Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1003.1973
A Definition of Background Independence
Sean B. Gryb
35 pages
(Submitted on 9 Mar 2010)
"We propose a definition for background (in)/dependence in dynamical theories of the evolution of configurations that have a continuous symmetry and test this definition on particle models and on gravity. Our definition draws from Barbour's best-matching framework developed for the purpose of implementing spatial and temporal relationalism. Among other interesting theories, general relativity can be derived within this framework in novel ways. We study the detailed canonical structure of a wide range of best matching theories and show that their actions must have a local gauge symmetry. When gauge theory is derived in this way, we obtain at the same time a conceptual framework for distinguishing between background dependent and independent theories. Gauge invariant observables satisfying Kuchar's criterion are identified and, in simple cases, explicitly computed. We propose a procedure for inserting a global background time into temporally relational theories. Interestingly, using this procedure in general relativity leads to unimodular gravity."

 

[atyy]

http://arxiv.org/abs/1003.1729
Exploring Topological Phases With Quantum Walks
Authors: Takuya Kitagawa, Mark S. Rudner, Erez Berg, Eugene Demler
(Submitted on 8 Mar 2010)
The quantum walk was originally proposed as a quantum mechanical analogue of the classical random walk, and has since become a powerful tool in quantum information science. In this paper, we show that discrete time quantum walks provide a versatile platform for studying topological phases, which are currently the subject of intense theoretical and experimental investigation. In particular, we demonstrate that recent experimental realizations of quantum walks simulate a non-trivial one dimensional topological phase. With simple modifications, the quantum walk can be engineered to realize all of the topological phases which have been classified in one and two dimensions. We further discuss the existence of robust edge modes at phase boundaries, which provide experimental signatures for the non-trivial topological character of the system.

 

[MTd2]

http://arxiv.org/abs/1003.1998

Entropic corrections to Newton's law

Leonardo Modesto, Andrew Randono
(Submitted on 9 Mar 2010)
It has been known for some time that there is a deep connection between thermodynamics and gravity, with perhaps the most dramatic implication that the Einstein equations can be viewed as a thermodynamic equation of state. Recently Verlinde has proposed a model for gravity with a simple statistical mechanical interpretation that is applicable in the non-relatvistic regime. After critically analyzing the construction, we present a strong consistency check of the model. Specifically, we consider two well-motivated corrections to the area-entropy relation, the log correction and the volume correction, and follow Verlinde's construction to derive corrections to Newton's law of gravitation. We show that the deviations from Newton's law stemming from the log correction have the same form as the lowest order quantum effects of perturbative quantum gravity, and the deviations stemming from the volume correction have the same form as some modified Newtonian gravity models designed to explain the anomalous galactic rotation curves.

http://arxiv.org/abs/1003.2049

Surface gravity and Hawking temperature from entropic force viewpoint

Ee Chang-Young, Myungseok Eune, Kyoungtae Kimm, Daeho Lee
(Submitted on 10 Mar 2010)
We consider a freely falling holographic screen for the Schwarzschild and Reissner-Nordstr\"om black holes and evaluate the entropic force \`a la Verlinde. When the screen crosses the event horizon, the temperature of the screen agrees to the Hawking temperature and the entropic force gives rise to the surface gravity for both of the black holes.http://arxiv.org/abs/1003.2056

From the Hamiltonian formalism to Spin-Foams

Marin Diego (University of Trento - Italy -, Department of Physics and Associated Group of INFN Trento, Padova Section)
(Submitted on 10 Mar 2010)
Starting from a BF-type formulation of General Relativity in the canonical formalism, we construct a physical scalar product with no restrictions for the boundary states that, for particular cases, reproduce the E.P.R. amplitude.

 

[MTd2]

http://arxiv.org/abs/1003.2412

Hamiltonian analysis of SO(4,1) constrained BF theory

R. Durka, J. Kowalski-Glikman
(Submitted on 11 Mar 2010)
In this paper we discuss canonical analysis of SO(4,1) constrained BF theory. The action of this theory contains topological terms appended by a term that breaks the gauge symmetry down to the Lorentz subgroup SO(3,1). The equations of motion of this theory turn out to be the vacuum Einstein equations. By solving the B field equations one finds that the action of this theory contains not only the standard Einstein-Cartan term, but also the Holst term proportional to the inverse of the Immirzi parameter, as well as a combination of topological invariants. We show that the structure of the constraints of a SO(4,1) constrained BF theory is exactly that of gravity in Holst formulation. We also briefly discuss quantization of the theory.

http://arxiv.org/abs/1003.2312

Statistical Origin of Gravity

Rabin Banerjee, Bibhas Ranjan Majhi
(Submitted on 11 Mar 2010)
Starting from the definition of entropy used in statistical mechanics we show that it is proportional to the gravity action. For a stationary black hole this entropy is expressed as $S = E/ 2T$, where $T$ is the Hawking temperature and $E$ is shown to be the Komar energy. This relation is also equivalent to the generalised Smarr formula for mass.

 

[marcus]

http://www.fuw.edu.pl/~jpa/qgqg3/schedule.html
This page has links to the PDF slide files for most or all of the talks given at the third European QG school at Zakopane 28 Feb thru 4 March. The name QGQG emphasizes that QG includes both quantum geometry and quantum gravity.

http://arxiv.org/abs/1003.2348
Oriented Matroids -- Combinatorial Structures Underlying Loop Quantum Gravity
Johannes Brunnemann, David Rideout
41 pages, 25 figures
(Submitted on 11 Mar 2010)
"We analyze combinatorial structures which play a central role in determining spectral properties of the volume operator in loop quantum gravity (LQG). These structures encode geometrical information of the embedding of arbitrary valence vertices of a graph in 3-dimensional Riemannian space, and can be represented by sign strings containing relative orientations of embedded edges. We demonstrate that these signature factors are a special representation of the general mathematical concept of an oriented matroid. Moreover, we show that oriented matroids can also be used to describe the topology (connectedness) of directed graphs. Hence the mathematical methods developed for oriented matroids can be applied to the difficult combinatorics of embedded graphs underlying the construction of LQG. As a first application we revisit the analysis of [4-5], and find that enumeration of all possible sign configurations used there is equivalent to enumerating all realizable oriented matroids of rank 3, and thus can be greatly simplified. We find that for 7-valent vertices having no coplanar triples of edge tangents, the smallest non-zero eigenvalue of the volume spectrum does not grow as one increases the maximum spin j_max at the vertex, for any orientation of the edge tangents. This indicates that, in contrast to the area operator, considering large j_max does not necessarily imply large volume eigenvalues. In addition we give an outlook to possible starting points for rewriting the combinatorics of LQG in terms of oriented matroids."

 

[MTd2]

http://arxiv.org/abs/1003.2311

3+1 decomposition in the new action for the Einstein Theory of Gravitation

L.D.Faddeev
(Submitted on 11 Mar 2010)
The action of recently proposed formulation of Einstein Theory of Gravitation is written according to 3+1 decomposition of the space-time variables. The result coincides with known formulation of Dirac and Arnowitt-Deser-Misner.

 

[MTd2]

http://arxiv.org/abs/1003.2510

Quantization of Black Holes

Xiao-Gang He, Bo-Qiang Ma
(Submitted on 12 Mar 2010)
We show that black holes can be quantized in an intuitive and elegant way with results in agreement with conventional knowledge of black holes by using Bohr's idea of quantizing the motion of an electron inside the atom in quantum mechanics. We find that properties of black holes can be also derived from an Ansatz of quantized entropy $\Delta S=4\pi k {\Delta R / \lambdabar}$, which was suggested in a previous work to unify the black hole entropy formula and Verlinde's conjecture to explain gravity as an entropic force. Such an Ansatz also explains gravity as an entropic force from quantum effect. This suggests a way to unify gravity with quantum theory. Several interesting and surprising results of black holes are given from which we predict the existence of primordial black holes ranging from Planck scale both is size and energy to big ones in size but

 

[marcus]

http://arxiv.org/abs/1003.3012
Saturating the holographic entropy bound
Raphael Bousso, Ben Freivogel, Stefan Leichenauer
36 pages, 10 figures
(Submitted on 15 Mar 2010)
"The covariant entropy bound states that the entropy, S, of matter on a light-sheet cannot exceed a quarter of its initial area, A, in Planck units. The gravitational entropy of black holes saturates this inequality. The entropy of matter systems, however, falls short of saturating the bound in known examples. This puzzling gap has led to speculation that a much stronger bound, S< A^3/4, may hold true. In this note, we exhibit light-sheets whose entropy exceeds A^3/4 by arbitrarily large factors. In open FRW universes, such light-sheets contain the entropy visible in the sky; in the limit of early curvature domination, the covariant bound can be saturated but not violated. As a corollary, we find that the maximum observable matter and radiation entropy in universes with positive (negative) cosmological constant is of order Lambda^-1 (Lambda^-2), and not |Lambda|^-3/4 as had hitherto been believed. Our results strengthen the evidence for the covariant entropy bound, while showing that the stronger bound S< A^3/4 is not universally valid. We conjecture that the stronger bound does hold for static, weakly gravitating systems."

 

[John86]

http://arxiv.org/abs/1003.2783

Experimental proposal for testing the Emergence of Environment Induced (EIN) Classical Selection rules with Biological Systems
Authors: Thomas Durt
(Submitted on 14 Mar 2010)

Abstract: According to the so-called Quantum Darwinist approach, the emergence of "classical islands" from a quantum background is assumed to obey a (selection) principle of maximal information. We illustrate this idea by considering the coupling of two oscillators (modes). As our approach suggests that the classical limit could have emerged throughout a long and progressive Evolution mechanism, it is likely that primitive living organisms behave in a "more quantum", "less classical" way than more evolved ones. This brings us to seriously consider the possibility to measure departures from classicality exhibited by biological systems. We describe an experimental proposal the aimed at revealing the presence of entanglement in the biophotonic radiation emitted by biological sources.

http://arxiv.org/abs/1003.2781

Crucial tests of the existence of a Time Operator
Authors: Thomas Durt
(Submitted on 14 Mar 2010)

Abstract: In the present paper we show that the Temporal Wave Function approach of e decay process, which is a multicomponent version of the Time Operator approach leads to new, non-standard, predictions concerning the statistical properties of decay time distributions of single kaons and entangled pairs of mesons. These results suggest crucial experimental tests for the existence of a Time Operator for the decay process to be realized in High Energy Physics or Quantum Optics.

 

[MTd2]

http://arxiv.org/abs/1003.3367

Black Hole Entropy Prediction without Immirzi Parameter

Brian Kong, Youngsub Yoon
(Submitted on 17 Mar 2010)
In our earlier paper "Corrections to the Bekenstein-Hawking entropy and the Hawking radiation spectrum", arXiv:0910.2755, we provided two concrete numerical evidences for the new area spectrum based on the Einstein-Kaufman pseudo tensor as opposed to the Ashtekar variables: namely, the reproduction of the Bekenstein-Hawking entropy without fixing Immirzi parameter and the reproduction of the Hawking radiation spectrum. In this article, we provide another concrete, numerical evidence for this new area spectrum; there was a constant in our earlier article, which was inversely proportional to the density of state, and which we could not fix a priori. Nevertheless, in our earlier article, we obtained this constant to be around 172~173 by fitting it to the Planck radiation spectrum. In this article, we calculate this value using another method. We obtain 172.87...which implies consistency.

 

[marcus]

http://arxiv.org/abs/1003.3483
Towards Spinfoam Cosmology
Eugenio Bianchi, Carlo Rovelli, Francesca Vidotto
8 pages
(Submitted on 17 Mar 2010)
"We compute the transition amplitude between coherent quantum-states of geometry peaked on homogeneous isotropic metrics. We use the holomorphic representations of loop quantum gravity and the Kaminski-Kisielowski-Lewandowski generalization of the new vertex, and work at first order in the vertex expansion, second order in the graph (multipole) expansion, and first order in 1/volume. We show that the resulting amplitude is in the kernel of a differential operator whose classical limit is the canonical hamiltonian of a Friedmann-Robertson-Walker cosmology. This result is an indication that the dynamics of loop quantum gravity defined by the new vertex yields the Friedmann equation in the appropriate limit."

 

[marcus]

http://arxiv.org/abs/1002.3966
Why all these prejudices against a constant?
Eugenio Bianchi, Carlo Rovelli
9 pages, 4 figures
(Submitted on 21 Feb 2010)
"The expansion of the observed universe appears to be accelerating. A simple explanation of this phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations. Arguments are commonly presented to the effect that this simple explanation is not viable or not sufficient, and therefore we are facing the 'great mystery' of the 'nature of a dark energy'. We argue that these arguments are unconvincing, or ill-founded."

 

[marcus]

http://arxiv.org/abs/1003.3802
On a Derivation of the Dirac Hamiltonian From a Construction of Quantum Gravity
Johannes Aastrup, Jesper M. Grimstrup, Mario Paschke
13 pages, two figures
(Submitted on 19 Mar 2010)
"The structure of the Dirac Hamiltonian in 3+1 dimensions is shown to emerge in a semi-classical approximation from a abstract spectral triple construction. The spectral triple is constructed over an algebra of holonomy loops, corresponding to a configuration space of connections, and encodes information of the kinematics of General Relativity. The emergence of the Dirac Hamiltonian follows from the observation that the algebra of loops comes with a dependency on a choice of base-point. The elimination of this dependency entails spinor fields and, in the semi-classical approximation, the structure of the Dirac Hamiltonian."

http://arxiv.org/abs/1003.3809
Warping with dilatation symmetry and self-tuning of the cosmological constant
C. Wetterich
28 pages
(Submitted on 19 Mar 2010)
"We investigate consequences of an ultraviolet fixed point in quantum gravity for the cosmological constant. For this purpose we perform dimensional reduction of a general dilatation symmetric effective action Gamma in dimension d>4 to an effective four-dimensional theory of gravity with a dilaton field. We find a stable flat phase in the space of extrema of Gamma which results in a vanishing four-dimensional cosmological constant Lambda. In order to understand the self-tuning mechanism leading to Lambda = 0 we discuss in detail the most general warped geometries with maximal four-dimensional symmetry and SO(d-4) isometry of internal space. While the solutions of the d-dimensional field equations admit singular spaces with arbitrary Lambda, the extremum condition for Gamma imposes additional restrictions which result in Lambda = 0. In cosmology, the dilatation symmetric fixed point may only be reached for asymptotic time t -> infinity. At finite t dilatation anomalies result in an effective potential and mass for the pseudo-dilaton or cosmon and in dark energy."

another part of the controversy over "dark energy" and the cosmological constant, joined in the recent Bianchi Rovelli paper:

http://arxiv.org/abs/1003.3845
Dark Energy and Projective Symmetry
G. W. Gibbons, C. M. Warnick
(Submitted on 19 Mar 2010)
"Nurowski [arXiv:1003.1503] has recently suggested a link between the observation of Dark Energy in cosmology and the projective equivalence of certain Friedman-Lemaitre-Robertson-Walker (FLRW) metrics. Specifically, he points out that two FLRW metrics with the same unparameterized geodesics have their energy densities differing by a constant. From this he queries whether the existence of dark energy is meaningful. We point out that physical observables in cosmology are not projectively invariant and we relate the projective symmetry uncovered by Nurowski to some previous work on projective equivalence in cosmology."

http://arxiv.org/abs/1003.1503
Is dark energy meaningless?
Pawel Nurowski
(Submitted on 7 Mar 2010)
"We show that there are isometrically nonequivalent Robertson-Walker metrics which have the same set of geodesics. While one of these metrics satisfies the Einstein equations of pure dust without a cosmological constant, all the other describe pure dust with additional energy momentum tensor of cosmological constant type. Since each of these metrics have the same geodesics it is not clear how to distinguish experimentally between the Universes whose energy momentum tensor includes or not the cosmological constant type term."

 

[atyy]

http://arxiv.org/abs/1003.3715
Spin and the Honeycomb Lattice: Lessons from Graphene
Matthew Mecklenburg, B. C. Regan
(Submitted on 19 Mar 2010)
Spin-1/2 particles such as the electron are described by the Dirac equation, which allows for two spin eigenvalues (up or down) and two types of energy eigenvalues (positive or negative, corresponding to the electron and the positron). A model of electrons hopping from carbon atom to carbon atom in graphene's honeycomb lattice gives low-energy electronic excitations that obey a relation formally identical to a two-dimensional Dirac equation. In graphene the spin equivalent, termed pseudospin, arises from the degeneracy introduced by the honeycomb lattice's two inequivalent atomic sites per unit cell. Previously it has been thought that the usual electron spin and the pseudospin indexing the graphene sublattice state are merely analogues. Here we show that the pseudospin is also a real, intrinsic angular momentum. This identification explains the suppression of electron backscattering in carbon nanotubes and the angular dependence of light absorption by graphene. Furthermore, it suggests that the half-integer spin of the quarks and leptons could derive from hidden substructure, not of the particles themselves, but rather of the space in which these particles live. In other words, the existence of spin might be interpreted as evidence that space consists of discrete points arranged in a non-cubic lattice.

 

[marcus]

http://arxiv.org/abs/1003.3942
Doubly-Special Relativity: Facts, Myths and Some Key Open Issues
Giovanni Amelino-Camelia
(Submitted on 20 Mar 2010)
"I report, emphasizing some key open issues and some aspects that are particularly relevant for phenomenology, on the status of the development of 'doubly-special' relativistic ('DSR') theories with both an observer-independent high-velocity scale and an observer-independent small-length/large-momentum scale, possibly relevant for the Planck-scale/quantum-gravity realm. I also give a true/false characterization of the structure of these theories. In particular, I discuss a DSR scenario without modification of the energy-momentum dispersion relation and without the kappa-Poincaré Hopf algebra, a scenario with deformed Poincaré symmetries which is not a DSR scenario, some scenarios with both an invariant length scale and an invariant velocity scale which are not DSR scenarios, and a DSR scenario in which it is easy to verify that some observable relativistic (but non-special-relativistic) features are insensitive to possible nonlinear redefinitions of symmetry generators."

 

[atyy]

http://arxiv.org/abs/1003.4134
Emergent Geometry and Gravity from Matrix Models: an Introduction
Harold Steinacker
(Submitted on 22 Mar 2010)
A introductory review to emergent noncommutative gravity within Yang-Mills Matrix models is presented. Space-time is described as a noncommutative brane solution of the matrix model, i.e. as submanifold of \R^D. Fields and matter on the brane arise as fluctuations of the bosonic resp. fermionic matrices around such a background, and couple to an effective metric interpreted in terms of gravity. Suitable tools are provided for the description of the effective geometry in the semi-classical limit. The relation to noncommutative gauge theory and the role of UV/IR mixing is explained. Several types of geometries are identified, in particular "harmonic" and "Einstein" type of solutions. The physics of the harmonic branch is discussed in some detail, emphasizing the non-standard role of vacuum energy. This may provide new approach to some of the big puzzles in this context. The IKKT model with D=10 and close relatives are singled out as promising candidates for a quantum theory of fundamental interactions including gravity.

http://arxiv.org/abs/1003.4132
Curvature and Gravity Actions for Matrix Models
Daniel N. Blaschke, Harold Steinacker
(Submitted on 22 Mar 2010)
We show how gravitational actions, in particular the Einstein-Hilbert action, can be obtained from additional terms in Yang-Mills matrix models. This is consistent with recent results on induced gravitational actions in these matrix models, realizing space-time as 4-dimensional brane solutions. It opens up the possibility for a controlled non-perturbative description of gravity through simple matrix models, with interesting perspectives for the problem of vacuum energy. The relation with UV/IR mixing and non-commutative gauge theory is discussed.

http://arxiv.org/abs/1003.4020
Averaging Spacetime: Where do we go from here?
R.J. van den Hoogen
(Submitted on 21 Mar 2010)
The construction of an averaged theory of gravity based on Einstein's General Relativity is very difficult due to the non-linear nature of the gravitational field equations. This problem is further exacerbated by the difficulty in defining a mathematically precise covariant averaging procedure for tensor fields over differentiable manifolds. Together, these two ideas have been called the averaging problem for General Relativity. In the first part of the talk, an attempt to review some the various approaches to this problem will be given, highlighting strengths, weaknesses, and commonalities between them. In the second part of the talk, an argument will be made, that if one wishes to develop a well-defined averaging procedure, one may choose to parallel transport along geodesics with respect to the Levi-Cevita connection or, use the Weitzenb\"ock connection and ensure the transportation is independent of path. The talk concludes with some open questions to generate further discussion.

 

[marcus]

http://arxiv.org/abs/1003.4485
An Invitation to Higher Gauge Theory
John C. Baez, John Huerta
60 pages, based on lectures at the 2nd School and Workshop on Quantum Gravity and Quantum Geometry at the 2009 Corfu Summer Institute
(Submitted on 23 Mar 2010)
"In this easy introduction to higher gauge theory, we describe parallel transport for particles and strings in terms of 2-connections on 2-bundles. Just as ordinary gauge theory involves a gauge group, this generalization involves a gauge '2-group'. We focus on 6 examples. First, every abelian Lie group gives a Lie 2-group; the case of U(1) yields the theory of U(1) gerbes, which play an important role in string theory and multisymplectic geometry. Second, every group representation gives a Lie 2-group; the representation of the Lorentz group on 4d Minkowski spacetime gives the Poincaré 2-group, which leads to a spin foam model for Minkowski spacetime. Third, taking the adjoint representation of any Lie group on its own Lie algebra gives a 'tangent 2-group', which serves as a gauge 2-group in 4d BF theory, which has topological gravity as a special case. Fourth, every Lie group has an 'inner automorphism 2-group', which serves as the gauge group in 4d BF theory with cosmological constant term. Fifth, every Lie group has an 'automorphism 2-group', which plays an important role in the theory of nonabelian gerbes. And sixth, every compact simple Lie group gives a 'string 2-group'. We also touch upon higher structures such as the 'gravity 3-group' and the Lie 3-superalgebra that governs 11-dimensional supergravity."

 

[marcus]

http://arxiv.org/abs/1003.4660
Inflation in loop quantum cosmology: dynamics and spectrum of gravitational waves
Jakub Mielczarek, Thomas Cailleteau, Julien Grain, Aurelien Barrau
11 pages, 14 figures
(Submitted on 24 Mar 2010)
"Loop quantum cosmology provides an efficient framework to study the evolution of the Universe beyond the classical Big Bang paradigm. Due to holonomy corrections, the singularity is replaced by a "bounce". The dynamics of the background is investigated into the details, as a function of the parameters of the model. In particular, the conditions required for inflation to occur are carefully considered and are shown to be generically met. The propagation of gravitational waves is then investigated in this framework. By both numerical and analytical approaches, the primordial tensor power spectrum is computed for a wide range of parameters. Several interesting features could be observationally probed."

http://arxiv.org/abs/1003.4701
Bi-metric theory of gravity from the non-chiral Plebanski action
Simone Speziale
28 pages
(Submitted on 24 Mar 2010)
"We study a modification of the Plebanski action for general relativity, which leads to a modified theory of gravity with eight degrees of freedom. We show how the action can be recasted as a bi-metric theory of gravity, and expanding around a bi-flat background we identify the six extra degrees of freedom with a second, massive graviton and a scalar mode."
Comments:

 

[atyy]

http://arxiv.org/abs/1003.4662
Quantum computation on the edge of a symmetry-protected topological order
Akimasa Miyake
(Submitted on 24 Mar 2010)
We elaborate the idea of quantum computation through measuring the correlation of a gapped ground state, while the bulk Hamiltonian is utilized to stabilize the resource. A simple computational primitive, by pulling out a single spin adiabatically from the bulk followed by its measurement, is shown to make any ground state of the one-dimensional isotropic Haldane phase useful ubiquitously as a quantum logical wire. The primitive is compatible with certain discrete symmetries that are crucial to protect this topological order, and the antiferromagnetic Heisenberg spin-1 chain of a finite length is practically a sufficient resource. Our approach manifests a holographic principle in that the logical information of a universal quantum computer can be written and processed perfectly on the edge state (i.e. boundary) of the system, supported by the persistent entanglement from the bulk even when the ground state and its evolution cannot be exactly analyzed.

 

[marcus]

http://arxiv.org/abs/1003.4811
Dark Energy, Black Hole Entropy, and the First Precision Measurement in Quantum Gravity
Niayesh Afshordi (Perimeter/Waterloo)
4 pages
(Submitted on 25 Mar 2010)
"The two apparently distinct phenomena of dark energy (or late-time cosmic acceleration) and quantum gravity dominate physics on extremely low, and extremely high energies, but do not seem to have any apparent empirical connection. Nevertheless, the two have a theoretical connection, through the cosmological constant problem. I argue that the finite temperature quantum gravitational corrections to black hole entropy yields a pressure for the gravitational vacuum (or gravitational aether). Assuming that the relative corrections are linear in horizon temperature (i.e. are suppressed by one power of Planck energy), the pressure is comparable to that of dark energy for astrophysical black holes. This implies that the observation of late-time cosmic acceleration may have provided us with the first precision measurement of quantum gravity, i.e. that of black hole entropy."

 

[atyy]

http://arxiv.org/abs/1003.4877
Translation-invariant noncommutative renormalization
Adrian Tanasa
(Submitted on 25 Mar 2010)
We review here the construction of a translation-invariant scalar model which was proved to be renormalizable on Moyal space. Some general considerations on non-local renormalizability are given. Finally, we present perspectives for generalizing these quantum field theoretical techniques to group field theory, a new setting for quantum gravity.

 

[atyy]

http://arxiv.org/abs/1003.4750
Oriented matroid theory and loop quantum gravity in (2+2) and eight dimensions
J. A. Nieto
(Submitted on 24 Mar 2010)
We establish a connection between oriented matroid theory and loop quantum gravity in (2+2) (two time and two space dimensions) and 8-dimensions. We start by observing that supersymmetry implies that the structure constants of the real numbers, complex numbers, quaternions and octonions can be identified with the chirotope concept. This means, among other things, that normed divisions algebras, which are only possible in 1,2, 4 or 8-dimensions, are linked to oriented matroid theory. Therefore, we argue that the possibility for developing loop quantum gravity in 8-dimensions must be taken as important alternative. Moreover, we show that in 4-dimensions, loop quantum gravity theories in the (1+3) or (0+4) signatures are not the only possibilities. In fact, we show that loop quantum gravity associated with the (2+2)-signature may also be an interesting physical structure.

 

[marcus]

http://arxiv.org/abs/1003.5129
Matter Induced Bimetric Actions for Gravity
Elisa Manrique, Martin Reuter, Frank Saueressig
34 pages
(Submitted on 26 Mar 2010)
"The gravitational effective average action is studied in a bimetric truncation with a nontrivial background field dependence, and its renormalization group flow due to a scalar multiplet coupled to gravity is derived. Neglecting the metric contributions to the corresponding beta functions, the analysis of its fixed points reveals that, even on the new enlarged theory space which includes bimetric action functionals, the theory is asymptotically safe in the large N expansion."

http://arxiv.org/abs/1003.5652
Spin foams with timelike surfaces
Florian Conrady (Perimeter Inst. Theor. Phys.)
22 pages
(Submitted on 29 Mar 2010)
"Spin foams of 4d gravity were recently extended from complexes with purely spacelike surfaces to complexes that also contain timelike surfaces. In this article, we express the associated partition function in terms of vertex amplitudes and integrals over coherent states. The coherent states are characterized by unit 3--vectors which represent normals to surfaces and lie either in the 2--sphere or the 2d hyperboloids. In the case of timelike surfaces, a new type of coherent state is used and the associated completeness relation is derived. It is also shown that the quantum simplicity constraints can be deduced by three different methods: by weak imposition of the constraints, by restriction of coherent state bases and by the master constraint."

 

[francesca]

http://arxiv.org/abs/1003.5245"
Searching for spacetime granularity: analyzing a concrete experimental setup
Yuri Bonder, Daniel Sudarsky
9 pages. For the proceedings of the VIII School of the Gravitation and Mathematical Physics Division of the Mexican Physical Society 'Speakable and unspeakable in gravitational physics: testing gravity from submillimeter to cosmic scale'.

"In this work we show that the spin pendulum techniques developed by the Eöt-Wash group could be used to put very stringent bounds on the free parameters of a Lorentz invariant phenomenological model of quantum gravity. The model is briefly described as well as the experimental setup that we have in mind."

 

[marcus]

http://arxiv.org/abs/1003.5665
Surface Density of Spacetime Degrees of Freedom from Equipartition Law in theories of Gravity
T. Padmanabhan
20 pages
(Submitted on 29 Mar 2010)
"I show that the principle of equipartition, applied to area elements of a surface which are in equilibrium at the local Davies-Unruh temperature, allows one to determine the surface number density of the microscopic spacetime degrees of freedom in any diffeomorphism invariant theory of gravity. The entropy associated with these degrees of freedom matches with the Wald entropy for the theory. This result also allows one to attribute an entropy density to the spacetime in a natural manner. The field equations of the theory can then be obtained by extremising this entropy. Moreover, when the microscopic degrees of freedom are in local thermal equilibrium, the spacetime entropy of a bulk region resides on its boundary."

http://arxiv.org/abs/1003.5890
Discovering the Discrete Universe
Joe Henson
24 pages, 4 figures. Based on a proceedings article for the "Foundations of Space and Time" conference, Cape Town, August 2009, in honour of George Ellis' 70th birthday.
(Submitted on 30 Mar 2010)
"This paper presents an brief review of some recent work on the causal set approach to quantum gravity. Causal sets are a discretisation of spacetime that allow the symmetries of GR to be preserved in the continuum approximation. One proposed application of causal sets is to use them as the histories in a quantum sum-over-histories, i.e. to construct a quantum theory of spacetime.
It is expected by many that quantum gravity will introduce some kind of 'fuzziness', uncertainty and perhaps discreteness into spacetime, and generic effects of this fuzziness are currently being sought. Applied as a model of discrete spacetime, causal sets can be used to construct simple phenomenological models which allow us to understand some of the consequences of this general expectation."

 

[marcus]

http://arxiv.org/abs/1003.6019
Motion in Quantum Gravity
Karim Noui
30 pages, to appear in the book "Mass and Motion in General Relativity", proceedings of the C.N.R.S. School in Orleans, France, eds. L. Blanchet, A. Spallicci and B. Whiting
(Submitted on 31 Mar 2010)
"We tackle the question of motion in Quantum Gravity: what does motion mean at the Planck scale? Although we are still far from a complete answer we consider here a toy model in which the problem can be formulated and resolved precisely. The setting of the toy model is three dimensional Euclidean gravity. Before studying the model in detail, we argue that Loop Quantum Gravity may provide a very useful approach when discussing the question of motion in Quantum Gravity."

http://arxiv.org/abs/1003.5952
Go with the Flow, Average Holographic Universe
George F. Smoot
14 pages
(Submitted on 31 Mar 2010)
"Gravity is a macroscopic manifestation of a microscopic quantum theory of space-time, just as the theories of elasticity and hydrodynamics are the macroscopic manifestation of the underlying quantum theory of atoms. The connection of gravitation and thermodynamics is long and deep. The observation that space-time has a temperature for accelerating observers and horizons is direct evidence that there are underlying microscopic degrees of freedom. The equipartition of energy, meaning of temperature, in these modes leads one to anticipate that there is also an entropy associated. When this entropy is maximized on a volume of space-time, then one retrieves the metric of space-time (i.e. the equations of gravity, e.g. GR). Since the metric satisfies the extremum in entropy on the volume, then the volume integral of the entropy can readily be converted to surface integral, via Gauss's Theorem. This surface integral is simply an integral of the macroscopic entropy flow producing the mean entropy holographic principle. This approach also has the added value that it naturally dispenses with the cosmological constant/vacuum energy problem in gravity except perhaps for second order quantum effects on the mean surface entropy."

[The abstract is sober and straightforward---tending to allay suspicion that might be raised by the exuberant title. But if you look into the article itself, you may find it a bit over-the-top. Given that it's early April, I'm not entirely certain how to take this article.
Readers can decide for themselves.]

 

[atyy]

http://arxiv.org/abs/1004.0055
On the consistency of the Horava Theory
Jorge Bellorin, Alvaro Restuccia
(Submitted on 1 Apr 2010)
The new proposal of Horava for a renormalizable theory of gravity has received a considerable amount of criticisms. It particular, it has been argued that the propagating physical degrees of freedom do not match with those of General Relativity. Moreover, it has been proposed that the only possibility for the lapse function is no other than to be zero everywhere, which would be catastrophic for the theory. With the goal of giving a test for the theoretical admissibility of the Horava Theory, we perform the Hamiltonian analysis of its effective action for large distances. This effective action has the same potential term of General Relativity, but the kinetic term is modified by the inclusion of an arbitrary coupling constant $\lambda$. Since this constant breaks the general space-time diffeomorphisms symmetry, it is believed that the model with arbitrary $\lambda$ deviates from General Relativity. Indeed, part of the computations done in previous papers to support the arguments against the Horava theory were performed using not the complete theory but precisely the effective model. In this paper we shown that this model is not a deviation at all, instead it is completely equivalent to General Relativity in a particular partial gauge fixing for it. In particular the physical degrees of freedom of both theories are identical. To show this, we identify a second class constraint of the model that was erroneously interpreted as a condition for the lapse function in previous papers. The lapse function is determined by another conservation equation following the Dirac approach, an equation that was already known from General Relativity and that has solutions with the correct asymptotic physical properties.

 

[MTd2]

http://arxiv.org/abs/1004.0352

Causal Dynamical Triangulations and the Quest for Quantum Gravity

J. Ambjorn, J. Jurkiewicz, R. Loll

Quantum Gravity by Causal Dynamical Triangulation has over the last few years emerged as a serious contender for a nonperturbative description of the theory. It is a nonperturbative implementation of the sum-over-histories, which relies on few ingredients and initial assumptions, has few free parameters and - crucially - is amenable to numerical simulations. It is the only approach to have demonstrated that a classical universe can be generated dynamically from Planckian quantum fluctuations. At the same time, it allows for the explicit evaluation of expectation values of invariants characterizing the highly nonclassical, short-distance behaviour of spacetime. As an added bonus, we have learned important lessons on which aspects of spacetime need to be fixed a priori as part of the background structure and which can be expected to emerge dynamically.

 

[marcus]

ALAIN CONNES RIDES AGAIN!
http://arxiv.org/abs/1004.0464
Noncommutative Geometry as a Framework for Unification of all Fundamental Interactions including Gravity. Part I
Ali H. Chamseddine, Alain Connes
56 pages
(Submitted on 3 Apr 2010)
"We examine the hypothesis that space-time is a product of a continuous four-dimensional manifold times a finite space. A new tensorial notation is developed to present the various constructs of noncommutative geometry. In particular, this notation is used to determine the spectral data of the standard model. The particle spectrum with all of its symmetries is derived, almost uniquely, under the assumption of irreducibility and of dimension 6 modulo 8 for the finite space. The reduction from the natural symmetry group SU(2)xSU(2)xSU(4) to U(1)xSU(2)xSU(3) is a consequence of the hypothesis that the two layers of space-time are finite distance apart but is non-dynamical. The square of the Dirac operator, and all geometrical invariants that appear in the calculation of the heat kernel expansion are evaluated. We re-derive the leading order terms in the spectral action. The geometrical action yields unification of all fundamental interactions including gravity at very high energies. We make the following predictions:

(i) The number of fermions per family is 16.

(ii) The symmetry group is U(1)xSU(2)xSU(3).

(iii) There are quarks and leptons in the correct representations.

(iv) There is a doublet Higgs that breaks the electroweak symmetry to U(1).

(v) Top quark mass of 170-175 Gev.

(vi) There is a right-handed neutrino with a see-saw mechanism.

Moreover, the zeroth order spectral action obtained with a cut-off function is consistent with experimental data up to few percent. We discuss a number of open issues. We prepare the ground for computing higher order corrections since the predicted mass of the Higgs field is quite sensitive to the higher order corrections. We speculate on the nature of the noncommutative space at Planckian energies and the possible role of the fundamental group for the problem of generations."

http://arxiv.org/abs/1004.0664
Classical paradoxes of locality and their possible quantum resolutions in deformed special relativity
Lee Smolin
19 pages
(Submitted on 5 Apr 2010)
"In deformed or doubly special relativity (DSR) the action of the lorentz group on momentum eigenstates is deformed to preserve a maximal momenta or minimal length, supposed equal to the Planck length. The classical and quantum dynamics of a particle propagating in kappa-Minkowski spacetime is discussed in order to examine an apparent paradox of locality which arises in the classical dynamics. This is due to the fact that the Lorentz transformations of spacetime positions of particles depend on their energies, so whether or not a local event, defined by the coincidence of two or more particles, takes place appears to depend on the frame of reference of the observer. Here it is proposed that the paradox arises only in the classical picture, and may be resolved when the quantum dynamics is taken into account. If so, the apparent paradoxes arise because it is inconsistent to study physics in which Planck's constant is zero but the Planck length is non-vanishing. This may be relevant for phenomenology such as observations by FERMI, because at leading order there is both a direct and a stochastic dependence of arrival time on energy, due to an additional spreading of wavepackets."

http://arxiv.org/abs/1004.0418
Bounds on an energy-dependent and observer-independent speed of light from violations of locality
Sabine Hossenfelder
short version of arXiv:0912.0090
(Submitted on 3 Apr 2010)
"We show that models with deformations of special relativity that have an energy-dependent speed of light have non-local effects. The requirement that the arising non-locality is not in conflict with known particle physics allows us to derive strong bounds on deformations of special relativity and rule out a modification to first order in energy over the Planck mass."

http://arxiv.org/abs/1004.0627
Entropic Corrections to Friedmann Equations
Ahmad Sheykhi
6 pages
(Submitted on 30 Mar 2010)
"Recently, Verlinde discussed that gravity can be understood as an entropic force caused by changes in the information associated with the positions of material bodies. In the Verlinde's argument, the area law of the black hole entropy plays a crucial role. However, the entropy-area relation can be modified from the inclusion of quantum effects, motivated from the loop quantum gravity. In this note, by employing this modified entropy-area relation, we derive corrections to Newton's law of gravitation as well as the modified Friedman equations by adopting the viewpoint that gravity can be emerged as an entropic force. Our study further supports the universality of the log correction and provides a strong consistency check on Verlinde's model."

http://arxiv.org/abs/1004.0648
Prospects for New Physics at the LHC
John Ellis
13 pages, 9 figures, Invited Talk at Conference in Honor of Murray Gell-Mann's 80th Birthday, on Quantum Mechanics, Elementary Particles, Quantum Cosmology and Complexity, Nanyang Executive Centre, NTU, Singapore, 24th-26th February 2010
(Submitted on 5 Apr 2010)
"High-energy collisions at the LHC are now starting. The new physics agenda of the LHC is reviewed, with emphasis on the hunt for the Higgs boson (or whatever replaces it) and supersymmetry. In particular, the prospects for discovering new physics in the 2010-2011 run are discussed."


brief mention:
http://arxiv.org/abs/1004.0525
Quantum Corrected Spherical Collapse: A Phenomenological Framework
Jonathan Ziprick, Gabor Kunstatter
(Submitted on 4 Apr 2010)

 

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http://arxiv.org/abs/1004.0476
Perturbative Quantum Gravity from Gauge Theory
Zvi Bern, John Joseph M. Carrasco, Henrik Johansson
(Submitted on 4 Apr 2010)
In a previous paper we observed that (classical) tree-level gauge theory amplitudes can be rearranged to display a duality between color and kinematics. Once this is imposed, gravity amplitudes are obtained using two copies of gauge-theory diagram numerators. Here we suggest that this duality persists to all quantum loop orders and can thus be used to obtain multi-loop gravity amplitudes easily from gauge-theory ones. As a non-trivial test, we show that the three-loop four-point amplitude of N=4 super-Yang-Mills theory can be arranged into a form satisfying the duality, and by taking double copies of the diagram numerators we obtain the corresponding amplitude of N=8 supergravity. We also remark on a non-supersymmetric two-loop test based on pure Yang-Mills theory resulting in gravity coupled to an anti-symmetric tensor and dilaton.

http://arxiv.org/abs/1004.0576
Galactic rotation curves in modified gravity with non-minimal coupling between matter and geometry
T. Harko
(Submitted on 5 Apr 2010)
We investigate the possibility that the behavior of the rotational velocities of test particles gravitating around galaxies can be explained in the framework of modified gravity models with non-minimal matter-geometry coupling. Generally, the dynamics of test particles around galaxies, as well as the corresponding mass deficit, is explained by postulating the existence of dark matter. The extra-terms in the gravitational field equations with geometry-matter coupling modify the equations of motion of test particles, and induce a supplementary gravitational interaction. Starting from the variational principle describing the particle motion in the presence of the non-minimal coupling, the expression of the tangential velocity of a test particle, moving in the vacuum on a stable circular orbit in a spherically symmetric geometry, is derived. The tangential velocity depends on the metric tensor components, as well as of the coupling function between matter and geometry. The Doppler velocity shifts are also obtained in terms of the coupling function. If the tangential velocity profile is known, the coupling term between matter and geometry can be obtained explicitly in an analytical form. The functional form of this function is obtained in two cases, for a constant tangential velocity, and for an empirical velocity profile obtained from astronomical observations, respectively. Therefore, these results open the possibility of directly testing the modified gravity models with non-minimal coupling between matter and geometry by using direct astronomical and astrophysical observations at the galactic or extra-galactic scale.

http://arxiv.org/abs/1004.0597
The Superfluid Universe
G.E. Volovik
(Submitted on 5 Apr 2010)
We discuss phenomenology of quantum vacuum. Phenomenology of macroscopic systems has three sources: thermodynamics, topology and symmetry. Thermodynamics of the self-sustained vacuum allows us to treat the problems related to the vacuum energy: the cosmological constant problems. The natural value of the energy density of the equilibrium the self-sustained vacuum is zero. Cosmology is discussed as the process of relaxation of vacuum towards the equilibrium state. The present value of the cosmological constant is very small compared to the Planck scale, because the present Universe is very old and thus is close to equilibrium. Momentum space topology determines the universality classes of fermionic vacua. The Standard Model vacuum both in its massless and massive states is topological medium. The vacuum in its massless state shares the properties of superfluid 3He-A, which is topological superfluid. It belongs to the Fermi-point universality class, which has topologically protected fermionic quasiparticles. At low energy they behave as relativistic massless Weyl fermions. Gauge fields and gravity emerge together with Weyl fermions at low energy. This allows us to treat the hierarchy problem in Standard Model: the masses of elementary particles are very small compared to the Planck scale because the natural value of the quark and lepton masses is zero. The small nonzero masses appear in the infrared region, where the quantum vacuum acquires the properties of another topological superfluid, 3He-B, and 3+1 topological insulators. The other topological media in dimensions 2+1 and 3+1 are also discussed. In most cases, topology is supported by discrete symmetry of the underlying microscopic system, which indicates the important role of discrete symmetry in Standard Model.

 

[MTd2]

http://arxiv.org/abs/1004.0628

Fractional Dynamics from Einstein Gravity, General Solutions, and Black Holes

Sergiu I. Vacaru
(Submitted on 5 Apr 2010)
We study the fractional gravity for spacetimes with non-integer dimensions. Our constructions are based on a geometric formalism with the fractional Caputo derivative and integral calculus adapted to nonolonomic distributions. This allows us to define a fractional spacetime geometry with fundamental geometric/physical objects and a generalized tensor calculus all being similar to respective integer dimension constructions. Such models of fractional gravity mimic the Einstein gravity theory and various Lagrange-Finsler and Hamilton-Cartan generalizations in nonholonomic variables. The approach suggests a number of new implications for gravity and matter field theories with singular, stochastic, kinetic, fractal, memory etc processes. We prove that the fractional gravitational field equations can be integrated in very general forms following the anholonomic deformation method for constructing exact solutions. Finally, we study some examples of fractional black hole solutions, fractional ellipsoid gravitational configurations and imbedding of such objects in fractional solitonic backgrounds.

 

[MTd2]

http://arxiv.org/abs/1004.0718

Causal random geometry from stochastic quantization

J. Ambjorn, R. Loll, W. Westra, S. Zohren
(Submitted on 5 Apr 2010)
In this short note we review a recently found formulation of two-dimensional causal quantum gravity defined through Causal Dynamical Triangulations and stochastic quantization. This procedure enables one to extract the nonperturbative quantum Hamiltonian of the random surface model including the sum over topologies. Interestingly, the generally fictitious stochastic time corresponds to proper time on the geometries.

http://arxiv.org/abs/1004.0877

A No-go Theorem Prohibiting Inflation in the Entropic Force Scenario

Miao Li, Yi Pang
(Submitted on 6 Apr 2010)
We show that to accommodate inflation in the entropic force scenario of Verlinde, it is necessary to introduce a negative temperature on a holographic screen, this will introduce several puzzles such as energy non-conservation. If one tries to modify the derivation of the Einstein equations to avoid a negative temperature, we prove that it is impossible to find a proper new definition of temperature to derive the Einstein equations.

http://arxiv.org/abs/1004.0693

Gravity as the Square of Gauge Theory

Zvi Bern, Tristan Dennen, Yu-tin Huang, Michael Kiermaier
(Submitted on 5 Apr 2010)
We explore consequences of the recently discovered duality between color and kinematics, which states that kinematic numerators in a diagrammatic expansion of gauge-theory amplitudes can be arranged to satisfy Jacobi-like identities in one-to-one correspondence to the associated color factors. Using on-shell recursion relations, we give a field-theory proof showing that the duality implies that diagrammatic numerators in gravity are just the product of two corresponding gauge-theory numerators, as previously conjectured. These squaring relations express gravity amplitudes in terms of gauge-theory ingredients, and are a recasting of the Kawai, Lewellen and Tye relations. Assuming that numerators of loop amplitudes can be arranged to satisfy the duality, our tree-level proof immediately carries over to loop level via the unitarity method. We then present a Yang-Mills Lagrangian whose diagrams through five points manifestly satisfy the duality between color and kinematics. The existence of such Lagrangians suggests that the duality also extends to loop amplitudes, as confirmed at two and three loops in a concurrent paper. By "squaring" the novel Yang-Mills Lagrangian we immediately obtain its gravity counterpart. We outline the general structure of these Lagrangians for higher points. We also write down various new representations of gauge-theory and gravity amplitudes that follow from the duality between color and kinematics.

 

[marcus]

http://arxiv.org/abs/1004.1063
New length operator for loop quantum gravity
Yongge Ma, Chopin Soo, Jinsong Yang
10 pages, 2 figures
(Submitted on 7 Apr 2010)
"An alternative expression for the length operator in loop quantum gravity is presented. The operator is background-independent, symmetric, positive semi-definite, and well-defined on the kinematical Hilbert space. The expression for the regularized length operator can moreover be understood both from a simple geometrical perspective as the average of a formula relating the length to area, volume and flux operators, and also consistently as the result of direct substitution of the densitized triad operator with the functional derivative operator into the regularized expression of the length. Both these derivations are discussed, and the origin of an undetermined overall factor in each case is also elucidated."

http://arxiv.org/abs/1004.1110
Interplay between curvature and Planck-scale effects in astrophysics and cosmology
Antonino Marciano, Giovanni Amelino-Camelia, Nicola Rossano Bruno, Giulia Gubitosi, Gianluca Mandanici, Alessandro Melchiorri
26 pages.
(Submitted on 7 Apr 2010)
"Several recent studies have considered the implications for astrophysics and cosmology of some possible nonclassical properties of spacetime at the Planck scale. The new effects, such as a Planck-scale-modified energy-momentum (dispersion) relation, are often inferred from the analysis of some quantum versions of Minkowski spacetime, and therefore the relevant estimates depend heavily on the assumption that there could not be significant interplay between Planck-scale and curvature effects. We here scrutinize this assumption, using as guidance a quantum version of de Sitter spacetime with known Inonu-Wigner contraction to a quantum Minkowski spacetime. And we show that, contrary to common (but unsupported) beliefs, the interplay between Planck-scale and curvature effects can be significant. Within our illustrative example, in the Minkowski limit the quantum-geometry deformation parameter is indeed given by the Planck scale, while in the de Sitter picture the parameter of quantization of geometry depends both on the Planck scale and the curvature scalar. For the much-studied case of Planck-scale effects that intervene in the observation of gamma-ray bursts we can estimate the implications of "quantum spacetime curvature" within robust simplifying assumptions. For cosmology at the present stage of the development of the relevant mathematics one cannot go beyond semiheuristic reasoning, and we here propose a candidate approximate description of a quantum FRW geometry, obtained by patching together pieces (with different spacetime curvature) of our quantum de Sitter. This semiheuristic picture, in spite of its limitations, provides rather robust evidence that in the early Universe the interplay between Planck-scale and curvature effects could have been particularly significant."

 

[MTd2]

http://arxiv.org/abs/1004.1268

Principle of Relativity, 24 possiblw kinematical algebras and new geometries with Poincaré symmetry

Chao-Guang Huang
(Submitted on 8 Apr 2010)
From the principle of relativity with two universal invariant parameters $c$ and $l$, 24 possible kinematical (including geometrical and static) algebras can be obtained. Each algebra is of 10 dimensional, generating the symmetry of a 4 dimensional homogeneous space-time or a pure space. In addition to the ordinary Poincar\'e algebra, there is another Poincar\'e algebra among the 24 algebras. New 4d geometries with the new Poincar\'e symmetry are presented. The motion of free particles on one of the new space-times is discussed.

http://arxiv.org/abs/1004.1226

Logic is to the quantum as geometry is to gravity

Rafael D. Sorkin (Perimeter Institute and Syracuse University)
(Submitted on 8 Apr 2010)
I will propose that the reality to which the quantum formalism implicitly refers is a kind of generalized history, the word history having here the same meaning as in the phrase sum-over-histories. This proposal confers a certain independence on the concept of event, and it modifies the rules of inference concerning events in order to resolve a contradiction between the idea of reality as a single history and the principle that events of zero measure cannot happen (the Kochen-Specker paradox being a classic expression of this contradiction). The so-called measurement problem is then solved if macroscopic events satisfy classical rules of inference, and this can in principle be decided by a calculation. The resulting conception of reality involves neither multiple worlds nor external observers. It is therefore suitable for quantum gravity in general and causal sets in particular.

 

[marcus]

http://arxiv.org/abs/1004.1780
A new look at loop quantum gravity
Carlo Rovelli
15 pages, 5 figures
(Submitted on 11 Apr 2010)
"I describe a possible perspective on the current state of loop quantum gravity, at the light of the developments of the last years. I point out that a theory is now available, having a well-defined background-independent kinematics and a dynamics allowing transition amplitudes to be computed explicitly in different regimes. I underline the fact that the dynamics can be given in terms of a simple vertex function, largely determined by locality, diffeomorphism invariance and local Lorentz invariance. I emphasize the importance of approximations. I list open problems."

http://arxiv.org/abs/1004.1856
The Hubble Constant
Wendy L. Freedman, Barry F. Madore
To be published in Annual Reviews of Astronomy and Astrophysics, Vol. 48, 2010, consisting of 79 pages, 13 figures, 2 tables
(Submitted on 11 Apr 2010)
"Considerable progress has been made in determining the Hubble constant over the past two decades. We discuss the cosmological context and importance of an accurate measurement of the Hubble constant, and focus on six high-precision distance-determination methods: Cepheids, tip of the red giant branch, maser galaxies, surface brightness fluctuations, the Tully-Fisher relation and Type Ia supernovae. We discuss in detail known systematic errors in the measurement of galaxy distances and how to minimize them. Our best current estimate of the Hubble constant is 73 +/-2 (random) +/-4 (systematic) km/s/Mpc. The importance of improved accuracy in the Hubble constant will increase over the next decade with new missions and experiments designed to increase the precision in other cosmological parameters. We outline the steps that will be required to deliver a value of the Hubble constant to 2% systematic uncertainty and discuss the constraints on other cosmological parameters that will then be possible with such accuracy."