Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1403.2080
The Soccer-ball Problem
S. Hossenfelder
(Submitted on 9 Mar 2014)
The idea that Lorentz-symmetry in momentum space could be modified but still remain observer-independent has received quite some attention in the recent years. Motivated by Loop Quantum Gravity, this modified Lorentz-symmetry is being used as a phenomenological model to test possibly observable effects of quantum gravity. The most pressing problem in these models is the treatment of multi-particle states, known as the "soccer-ball problem." This article briefly reviews the problem and the status of existing solution attempts.
9 pages, prepared for the SIGMA Special Issue on Deformations of Space-Time

 

[MTd2]

http://backreaction.blogspot.com.br/2014/03/what-is-asymptotically-safe-gravity-and.html

Bee's blog:

What is asymptotically safe gravity and what does it save?

 

[marcus]

http://arxiv.org/abs/1403.2815
FRW cosmology of the generalized model of LQG
S. Chattopadhyay, A. Ashurov, M. Khurshudyan, K. Myrzakulov, A. Pasqua, R. Myrzakulov
(Submitted on 12 Mar 2014)
In this paper, we study the main cosmological properties of the classical Friedmann equations in the case of homogeneous and isotropic Friedmann-Robertson-Walker Universe and we also generalized the expression of the Friedmann equation in the case of Loop Quantum Cosmology (LQC). Considering the M₃₅-model, we found the solutions of the equations considered for two particular cases, i.e. Q=0 (i.e., the de Sitter solution) and Q>0. Moreover, we considered and studied two exact cosmological solutions of the M₃₅-model, in particular the power-law and the exponential ones. Futhermore, we also considered a third more complicated case and we derived the solution for an arbitrary function of the time f(t). A scalar field description of the model is presented by constructing its self-interacting potential.
16 pages

http://arxiv.org/abs/1403.2974
Separable Hilbert space for loop quantization
J. Fernando Barbero G., Tomasz Pawlowski, Eduardo J. S. Villasenor
(Submitted on 12 Mar 2014)
We discuss, within the simplified context provided by the polymeric harmonic oscillator, a construction leading to a separable Hilbert space that preserves some of the most important features of the spectrum of the Hamiltonian operator. This construction can be generalized to loop quantum cosmology and is helpful to sidestep some of the issues that appear in that context. In particular those related to superselection and the definition of suitable ensembles for the statistical mechanics of these types of systems.
5 pages

 

[marcus]

http://arxiv.org/abs/1403.3190
A curvature operator for LQG
Emanuele Alesci, Mehdi Assanioussi, Jerzy Lewandowski
(Submitted on 13 Mar 2014)
We introduce a new operator in Loop Quantum Gravity - the 3D curvature operator - related to the 3-dimensional scalar curvature. The construction is based on Regge Calculus. We define it starting from the classical expression of the Regge curvature, then we derive its properties and discuss some explicit checks of the semi-classical limit.
20 pages.

 

[marcus]

http://arxiv.org/abs/1403.3879
Geometric time in quantum cosmology
Tomasz Pawłowski
(Submitted on 16 Mar 2014)
Various choices of the geometry degrees of freedom as the emergent time are tested on the model of an isotropic universe with a scalar field of ϕ2 potential. Potential problems with each choices as well as possible applications in loop quantization are discussed.
3 pages, To appear in Proceedings of the 13th Marcel Grossmann Meeting (MG13), Stockholm, Sweden, 1-7 July 2012

BICEP report (g-waves observed in CMB polarization map), plus supplementary material:
http://bicepkeck.org/index.html#papers

interesting dark matter paper that Chronos spotted (by Avi Loeb et al ruling out primordial BH as dark matter)
http://arxiv.org/abs/1401.3025
Exclusion of the remaining mass window for primordial black holes as the dominant constituent of dark matter
Abraham Loeb, Paolo Pani

also possibly of general interest:
http://arxiv.org/abs/1403.4110
A new approach to understanding dark matter
Christian G. Boehmer, Nicola Tamanini, Matthew Wright
(Submitted on 17 Mar 2014)
We consider a modification of General Relativity motivated by the treatment of anisotropies in Continuum Mechanics. The Newtonian limit of the theory is formulated and applied to galactic rotation curves. By assuming that the additional structure of spacetime behaves like a Newtonian gravitational potential for small deviations from isotropy, we are able to recover the Nevarro-Frenk-White profile of dark matter halos by a suitable identification of constants.
8 pages

briefest mention:
http://arxiv.org/abs/1403.4226
Agravity
Alberto Salvio, Alessandro Strumia

 

[John86]

http://arxiv.org/abs/1403.3822
Entropic Dynamics: an Inference Approach to Quantum Theory, Time and Measurement
Ariel Caticha
(Submitted on 15 Mar 2014)
We review the derivation of quantum theory as an application of entropic methods of inference. The new contribution in this paper is a streamlined derivation of the Schr\"odinger equation based on a different choice of microstates and constraints.

http://arxiv.org/abs/1403.3961
Cosmology with Mimetic Matter
Ali H. Chamseddine, Viatcheslav Mukhanov, Alexander Vikman
(Submitted on 16 Mar 2014)
We consider minimal extensions of the recently proposed Mimetic Dark Matter and show that by introducing a potential for the mimetic non-dynamical scalar field we can mimic nearly any gravitational properties of the normal matter. In particular, the mimetic matter can provide us with inflaton, quintessence and even can lead to a bouncing nonsingular universe. We also investigate the behaviour of cosmological perturbations due to a mimetic matter. We demonstrate that simple mimetic inflation can produce red-tilted scalar perturbations which are largely enhanced over gravity waves.

 

[marcus]

http://arxiv.org/abs/1403.4529
How can holonomy corrections be introduced in f(R) gravity?
Jaume de Haro
(Submitted on 18 Mar 2014)
We study the introduction of holonomy corrections in f(R) gravity. We will show that there are infinitely many ways, as many as canonical transformations, to introduce this kind of corrections, depending on the canonical variables (two coordinates and its conjugate momenta) used to obtain the Hamiltonian. In each case, these corrections lead, at effective level, to different modified holonomy corrected Friedmann equations in f(R) gravity, which are in practice analytically unworkable, i.e. only numerical analysis can be used to understand its dynamics. Finally, we give arguments in favour of one preferred set of variables, the one that conformally maps f(R) to Einstein gravity, because for these variables the dynamics of the system has a clear physical meaning: the same as in standard Loop Quantum Cosmology, where the effective dynamics of a system can be analytically studied.
11 pages

 

[atyy]

http://arxiv.org/abs/1403.1232
Global Flows in Quantum Gravity
Nicolai Christiansen, Benjamin Knorr, Jan M. Pawlowski, Andreas Rodigast
(Submitted on 5 Mar 2014)
We study four-dimensional quantum gravity using non-perturbative renormalization group methods. We solve the corresponding equations for the fully momentum-dependent propagator, Newton's coupling and the cosmological constant. For the first time, we obtain a global phase diagram where the non-Gaussian ultraviolet fixed point of asymptotic safety is connected via smooth trajectories to a classical infrared fixed point. The theory is therefore ultraviolet complete and deforms smoothly into classical gravity as the infrared limit is approached.

 

[atyy]

http://arxiv.org/abs/1402.6406
Renormalizability of Supersymmetric Group Field Cosmology
Sudhaker Upadhyay
(Submitted on 26 Feb 2014)
In this paper we consider the gauge invariant third quantized model of supersymmetric group field cosmology. The supersymmetric BRST invariance for such theory in non-linear gauge is also analysed. The path integral formulation to the case of a multiverse made up of homogeneous and isotropic spacetimes filled with a perfect fluid is presented. The renormalizability for the scattering of universes in multiverse are established with suitably constructed master equations for connected diagrams and proper vertices. The Slavnov-Taylor identities for this theory hold to all orders of radiative corrections.

 

[marcus]

http://arxiv.org/abs/1403.5625
Big Bounce Genesis
Changhong Li, Robert H. Brandenberger, Yeuk-Kwan E. Cheung
(Submitted on 22 Mar 2014)
We report on the possibility to use dark matter mass and its interaction cross section as a smoking gun signal of the existence of a big bounce at the early stage in the evolution of our currently observed universe. A model independent study of dark matter production in the contraction and expansion phases of the bounce universe reveals a new venue for achieving the observed relic abundance in which a significantly smaller amount of dark matter--compared to the standard cosmology--is produced and survives until today, diluted only by the cosmic expansion since the radiation dominated era. Once DM mass and its interaction strength with ordinary matter are determined by experiments, this alternative route becomes a signature of the bounce universe scenario.
5 pages; 3 figures

 

[MTd2]

http://arxiv.org/abs/1403.5940

The effective action in 4-dim CDT. The transfer matrix approach

Jan Ambjorn, Jakub Gizbert-Studnicki, Andrzej Görlich, Jerzy Jurkiewicz
(Submitted on 24 Mar 2014)
We measure the effective action in all three phases of 4-dimensional Causal Dynamical Triangulations (CDT) using the transfer matrix method. The transfer matrix is parametrized by the total 3-volume of the CDT universe at a given (discrete) time. We present a simple effective model based on the transfer matrix measured in the de Sitter phase. It allows us to reconstruct the results of full CDT in this phase. We argue that the transfer matrix method is valid not only inside the de Sitter phase ('C') but also in the other two phases. A parametrization of the measured transfer matrix / effective action in the 'A' and 'B' phases is proposed and the relation to phase transitions is explained. We discover a potentially new 'bifurcation' phase separating the de Sitter phase ('C') and the 'collapsed' phase ('B').

 

[MTd2]

http://arxiv.org/abs/1403.5940

The effective action in 4-dim CDT. The transfer matrix approach

Jan Ambjorn, Jakub Gizbert-Studnicki, Andrzej Görlich, Jerzy Jurkiewicz
(Submitted on 24 Mar 2014)
We measure the effective action in all three phases of 4-dimensional Causal Dynamical Triangulations (CDT) using the transfer matrix method. The transfer matrix is parametrized by the total 3-volume of the CDT universe at a given (discrete) time. We present a simple effective model based on the transfer matrix measured in the de Sitter phase. It allows us to reconstruct the results of full CDT in this phase. We argue that the transfer matrix method is valid not only inside the de Sitter phase ('C') but also in the other two phases. A parametrization of the measured transfer matrix / effective action in the 'A' and 'B' phases is proposed and the relation to phase transitions is explained. We discover a potentially new 'bifurcation' phase separating the de Sitter phase ('C') and the 'collapsed' phase ('B').

 

[marcus]

http://arxiv.org/abs/1403.6396
Viability of the matter bounce scenario in Loop Quantum Cosmology from BICEP2 last data
Jaume de Haro, Jaume Amorós
(Submitted on 25 Mar 2014)
The CMB map provided by the Planck project constrains the value of the ratio of tensor-to-scalar perturbations, namely r, to be smaller than 0.11 (95% CL). This bound rules out the simplest models of inflation. However, recent data from BICEP2 is in strong tension with this constrain, as it finds a value r=0.20^+0.07_−0.05 with r=0 disfavored at 7.0σ, which allows these simplest inflationary models to survive. The remarkable fact is that, even though the BICEP2 experiment was conceived to search for evidence of inflation, its experimental data matches correctly theoretical results coming from the matter bounce scenario (the alternative model to the inflationary paradigm). More precisely, most bouncing cosmologies do not pass Planck's constrains due to the smallness of the value of the tensor/scalar ratio r≤0.11, but with new BICEP2 data some of them fit well with experimental data. This is the case with the matter bounce scenario in the teleparallel version of Loop Quantum Cosmology.
4 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1403.7121
Towards the Turaev-Viro amplitudes from a Hamiltonian constraint
Valentin Bonzom, Maité Dupuis, Florian Girelli
(Submitted on 27 Mar 2014)
3D Loop Quantum Gravity with a vanishing cosmological constant can be related to the quantization of the SU(2) BF theory discretized on a lattice. At the classical level, this discrete model characterizes discrete flat geometries and its phase space is built from T*SU(2). In a recent paper [arXiv:1402.2323], this discrete model was deformed using the Poisson-Lie group formalism and was shown to characterize discrete hyperbolic geometries while being still topological. Hence, it is a good candidate to describe the discretization of SU(2) BF theory with a (negative) cosmological constant. We proceed here to the quantization of this model. At the kinematical level, the Hilbert space is spanned by spin networks built on [the quantum algebra] Uq(su(2)) (with q real). In particular, the quantization of the discretized Gauss constraint leads naturally to Uq(su(2)) intertwiners. We also quantize the Hamiltonian constraint on a face of degree 3 and show that physical states are proportional to the quantum 6j-symbol. This suggests that the Turaev-Viro amplitude with q real is a solution of the quantum Hamiltonian. This model is therefore a natural candidate to describe 3D loop quantum gravity with a (negative) cosmological constant.
24 pages, 6 figures

 

[marcus]

http://arxiv.org/abs/1403.7482
Deformed Spinor Networks for Loop Gravity: Towards Hyperbolic Twisted Geometries
Maité Dupuis, Florian Girelli, Etera R. Livine
(Submitted on 28 Mar 2014)
In the context of a canonical quantization of general relativity, one can deform the loop gravity phase space on a graph by replacing the T*SU(2) phase space attached to each edge by SL(2,C) seen as a phase space. This deformation is supposed to encode the presence of a non-zero cosmological constant. Here we show how to parametrize this phase space in terms of spinor variables, thus obtaining deformed spinor networks for loop gravity, with a deformed action of the gauge group SU(2) at the vertices. These are to be formally interpreted as the generalization of loop gravity twisted geometries to a hyperbolic curvature.
15 pages, 4 figures

general interest:
http://arxiv.org/abs/1403.7377
The Confrontation between General Relativity and Experiment
Clifford M. Will
(Submitted on 28 Mar 2014)
The status of experimental tests of general relativity and of theoretical frameworks for analyzing them are reviewed and updated. Einstein's equivalence principle (EEP) is well supported by experiments such as the Eotvos experiment, tests of local Lorentz invariance and clock experiments. Ongoing tests of EEP and of the inverse square law are searching for new interactions arising from unification or quantum gravity. Tests of general relativity at the post-Newtonian level have reached high precision, including the light deflection, the Shapiro time delay, the perihelion advance of Mercury, the Nordtvedt effect in lunar motion, and frame-dragging. Gravitational-wave damping has been detected in an amount that agrees with general relativity to better than half a percent using the Hulse-Taylor binary pulsar, and a growing family of other binary pulsar systems is yielding new tests, especially of strong-field effects. Current and future tests of relativity will center on strong gravity and gravitational waves.
113 pages, 10 figures, an update of the 2006 Living Review arXiv:gr-qc/0510072 ; submitted to Living Reviews in Relativity

 

[marcus]

http://arxiv.org/abs/1403.8062
Observables for General Relativity related to geometry
Paweł Duch, Wojciech Kamiński, Jerzy Lewandowski, Jedrzej Świeżewski
(Submitted on 31 Mar 2014)
We present a new scheme of defining invariant observables for general relativistic systems. The scheme is based on the introduction of an observer which endowes the construction with a straightforward physical interpretation. The observables are invariant with respect to spatial diffeomorphisms which preserve the observer. The limited residual spatial gauge freedom is studied and fully understood. A full canonical analysis of the observables is presented: we analyze their variations, Poisson algebra and discuss their dynamics. Lastly, the observables are used to solve the vector constraint, which triggers a possible considerable reduction of the degrees of freedom of general relativistic theories.
33 pages, 1 figure

general interest:
http://arxiv.org/abs/1403.8049
Neutrinos help reconcile Planck measurements with both Early and Local Universe
Cora Dvorkin, Mark Wyman, Douglas H. Rudd, Wayne Hu
(Submitted on 31 Mar 2014)
In light of the recent BICEP2 B-mode polarization detection, which implies a large inflationary tensor-to-scalar ratio r=0.2^+0.07_-0.05, we re-examine the evidence for an extra sterile massive neutrino, originally invoked to account for the tension between the cosmic microwave background (CMB) temperature power spectrum and local measurements of the expansion rate H₀ and cosmological structure. With only the standard active neutrinos and power-law scalar spectra, this detection is in tension with the upper limit of r<0.11 (95% confidence) from the lack of a corresponding low multipole excess in the temperature anisotropy from gravitational waves. An extra sterile species with the same energy density as is needed to reconcile the CMB data with H₀ measurements can also alleviate this new tension. By combining data from the Planck and ACT/SPT temperature spectra, WMAP9 polarization, H₀, baryon acoustic oscillation and local cluster abundance measurements with BICEP2 data, we find the joint evidence for a sterile massive neutrino increases to ΔN_eff=0.81± 0.25 for the effective number and m_s=0.47± 0.13 eV for the effective mass or 3.2 σ and 3.6 σ evidence respectively.
7 pages, 7 figures

 

[MTd2]

http://arxiv.org/abs/1403.7623v1

Evidence for bouncing evolution before inflation after BICEP2

Jun-Qing Xia, Yi-Fu Cai, Hong Li, Xinmin Zhang
(Submitted on 29 Mar 2014)
The BICEP2 collaboration reports a detection of primordial cosmic microwave background (CMB) B-mode with a tensor-scalar ratio r=0.20+0.07−0.05 (68% C.L.). However, this result is in tension with the recent Planck limit, r<0.11 (95% C.L.), on constraining inflation models. In this Letter we consider an inflationary cosmology with a preceding nonsingular bounce which gives rise to observable signatures on primordial perturbations. One interesting phenomenon is that both the primordial scalar and tensor modes can have a step feature on their power spectra, which nicely cancels the tensor excess power on the CMB temperature power spectrum. By performing a global analysis, we obtain the 68% C.L. constraints on the parameters of the model from the Planck+WP and BICEP2 data together: the jump scale log10(kb/Mpc−1)=−2.4±0.2 and the spectrum amplitude ratio of bounce-to-inflation rB≡Pm/As=0.71±0.09. Our result reveals that the bounce inflation scenario can simultaneously explain the Planck and BICEP2 observations better than the standard ΛCDM model, and can be verified by the future CMB polarization measurements.

 

[MTd2]

http://arxiv.org/abs/1404.0228v1

Big Bang as a critical point

Jakub Mielczarek
(Submitted on 1 Apr 2014)
This essay addresses the issue of gravitational phase transitions in the early universe. We suggest that a second order phase transition observed in the Causal Dynamical Triangulations approach to quantum gravity may have a cosmological relevance. The phase transition interpolates between a non-geometric crumpled phase of gravity, and an extended phase with classical properties. Transitions of this kind have been postulated earlier in the context of geometrogenesis in Quantum Graphity. We show that critical behavior may also be associated with a signature change event in Loop Quantum Cosmology. In both cases, classical spacetime originates at the critical point associated with a second order phase transition.

 

[marcus]

http://arxiv.org/abs/1404.0602
Entanglement entropy and negative-energy fluxes in two-dimensional spacetimes
Eugenio Bianchi, Matteo Smerlak
(Submitted on 2 Apr 2014)
It is well known that quantum effects can violate the positive energy conditions, if only for a limited time. Here we show in the context of two-dimensional conformal field theory that such violations are generic, and can be related to the entanglement structure of the conformal vacuum. Specifically, we prove that the renormalized energy flux F and entanglement entropy S at future null infinity satisfy
$\int_{\mathcal{I}^{+}}d\lambda\, F(\lambda)\exp[6 S(\lambda)/c]=0$
where c is the central charge (c=1 for the free scalar). When applied to unitary black hole evaporation, this identity implies that the semiclassical retarded mass (classical ADM mass minus vacuum outgoing energy) cannot be monotonically decreasing.
5 pages, 3 figuresgeneral interest:
http://arxiv.org/abs/1404.0634
From B Modes to Quantum Gravity and Unification of Forces
Lawrence M. Krauss, Frank Wilczek
(Submitted on 2 Apr 2014)
It is commonly anticipated that gravity is subject to the standard principles of quantum mechanics. Yet some (including Einstein) have questioned that presumption, whose empirical basis is weak. Indeed, recently Freeman Dyson has emphasized that no conventional experiment is capable of detecting individual gravitons. However, as we describe, if inflation occurred, the Universe, by acting as an ideal graviton amplifier, affords such access. It produces a classical signal, in the form of macroscopic gravitational waves, in response to spontaneous (not induced) emission of gravitons. Thus recent BICEP2 observations of polarization in the cosmic microwave background will, if confirmed, provide empirical evidence for the quantization of gravity. Their details also support quantitative ideas concerning the unification of strong, electromagnetic, and weak forces, and of all these with gravity.
4 pages

http://arxiv.org/abs/1404.0476
Λ may not be vacuum energy, after all
Luciano Vanzo
(Submitted on 2 Apr 2014)
We suggest the possibility that the mysterious dark energy component driving the acceleration of the Universe is the leading term, in the de Sitter temperature, of the free energy density of space-time seen as a quantum gravity coherent state of the gravitational field. The corresponding field theory classically has positive pressure, and can be considered as living on the Hubble horizon, or, alternatively, within the non compact part of the Robertson-Walker metric, both manifolds being characterized by the same scale and degrees of freedom. The equation of state is then recovered via the conformal anomaly. No such interpretation seems to be available for negative Λ.
5 pages, Essay written for the Gravity Research Foundation 2014 Contest

http://arxiv.org/abs/1404.0535
Eternal Universe
C. Wetterich
(Submitted on 2 Apr 2014)
10 pages, 1 figure

 

[MTd2]

http://arxiv.org/abs/1404.0622

End of the cosmic neutrino energy spectrum

L. A. Anchordoqui, V. Barger, H. Goldberg, J. G. Learned, D. Marfatia, S. Pakvasa, T. C. Paul, T. J. Weiler
(Submitted on 2 Apr 2014)
There may be a high-energy cutoff of neutrino events in IceCube data. In particular, IceCube does not observe the Standard Model Glashow-resonance events expected at 6.3 PeV. There are also no higher-energy neutrino signatures in the ANITA and Auger experiments. This absence of high-energy neutrino events motivates models with a fundamental restriction on neutrino energies above a few PeV. The simplest scenario to terminate the neutrino spectrum is Lorentz-invariance violating with a limiting neutrino velocity that is smaller than the speed of light. A consequence is that charged pions are stable above four times the maximum neutrino energy and may serve as a cosmic ray primary.

 

[marcus]

http://arxiv.org/abs/1404.1018
Anomaly-free cosmological perturbations in effective canonical quantum gravity
Aurelien Barrau, Martin Bojowald, Gianluca Calcagni, Julien Grain, Mikhail Kagan
(Submitted on 3 Apr 2014)
This article lays out a complete framework for an effective theory of cosmological perturbations with corrections from canonical quantum gravity. Since several examples exist for quantum-gravity effects that change the structure of space-time, the classical perturbative treatment must be rethought carefully. The present discussion provides a unified picture of several previous works, together with new treatments of higher-order perturbations and the specification of initial states.
56 pages

disfavoring large extra dimensions:
http://arxiv.org/abs/1404.0745
Does the BICEP2 Observation of Cosmological Tensor Modes Imply an Era of Nearly Planckian Energy Densities?
Chiu Man Ho, Stephen D. H. Hsu
(Submitted on 3 Apr 2014)
BICEP2 observations, interpreted most simply, suggest an era of inflation with energy densities of order (10¹⁶GeV)⁴, not far below the Planck density. However, models of TeV gravity with large dimensions might allow a very different interpretation involving much more modest energy scales. We discuss the viability of inflation in such models, and conclude that existing scenarios do not provide attractive alternatives to single field inflation in four dimensions. Because the detection of tensor modes strengthens our confidence that inflation occurred, it disfavors models of large extra dimensions, at least for the moment.
4 pages

 

[marcus]

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective
T. Padmanabhan, Hamsa Padmanabhan
(Submitted on 8 Apr 2014)
Observations indicate that our universe is characterized by a late-time accelerating phase, possibly driven by a cosmological constant Λ, with the dimensionless parameter ΛL_P² ≃ 10⁻¹²², where L_P=(Gℏ/c³)^1/2 is the Planck length. In this review, we describe how the emergent gravity paradigm provides a new insight and a possible solution to the cosmological constant problem. After reviewing the necessary background material, we identify the necessary and sufficient conditions for solving the cosmological constant problem. We show that these conditions are naturally satisfied in the emergent gravity paradigm in which (i) the field equations of gravity are invariant under the addition of a constant to the matter Lagrangian and (ii) the cosmological constant appears as an integration constant in the solution. The numerical value of this integration constant can be related to another dimensionless number (called CosMIn) that counts the number of modes inside a Hubble volume that cross the Hubble radius during the radiation and the matter dominated epochs of the universe. The emergent gravity paradigm suggests that CosMIn has the numerical value 4π, which, in turn, leads to the correct, observed value of the cosmological constant. Further, the emergent gravity paradigm provides an alternative perspective on cosmology and interprets the expansion of the universe itself as a quest towards holographic equipartition. We discuss the implications of this novel and alternate description of cosmology.
48 pages; 5 figures. Invited review to appear in Int. Jour. Mod. Phys. D

general interest:
http://arxiv.org/abs/1404.2591
Broaden the search for dark matter
Mario Livio, Joe Silk
(Submitted on 9 Apr 2014)
Bold strategies are needed to identify most of the elusive particles that should make up most of the Universe's mass
12 pages, 1 figure, published in Nature, v. 507, p.29 (2014)

 

[marcus]

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen
(Submitted on 7 Apr 2014)
Following earlier insights by Livine and Terno, we develop a technique for describing quantum states of the gravitational field in terms of coarse grained spin networks. We show that the number of nodes and links and the values of the spin depend on the observables chosen for the description of the state. Hence the question in the title of this paper is ill posed, unless further information about what is being measured is given.
16 pages, 9 figures

http://arxiv.org/abs/arXiv:1404.2803
General dissipative coefficient in warm intermediate inflation in loop quantum cosmology in light of Planck and BICEP2
Ramón Herrera, Marco Olivares, Nelson Videla
(Submitted on 10 Apr 2014)
In this paper, we study a warm intermediate inflationary model with a general form for the dissipative coefficient Γ(T,ϕ)=C_ϕT^m/ϕ^m−1 in the context of loop quantum cosmology. We examine this model in the weak and strong dissipative regimes. In general, we discuss in great detail the characteristics of this model in the slow-roll approximation. In this approach, we use recent astronomical observations from Planck and BICEP2 experiments to restrict the parameters in our model.
19 pages, 3 figures

 

[marcus]

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen
(Submitted on 10 Apr 2014)
Spatially homogeneous universes can be described in (loop) quantum gravity as condensates of elementary excitations of space. Their treatment is easiest in the second-quantised group field theory formalism which allows the adaptation of techniques from the description of Bose-Einstein condensates in condensed matter physics. Dynamical equations for the states can be derived directly from the underlying quantum gravity dynamics. The analogue of the Gross-Pitaevskii equation defines an anisotropic quantum cosmology model, in which the condensate wavefunction becomes a quantum cosmology wavefunction on minisuperspace. To illustrate this general formalism, we give a mapping of the gauge-invariant geometric data for a tetrahedron to a minisuperspace of homogeneous anisotropic 3-metrics. We then study an example for which we give the resulting quantum cosmology model in the general anisotropic case and derive the general analytical solution for isotropic universes. We discuss the interpretation of these solutions and comment on the validity of the WKB approximation used in previous studies.
20 pages

 

[marcus]

http://arxiv.org/abs/1404.3696
Lorentz Invariance in Shape Dynamics
S. Carlip, H. Gomes
(Submitted on 14 Apr 2014)
Shape dynamics is a reframing of canonical general relativity in which time reparametrization invariance is "traded" for a local conformal invariance. We explore the emergence of Lorentz invariance in this model in three contexts: as a maximal symmetry, an asymptotic symmetry, and a local invariance.
13 pages

 

[marcus]

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing
(Submitted on 15 Apr 2014)
We study the loop quantum cosmology of a flat Friedmann-Lemaitre-Robertson-Walker space-time with a Maxwell field. We show that many of the qualitative properties derived for the case of a massless scalar field also hold for a Maxwell field. In particular, the big-bang singularity is replaced by a quantum bounce, and the operator corresponding to the matter energy density is bounded above by the same critical energy density. We also numerically study the evolution of wave functions that are sharply peaked in the low energy regime, and derive effective equations which very closely approximate the full quantum dynamics of sharply peaked states at all times, including the near-bounce epoch.
27 pages, 6 figures

not QG, but of possible interest:
http://arxiv.org/abs/1404.3729
3.5 keV X-rays as the "21 cm line" of dark atoms, and a link to light sterile neutrinos
James M. Cline, Yasaman Farzan, Zuowei Liu, Wei Xue
(Submitted on 14 Apr 2014)
The recently discovered 3.5 keV X-ray line from extragalactic sources may be evidence of dark matter scatterings or decays. We show that dark atoms can be the source of the emission, through their hyperfine transitions, which would be the analog of 21 cm radiation from a dark sector. We identify two families of dark atom models that match the X-ray observations and are consistent with other constraints. In the first, the hyperfine excited state is long-lived compared to the age of the universe, and the dark atom mass is relatively unconstrained; dark atoms could be strongly self-interacting in this case. In the second, the excited state is short-lived and viable models are parameterized by the value of the dark proton-to-electron mass ratio R: for R=10²−10⁴, the dark atom mass is predicted be in the range 350−1300 GeV, with fine structure constant α′≅0.1−0.6. In either class of models, the dark photon must be massive with mγ′≳ 1 MeV and decay into e+e−. Evidence for the model could come from direct detection of the dark atoms. In a natural extension of this framework, the dark photon could decay predominantly into invisible particles, for example ∼0.5 eV sterile neutrinos, explaining the extra radiation degree of freedom recently suggested by data from BICEP2.
5 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale
(Submitted on 16 Apr 2014)
I give a brief introduction to the relation between loop quantum gravity and twistor theory, and comment on some perspectives on the problem of time.
10 pages, invited lecture to the "2nd International Conference on New Frontiers in Physics 2013" (ICNFP 2013), to be published in EPJ Web of Conferences vol. 71

 

[MTd2]

As Marcus said, general interest, but not QG(or is it?):

http://arxiv.org/abs/1404.4044

Breakdown of the equivalence between gravitational mass and energy for a composite quantum body

Andrei G. Lebed
(Submitted on 14 Apr 2014)
The simplest quantum composite body, a hydrogen atom, is considered in the presence of a weak external gravitational field. We define an operator for the passive gravitational mass of the atom in the post-Newtonian approximation of the general relativity and show that it does not commute with its energy operator. Nevertheless, the equivalence between the expectation values of the mass and energy is shown to survive at a macroscopic level for stationary quantum states. Breakdown of the equivalence between passive gravitational mass and energy at a microscopic level for stationary quantum states can be experimentally detected by studying unusual electromagnetic radiation, emitted by the atoms, supported by and moving in the Earth's gravitational field with constant velocity, using spacecraft or satellite.

http://arxiv.org/abs/1404.3765

Does the Equivalence between Gravitational Mass and Energy Survive for a Composite Quantum Body?

Andrei G. Lebed
(Submitted on 14 Apr 2014)
We define passive and active gravitational mass operators of the simplest composite quantum body - a hydrogen atom. Although they do not commute with its energy operator, the equivalence between the expectation values of passive and active gravitational masses and energy is shown to survive for stationary quantum states. In our calculations of passive gravitational mass operator, we take into account not only kinetic and Coulomb potential energies but also the so-called relativistic corrections to electron motion in a hydrogen atom. Inequivalence between passive and active gravitational masses and energy at a macroscopic level is demonstrated to reveal itself as time dependent oscillations of the expectation values of the gravitational masses for superpositions of stationary quantum states. Breakdown of the equivalence between passive gravitational mass and energy at a microscopic level reveals itself as unusual electromagnetic radiation, emitted by macroscopic ensemble of hydrogen atoms, moved by small spacecraft with constant velocity in the Earth's gravitational field. We suggest the corresponding experiment on the Earth's orbit to detect this radiation, which would be the first direct experiment where quantum effects in general relativity are observed.

 

[MTd2]

Non QG:

http://arxiv.org/abs/1404.4252

The Riemann zeros as energy levels of a Dirac fermion in a potential built from the prime numbers in Rindler spacetime

German Sierra
(Submitted on 16 Apr 2014)
We construct a Hamiltonian H whose discrete spectrum contains, in a certain limit, the Riemann zeros. H is derived from the action of a massless Dirac fermion living in a domain of Rindler spacetime, in 1+1 dimensions, that has a boundary given by the world line of a uniformly accelerated observer.
The action contains a sum of delta function potentials that can be viewed as partially reflecting moving mirrors. An appropriate choice of the accelerations of the mirrors, provide primitive periodic orbits associated to the prime numbers p, whose periods, measured by the observer's clock, are log p. Acting on the chiral components of the fermion, H becomes the Berry-Keating Hamiltonian (x p + p x)/2, where x is identified with the Rindler spatial coordinate and p with the conjugate momentum.
The delta function potentials give the matching conditions of the fermion wave functions on both sides of the mirrors. There is also a phase shift for the reflection of the fermions at the boundary where the observer sits. The eigenvalue problem is solved by transfer matrix methods in the limit where the reflection amplitudes become infinitesimally small. We find that for generic values of the phase shift the spectrum is a continuum, where the Riemann zeros are missing, as in the adelic Connes model. However, for some values of phase shift, related to the phase of the zeta function, the Riemann zeros appear as discrete eigenvalues immersed in the continuum.
We generalize this result to the zeros of Dirichlet L-functions, associated to primitive characters, that are encoded in the reflection coefficients of the mirrors. Finally, we show that the Hamiltonian associated to the Riemann zeros belongs to class AIII, or chiral GUE, of Random Matrix Theory.

 

[marcus]

http://arxiv.org/abs/1404.4364
Nonsingular bouncing cosmologies in light of BICEP2
Yi-Fu Cai, Jerome Quintin, Emmanuel N. Saridakis, Edward Wilson-Ewing
(Submitted on 16 Apr 2014)
We confront various nonsingular bouncing cosmologies with the recently released BICEP2 data and investigate the observational constraints on their parameter space. In particular, within the context of the effective field approach, we analyze the constraints on the matter bounce curvaton scenario with a light scalar field, and the new matter bounce cosmology model in which the universe successively experiences a period of matter contraction and an ekpyrotic phase. Additionally, we consider three nonsingular bouncing cosmologies obtained in the framework of modified gravity theories, namely the Horava-Lifshitz bounce model, the f(T) bounce model, and loop quantum cosmology.
8 pages, 3 figures

Refers to this February paper by two of the authors, Yi-Fu Cai and Edward Wilson-Ewing:
http://arxiv.org/abs/1402.3009
which in turn refers to this by Wilson-Ewing:
http://arxiv.org/abs/1306.6582

very brief mention:
http://arxiv.org/abs/1404.4614
Negative running prevents eternal inflation
William H. Kinney (Univ. at Buffalo, SUNY), Katherine Freese (Univ. of Michigan)
(Submitted on 17 Apr 2014)
Current data from the Planck satellite and the BICEP2 telescope favor, at around the 2σ level, negative running of the spectral index of curvature perturbations...

 

[atyy]

http://arxiv.org/abs/1404.4537
En route to Background Independence: Broken split-symmetry, and how to restore it with bi-metric average actions
Daniel Becker, Martin Reuter
(Submitted on 17 Apr 2014)
The most momentous requirement a quantum theory of gravity must satisfy is Background Independence, necessitating in particular an ab initio derivation of the arena all non-gravitational physics takes place in, namely spacetime. Using the background field technique, this requirement translates into the condition of an unbroken split-symmetry connecting the (quantized) metric fluctuations to the (classical) background metric. If the regularization scheme used violates split-symmetry during the quantization process it is mandatory to restore it in the end at the level of observable physics. In this paper we present a detailed investigation of split-symmetry breaking and restoration within the Effective Average Action (EAA) approach to Quantum Einstein Gravity (QEG) with a special emphasis on the Asymptotic Safety conjecture. In particular we demonstrate for the first time in a non-trivial setting that the two key requirements of Background Independence and Asymptotic Safety can be satisfied simultaneously. Carefully disentangling fluctuation and background fields, we employ a `bi-metric' ansatz for the EAA and project the flow generated by its functional renormalization group equation on a truncated theory space spanned by two separate Einstein-Hilbert actions for the dynamical and the background metric, respectively. A new powerful method is used to derive the corresponding renormalization group (RG) equations for the Newton- and cosmological constant, both in the dynamical and the background sector. We classify and analyze their solutions in detail, determine their fixed point structure, and identify an attractor mechanism which turns out instrumental in the split-symmetry restoration. We show that there exists a subset of RG trajectories which are both asymptotically safe and split-symmetry restoring. We conclude that the next generation of truncations must be bi-metric.

 

[marcus]

http://arxiv.org/abs/1404.5288
Discretization independence implies non-locality in 4D discrete quantum gravity
Bianca Dittrich, Wojciech Kaminski, Sebastian Steinhaus
(Submitted on 21 Apr 2014)
The 4D Regge action is invariant under 5--1 and 4--2 Pachner moves, which define a subset of (local) changes of the triangulation. Given this fact one might hope to find a local path integral measure that makes the quantum theory invariant under these moves and hence makes the theory partially triangulation invariant. We show that such a local invariant path integral measure does not exist for the 4D linearized Regge theory.
To this end we uncover an interesting geometric interpretation for the Hessian of the 4D Regge action. This geometric interpretation will allow us to prove that the determinant of the Hessian of the 4D Regge action does not factorize over 4--simplices or subsimplices. It furthermore allows to determine configurations where this Hessian vanishes, which only appears to be the case in degenerate backgrounds or if one allows for different orientations of the simplices.
We suggest a non--local measure factor that absorbs the non--local part of the determinant of the Hessian under 5--1 moves as well as a local measure factor that is preserved for very special configurations.
18 pages, 6 figures

http://arxiv.org/abs/1404.5284
Fluctuation energies in quantum cosmology
Martin Bojowald
(Submitted on 21 Apr 2014)
Quantum fluctuations or other moments of a state contribute to energy expectation values and can imply interesting physical effects. In quantum cosmology, they turn out to be important for a discussion of density bounds and instabilities of initial-value problems in the presence of signature change in loop-quantized models. This article provides an effective description of these issues, accompanied by a comparison with existing numerical results and an extension to squeezed states. The comparison confirms that canonical effective methods are well-suited for computations of properties of physical states. As a side product, an example is found for a simple state in which quantum fluctuations can cancel holonomy modifications of loop quantum cosmology.
35 pages, 6 figures

 

[MTd2]

http://arxiv.org/abs/1404.5821

Planck star phenomenology

Aurelien Barrau, Carlo Rovelli
(Submitted on 23 Apr 2014)
It is possible that black holes hide a core of Planckian density, sustained by quantum-gravitational pressure. As a black hole evaporates, the core remembers the initial mass and the final explosion occurs at macroscopic scale. We investigate possible phenomenological consequences of this idea. Under several rough assumptions, we estimate that up to several short gamma-ray bursts per day, around 10 MeV, with isotropic distribution, can be expected coming from a region of a few hundred light years around us.

Related:

http://arxiv.org/abs/1404.5635

Proof of a Quantum Bousso Bound

Raphael Bousso, Horacio Casini, Zachary Fisher, Juan Maldacena
(Submitted on 22 Apr 2014)
We prove the generalized Covariant Entropy Bound, ΔS≤(A−A′)/4Gℏ, for light-sheets with initial area A and final area A′. The entropy ΔS is defined as a difference of von Neumann entropies of an arbitrary state and the vacuum, with both states restricted to the light-sheet under consideration. The proof applies to free fields, in the limit where gravitational backreaction is small. We do not assume the null energy condition. In regions where it is violated, we find that the bound is protected by the defining property of light-sheets: that their null generators are nowhere expanding.

 

[marcus]

http://arxiv.org/abs/1404.6797
Quantum Gravity models - brief conceptual summary
Jerzy Lukierski
(Submitted on 27 Apr 2014)
After short historical overview we describe the difficulties with application of standard QFT methods in quantum gravity (QG). The incompatibility of QG with the use of classical continuous space-time required conceptually new approach. We present briefly three proposals:

• loop quantum gravity (LQG),
• the field-theoretic framework on noncommutative space-time and
• QG models formulated on discretized (triangularized) space-time.

We evaluate these models as realizing expected important properties of QG: background independence, consistent quantum diffeomorphisms, noncommutative or discrete structure of space-time at very short distances, finite/renormalizable QG corrections. We only briefly outline an important issue of embedding QG into larger geometric and dynamical frameworks (e.g. supergravity, (super)strings, p-branes, M-theory), with the aim to achieve full unification of all fundamental interactions.
24 pages; invited article for the book "Mathematical Structure of the Universe", publ. Copernicus Center Press, Copernicus Center for Interdisciplinary Studies, Cracow 2014

 

[marcus]

general interest (observational cosmology/astrophysics are potential windows on QG):
http://arxiv.org/abs/1404.7266
Cosmological constraints on dark energy
Tamara M. Davis
(Submitted on 29 Apr 2014)
23 pages, 8 figures.