, being rapidly turned into black holes [3]. Studying the crossing of null shells on geometries describing the black to white hole transition, we obtain the conditions for the instability to develop in terms of the parameters of these geometries. We conclude that transitions with long characteristic time scales are pathologically unstable: occasional perturbations away from the perfect vacuum around these compact objects, even if being imperceptibly small, suffocate the white hole explosion. On the other hand, geometries with short characteristic time scales are shown to be robust against perturbations, so that the corresponding processes could take place in real astrophysical scenarios. This motivates a conjecture about the transition amplitudes of different decay channels for black holes in a suitable ultraviolet completion of general relativity.
24 pages, 3 figures

 

[Raul Carballo]

possible interest, I don't think Eardley's instability (1974) applies in the case of LQG BH bounce where critical density has been achieved and quantum corrections make gravity repel, but the authors apply it anyway:
http://arxiv.org/abs/1511.00633
Black holes turn white fast, otherwise stay black: no half measures
Carlos Barceló, Raúl Carballo-Rubio, Luis J. Garay
(Submitted on 2 Nov 2015)
Recently, various authors have proposed that the first ultraviolet effect on the gravitational collapse of massive stars to black holes is the transition between a black-hole geometry and a white-hole geometry, though their proposals are radically different in terms of their physical interpretation and characteristic time scales [1,2]. Several decades ago, it was shown by Eardley that white holes are highly unstable to the accretion of small amounts of matter, being rapidly turned into black holes [3]. Studying the crossing of null shells on geometries describing the black to white hole transition, we obtain the conditions for the instability to develop in terms of the parameters of these geometries. We conclude that transitions with long characteristic time scales are pathologically unstable: occasional perturbations away from the perfect vacuum around these compact objects, even if being imperceptibly small, suffocate the white hole explosion. On the other hand, geometries with short characteristic time scales are shown to be robust against perturbations, so that the corresponding processes could take place in real astrophysical scenarios. This motivates a conjecture about the transition amplitudes of different decay channels for black holes in a suitable ultraviolet completion of general relativity.
24 pages, 3 figures

Dear marcus,
Many thanks for posting our paper. As you can read in the body of the text, we have been very careful in using arguments that only rely on the properties of regions of spacetime in which general relativity is expected to hold. The subsequent results are independent of the fine details (and the nature) of the regularization that leads to the bounce. That using this approach permits to obtain nontrivial model-independent conclusions is one of the outcomes we are reporting.

 

[marcus]

Dear marcus,
Many thanks for posting our paper. As you can read in the body of the text, we have been very careful in using arguments that only rely on the properties of regions of spacetime in which general relativity is expected to hold. The subsequent results are independent of the fine details (and the nature) of the regularization that leads to the bounce. That using this approach permits to obtain nontrivial model-independent conclusions is one of the outcomes we are reporting.

Dear Raul, glad you noticed we picked up your paper in our informal QG bibliography. Maybe someone will start a discussion thread about it or a related topic. Feel free to start one if you'd like.

I tend to avoid discussion in the biblio context (don't want to crowd the bibliography) and mostly just add titles. Here's one that came up today:

http://arxiv.org/abs/1511.02085
Discrepancies between observational data and theoretical forecast in single field slow roll inflation
Jaume Amorós, Jaume de Haro
(Submitted on 6 Nov 2015)
The PLANCK collaboration has determined values for the spectral parameters of the CMB radiation, namely the spectral index ns, its running αs, the running of the running βs, using a growing body of measurements of CMB anisotropies by the Planck satellite and other missions. These values do not follow the hierarchy of sizes predicted by single field, slow roll inflationary theory, and are thus difficult to fit for such inflation models.
In this work we present first a study of 49 single field, slow roll inflationary potentials in which we assess the likelyhood of these models fitting the spectral parameters to their currently most accurate determination given by the PLANCK collaboration. We check numerically with a MATLAB program the spectral parameters that each model can yield for a very broad, comprehensive list of possible parameter and field values. The comparison of spectral parameter values supported by the models with their determinations by the PLANCK collaboration leads to the conclusion that the data provided by PLANCK2015 TT+lowP and PLANCK2015 TT,TE,EE+lowP taking into account the running of the running disfavours 40 of the 49 models with confidence level at least 92.8\%.
Next, we discuss the reliability of the current computations of these spectral parameters. We identify a bias in the method of determination of the spectral parameters by least residue parameter fitting (using MCMC or any other scheme) currently used to reconstruct the power spectrum of scalar perturbations. This bias can explain the observed contradiction between theory and observations. Its removal is computationally costly, but necessary in order to compare the forecasts of single field, slow roll theories with observations.
17 pages, 1 figure.

here's something which though not QG-related might be of general interest as it may lead to new results in observational cosmology:
http://arxiv.org/abs/1511.02232
Flaring of tidally compressed dark-matter clumps
Yacine Ali-Haïmoud, Ely D. Kovetz, Joseph Silk
(Submitted on 6 Nov 2015)
We explore the physics and observational consequences of tidal compression events (TCEs) of dark-matter clumps (DMCs) by supermassive black holes (SMBHs). Our analytic calculations show that a DMC approaching a SMBH much closer than the tidal radius undergoes significant compression along the axis perpendicular to the orbital plane, shortly after pericenter passage. For DMCs composed of self-annihilating dark-matter particles, we find that the boosted DMC density and velocity dispersion lead to a flaring of the annihilation rate, most pronounced for a velocity- dependent annihilation cross section. If the end products of the annihilation are photons, this results in a gamma-ray flare, detectable (and possibly already detected) by the Fermi telescope for a range of model parameters. If the end products of dark-matter annihilation are relativistic electrons and positrons and the local magnetic field is large enough, TCEs of DMCs can lead to flares of synchrotron radiation. Finally, TCEs of DMCs lead to a burst of gravitational waves, in addition to the ones radiated by the orbital motion alone, and with a different frequency spectrum. These transient phenomena provide interesting new avenues to explore the properties of dark matter.
11 pages, 6 figures. To be submitted to PRD
http://inspirehep.net/record/1403490?ln=en
http://inspirehep.net/author/profile/J.I.Silk.1
(Inspire database has an error. The author is J.I.Silk, see his profile)

another interesting one in today's batch, just a brief review:
http://arxiv.org/abs/1511.02588
The evolution of high-redshift massive black holes
Marta Volonteri, Melanie Habouzit, Fabio Pacucci, Michael Tremmel
(Submitted on 9 Nov 2015)
Massive black holes (MBHs) are nowadays recognized as integral parts of galaxy evolution. Both the approximate proportionality between MBH and galaxy mass, and the expected importance of feedback from active MBHs in regulating star formation in their host galaxies point to a strong interplay between MBHs and galaxies. ...
... While current observational data only probe the most massive and luminous MBHs, the tip of the iceberg, we will soon be able to test theoretical models of MBH evolution on more "normal" MBHs: the MBHs that are indeed relevant in building the population that we observe in local galaxies, including our own Milky Way.
8 pages. Brief review to appear in Galaxies at High Redshift and Their Evolution over Cosmic Time, Proceedings IAU Symposium No. 319, 2015

in case there's interest in an online sophomore level textbook in Special Rel:
http://arxiv.org/abs/1511.02121
Theory of Special Relativity
Nadia L. Zakamska (Johns Hopkins University)
(Submitted on 4 Nov 2015)
Special Relativity is taught to physics sophomores at Johns Hopkins University in a series of eight lectures. Lecture 1 covers the principle of relativity and the derivation of the Lorentz transform. Lecture 2 covers length contraction and time dilation. Lecture 3 covers Minkowski diagrams, simultaneous events and causally connected events, as well as velocity transforms. Lecture 4 covers energy and momentum of particles and introduces 4-vectors. Lecture 5 covers energy and momentum of photons and collision problems. Lecture 6 covers Doppler effect and aberration. Lecture 7 covers relativistic dynamics. Optional Lecture 8 covers field transforms. The main purpose of these notes is to introduce 4-vectors and the matrix notation and to demonstrate their use in solving standard problems in Special Relativity. The pre-requisites for the class are calculus-based Classical Mechanics and Electricity & Magnetism, and Linear Algebra is highly recommended.
98 pages with color figures and hyperlinks to online materials. The author will appreciate reports of typos and other problems. This textbook is not being published elsewhere

 

[marcus]

http://arxiv.org/abs/1511.03644
Hessian and graviton propagator of the proper vertex
Atousa Chaharsough Shirazi, Jonathan Engle, Ilya Vilensky
(Submitted on 11 Nov 2015)
The proper spin-foam vertex amplitude is obtained from the EPRL vertex by projecting out all but a single gravitational sector, in order to achieve correct semi-classical behavior. In this paper we calculate the gravitational two-point function predicted by the proper spin-foam vertex to lowest order in the vertex expansion. We find the same answer as in the EPRL case in the `continuum spectrum' limit, so that the theory is consistent with the predictions of linearized gravity in the regime of small curvature. The method for calculating the two-point function is similar to that used in prior works: we cast it in terms of an action integral and to use stationary phase methods. Thus, the calculation of the Hessian matrix plays a key role. Once the Hessian is calculated, it is used not only to calculate the two-point function, but also to calculate the coefficient appearing in the semi-classical limit of the proper vertex amplitude itself. This coefficient is the effective discrete "measure factor" encoded in the spin-foam model. Through a non-trivial cancellation of different factors, we find that this coefficient is the same as the coefficient in front of the term in the asymptotics of the EPRL vertex corresponding to the selected gravitational sector.
20 pages

http://arxiv.org/abs/1511.03278
Planckian Interacting Massive Particles as Dark Matter
Mathias Garny, McCullen Sandora, Martin S. Sloth
(Submitted on 10 Nov 2015)
The Standard Model could be self-consistent up to the Planck scale according to the present measurements of the Higgs mass and top quark Yukawa coupling. It is therefore possible that new physics is only coupled to the Standard Model through Planck suppressed higher dimensional operators. In this case the WIMP miracle is a mirage, and instead minimality as dictated by Occam's razor would indicate that dark matter is related to the Planck scale, where quantum gravity is anyway expected to manifest itself. Assuming within this framework that dark matter is a Planckian Interacting Massive Particle, we show that the most natural mass larger than 0.01M_p is already ruled out by the absence of tensor modes in the CMB. This also indicates that we expect tensor modes in the CMB to be observed soon for this type of minimal dark matter model. Finally, we touch upon the KK graviton mode as a possible realization of this scenario within UV complete models, as well as further potential signatures and peculiar properties of this type of dark matter candidate. This paradigm therefore leads to a subtle connection between quantum gravity, the physics of primordial inflation, and the nature of dark matter.
5 pages, 1 figure

http://arxiv.org/abs/1511.03076
Current observations with a decaying cosmological constant allow for chaotic cyclic cosmology
George F.R. Ellis, Emma Platts, David Sloan, Amanda Weltman
(Submitted on 10 Nov 2015)
We use the phase plane analysis technique of Madsen and Ellis to consider a universe with a true cosmological constant as well as a cosmological "constant" that is decaying. Time symmetric dynamics for the inflationary era allows eternally bouncing models to occur. Allowing for scalar field dynamic evolution, we find that if dark energy decays in the future, chaotic cyclic universes exist provided the spatial curvature is positive. This is particularly interesting in light of current observations which do not yet rule out either closed universes or possible evolution of the cosmological constant. We present only a proof of principle, with no definite claim on the physical mechanism required for the present dark energy to decay.
23 pages, 11 figures
(see page 18 in conclusions where LQC cyclic enters discussion as an alternative)

very strange paper, sounds to me as if it might be too good to be true:
http://arxiv.org/abs/1511.03396
Von Neumann's Quantization of General Relativity
A.B. Arbuzov, A.Yu. Cherny, D.J. Cirilo-Lombardo, R.G. Nazmitdinov, Nguyen Suan Han, A.E. Pavlov, V.N. Pervushin, A.F. Zakharov
(Submitted on 11 Nov 2015)
Von Neumann's procedure is applied for quantization of General Relativity. We quantize the initial data of dynamical variables at the Planck epoch, where the Hubble parameter coincides with the Planck mass. These initial data are defined via the Fock simplex in the tangent Minkowskian space-time, the Dirac conformal interval. The Einstein cosmological principle is applied for the average of the spatial metric determinant logarithm over the spatial volume of the visible Universe. We derive the splitting of the general coordinate transformations into the diffeomorphisms (as the object of the second Noether theorem) and the initial data transformations (as objects of the first Noether theorem). Following von Neumann, we suppose that the vacuum state is a quantum ensemble. The vacuum state is degenerated with respect to quantum numbers of non-vacuum states with the distribution function that yields the Casimir effect in gravidynamics in analogy to the one in electrodynamics. The generation functional of the perturbation theory in gravidynamics is given as a solution of the quantum energy constraint. We discuss the region of applicability of gravidynamics and its possible predictions for explanation of the modern observational and experimental data.
12 pages

 

[marcus]

The International LQG Seminar talks for the current semester are a useful resource, providing a window on current research interests and recent results. Here are the links to audio and slides PDF. It's convenient to first get the slides in a separate window before you start the audio. The speakers usually indicate when they move to the next slide--making it easy to follow the talk online.
ILQGS still has two more talks coming up, see the schedule here:
http://relativity.phys.lsu.edu/ilqgs/
Nov. 24 On deparametrized models in LQG Mehdi Assanioussi Univ Warszaw
Dec. 8 Quantum symmetry reductions based on classical gauge fixings Norbert Bodendorfer Univ. Warszaw
Here are the talks that have already been presented and are available online:
==============================
Tuesday, Nov. 10th
Jedrzej Swiezewski, Univ. Warszaw
Title: Developments on the radial gauge
PDF of the talk (4MB)
Audio [.mp3 40MB]
Tuesday, Oct. 27th
Eyo Eyo Ita, USNA
Title: Intrinsic time in quantum geometrodynamics
PDF of the talk (350k)
Audio [.wav 20MB]
Tuesday, Oct. 13th
Ivan Agulló, LSU
Title: Loop Quantum Cosmology, Non-Gaussianity, and CMB anomalies
PDF of the talk (8MB)
Audio [.mp3 23MB]
Tuesday, Sept. 29th
Eugenio Bianchi, PennState
Title: Squeezed spin-networks and entanglement
PDF of the talk (7MB)
Audio [.mp3 30MB]
Tuesday, Sept. 15th
Bianca Dittrich, Perimeter Institute
Title: 3D holography
PDF of the talk (15MB)
Audio [.wav 38MB]
Tuesday, Sept. 1st
Benjamin Bahr, DESY
Title: Numerical investigations of the EPRL model
PDF of the talk (2MB)
Audio [.wav 41MB]
======================
An unrelated matter---this paper was published in Astrophysics and Space Science but was not posted on arxiv. It's behind paywall. I'd be interested to know if anyone with institutional access to that journal has looked at it.
http://link.springer.com/article/10.1007/s10509-015-2553-7
Quantum gravitational corrections to the Friedmann equations in FRW universe
Mehdi Dehghani
(Online: 09 November 2015)
Abstract The possible corrections to the entropy and Friedmann equations in the 4-dimensional Friedmann-Robertson-Walker (FRW) universe have been investigated within the generalized uncertainty principle (GUP), modified dispersion relation (MDR) and loop quantum gravity (LQG) separately. The corrected entropies stem from GUP, MDR and LQG analysis have been used separately to obtain the quantum gravitational corrections to the Friedmann equations in FRW universe. The explicit form of the corrections are worked out up to the sixth power of the Planck length. Through comparison of the corrections obtained from GUP, MDR and LQG approaches it is found that the results of these alternative approaches should be identical if one uses the suitable expansion coefficients.
Published Astrophysics and Space Science December 2015, 360:45

 

[Leef]

OK no idea at the level you guys are talking but I am sure you can simply answer my question in 2 or 3 lines. I have not checked back with the physics world for 5 years or more. Started when I was 12. I see clearly in my head the direct connection / cause for inertial force and gravity with a direct undeniable common thread. Seems like it should be good from Black holes to subatomic. Is there still no excepted understanding of these forces? I can't say more or I would be speculating and well had one warning on that. Maybe I am just a dreamer.

 

[marcus]

Leef, it's fine to be a dreamer if so you happen to be. But if you want to stay active at PF you have to develop different skills. The surest way to proceed is to find some current research paper(s) that you find interesting and which you think fit in somehow with your thinking, and start a discussion thread by:
(1) giving a link to one of the papers.
(2) asking a question
Or maybe the question comes first and then you give the link.

All the new physics and cosmology papers (99% anyway) are on arXiv.org

 

[marcus]

This thread is a bibliography thread, meaning that we don't ordinarily have Q and A or discussion. We just log online resources as they become available, mainly research papers. Almost entirely research papers, in fact, plus occasionally some seminar talks. It you want to DISCUSS any of the papers, the thing to do is to start a thread specifically about that paper in the appropriate forum.

For example back a ways on the previous page, if you scroll up, you see this paper listed:
http://arxiv.org/abs/1510.00699
A Perfect Bounce
Steffen Gielen, Neil Turok
(Submitted on 2 Oct 2015)
We study the quantum cosmology of a universe with conformal matter comprising a perfect radiation fluid and a number of conformally coupled scalar fields. For FRW backgrounds, we are able to perform the quantum gravity path integral exactly. We find the evolution to describe a "perfect bounce," in which the universe passes smoothly through the singularity. The Feynman path integral amplitude is precisely that of a relativistic oscillator, for which the scale factor of the universe is the time and the scalar fields are the spatial coordinates. This picture provides natural, unitary quantum mechanical evolution across a bounce. We also study the quantum evolution of anisotropies and of inhomogeneous perturbations, at linear and nonlinear order. We provide evidence for a semiclassical description in which all fields pass "around" the cosmological singularity along complex classical paths.
5 pages.
Neil Turok is director of the Perimeter Institute. The idea is a singularity never actually happens, the universe is contracting, it rebounds at extreme density, and becomes the expanding universe that we see and live in. In my view it's an interesting idea. If someone wanted to discuss it they could read whatever parts of the paper they could understand, and think of a question, and start a thread in COSMOLOGY forum. That would be the appropriate forum, I think.

 

[Leef]

Thanks Marcus, good advice. I get it PF has become a valued dependable reference for me. Nice job on keeping the Pseudoscience out. So give me a little heads up on this. I came from a world of engineering, patentable things, ladder climbers, back stabber claiming it was there idea first …. Politics of business. Do you get pretty much the same thing in high level physics?

 

[marcus]

Let's not continue digressing here in this particular thread. It's just for bibliography---i.e. listing new research papers etc. You could start a thread in general discussion forum about priority fights in the Academic world. Has anyone ever witnessed one? Do they ever turn nasty? Etc etc.
It has to do not so much with money as with honor and reputation. Let's stop here so this thread does not get loaded with off-topic stuff. BTW I'm not a physicist, just a retired guy who likes to follow developments in QG and cosmology research.

 

[marcus]

http://arxiv.org/abs/1511.05441
3D holography: from discretum to continuum
Valentin Bonzom, Bianca Dittrich
(Submitted on 17 Nov 2015)
We study the one-loop partition function of 3D gravity without cosmological constant on the solid torus with arbitrary metric fluctuations on the boundary. To this end we employ the discrete approach of (quantum) Regge calculus. In contrast with similar calculations performed directly in the continuum, we work with a boundary at finite distance from the torus axis. We show that after taking the continuum limit on the boundary - but still keeping finite distance from the torus axis - the one-loop correction is the same as the one recently found in the continuum in Barnich et al. for an asymptotically flat boundary. The discrete approach taken here allows to identify the boundary degrees of freedom which are responsible for the non-trivial structure of the one-loop correction. We therefore calculate also the Hamilton-Jacobi function to quadratic order in the boundary fluctuations both in the discrete set-up and directly in the continuum theory. We identify a dual boundary field theory with a Liouville type coupling to the boundary metric. The discrete set-up allows again to identify the dual field with degrees of freedom associated to radial bulk edges attached to the boundary. Integrating out this dual field reproduces the (boundary diffeomorphism invariant part of the) quadratic order of the Hamilton-Jacobi functional. The considerations here show that bulk boundary dualities might also emerge at finite boundaries and moreover that discrete approaches are helpful in identifying such dualities.
42 pages

http://arxiv.org/abs/1511.04316
Particle creation rate for general black holes
Javad T. Firouzjaee, George F R Ellis
(Submitted on 13 Nov 2015)
We present the particle creation probability rate around a general black hole as an outcome of quantum fluctuations. Using the uncertainty principle for these fluctuation, we derive a new ultraviolet frequency cutoff for the radiation spectrum of a dynamical black hole. Using this frequency cutoff, we define the probability creation rate function for such black holes. We consider a dynamical Vaidya model, and calculate the probability creation rate for this case when its horizon is in a slowly evolving phase. Our results show that one can expect the usual Hawking radiation emission process in the case of a dynamical black hole when it has a slowly evolving horizon. Moreover, calculating the probability rate for a dynamical black hole gives a measure of when Hawking radiation can be killed off by an incoming flux of matter or radiation. Our result strictly suggests that we have to revise the Hawking radiation expectation for primordial black holes that have grown substantially since they were created in the early universe.
10 pages, 1 figure

http://arxiv.org/abs/1511.04427
Singularities, horizons, firewalls, and local conformal symmetry
Gerard 't Hooft
(Submitted on 13 Nov 2015)
The Einstein-Hilbert theory of gravity can be rephrased by focusing on local conformal symmetry as an exact, but spontaneously broken symmetry of nature. The conformal component of the metric field is then treated as a dilaton field with only renormalizable interactions. This imposes constraints on the theory, which can also be viewed as demanding regularity of the action as the dilaton field variable tends to 0. In other words, we have constraints on the small distance behaviour. Our procedure appears to turn a black hole into a regular, topologically trivial soliton without singularities, horizons or firewalls, but many questions remain.
12 pages (incl. title page), 3 figures. Plenary lecture presented at the 2nd Karl Schwarzschild Meeting on Gravitational Physics, Frankfurt, July 23, 2015.

possible side interest:
http://arxiv.org/abs/1511.06176
Quantum Thermodynamics, Entropy of the Universe, Free Energy, and the Second Law
George L. Barnes, Michael E. Kellman
(Submitted on 19 Nov 2015)
We take the view that the standard von Neumann definition, in which the entropy S^vN of a pure state is zero, is in evident conflict with the statement of the second law that the entropy of the universe S_univ increases in spontaneous processes, ΔS_univ>0. Here we seek an alternative entropy of the universe S_univ that is in accord with the second law, in a spirit not dissimilar to von Neumann himself in lesser-known work. We perform simulations of time dependent dynamics for a previously developed [1] model quantum system becoming entangled with a quantum environment. We test the new definition of the entropy of the system-environment "universe" against the standard thermodynamic relation ΔF_sys=−TΔS_univ, calculating system properties using the reduced density matrix and standard von Neumann entropy. Good agreement is obtained, showing the compatibility of an entropy for a pure state of a universe with the statement of the second law and the concept of free energy. Interesting deviation from microcanonical behavior within the zero order energy shell is observed in a context of effectively microcanonical behavior within the much larger total basis of the time dependent universe.
24 pages, 4 figures, and 1 table

http://arxiv.org/abs/1511.04256
Quantum estimation of physical parameters in the spacetime of a rotating planet
Jan Kohlrus, David Edward Bruschi, Jorma Louko, Ivette Fuentes
(Submitted on 13 Nov 2015)
We employ quantum estimation techniques to obtain ultimate bounds on precision measurements of gravitational parameters of the spacetime outside a rotating planet. Spacetime curvature affects the frequency distribution of a photon sent from Earth to a satellite, and this change encodes parameters of the spacetime. This allows us to achieve precise measurements of parameters of Earth such as its Schwarzschild radius and equatorial angular velocity. We then are able to provide a comparison with the state-of-the-art in parameter estimation obtained through classical means. Extensions and future directions are also discussed.
7 pages, 1 figure.

http://arxiv.org/abs/1511.04297
Calculating the Potato Radius of Asteroids using the Height of Mt. Everest
M. E. Caplan
(Submitted on 7 Nov 2015)
At approximate radii of 200-300 km, asteroids transition from oblong `potato' shapes to spheres. This limit is known as the Potato Radius, and has been proposed as a classification for separating asteroids from dwarf planets. The Potato Radius can be calculated from first principles based on the elastic properties and gravity of the asteroid. Similarly, the tallest mountain that a planet can support is also known to be based on the elastic properties and gravity. In this work, a simple novel method of calculating the Potato Radius is presented using what is known about the maximum height of mountains and Newtonian gravity for a spherical body. This method does not assume any knowledge beyond high school level mechanics, and may be appropriate for students interested in applications of physics to astronomy.

to look at later:
http://arxiv.org/abs/1511.05205

 

[marcus]

http://arxiv.org/abs/1511.09084
Polchinski's exact renormalisation group for tensorial theories: Gaussian universality and power counting
Thomas Krajewski, Reiko Toriumi
(Submitted on 29 Nov 2015)
In this paper, we use the exact renormalisation in the context of tensor models and tensorial group field theories. As a byproduct, we rederive Gaussian universality for random tensors and provide a general power counting for Abelian tensorial field theories with a closure constraint, leading us to a only five renormalizable theories.
22 pages, 4 figures

http://arxiv.org/abs/1511.07332
Towards self dual Loop Quantum Gravity
Jibril Ben Achour
(Submitted on 23 Nov 2015)
In this PhD thesis, we introduced a new strategy to investigate the kinematical and physical predictions of self dual Loop Quantum Gravity (LQG) and by-passed the old problem of implementing quantum mechanically the so called reality conditions inherent to the self dual Ashtekar's phase space.
We first review the loop quantization of the spherically isolated horizon and the computation of its micro-canonical entropy. Then we present the so called gas of punctures model for the quantum horizon, discussing its results in the canonical and grand-canonical ensembles and its limits.
The fourth chapter is devoted to studying to what extend the loop quantization based on the self dual variables could cure those problems. We introduce a new strategy, based on an analytic continuation of the degeneracy from γ∈R to γ=±i. We review in details the construction of the procedure, and present the results. At the leading term, we recover exactly the Bekenstein-Hawking area law.
The fifth chapter is devoted to understanding more precisely the interplay between the status of the Immirzi parameter. In order to do this, we introduce from a new toy model describing 2+1 gravity which depends explicitly on the Immirzi parameter.
Finally, the sixth chapter is devoted to applying our procedure to the simplest Loop Quantum Cosmology model. By first constructing the LQC dynamics for any arbitrary spin j and then implementing our analytic continuation, we show that our procedure preserves the key features of the LQC models, i.e. we obtain a bouncing universe which admits the right semi classical limit after the bounce.
PhD Thesis - 254 pages

http://arxiv.org/abs/1511.06460
Thermodynamic Origin of the Null Energy Condition
Maulik Parikh, Andrew Svesko
(Submitted on 20 Nov 2015)
We derive the classical null energy condition, understood as a constraint on the Ricci tensor, from the second law of thermodynamics applied to Bekenstein-Hawking entropy. The derivation provides evidence that the null energy condition, which has usually been regarded as a condition on matter, is fundamentally a property of gravity.
9 pages.

http://arxiv.org/abs/1511.06560
Essential nature of Newton's constant in unimodular gravity
Dario Benedetti
(Submitted on 20 Nov 2015)
We point out that in unimodular gravity Newton's constant is an essential coupling, i.e. it is independent of field redefinitions. We illustrate the consequences of this fact by a calculation in a standard simple approximation, showing that in this case the renormalization group flow of Newton's constant is gauge and parametrization independent.
15 pages

http://arxiv.org/abs/1511.09197
Emergence of Spacetime in Quantum Shape Dynamics
Furkan Semih Dündar, Baris Tamer Tonguc
(Submitted on 30 Nov 2015)
We study kinematics of atoms and molecules in quantum shape dynamics. We analyzed a model universe where there is only electrical force between protons and electrons. In ref. \cite{sd-prl} a similar model where there is only gravitational attraction between masses is investigated. Our results is an expansion of the ideas there. We found that hydrogen atoms can form when they are entangled in singlet pairs. On the other hand if there is single hydrogen atom in the universe, it occupies the entire universe. What is more, in the investigation of hydrogen molecule, we found that absolute spacetime emerges by coarse graining the quantum degrees of freedom. It may be that spacetime only exists because of the presence of quantum degrees of freedom. This is especially important from the quantum gravity perspective.
7 pages

http://arxiv.org/abs/1511.08221
Hawking radiation, the Stefan-Boltzmann law, and unitarization
Steven B. Giddings
(Submitted on 25 Nov 2015)
Where does Hawking radiation originate? A common picture is that it arises from excitations very near or at the horizon, and this viewpoint has supported the "firewall" argument and arguments for a key role for the UV-dependent entanglement entropy in describing the quantum mechanics of black holes. However, closer investigation of both the total emission rate and the stress tensor of Hawking radiation supports the statement that its source is a near-horizon quantum region, or "atmosphere," whose radial extent is set by the horizon radius scale. This is potentially important, since Hawking radiation needs to be modified to restore unitarity, and a natural assumption is that the scales relevant to such modifications are comparable to those governing the Hawking radiation. Moreover, related discussion suggests a resolution to questions regarding extra energy flux in "nonviolent" scenarios, that does not spoil black hole thermodynamics as governed by the Bekenstein-Hawking entropy.
7 pages + references

http://arxiv.org/abs/1511.08517
The KLT relations in unimodular gravity
Daniel J Burger, George F. R. Ellis, Jeff Murugan, Amanda Weltman
(Submitted on 26 Nov 2015)
With this article, we initiate a systematic study of some of the symmetry properties of unimodular gravity, building on much of the known structure of general relativity, and utilising the powerful technology developed in that context. In particular, we show, up to four-points and tree-level, that the KLT relations of perturbative gravity hold for tracefree or unimodular gravity.
28 pages, 2 figures

 

[kodama]

Gravitational axial perturbations and quasinormal modes of loop quantum black holes
M.B. Cruz, C.A.S. Silva, F.A. Brito
(Submitted on 26 Nov 2015)
Gravitational waves can be used as a way to investigate the structure of spacetime. Loop Quantum Gravity is a theory that propose a way to model the behavior of spacetime in situations where its atomic characteristic arises. Among these situations, the spacetime behavior near the Big Bang or black hole's singularity. A recent prediction of loop quantum gravity is the existence of sub-Planckian black holes called loop quantum black holes (LQBH) or self-dual black holes which correspond to a quantized version of Schwarzschild black hole. In this work, we study the gravitational waves spectrum emitted by a LQBH through the analysis of its the quasinormal modes. From the results obtained, loop quantum black holes have been shown stable under axial gravitational perturbations.
Comments: 9 pages, 4 figures, 2 tables
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1511.08263 [gr-qc]
(or arXiv:1511.08263v1 [gr-qc] for this version)

 

[marcus]

http://arxiv.org/abs/1512.00221
On the relation between reduced quantisation and quantum reduction for spherical symmetry in loop quantum gravity
Norbert Bodendorfer, Antonia Zipfel
(Submitted on 1 Dec 2015)
Building on a recent proposal for a quantum reduction to spherical symmetry from full loop quantum gravity, we investigate the relation between a quantisation of spherically symmetric general relativity and a reduction at the quantum level. To this end, we generalise the previously proposed quantum reduction by dropping the gauge fixing condition on the radial diffeomorphisms, thus allowing to make direct contact between previous work on reduced quantisation. A dictionary between spherically symmetric variables and observables with respect to the reduction constraints in the full theory is discussed, as well as an embedding of reduced quantum states to a sub sector of the quantum symmetry reduced full theory states. On this full theory sub sector, the quantum algebra of the mentioned observables is computed and shown to qualitatively reproduce the quantum algebra of the reduced variables in the large quantum number limit for a specific choice of regularisation. Insufficiencies in recovering the reduced algebra quantitatively from the full theory are attributed to the oversimplified full theory quantum states we use.
34 pages

http://arxiv.org/abs/1512.00605
A note on asymptotically anti-de Sitter quantum spacetimes in loop quantum gravity
Norbert Bodendorfer
(Submitted on 2 Dec 2015)
A framework conceptually based on the conformal techniques employed to study the structure of the gravitational field at infinity is set up in the context of loop quantum gravity to describe asymptotically anti-de Sitter quantum spacetimes. A conformal compactification of the spatial slice is performed, which, in terms of the rescaled metric, has now finite volume, and can thus be conveniently described by spin networks states. The conformal factor used is a physical scalar field, which has the necessary asymptotics for many asymptotically AdS black hole solutions.
7 pages

http://arxiv.org/abs/1512.00713
An embedding of loop quantum cosmology in (b, v) variables into a full theory context
Norbert Bodendorfer
(Submitted on 2 Dec 2015)
Loop quantum cosmology in (b, v) variables, which is governed by a unit step size difference equation, is embedded into a full theory context based on similar variables. From the full theory perspective, the symmetry reduction is characterised by the vanishing of certain phase space functions which are implemented as operator equations in the quantum theory. The loop quantum cosmology dynamics arise as the action of the full theory Hamiltonian on maximally coarse states in the kernel of the reduction constraints. An application of this reduction procedure to spherical symmetry is also sketched, with similar results, but only one canonical pair in (b, v) form.
17 pages

http://arxiv.org/abs/1512.01589
Asymptotic safety in an interacting system of gravity and scalar matter
Pietro Donà, Astrid Eichhorn, Peter Labus, Roberto Percacci
(Submitted on 4 Dec 2015)
Asymptotic safety is an attractive scenario for the dynamics of quantum spacetime. Here, we work from a phenomenologically motivated point of view and emphasize that a viable dynamics for quantum gravity in our universe must account for the existence of matter. In particular, we explore the scale-dependence of a scalar matter-gravity-vertex, and investigate whether an interacting fixed point exists for the so-defined Newton coupling. We find a viable fixed point in the pure-gravity system, disregarding scalar quantum fluctuations. We explore its extensions to the case with dynamical scalars, and find indications of asymptotic safety in the matter-gravity system. We moreover examine the anomalous dimensions for different components of the metric fluctuations, and find significant differences between the transverse traceless and scalar component.
15 pages, 14 figures

 

[marcus]

http://arxiv.org/abs/1512.02304
Oscillatory Attractors: A New Cosmological Phase
Jasdeep S. Bains, Mark P. Hertzberg, Frank Wilczek
(Submitted on 8 Dec 2015)
In expanding FRW spacetimes, it is usually the case that homogeneous scalar fields redshift and their amplitudes approach limiting values: Hubble friction usually ensures that the field relaxes to its minimum energy configuration, which is usually a static configuration. Here we discover a class of relativistic scalar field models in which the attractor behavior is the field oscillating indefinitely, with finite amplitude, in an expanding FRW spacetime, despite the presence of Hubble friction. This is an example of spontaneous breaking of time translation symmetry. We find that the effective equation of state of the field has average value ⟨w⟩=−1, implying that the field itself could drive an inflationary or dark energy dominated phase. This behavior is reminiscent of ghost condensate models, but in the new models, unlike in the ghost condensate models, the energy-momentum tensor is time dependent, so that these new models embody a more definitive breaking of time translation symmetry. We explore (quantum) fluctuations around the homogeneous background solution, and find that low k-modes can be stable, while high k-modes are typically unstable. We discuss possible interpretations and implications of that instability.
36 pages, 15 figures

http://arxiv.org/abs/1512.03100
Gravitationally induced adiabatic particle productions: From Big Bang to de Sitter
Jaume de Haro, Supriya Pan
(Submitted on 7 Dec 2015)
In the background of a flat homogeneous and isotropic space-time, we consider a scenario of the universe driven by the gravitationally induced constant `adiabatic' particle productions. We have shown that this universe attains a big bang singularity in the past and at late-time, it asymptotically becomes de Sitter. To clarify this model universe, we perform a dynamical analysis. Further, we discussed the possible effects of this particle creations in the context of loop quantum cosmology.
8 pages, 2 figures

 

[marcus]

http://arxiv.org/abs/1512.03684
Anisotropic loop quantum cosmology with self-dual variables
Edward Wilson-Ewing
(Submitted on 11 Dec 2015)
A loop quantization of the diagonal class A Bianchi models starting from the complex-valued self-dual connection variables is presented in this paper. The basic operators in the quantum theory correspond to areas and generalized holonomies of the Ashtekar connection and the reality conditions are implemented via the choice of the inner product on the kinematical Hilbert space. The action of the Hamiltonian constraint operator is given explicitly for the case when the matter content is a massless scalar field (in which case the scalar field can be used as a relational clock), and it is shown that the big-bang and big-crunch singularities are resolved in the sense that singular and non-singular states decouple under the action of the Hamiltonian constraint operator.
16 pages

http://arxiv.org/abs/1512.05331
A simpler way of imposing simplicity constraints
Andrzej Banburski, Lin-Qing Chen
(Submitted on 16 Dec 2015)
We investigate a way of imposing simplicity constraints in a holomorphic Spin Foam model that we recently introduced. Rather than imposing the constraints on the boundary spin network, as is usually done, one can impose the constraints directly on the Spin Foam propagator. We find that the two approaches have the same leading asymptotic behaviour, with differences appearing at higher order. This allows us to obtain a model that greatly simplifies calculations, but still has Regge Calculus as its semi-classical limit.
16 pages

http://arxiv.org/abs/1512.04566
Improved Black Hole Fireworks: Asymmetric Black-Hole-to-White-Hole Tunneling Scenario
Tommaso De Lorenzo, Alejandro Perez
(Submitted on 14 Dec 2015)
A new scenario for gravitational collapse has been recently proposed by Haggard and Rovelli. Presenting the model under the name of black hole fireworks, they claimed that the accumulation of quantum gravitational effects outside the horizon can cause the tunneling of geometry from a black hole to a white hole, allowing a bounce of the collapsing star which can eventually go back to infinity. In this paper we discuss the instabilities of this model and propose a simple minimal modification which eliminates them, as well as other related instabilities discussed in the literature. The new scenario is a time-asymmetric version of the original model with a time-scale for the final explosion that is shorter than m log m in Planck units. Our analysis highlights the importance of irreversibility in gravitational collapse which, in turn, uncovers important issues that cannot be addressed in detail without a full quantum gravity treatment.
18 Pages, 6 Figures

likely of general interest:
http://arxiv.org/abs/1512.04993
Complexity, Action, and Black Holes
Adam Brown, Daniel A. Roberts, Leonard Susskind, Brian Swingle, Ying Zhao
(Submitted on 15 Dec 2015)
Our earlier paper "Complexity Equals Action" conjectured that the quantum computational complexity of a holographic state is given by the classical action of a region in the bulk (the `Wheeler-DeWitt' patch). We provide calculations for the results quoted in that paper, explain how it fits into a broader (tensor) network of ideas, and elaborate on the hypothesis that black holes are fastest computers in nature.
55+14 pages, many figures

http://arxiv.org/abs/1512.03808
Background-Independence from the Perspective of Gauge Theory
Casey Cartwright, Alex Flournoy
(Submitted on 11 Dec 2015)

 

[marcus]

http://arxiv.org/abs/1512.05743
Evolution of the tensor-to-scalar ratio across the loop quantum cosmology bounce
Edward Wilson-Ewing
(Submitted on 17 Dec 2015)
I review how the separate universe approach to cosmological perturbation theory can be used to study the dynamics of long wavelength perturbations in loop quantum cosmology, and use it to calculate how the long wavelength perturbations evolve across the bounce assuming a constant equation of state. An interesting result is that the tensor-to-scalar ratio can be damped or amplified by quantum gravity effects during the bounce, depending on the equation of state of the matter field dominating the dynamics. In particular, if the equation of state is greater than -1/3 the value of the tensor-to-scalar ratio will be damped during the bounce, in some cases significantly.
12 pages. Prepared for submission to IJMPD special issue on Loop Quantum Cosmology

So the International Journal of Modern Physics series D is going to do a special issue on LQC. Nice!

 

[atyy]

http://arxiv.org/abs/1512.06252
Categorical generalization of spinfoam models
Aleksandar Mikovic, Marko Vojinovic
(Submitted on 19 Dec 2015)
We give a brief review of the problem of quantum gravity. After the discussion of the nonrenormalizability of general relativity, we briefly mention the main research directions which aim to resolve this problem. Our attention then focuses on the approach of Loop Quantum Gravity, specifically spinfoam models. These models have some issues concerning the semiclassical limit and coupling of matter fields. The recent developments in category theory provide us with the necessary formalism to introduce a new action for general relativity and perform covariant quantization so that the issues of spinfoam models are successfully resolved.

http://arxiv.org/abs/1512.06546
Gravity and/is Thermodynamics
T. Padmanabhan
(Submitted on 21 Dec 2015)
The equations of motion describing all physical systems, except gravity, remain invariant if a constant is added to the Lagrangian. In the conventional approach, gravitational theories break this symmetry exhibited by all other physical systems. Restoring this symmetry to gravity and demanding that gravitational field equations should also remain invariant under the addition of a constant to a Lagrangian, leads to the interpretation of gravity as the thermodynamic limit of the kinetic theory of atoms of space. This approach selects, in a very natural fashion, Einstein's general relativity in d=4. Developing this paradigm at a deeper level, one can obtain the distribution function for the atoms of space and connect it up with the thermodynamic description of spacetime. This extension relies on a curious fact that the quantum spacetime endows each event with a finite area but zero volume. This approach allows us determine the numerical value of the cosmological constant and suggests a new perspective on cosmology.

http://arxiv.org/abs/1512.06206
Finite Entanglement Entropy of Black Holes
Stefano Giaccari, Leonardo Modesto, Leslaw Rachwal, Yiwei Zhu
(Submitted on 19 Dec 2015)
We compute the area term contribution to the black holes' entanglement entropy for a class of local or weakly nonlocal renormalizable gravitational theories coupled to matter. For the case of super-renormalizable theories, we can get a finite conical entropy expressed only in terms of the classical Newton constant either by completing the theory to a finite one in dimensional regularization or by removing the quadratic divergences in the cut-off regularization by the introduction of additional interaction terms. Therefore, our result is independent from the renormalization scheme. We also propose a theory in which the renormalization of the Newton constant is entirely due to the standard model matter, arguing that such a contribution does not give the usual interpretational problems of conical entropy discussed in the literature.

Edit: Linked fixed. Thanks kodama and marcus!

 

[kodama]

http://arxiv.org/abs/1512.06252
Categorical generalization of spinfoam models
Aleksandar Mikovic, Marko Vojinovic
(Submitted on 19 Dec 2015)
We give a brief review of the problem of quantum gravity. After the discussion of the nonrenormalizability of general relativity, we briefly mention the main research directions which aim to resolve this problem. Our attention then focuses on the approach of Loop Quantum Gravity, specifically spinfoam models. These models have some issues concerning the semiclassical limit and coupling of matter fields. The recent developments in category theory provide us with the necessary formalism to introduce a new action for general relativity and perform covariant quantization so that the issues of spinfoam models are successfully resolved.

link does not lead to the paper

 

[marcus]

This should do it:
http://arxiv.org/abs/1512.06252
Atyy will probably fix the link. Thanks for catching that. At present in post#2372 the link intended to be to the Mikovic et al, points instead to the Ciacarri et al paper. If Atyy notices, and fixes the link then this post will not be needed and I will delete it.

Fascinating idea presented in the abstract! I haven't yet gotten around to looking the Mikovic et al paper itself. Hopefully you or others have.

BTW the historical perspective on GR that Thanu Padmanabhan posted today, I think is wonderful: concise, entertaining, well written, presents the core GR issues remarkably simply and clearly.
http://arxiv.org/abs/1512.06672
One hundred years of General Relativity: Summary, Status and Prospects
T. Padmanabhan
(Submitted on 21 Dec 2015)
General Relativity (GR) revolutionized the way we thought about gravity. After briefly describing the key successes of GR and its impact, I will discuss the major conceptual challenges it faces today. I conclude by outlining the prospective future directions of development, which hold the promise of deepening our understanding of the nature of gravity.
13 pages. Extended version of: (a) Guest Editorial written for Current Science and (b) Plenary Talk at the 28th Texas Symposium on Relativistic Astrophysics

http://arxiv.org/abs/1512.07110
Deformations of GR and BH thermodynamics
Kirill Krasnov
(Submitted on 22 Dec 2015)
In four space-time dimensions General Relativity can be non-trivially deformed. Deformed theories continue to describe two propagating degrees of freedom, as GR. We study Euclidean black hole thermodynamics in these deformations. We use the recently developed formulation that works with SO(3) connections as well as certain matrices M of auxiliary fields. We show that the black hole entropy is given by one quarter of the horizon area as measured by the Lie algebra valued two-form MF, where F is the connection curvature. This coincides with the horizon area as measured by the metric only for the case of General Relativity.
18 pages.

http://arxiv.org/abs/1512.07223
Unimodular F(R) Gravity
S. Nojiri, S.D. Odintsov, V.K. Oikonomou
(Submitted on 22 Dec 2015)
We extend the formalism of the Einstein-Hilbert unimodular gravity in the context of modified F(R) gravity. After appropriately modifying the Friedmann-Robertson-Walker metric in a way that it becomes compatible to the unimodular condition of having a constant metric determinant, we derive the equations of motion of the unimodular F(R) gravity by using the metric formalism of modified gravity with Lagrange multiplier constraint. The resulting equations are studied in frames of reconstruction method, which enables us to realize various cosmological scenarios, which was impossible to realize in the standard Einstein-Hilbert unimodular gravity. Several unimodular F(R) inflationary scenarios are presented, and in some cases, concordance with Planck and BICEP2 observational data can be achieved.

 

[marcus]

http://arxiv.org/abs/1512.07690
SL(2,C) Chern-Simons Theory, Flat Connections, and Four-dimensional Quantum Geometry
Hal M. Haggard, Muxin Han, Wojciech Kaminski, Aldo Riello
(Submitted on 24 Dec 2015)
The present paper analyze SL(2,ℂ) Chern-Simons theory on a class of graph complement 3-manifolds, and its relation with classical and quantum geometries on 4-dimensional manifolds. In classical theory, we explain the correspondence between a class of SL(2,ℂ) flat connections on 3-manifold and the Lorentzian simplicial geometries in 4 dimensions. The class of flat connections on the graph complement 3-manifold is specified by a certain boundary condition. The corresponding simplicial 4-dimensional geometries are made by constant curvature 4-simplices. The quantization of 4d simplicial geometry can be carried out via the quantization of flat connection on 3-manifold in Chern-Simons theory. In quantum SL(2,ℂ) Chern-Simons theory, a basis of physical wave functions is the class of (holomorphic) 3d block, defined by analytically continued Chern-Simons path integral over Lefschetz thimbles. Here we propose that the (holomorphic) 3d block with the proper boundary condition imposed gives the quantization of simplicial 4-dimensional geometry. Interestingly in the semiclassical asymptotic expansion of (holomorphic) 3d block, the leading contribution gives the classical action of simplicial Einstein-Hilbert gravity in 4 dimensions, i.e. Lorentzian 4d Regge action on constant curvature 4-simplices with a cosmological constant. Such a result suggests a relation between SL(2,ℂ) Chern-Simons theory on a class of 3-manifolds and simplicial quantum gravity on 4-dimensional manifolds. This paper presents the details for the results reported in arXiv:1509.00458.
43+11 pages, 9 figures

 

[marcus]

http://arxiv.org/abs/1512.08346
Quantum black hole without singularity
Claus Kiefer
(Submitted on 28 Dec 2015)
We discuss the quantization of a spherical dust shell in a rigorous manner. Classically, the shell can collapse to form a black hole with a singularity. In the quantum theory, we construct a well-defined self-adjoint extension for the Hamilton operator. As a result, the evolution is unitary and the singularity is avoided. If we represent the shell initially by a narrow wave packet, it will first contract until it reaches the region where classically a black hole would form, but then re-expands to infinity. In a way, the state can be interpreted as a superposition of a black hole with a white hole.
5 pages, invited contribution to the BH6 session at the Marcel Grossmann Conference MG14

http://arxiv.org/abs/1512.08997
Phenomenology of Causal Dynamical Triangulations
Jakub Mielczarek
(Submitted on 30 Dec 2015)
The four dimensional Causal Dynamical Triangulations (CDT) approach to quantum gravity is already more than ten years old theory with numerous unprecedented predictions such as non-trivial phase structure of gravitational field and dimensional running. Here, we discuss possible empirical consequences of CDT derived based on the two features of the approach mentioned above. A possibility of using both astrophysical and cosmological observations to test CDT is discussed. We show that scenarios which can be ruled out at the empirical level exist.
4 pages, 1 figure. Presented at the 14th Marcel Grossman Meeting, Rome, July 12-18, 2015

http://arxiv.org/abs/1512.08959
Entanglement time in the primordial universe
Eugenio Bianchi, Lucas Hackl, Nelson Yokomizo
(Submitted on 30 Dec 2015)
We investigate the behavior of the entanglement entropy of space in the primordial phase of the universe before the beginning of cosmic inflation. We argue that in this phase the entanglement entropy of a region of space grows from a zero-law to an area-law. This behavior provides a quantum version of the classical BKL conjecture that spatially separated points decouple in the approach to a cosmological singularity. We show that the relational growth of the entanglement entropy with the scale factor provides a new statistical notion of arrow of time in quantum gravity. The growth of entanglement in the pre-inflationary phase provides a mechanism for the production of the quantum correlations present at the beginning of inflation and imprinted in the CMB sky.
12 pages, 3 figures, Honorable Mention in the Gravity Research Foundation 2015 Essay Competition

http://arxiv.org/abs/1512.09010
Implications of Planck2015 for inflationary, ekpyrotic and anamorphic bouncing cosmologies
Anna Ijjas, Paul J. Steinhardt
(Submitted on 30 Dec 2015)
The results from Planck2015, when combined with earlier observations from WMAP, ACT, SPT and other experiments, were the first observations to disfavor the "classic" inflationary paradigm. To satisfy the observational constraints, inflationary theorists have been forced to consider plateau-like inflaton potentials that introduce more parameters and more fine-tuning, problematic initial conditions, multiverse-unpredictability issues, and a new 'unlikeliness problem.' Some propose turning instead to a "postmodern" inflationary paradigm in which the cosmological properties in our observable universe are only locally valid and set randomly, with completely different properties (and perhaps even different physical laws) existing in most regions outside our horizon. By contrast, the new results are consistent with the simplest versions of ekpyrotic cyclic models in which the universe is smoothed and flattened during a period of slow contraction followed by a bounce, and another promising bouncing theory, anamorphic cosmology, has been proposed that can produce distinctive predictions.
9 pages and 2 figures. Invited review article for the focus issue "Planck and the fundamentals of cosmology" in Classical and Quantum Gravity. Accepted for publication.

http://arxiv.org/abs/1512.08979
Evidence for Matter Bounce Cosmology in Low Redshift Observations
Yi-Fu Cai, Francis Duplessis, Damien A. Easson, Dong-Gang Wang
(Submitted on 30 Dec 2015)
The Matter Bounce scenario allows for a sizable parameter space where cosmological fluctuations originally exit the Hubble radius when the background energy density was small. In this scenario and its extended versions, the low energy degrees of freedom are likely responsible for the statistical properties of the cosmic microwave background (CMB) power spectrum at large length scales. An interesting consequence is that these modes might be observable only at relatively late times. Therefore low redshift observations could provide evidence for, or even falsify, various bouncing models. We provide an example where a recently hinted potential deviation from Λ-Cold-Dark-Matter (ΛCDM) cosmology results from a dark matter (DM) and dark energy (DE) interaction. The same interaction allows Matter Bounce models to generate a red tilt for the primordial curvature perturbations in corroboration with CMB experiments.
11 pages, 3 figures

http://arxiv.org/abs/1512.09076
Geometry from Information Geometry
Ariel Caticha
(Submitted on 30 Dec 2015)
We use the method of maximum entropy to model physical space as a curved statistical manifold. It is then natural to use information geometry to explain the geometry of space. We find that the resultant information metric does not describe the full geometry of space but only its conformal geometry -- the geometry up to local changes of scale. Remarkably, this is precisely what is needed to model "physical" space in general relativity.
Presented at MaxEnt 2015, the 35th International Workshop on Bayesian Inference and Maximum Entropy Methods in Science and Engineering (July 19-24, 2015, Potsdam NY, USA)

 

[marcus]

http://arxiv.org/abs/1601.05688
Quantum self-gravitating collapsing matter in a quantum geometry
Miguel Campiglia, Rodolfo Gambini, Javier Olmedo, Jorge Pullin
(Submitted on 21 Jan 2016)
The problem of how space-time responds to gravitating quantum matter in full quantum gravity has been one of the main questions that any program of quantization of gravity should address. Here we analyze this issue by considering the quantization of a collapsing null shell coupled to spherically symmetric loop quantum gravity. We show that the constraint algebra of canonical gravity is Abelian both classically and when quantized using loop quantum gravity techniques. The Hamiltonian constraint is well defined and suitable Dirac observables characterizing the problem were identified at the quantum level. We can write the metric as a parameterized Dirac observable at the quantum level and study the physics of the collapsing shell and black hole formation. We show how the singularity inside the black hole is eliminated by loop quantum gravity and how the shell can traverse it. The construction is compatible with a scenario in which the shell tunnels into a baby universe inside the black hole or one in which it could emerge through a white hole.
4 pages

http://arxiv.org/abs/1601.05707
Projective quantum states for Loop Quantum Gravity coupled to tensor fields
Andrzej Okolow
(Submitted on 21 Jan 2016)
We present a construction of kinematic quantum states for theories of tensor fields of an arbitrary sort. The construction is based on projective techniques by Kijowski. Applying projective quantum states for Loop Quantum Gravity obtained by Lanery and Thiemann we construct quantum states for LQG coupled to tensor fields.
23 pages.

http://arxiv.org/abs/1601.05531
Invariant Connections and Symmetry Reduction in Loop Quantum Gravity
Maximilian Hanusch
(Submitted on 21 Jan 2016)
The intention of this thesis is to provide general tools and concepts that allow to perform a mathematically substantiated symmetry reduction in (quantum) gauge field theories. Here, the main focus is on the framework of loop quantum gravity (LQG), where we concentrate on the reduction of the quantum configuration space, and the construction of a normalized Radon measures on the reduced one. More precisely, we introduce a new way to symmetry reduce the LQG-configuration space directly on the quantum level, and then show that this always leads to a (strictly) larger reduced space than quantizing the classical configuration space of invariant connections (traditional approach). We prove a general classification theorem for such invariant connections, which we then use to calculate the classical configuration space for the homogeneous and the spherically symmetric case. Here, the backbone of the introduced reduction concept is a lifting result for group actions on sets to spectra of C∗-subalgebras of the bounded functions thereon; and as a further application of this, we single out the standard kinematical Hilbert space of homogeneous isotropic loop quantum cosmology by means of the same invariance condition for both the standard configuration space ℝBohr, as well as for the Fleischhack one ℝ⊔ℝBohr. Along the way, symmetries of embedded analytic curves under a given analytic Lie group action are investigated, and a first classification result is proven for the case that the action is proper or pointwise proper and transitive, and only admits normal stabilizers.
190 pages. PhD thesis, University of Paderborn, December 2014 (supervisor: Ch. Fleischhack)

possibly of general interest:
http://arxiv.org/abs/1601.05956
Higher prequantum geometry
[URL='https://www.physicsforums.com/insights/author/urs-schreiber/']Urs Schreiber[/URL]
(Submitted on 22 Jan 2016)
This is a survey of motivations, constructions and applications of higher prequantum geometry. In section 1 we highlight the open problem of prequantizing local field theory in a local and gauge invariant way, and we survey how a solution to this problem exists in higher differential geometry. In section 2 we survey examples and problems of interest. In section 3 we survey the abstract cohesive homotopy theory that serves to make all this precise and tractable.
68 pages, many figures. expanded version of my contribution to Catren, Anel (eds.) "New Spaces in Mathematics and Physics" (ercpqg-espace.sciencesconf.org)

http://arxiv.org/abs/1601.05473
The Early Growth of the First Black Holes
Jarrett L. Johnson (LANL), Francesco Haardt (Universita dell'Insubria)
(Submitted on 20 Jan 2016)
With detections of quasars powered by increasingly massive black holes (BHs) at increasingly early times in cosmic history over the past decade, there has been correspondingly rapid progress made on the theory of early BH formation and growth. Here we review the emerging picture of how the first massive BHs formed from the primordial gas and then grew to supermassive scales. We discuss the initial conditions for the formation of the progenitors of these seed BHs, the factors dictating the initial masses with which they form, and their initial stages of growth via accretion, which may occur at super-Eddington rates. Finally, we briefly discuss how these results connect to large-scale simulations of the growth of supermassive BHs over the course of the first billion years following the Big Bang.
13 pages, 9 figures, invited review submitted to PASA

http://arxiv.org/abs/1601.06831
Gravity-Matter Entanglement in Regge Quantum Gravity
Nikola Paunković, Marko Vojinović
(Submitted on 22 Jan 2016)
We argue that Hartle-Hawking states in the Regge quantum gravity model generically contain non-trivial entanglement between gravity and matter fields. Generic impossibility to talk about "matter in a point of space" is in line with the idea of an emergent spacetime, and as such could be taken as a possible candidate for a criterion for a plausible theory of quantum gravity. Finally, this new entanglement could be seen as an additional "effective interaction", which could possibly bring corrections to the weak equivalence principle.
2 pages. Proceedings of the EmQM15 conference, to appear in J. Phys. Conf. Ser. 2 pages

http://arxiv.org/abs/1601.06932
Creation of Matter in a Noncommutative Universe
T. Miller, M. Heller
(Submitted on 26 Jan 2016)
The dark matter and dark energy problem, that is now dominating the research in cosmology, makes the question of the origin of mass-energy content of the universe more urgent than ever. There are two philosophies regarding this question: according to Mach's principle it is matter that generates geometry of space-time, and according to Wheeler's geometrodynamics some configurations of space-time geometry are to be interpreted as its material content. Neither of these philosophies has led to success. In the present paper, we show that there exists an algebraic generalisation of geometry that reconciles, in a sense, these two seemingly opposite standpoints. The geometry is constructed with the help of a noncommutative algebra of smooth functions on a groupoid and its derivations. The groupoid in question has a nice physical interpretation: it can be regarded as a space of Lorentz rotations. In this way, Lorentz symmetries are inherent to the generalised geometry of space-time. We define the action for this geometry and, by varying it, obtain generalised vacuum Einstein equations (for a simplified model). It turns out that these equations contain additional terms (with respect to the standard vacuum Einstein equations) which are naturally interpreted as the components of the energy-momentum tensor. Matter is thus created out of purely geometric degrees of freedom. We find two exact solutions (for even more simplified case). We argue that the creation of matter, being a global effect, makes the contrast between Mach and Wheeler philosophies ineffective.
18 pages

http://arxiv.org/abs/1601.07057
Unimodular-Mimetic Cosmology
S. Nojiri, S.D. Odintsov, V.K. Oikonomou
(Submitted on 26 Jan 2016)
We combine the unimodular gravity and mimetic gravity theories into a unified theoretical framework, which is proposed to solve the cosmological constant problem and the dark matter issue. After providing the formulation of the unimodular mimetic gravity and investigating all the new features that the vacuum unimodular gravity implies, by using the underlying reconstruction method, we realize some well known cosmological evolutions, with some of these being exotic for the ordinary Einstein-Hilbert gravity. Specifically we provide the vacuum unimodular mimetic gravity description of the de Sitter cosmology, of the perfect fluid with constant equation of state cosmology, of the Type IV singular cosmology and of the R2 inflation cosmology. Moreover, we investigate how cosmologically viable cosmologies, which are compatible with the recent observational data, can be realized by the vacuum unimodular mimetic gravity. Since in some cases, the graceful exit from inflation problem might exist, we provide a qualitative description of the mechanism that can potentially generate the graceful exit from inflation in these theories, by searching for unstable de Sitter solutions in the context of unimodular mimetic theories of gravity.
14 pages.

 

[marcus]

http://arxiv.org/abs/1602.00106
A Note on Entanglement Entropy, Coherent States and Gravity
Madhavan Varadarajan
(Submitted on 30 Jan 2016)
The entanglement entropy of a free quantum field in a coherent state is independent of its stress energy content. We use this result to highlight the fact that while the Einstein equations for first order variations about a locally maximally symmetric vacuum state of geometry and quantum fields seem to follow from Jacobson's principle of maximal vacuum entanglement entropy, their possible derivation from this principle for the physically relevant case of finite but small variations remains an open issue. We also apply this result to the context of Bianchi's identification, independent of unknown Planck scale physics, of the first order variation of Bekenstein Hawking area with that of vacuum entanglement entropy. We argue that under certain technical assumptions this identification seems not to be extendible to the context of finite but small variations to coherent states. Our particular method of estimation of entanglement entropy variation reveals the existence of certain contributions over and above those appearing in Jacobson's and Bianchi's works. We discuss the sense in which these contributions may be subleading to those already present in these works.
15 pages.

http://arxiv.org/abs/1602.00608
Quantum theory of the Generalised Uncertainty Principle and the existence of a Minimal Length
Jean-Philippe Bruneton, Julien Larena
(Submitted on 1 Feb 2016)
We extend significantly previous works on the Hilbert space representations of the Generalized Uncertainty Principle (GUP) in 3+1 dimensions of the form [X_i,P_j]=iF_ij where Fij=f(P²)δ_ij+g(P²)P_iP_j for any functions f. However, we restrict our study to the case of commuting X's. We focus in particular on the symmetries of the theory, and the minimal length that emerge in some cases. We first show that, at the algebraic level, there exists an unambiguous mapping between the GUP with a deformed quantum algebra and a quadratic Hamiltonian into a standard, Heisenberg algebra of operators and an aquadratic Hamiltonian, provided the boost sector of the symmetries is modified accordingly. The theory can also be mapped to a completely standard Quantum Mechanics with standard symmetries, but with momentum dependent position operators. Next, we investigate the Hilbert space representations of these algebraically equivalent models, and focus, specifically on whether they exhibit a minimal length. We carry the functional analysis of the various operators involved, and show that the appearance of a minimal length critically depends on the relationship between the generators of translations and the physical momenta. In particular, because this relationship is preserved by the algebraic mapping presented in this paper, when a minimal length is present in the standard GUP, it is also present in the corresponding Aquadratic Hamiltonian formulation, despite the perfectly standard algebra of this model. In general, a minimal length requires bounded generators of translations, i.e. a specific kind of quantization of space, and this depends on the precise shape of the function fdefined previously. This result provides an elegant and unambiguous classification of which universal quantum gravity corrections lead to the emergence of a minimal length.
11 pages.

http://arxiv.org/abs/1601.08211
Functional Renormalisation Group analysis of Tensorial Group Field Theories on ℝ^d
Joseph Ben Geloun, Riccardo Martini, Daniele Oriti
(Submitted on 29 Jan 2016)
Rank-d Tensorial Group Field Theories are quantum field theories defined on a group manifold G^×d, which represent a non-local generalization of standard QFT, and a candidate formalism for quantum gravity, since, when endowed with appropriate data, they can be interpreted as defining a field theoretic description of the fundamental building blocks of quantum spacetime. Their renormalisation analysis is crucial both for establishing their consistency as quantum field theories, and for studying the emergence of continuum spacetime and geometry from them. In this paper, we study the renormalisation group flow of two simple classes of TGFTs, defined for the group G=ℝ for arbitrary rank, both without and with gauge invariance conditions, by means of functional renormalisation group techniques. The issue of IR divergences is tackled by the definition of a proper thermodynamic limit for TGFTs. We map the phase diagram of such models, in a simple truncation, and identify both UV and IR fixed points of the RG flow. Encouragingly, for all the models we study, we find evidence for the existence of a phase transition of condensation type.
43 pages, 9 figures

http://arxiv.org/abs/1601.08213
Renormalizable Tensor Field Theories
Joseph Ben Geloun
(Submitted on 29 Jan 2016)
Extending tensor models at the field theoretical level, tensor field theories are nonlocal quantum field theories with Feynman graphs identified with simplicial complexes. They become relevant for addressing quantum topology and geometry in any dimension and therefore form an interesting class of models for studying quantum gravity. We review the class of perturbatively renormalizable tensor field theories and some of their features.
9 pages, 2 figures. Based on the talk Tensor Models and Renormalization given at the International Congress on Mathematical Physics, ICMP2015, 27th July - 1st August, 2015, Santiago de Chile.

http://arxiv.org/abs/1602.01158
When Isolated Horizons met Near Horizon Geometries
Jerzy Lewandowski, Adam Szereszewski, Piotr Waluk
(Submitted on 2 Feb 2016)
There are two mathematical relativity frameworks generalizing the black hole theory: the theory of isolated horizons (IH) and the theory of near horizon geometries (NHG). We outline here and discuss the derivation of the NHG from the theory of IH by composing spacetimes from IH. The simplest but still quite general class of solutions to Einstein's equations of this type defines spacetimes foliated by Killing horizons emanating from extremal horizons. That derivation, clearly being a link between the two frameworks, seems to be unknown to the NHG researchers and is hardly acknowledged in reviews on the IH. This lecture was a contribution to the Mathematical Structures session of the 2nd LeCosPA International Symposium "Everything about Gravity" celebrating the centenary of Einstein's General Relativity on December 14-18, 2015 in Taipei.
6 pages

http://arxiv.org/abs/1602.03003
Why three generations?
Masahiro Ibe, Alexander Kusenko, Tsutomu T. Yanagida
We discuss an anthropic explanation of why there exist three generations of fermions. If one assumes that the right-handed neutrino sector is responsible for both the matter-antimatter asymmetry and the dark matter, then anthropic selection favors three or more families of fermions. For successful leptogenesis, at least two right-handed neutrinos are needed, while the third right-handed neutrino is invoked to play the role of dark matter. The number of the right-handed neutrinos is tied to the number of generations by the anomaly constraints of the U(1)_B-L gauge symmetry. Combining anthropic arguments with observational constraints, we obtain predictions for the X-ray observations, as well as for neutrinoless double-beta decay.
7pages, 2 figures
http://inspirehep.net/author/profile/A.Kusenko.1

​

noted: http://arxiv.org/abs/1601.08205

 

[marcus]

http://arxiv.org/abs/1602.03237
Can chaos be observed in quantum gravity?
Bianca Dittrich, Philipp A. Hoehn, Tim A. Koslowski, Mike I. Nelson
(Submitted on 10 Feb 2016)
Full general relativity is almost certainly non-integrable and likely chaotic and therefore almost certainly possesses neither differentiable Dirac observables nor a reduced phase space. It follows that the standard notion of observable has to be extended to include non-differentiable or even discontinuous generalized observables. These cannot carry Poisson-algebraic structures and do not admit a standard quantization. This has deep consequences for a quantum theory of gravity, which we investigate in a simple model for a system with Hamiltonian constraint that fails to be completely integrable. We show that basing the quantization on standard topology precludes a semiclassical limit and can even prohibit any solutions to the quantum constraints. Our proposed solution to this problem is to refine topology such that a complete set of Dirac observables becomes continuous. In the toy model it turns out that a refinement to a polymer-type topology, as e.g. used in loop quantum cosmology, is sufficient. Basing quantization of the toy model on this finer topology, we find a complete set of quantum Dirac observables and a suitable semiclassical limit.
4 pages + references = 6 pages
http://inspirehep.net/record/1420693?ln=en
IMHO: ausgezeichnet!
I suspect this insight opens the way for a significant advance in qg.

http://arxiv.org/abs/1602.04452
Conceptual issues in loop quantum cosmology
Aurélien Barrau, Boris Bolliet
(Submitted on 14 Feb 2016 )
Loop quantum gravity is a mature theory. To proceed to explicit calculations in cosmology, it is necessary to make assumptions and simplifications based on the symmetries of the cosmological setting. Symmetry reduction is especially critical when dealing with cosmological perturbations. The present article reviews several approaches to the problem of building a consistent formalism that describes the dynamics of perturbations on a quantum spacetime and tries to address their respective strengths and weaknesses. We also review the main open issues in loop quantum cosmology.
Invited article for an IJMP volume dedicated to loop quantum gravity

http://arxiv.org/abs/1602.01861
Twisted geometries, twistors and conformal transformations
Miklos Långvik, Simone Speziale
(Submitted on 4 Feb 2016)
The twisted geometries of spin network states are described by simple twistors, isomorphic to null twistors with a time-like direction singled out. The isomorphism depends on the Immirzi parameter, and reduces to the identity when the parameter goes to infinity. Using this twistorial representation we study the action of the conformal group SU(2,2) on the classical phase space of loop quantum gravity, described by twisted geometry. The generators of translations and conformal boosts do not preserve the geometric structure, whereas the dilatation generator does. It corresponds to a 1-parameter family of embeddings of T*SL(2,C) in twistor space, and its action preserves the intrinsic geometry while changing the extrinsic one - that is the boosts among polyhedra. We discuss the implication of this action from a dynamical point of view, and compare it with a discretisation of the dilatation generator of the continuum phase space, given by the Lie derivative of the group character. At leading order in the continuum limit, the latter reproduces the same transformation of the extrinsic geometry, while also rescaling the areas and volumes and preserving the angles associated with the intrinsic geometry. Away from the continuum limit its action has an interesting non-linear structure, but is in general incompatible with the closure constraint needed for the geometric interpretation. As a side result, we compute the precise relation between the extrinsic geometry used in twisted geometries and the one defined in the gauge-invariant parametrization by Dittrich and Ryan, and show that the secondary simplicity constraints they posited coincide with those dynamically derived in the toy model of [1409.0836].
20 pages

 

[torsten]

http://arxiv.org/abs/1601.06436
Smooth quantum gravity: Exotic smoothness and Quantum gravity
Torsten Asselmeyer-Maluga
(Submitted on 24 Jan 2016)
Over the last two decades, many unexpected relations between exotic smoothness, e.g. exotic R4, and quantum field theory were found. Some of these relations are rooted in a relation to superstring theory and quantum gravity. Therefore one would expect that exotic smoothness is directly related to the quantization of general relativity. In this article we will support this conjecture and develop a new approach to quantum gravity called \emph{smooth quantum gravity} by using smooth 4-manifolds with an exotic smoothness structure. In particular we discuss the appearance of a wildly embedded 3-manifold which we identify with a quantum state. Furthermore, we analyze this quantum state by using foliation theory and relate it to an element in an operator algebra. Then we describe a set of geometric, non-commutative operators, the skein algebra, which can be used to determine the geometry of a 3-manifold. This operator algebra can be understood as a deformation quantization of the classical Poisson algebra of observables given by holonomies. The structure of this operator algebra induces an action by using the quantized calculus of Connes. The scaling behavior of this action is analyzed to obtain the classical theory of General Relativity (GRT) for large scales. This approach has some obvious properties: there are non-linear gravitons, a connection to lattice gauge field theory and a dimensional reduction from 4D to 2D. Some cosmological consequences like the appearance of an inflationary phase are also discussed. At the end we will get the simple picture that the change from the standard R4 to the exotic R4 is a quantization of geometry.