Loop-and-allied QG bibliography

[MTd2]

http://arxiv.org/abs/0909.2829

Further Improvements in the Understanding of Isotropic Loop Quantum Cosmology

Mercedes Martin-Benito, Guillermo A. Mena Marugan, Javier Olmedo
(Submitted on 15 Sep 2009)
The flat, homogeneous, and isotropic universe with a massless scalar field is a paradigmatic model in Loop Quantum Cosmology. In spite of the prominent role that the model has played in the development of this branch of physics, there still remain some aspects of its quantization which deserve a more detailed discussion. These aspects include the kinematical resolution of the cosmological singularity, the precise relation between the solutions of the densitized and non-densitized versions of the quantum Hamiltonian constraint, the possibility of identifying superselection sectors which are as simple as possible, and a clear comprehension of the Wheeler-DeWitt (WDW) limit associated with the theory in those sectors. We propose an alternative operator to represent the Hamiltonian constraint which is specially suitable to deal with these issues in a satisfactory way. In particular, with our constraint operator, the singularity decouples in the kinematical Hilbert space and can be removed already at this level. Thanks to this fact, we can densitize the quantum Hamiltonian constraint in a rigorous manner. Besides, together with the physical observables, this constraint superselects simple sectors for the universe volume, with a support contained in a single semiaxis of the real line and for which the basic functions that encode the information about the geometry possesses optimal physical properties. Namely, they provide a no-boundary description around the cosmological singularity and admit a well-defined WDW limit in terms of standing waves. Both properties explain the presence of a generic quantum bounce replacing the singularity at a fundamental level, in contrast with previous studies where the bounce was proved in concrete regimes and focusing on states with a marked semiclassical behavior.

http://arxiv.org/abs/0909.2821

Stability of the Einstein static universe in Hořava-Lifgarbagez gravity

Puxun Wu, Hongwei Yu
(Submitted on 15 Sep 2009)
We study the stability of Einstein static universe in the Ho\v{r}ava-Lifgarbagez (HL) gravity with the detailed-balance condition, where the Friedmann equation gets corrected by a $1/{a^4}$ term. We find that, if the cosmological constant $\Lambda$ is negative, there exists a stable Einstein static state. The universe can stay at this stable state eternally and thus the big bang singularity can be avoided. However, in this case, it is difficult for the universe to break this stable state and then enter an inflationary era. For a positive $\Lambda$, the system has both an unstable state and a stable one. The former corresponds to an exponentially expanding phase. The universe can stay at this stable state past-eternally. Once the equation of state $w$ reaches infinity: $w\to\infty$ or $w\to-\infty$, this stable critical point coincides with the unstable one. Thus the stable state is broken and then the universe enters an inflationary era. Therefore, the big bang singularity can be avoided and a subsequent inflation can occur.

 

[MTd2]

http://arxiv.org/abs/0909.3265

Four-derivative interactions in asymptotically safe gravity

Dario Benedetti, Pedro F. Machado, Frank Saueressig
(Submitted on 17 Sep 2009)
We summarize recent progress in understanding the role of higher-derivative terms in the asymptotic safety scenario of gravity. Extending previous computations based on the functional renormalization group approach by including a Weyl-squared term in the ansatz for the effective action, further evidence for the existence of a non-Gaussian fixed point is found. The fixed point also persists upon including a minimally coupled free scalar field, providing an explicit example of perturbative counterterms being non-hazardous for asymptotic safety.

 

[marcus]

http://arxiv.org/abs/0909.3426
Quantum gravity without space-time singularities or horizons
Gerard 't Hooft
10 pages, 3 figures. Presented at the Erice Summerschool of Subnuclear Physics 2009
(Submitted on 18 Sep 2009)
"In an attempt to re-establish space-time as an essential frame for formulating quantum gravity - rather than an 'emergent' one -, we find that exact invariance under scale transformations is an essential new ingredient for such a theory. Use is made of the principle of 'black hole complementarity', the notion that observers entering a black hole describe its dynamics in a way that appears to be fundamentally different from the description by an outside observer. These differences can be boiled down to conformal transformations. If we add these to our set of symmetry transformations, black holes, space-time singularities, and horizons disappear, while causality and locality may survive as important principles for quantum gravity."

http://arxiv.org/abs/0909.3456
Antigravitation
Sabine Hossenfelder
This is the summary of a talk given at the 17th International Conference on Supersymmetry and the Unification of Fundamental Interactions in Boston, June 2009
(Submitted on 18 Sep 2009)
"We discuss why there are no negative gravitational sources in General Relativity and show that it is possible to extend the classical theory such that repulsive gravitational interaction occurs."

 

[MTd2]

This article is necessary to understand the conclusion of the above article from t'Hooft:

http://arxiv.org/abs/0908.3408

Entangled quantum states in a local deterministic theory

Gerard 't Hooft
(Submitted on 24 Aug 2009)
Investigating a class of models that is familiar in studies of cellular automata, we find that quantum operators can be employed to describe their long distance behavior. These operators span a Hilbert space that appears to turn such a model into a genuine quantum field theory, obeying the usual conditions of locality in terms of its quantum commutators. Entangled states can be constructed exactly as in quantum theories.
This raises the question whether such models allow Bell's inequalities to be violated. Being a local, deterministic theory, one would argue that this is impossible, but since at large distance scales the model does not seem to differ from real quantum field theories, there is reason to wonder why it should not allow entangled states. The standard arguments concerning Bell's inequalities are re-examined in this light.

On the topic of the convergence of several approaches to quantum gravity on Planck scale, due dimensional reduction, this is also very interesting:

http://arxiv.org/abs/0909.3329

Spontaneous Dimensional Reduction in Short-Distance Quantum Gravity?

Steven Carlip
(Submitted on 17 Sep 2009)
Several lines of evidence suggest that quantum gravity at very short distances may behave effectively as a two-dimensional theory. I summarize these hints, and offer an additional argument based on the strong-coupling limit of the Wheeler-DeWitt equation. The resulting scenario suggests a novel approach to quantum gravity at the Planck scale.

 

[marcus]

http://arxiv.org/abs/0909.3636
Testing Effective Quantum Gravity with Gravitational Waves from Extreme-Mass-Ratio Inspirals
Nicolas Yunes, C. F. Sopuerta
10 pages, 3 figures, invited paper for the Amaldi 8 Proceedings submitted to Class. Quant. Gravity
(Submitted on 20 Sep 2009)
"Testing deviation of GR is one of the main goals of the proposed Laser Interferometer Space Antenna, a space-based gravitational-wave observatory. For the first time, we consistently compute the generation of gravitational waves from extreme-mass ratio inspirals (stellar compact objects into supermassive black holes) in a well-motivated alternative theory of gravity, that to date remains weakly constrained by double binary pulsar observations. The theory we concentrate on is Chern-Simons (CS) modified gravity, a 4-D, effective theory that is motivated both from string theory and loop-quantum gravity, and which enhances the Einstein-Hilbert action through the addition of a dynamical scalar field and the parity-violating Pontryagin density. We show that although point particles continue to follow geodesics in the modified theory, the background about which they inspiral is a modification to the Kerr metric, which imprints a CS correction on the gravitational waves emitted. CS modified gravitational waves are sufficiently different from the General Relativistic expectation that they lead to significant dephasing after 3 weeks of evolution, but such dephasing will probably not prevent detection of these signals, but instead lead to a systematic error in the determination of parameters. We end with a study of radiation-reaction in the modified theory and show that, to leading-order, energy-momentum emission is not CS modified, except possibly for the subdominant effect of scalar-field emission. The inclusion of radiation-reaction will allow for tests of CS modified gravity with space-borne detectors that might be two orders of magnitude larger than current binary pulsar bounds."

http://arxiv.org/abs/0909.3767
Does time exist in quantum gravity?
Claus Kiefer
10 pages, second prize of the FQXi "The Nature of Time" essay contest
(Submitted on 21 Sep 2009)
"Time is absolute in standard quantum theory and dynamical in general relativity. The combination of both theories into a theory of quantum gravity leads therefore to a 'problem of time'. In my essay I shall investigate those consequences for the concept of time that may be drawn without a detailed knowledge of quantum gravity. The only assumptions are the experimentally supported universality of the linear structure of quantum theory and the recovery of general relativity in the classical limit. Among the consequences are the fundamental timelessness of quantum gravity, the approximate nature of a semiclassical time, and the correlation of entropy with the size of the Universe."

http://arxiv.org/abs/0909.3834
Analogue Models for Emergent Gravity
Stefano Liberati, Florian Girelli, Lorenzo Sindoni
47 pages. To appear in the Proceedings of the XVIII SIGRAV Conference, Cosenza, September 22-25, 2008
(Submitted on 21 Sep 2009)
"Gravity stands out among the fundamental interactions because of its apparent incompatibility with having a quantum description. Moreover, thermodynamic aspects of gravitation theory appears as puzzling features of some classical solutions such as black holes. These and other aspects of gravitational theories have recently lead to the proposal that gravity might not be a fundamental interaction but rather an emergent phenomenon, a sort of hydrodynamic limit of some more fundamental theory. In order to further explore this possibility we shall here discuss two systems where such emergence of a gravitational dynamics is observed. We shall consider first the case of a non-relativistic Bose-Einstein condensate and then a more abstract model based on scalar fields living on a Riemannian manifold. This will allow us to put in evidence the general issues related to emergent gravity scenarios with a particular attention to the role and nature of Lorentz and diffeomorphism invariance."

 

[atyy]

So one coefficient is unconstrained?

http://physics.aps.org/viewpoint-for/10.1103/PhysRevD.80.016002
Catching relativity violations with atoms
Quentin G. Bailey
Chung et al. combined their interferometer analysis with the lunar laser ranging results to tabulate the best constraints yet on gravity coefficients in the standard-model extension. The results show that five of nine possible coefficients are constrained to a value of zero to less than parts per billion, while another three of these nine coefficients are constrained at the level of parts per million.

 

[marcus]

http://arxiv.org/abs/0909.3983
A Larger Estimate of the Entropy of the Universe
Chas A. Egan, Charles H. Lineweaver
(Submitted on 22 Sep 2009)
"Using recent measurements of the supermassive black hole mass function we find that supermassive black holes are the largest contributor to the entropy of the observable Universe, contributing at least an order of magnitude more entropy than previously estimated. The total entropy of the observable Universe is correspondingly higher, and is S_{obs} = 3.1^{+3.0}_{-1.7}\times 10^{104} k. We calculate the entropy of the current cosmic event horizon to be S_{CEH} = 2.6 \pm 0.3 \times 10^{122} k, dwarfing the entropy of its interior, S_{CEH int} = 1.2^{+1.1}_{-0.7}\times 10^{103} k. We make the first tentative estimate of the entropy of dark matter within the observable Universe, S_{dm} = 10^{88\pm1} k. We highlight several caveats pertaining to these estimates and make recommendations for future work."

http://arxiv.org/abs/0909.4221
Loop Quantum Cosmology and Spin Foams
Abhay Ashtekar, Miguel Campiglia, Adam Henderson
11 pages
(Submitted on 23 Sep 2009)
"Loop quantum cosmology (LQC) is used to provide concrete evidence in support of the general paradigm underlying spin foam models (SFMs). Specifically, it is shown that: i) the physical inner product in the timeless framework equals the transition amplitude in the deparameterized theory; ii) this quantity admits a convergent vertex expansion a la SFMs in which the M-th term refers just to M volume transitions, without any reference to the time at which the transition takes place; iii) the exact physical inner product is obtained by summing over just the discrete geometries; no 'continuum limit' is involved; and, iv) the vertex expansion can be interpreted as a perturbative expansion in the spirit of group field theory. This sum over histories reformulation of LQC also addresses certain other issues which are briefly summarized."

http://arxiv.org/abs/0909.4211
Bianchi I model in terms of non-standard LQC: Classical dynamics
Piotr Dzierzak, Wlodzimierz Piechocki
12 pages
(Submitted on 23 Sep 2009)
"The cosmological singularities of the Bianchi I universe are analyzed in the setting of loop geometry underlying the loop quantum cosmology. We solve the Hamiltonian constraint of the theory and find the Lie algebra of elementary observables. Physical compound observables are defined in terms of elementary ones. Modification of classical theory by holonomy around a loop removes the singularities. However, our model has a free parameter that cannot be determined within our method. Testing the model by the data of observational cosmology may be possible after quantization of our modified classical theory."

 

[MTd2]

http://arxiv.org/abs/0909.4157

Revisiting the semiclassical gravity scenario for gravitational collapse

C. Barcelo, S. Liberati, S. Sonego, M. Visser
(Submitted on 23 Sep 2009)
The existence of extremely dark and compact astronomical bodies is by now a well established observational fact. On the other hand, classical General Relativity predicts the existence of black holes which fit very well with the observations, but do lead to important conceptual problems. In this contribution we ask ourselves the straightforward question: Are the dark and compact objects that we have observational evidence for black holes in the sense of General Relativity? By revising the semiclassical scenario of stellar collapse we find out that as the result of a collapse some alternative objects could be formed which might supplant black holes.

http://arxiv.org/abs/0909.4194

Non-equilibrium Thermodynamics of Spacetime: the Role of Gravitational Dissipation

G. Chirco, S. Liberati
(Submitted on 23 Sep 2009)
In arXiv:gr-qc/9504004 it was shown that the Einstein equation can be derived as a local constitutive equation for an equilibrium spacetime thermodynamics. More recently, in the attempt to extend the same approach to the case of $f(R)$ theories of gravity, it was found that a non-equilibrium setting is indeed required in order to fully describe both this theory as well as classical GR (arXiv:gr-qc/0602001). Here, elaborating on this point, we show that the dissipative character leading to a non-equilibrium spacetime thermodynamics is actually related -- both in GR as well as in $f(R)$ gravity -- to non-local heat fluxes associated with the purely gravitational/internal degrees of freedom of the theory. In particular, in the case of GR we show that the internal entropy production term is identical to the so called tidal heating term of Hartle-Hawking. Similarly, for the case of $f(R)$ gravity, we show that dissipative effects can be associated with the generalization of this term plus a scalar contribution whose presence is clearly justified within the scalar-tensor representation of the theory. Finally, we show that the allowed gravitational degrees of freedom can be fixed by the kinematics of the local spacetime causal structure, through the specific Equivalence Principle formulation. In this sense, the thermodynamical description seems to go beyond Einstein's theory as an intrinsic property of gravitation.

http://arxiv.org/abs/0909.4211

Bianchi I model in terms of non-standard LQC: Classical dynamics

Piotr Dzierzak, Wlodzimierz Piechocki
(Submitted on 23 Sep 2009)
The cosmological singularities of the Bianchi I universe are analyzed in the setting of loop geometry underlying the loop quantum cosmology. We solve the Hamiltonian constraint of the theory and find the Lie algebra of elementary observables. Physical compound observables are defined in terms of elementary ones. Modification of classical theory by holonomy around a loop removes the singularities. However, our model has a free parameter that cannot be determined within our method. Testing the model by the data of observational cosmology may be possible after quantization of our modified classical theory.

http://arxiv.org/abs/0909.4221

Loop Quantum Cosmology and Spin Foams

Abhay Ashtekar, Miguel Campiglia, Adam Henderson
(Submitted on 23 Sep 2009)
Loop quantum cosmology (LQC) is used to provide concrete evidence in support of the general paradigm underlying spin foam models (SFMs). Specifically, it is shown that: i) the physical inner product in the timeless framework equals the transition amplitude in the deparameterized theory; ii) this quantity admits a %convergent vertex expansion a la SFMs in which the $M$-th term refers just to $M$ volume transitions, without any reference to the time at which the transition takes place; iii) the exact physical inner product is obtained by summing over just the discrete geometries; no `continuum limit' is involved; and, iv) the vertex expansion can be interpreted as a perturbative expansion in the spirit of group field theory. This sum over histories reformulation of LQC also addresses certain other issues which are briefly summarized.

http://arxiv.org/abs/0909.4238

2+1 Quantum Gravity with Barbero-Immirzi like parameter on Toric Spatial Foliation

Rudranil Basu, Samir K Paul
(Submitted on 23 Sep 2009)
We consider gravity in 2+1 space-time dimensions, with negative cosmological constant and a `Barbero-Immirzi' (B-I) like parameter, when the space-time topology is of the form $ T^2 \times \mathbbm{R}$. The phase space structure, both in covariant and canonical framework is analyzed. Full quantization of the theory in the 'constrain first' approach reveals a finite dimensional physical Hilbert space. An explicit construction of wave functions is presented. The dimension of the Hilbert space is found to depend on the `Barbero-Immirzi' like parameter in an interesting fashion.

http://arxiv.org/abs/0909.3841

Anisotropic Conformal Infinity

Petr Horava, Charles M. Melby-Thompson
(Submitted on 21 Sep 2009)
We generalize Penrose's notion of conformal infinity of spacetime, to situations with anisotropic scaling. This is relevant not only for Lifgarbagez-type anisotropic gravity models, but also in standard general relativity and string theory, for spacetimes exhibiting a natural asymptotic anisotropy. Examples include the Lifgarbagez and Schrodinger spaces (proposed as AdS/CFT duals of nonrelativistic field theories), warped AdS_3, and the near-horizon extreme Kerr geometry. The anisotropic conformal boundary appears crucial for resolving puzzles of holographic renormalization in such spacetimes.

 

[MTd2]

http://arxiv.org/abs/0909.4472

Gravitation as a Plastic Distortion of the Lorentz Vaccum

Virginia V. Fernandez, Waldyr A. Rodrigues Jr
(Submitted on 24 Sep 2009)
In this paper we present a theory of the gravitational field where this field (a kind of square root of g) is represented by a (1,1)-extensor field h describing a plastic distortion of the Lorentz vacuum (a real substance that lives in a Minkowski spacetime) due to the presence of matter. The field h distorts the Minkowski metric extensor in an appropriate way (see below) generating what may be interpreted as an effective Lorentzian metric extensor g and also it permits the introduction of different kinds of parallelism rules on the world manifold, which may be interpreted as distortions of the parallelism structure of Minkowski spacetime and which may have non null curvature and/or torsion and/or nonmetricity tensors. We thus have different possible effective geometries which may be associated to the gravitational field and thus its description by a Lorentzian geometry is only a possibility, not an imposition from Nature. Moreover, we developed with enough details the theory of multiform functions and multiform functionals that permitted us to successfully write a Lagrangian for h and to obtain its equations of motion, that results equivalent to Einstein field equations of General Relativity (for all those solutions where the manifold M is diffeomorphic to R^4. However, in our theory, differently from the case of General Relativity, trustful energy-momentum and angular momentum conservation laws exist. We express also the results of our theory in terms of the gravitational potential 1-form fields (living in Minkowski spacetime) in order to have results which may be easily expressed with the theory of differential forms. The Hamiltonian formalism for our theory (formulated in terms of the potentials) is also discussed. The paper contains also several important Appendices that complete the material in the main text.

 

[marcus]

http://arxiv.org/abs/0909.4771
Indications of de Sitter Spacetime from Classical Sequential Growth Dynamics of Causal Sets
16 pages, 10 figures
Maqbool Ahmed, David Rideout
(Submitted on 25 Sep 2009)
"A large class of the dynamical laws for causal sets described by a classical process of sequential growth yield a cyclic universe, whose cycles of expansion and contraction are punctuated by single 'origin elements' of the causal set. We present evidence that the effective dynamics of the immediate future of one of these origin elements, within the context of the sequential growth dynamics, yields an initial period of de Sitter like exponential expansion, and argue that the resulting picture has many attractive features as a model of the early universe, with the potential to solve some of the standard model puzzles without any fine tuning."

 

[atyy]

http://arxiv.org/abs/0909.4621
On the Newtonian limit of emergent NC gravity and long-distance corrections
Harold Steinacker
(Submitted on 25 Sep 2009)
We show how Newtonian gravity emerges on 4-dimensional non-commutative spacetime branes in Yang-Mills matrix models. Large matter clusters such as galaxies are embedded in large-scale harmonic deformations of the space-time brane, which screen gravity for long distances. On shorter scales, the local matter distribution reproduces Newtonian gravity via local deformations of the brane and its metric. The harmonic ``gravity bag'' acts as a halo with effective positive energy density. This leads in particular to a significant enhancement of the orbital velocities around galaxies at large distances compared with the Newtonian case, before dropping to zero as the geometry merges with a Milne-like cosmology. Besides these ``harmonic'' solutions, there is another class of solutions which is more similar to Einstein gravity. Thus the IKKT model provides an accessible candidate for a quantum theory of gravity.

 

[John86]

http://arxiv.org/abs/0907.4887
Possible solution of the cosmological constant problem
Authors: F.R. Klinkhamer, G.E. Volovik
(Submitted on 28 Jul 2009 (v1), last revised 26 Sep 2009 (this version, v3))

Abstract: An extension of the standard model of elementary particle physics and the theory of general relativity is given, which is based on the appropriate introduction of a four-form field strength. The extended theory has, without fine-tuning, a Minkowski-type solution with spacetime-independent fields and provides, therefore, a solution of the main cosmological constant problem.

http://arxiv.org/abs/0909.3084
Topological invariant for superfluid 3He-B and quantum phase transitions
Authors: G.E. Volovik
(Submitted on 16 Sep 2009 (v1), last revised 22 Sep 2009 (this version, v4))

Abstract: We consider topological invariant describing the vacuum states of superfluid 3He-B, which belongs to the special class of time-reversal invariant topological insulators and superfluids. Discrete symmetries important for classification of the topologically distinct vacuum states are discussed. One of them leads to the additional subclasses of 3He-B states and is responsible for the finite density of states of Majorana fermions living on the diffusive wall. Integer valued topological invariant is expressed in terms of the Green's function, which allows us to consider systems with interaction.

 

[MTd2]

http://golem.ph.utexas.edu/category/2009/09/this_weeks_finds_in_mathematic_41.html

This Week’s Finds in Mathematical Physics (Week 280)
Posted by John Baez


In week280 of This Week’s Finds, hear about the courses taught by Ashtekar and Rovelli at the quantum gravity summer school in Corfu. Ashtekar spoke about loop quantum cosmology, and how it could turn the Big Bang into a Big Bounce. Rovelli spoke about spin foam models, and how the new EPRL model cures many problems of the old Barrett-Crane model. The graviton propagator seems to work!

 

[MTd2]

http://arxiv.org/abs/0909.5421

In, Trough and Beyond the Planck Scale

Leonardo Modesto, Isabeau Prémont-Schwarz
(Submitted on 29 Sep 2009)
In this paper we have recalled the semiclassical metric obtained from a classical analysis of the loop quantum black hole (LQBH). We show that the regular Reissner-Nordstr\"om-like metric is self-dual in the sense of T-duality: the form of the metric is invariant under the exchange r -> a0/r where a0 is proportional to the minimum area in LQG. Of particular interest, the symmetry imposes that if an observer at infinity sees a black hole of mass m an observer in the other asymptotic infinity beyond the horizon (near r=0) sees a dual mass proportional to m_P^2/m. We then show that small LQBHs are stable and could be a component of dark matter. Ultra-light LQBHs created shortly after the Big Bang would now have a mass of approximately 10^(-5) m_P and emit radiation with a typical energy of about 10^(13) - 10^(14) eV but they would also emit cosmic rays of much higher energies, albeit few of them. If these small LQBHs form a majority of the dark matter of the Milky Way's Halo, the production rate of ultra-high-energy-cosmic-rays (UHECR) by these ultra light black holes would be compatible with the observed rate of the Auger detector.http://arxiv.org/abs/0909.5405
Particle Kinematics in Horava-Lifgarbagez Gravity

Dario Capasso, Alexios P. Polychronakos
(Submitted on 29 Sep 2009)
We study the deformed kinematics of point particles in the Horava theory of gravity. This is achieved by considering particles as the optical limit of fields with a generalized Klein-Gordon action. We derive the deformed geodesic equation and study in detail the cases of flat and spherically symmetric (Schwarzschild-like) spacetimes. As the theory is not invariant under local Lorenz transformations, deviations from standard kinematics become evident even for flat manifolds, supporting superluminal as well as massive luminal particles. These deviations from standard behavior could be used for experimental tests of this modified theory of gravity.http://arxiv.org/abs/0909.4833
Notes on Matter in Horava-Lifgarbagez Gravity

Takao Suyama
(Submitted on 26 Sep 2009)
We investigate the dynamics of a scalar field governed by the Lifgarbagez-type action which should appear naturally in Horava-Lifgarbagez gravity. The wave of the scalar field may propagate with any speed without an upper bound. To preserve the causality, the action cannot have a generic form. Due to the superluminal propagation, a formation of a singularity may cause the breakdown of the predictability of the theory. To check whether such a catastrophe could occur in Horava-Lifgarbagez gravity, we investigate the dynamics of a dust. It turns out that the dust does not collapse completely to form a singularity in a generic situation, but expands again after it attains a maximum energy density.

 

[John86]

http://arxiv.org/abs/0909.5391
Emergent gravitational dynamics in Bose-Einstein condensates
Authors: Lorenzo Sindoni, Florian Girelli, Stefano Liberati
(Submitted on 29 Sep 2009)

Abstract: We discuss a toy model for an emergent non-relativistic gravitational theory. Within a certain class of Bose-Einstein condensates, it is possible to show that, in a suitable regime, a modified version of non-relativistic Newtonian gravity does effectively describes the low energy dynamics of the coupled system condensate/quasi-particles.

 

[marcus]

http://arxiv.org/abs/0909.5435
de Sitter Spaces
Andrew Randono
(Submitted on 29 Sep 2009)
"We exploit an interpretation of gravity as the symmetry broken phase of a de Sitter gauge theory to construct new solutions to the first order field equations. The new solutions are constructed by performing large Spin(4,1) gauge transformations on the ordinary de Sitter solution and extracting first the tetrad, then the induced metric. The class of metrics so obtained is an infinite class labelled by an integer, q. Each solution satisfies the local field equations defining constant positive curvature, and is therefore locally isometric to de Sitter space wherever the metric is non-degenerate. The degeneracy structure of the tetrad and metric reflects the topological differences among the solutions with different q. By topological arguments we show that the solutions are physically distinct with respect to the symmetries of Einstein-Cartan theory. Ultimately, the existence of solutions of this type may be a distinguishing characteristic of gravity as a metric theory versus gravity as a gauge theory."

http://arxiv.org/abs/0909.5631
Algebraic structures in quantum gravity
Adrian Tanasa
17 pages, 4 figues
(Submitted on 30 Sep 2009)
"Starting from a recently-introduced algebraic structure on spin foam models, we define a Hopf algebra by dividing with an appropriate quotient. The structure, thus defined, naturally allows for a mirror analysis of spin foam models with quantum field theory, from a combinatorial point of view. A grafting operator is introduced allowing for the equivalent of a Dyson-Schwinger equation to be written. Non-trivial examples are explicitly worked out. Finally, the physical significance of the results is discussed."

http://arxiv.org/abs/0909.5688
Breaking and restoring of diffeomorphism symmetry in discrete gravity
B. Bahr, B. Dittrich
To appear in the Proceedings of the XXV Max Born Symposium "The Planck Scale", Wroclaw, 29 June - 3 July, 2009
(Submitted on 30 Sep 2009)
"We discuss the fate of diffeomorphism symmetry in discrete gravity. Diffeomorphism symmetry is typically broken by the discretization. This has repercussions for the observable content and the canonical formulation of the theory. It might however be possible to construct discrete actions, so--called perfect actions, with exact symmetries and we will review first steps towards this end."

http://arxiv.org/abs/0910.0181
Cosmological consequences of the NonCommutative Geometry Spectral Action
Mairi Sakellariadou
7 pages. Invited talk at the XXV Max Bonn Symposium "Physics at the Planck Scale"
(Submitted on 1 Oct 2009)
"Cosmological consequences of the noncommutative geometry spectral action are presented. Neglecting the nonminimal coupling of the Higgs field to the curvature, background cosmology remains unchanged, and only the inhomogeneous perturbations will evolve differently from the equivalent classical system. However, considering the nonminimal coupling, corrections will be obtained even at the level of the background cosmologies. Finally, the Higgs field may act as an inflaton field, due to its nonminimal coupling with geometry."

 

[MTd2]

http://arxiv.org/abs/0910.0282

Quantum Gravity Effects on Unstable Orbits in Schwarzschild Space-time

Arundhati Dasgupta
(Submitted on 1 Oct 2009)
We study semiclassical corrections to the Schwarzschild metric, and their effects on unstable orbits

 

[marcus]

http://arxiv.org/abs/0910.0490
Planck Scale Cosmology and Resummed Quantum Gravity
B.F.L. Ward (Department of Physics, Baylor University, Waco, TX, USA)
6 pages, 2 figures. To be published in the proceedings of DPF-2009, Detroit, MI, July 2009
(Submitted on 2 Oct 2009)
"We show that, by using amplitude-based resummation techniques for Feynman's formulation of Einstein's theory, we get quantum field theoretic 'first principles' predictions for the UV fixed-point values of the dimensionless gravitational and cosmological constants. Connections to the phenomenological asymptotic safety analysis of Planck scale cosmology by Bonanno and Reuter are discussed."

B. Ward is one of those invited to the Perimeter workshop on AsymSafe to be held next month, in early November. He has other writings on AsymSafe as well.

http://arxiv.org/abs/0910.0623
Preferred foliation effects in Quantum General Relativity
Tim Koslowski, Alexander Schenkel
12 pages
(Submitted on 4 Oct 2009)
"We investigate the infrared (IR) effects of Lorentz violating terms in the gravitational sector using functional renormalization group methods similar to Reuter and collaborators. The model we consider consists of pure quantum gravity coupled to a preferred foliation, described effectively via a scalar field with non-standard dynamics. We find that vanishing Lorentz violation is a UV attractive fixed-point of this model. Thus, this model can only be made phenomenologically acceptable with a certain amount of fine-tuning. We also study effects of additional matter fields on the RG running of the Lorentz violating term and provide a general argument why they are small."

http://arxiv.org/abs/0910.0673
Light, Links and Causal Sets
Rafael D. Sorkin
18 pages
(Submitted on 5 Oct 2009)
"After sketching a context in which to seek observable signals of spatio-temporal discreteness, I briefly review the status of the causal set program for quantum gravity, concluding with a simple model for the field produced by a moving charge in a background causal set."

http://arxiv.org/abs/0910.0834
The Return of the Phoenix Universe
Jean-Luc Lehners, Paul J. Steinhardt, Neil Turok
5 pages, Honorable Mention at the 2009 Gravity Research Foundation essay competition
(Submitted on 5 Oct 2009)
"Georges Lemaitre introduced the term 'phoenix universe' to describe an oscillatory cosmology with alternating periods of gravitational collapse and expansion. This model is ruled out observationally because it requires a supercritical mass density and cannot accommodate dark energy. However, a new cyclic theory of the universe has been proposed that evades these problems. In a recent elaboration of this picture, almost the entire universe observed today is fated to become entrapped inside black holes, but a tiny region will emerge from these ashes like a phoenix to form an even larger smooth, flat universe filled with galaxies, stars, planets, and, presumably, life. Survival depends crucially on dark energy and suggests a reason why its density is small and positive today."

 

[John86]

http://arxiv.org/abs/0910.0790
A note on the analogy between superfluids and cosmology
Authors: A. Naddeo, G. Scelza
(Submitted on 5 Oct 2009)
Abstract: A new analogy between superfluid systems and cosmology is here presented, which relies strongly on the following ingredient: the back-reaction of the vacuum to the quanta of sound waves. We show how the presence of thermal phonons, the excitations above the quantum vacuum for $T> 0$, enable us to deduce an hydrodynamical equation formally similar to the one obtained for a perfect fluid in a Universe obeying the Friedmann-Robertson-Walker metric.

http://arxiv.org/abs/0910.0181
Cosmological consequences of the NonCommutative Geometry Spectral Action
Authors: Mairi Sakellariadou
(Submitted on 1 Oct 2009)
Abstract: Cosmological consequences of the noncommutative geometry spectral action are presented. Neglecting the nonminimal coupling of the Higgs field to the curvature, background cosmology remains unchanged, and only the inhomogeneous perturbations will evolve differently from the equivalent classical system. However, considering the nonminimal coupling, corrections will be obtained even at the level of the background cosmologies. Finally, the Higgs field may act as an inflaton field, due to its nonminimal coupling with geometry.

 

[marcus]

http://arxiv.org/abs/0910.0839
A Dialogue on the Nature of Gravity
T. Padmanabhan
32 pages,1 figure; based on the annual Vaidya-Raychaudhuri lecture given at Kolkata, Jan 09, and other lectures--adapted to make a dialogue.
(Submitted on 5 Oct 2009)
"I describe the conceptual and mathematical basis of an approach which describes gravity as an emergent phenomenon. Combining principle of equivalence and principle of general covariance with known properties of local Rindler horizons, perceived by observers accelerated with respect to local inertial frames, one can prove that the field equations describing gravity in any diffeomorphism invariant theory can be given a thermodynamic re-interpretation. This fact, in turn, leads us to the possibility of deriving the field equations of gravity by maximising a suitably defined entropy functional, without using the metric tensor as a dynamical variable. The approach synthesises concepts from quantum theory, thermodynamics and gravity leading to a fresh perspective on the nature of gravity. The description is presented here in the form of a dialogue, thereby addressing several frequently-asked-questions."

http://arxiv.org/abs/0910.0851
One loop beta functions and fixed points in Higher Derivative Sigma Models
R. Percacci, O. Zanusso
26 pages
(Submitted on 6 Oct 2009)
"We calculate the one loop beta functions for nonlinear sigma models in four dimensions containing general two and four derivative terms. In the O(N) model there are four such terms and nontrivial fixed points exist for all N >= 4. In the chiral SU(N) models there are in general six couplings, but only five for N=3 and four for N=2; we find fixed points only for N=2,3. In the approximation considered, the four derivative couplings are asymptotically free but the coupling in the two derivative term has a nonzero limit. These results support the hypothesis that certain sigma models may be asymptotically safe."

Percacci investigates the asymptotic safety of a non-gravity model with the intention of shedding light on asymptotic safety in the case of gravity (about which he has several papers).
He is another of those who will be participating in the Perimeter workshop on asymptotic safety around 3 weeks from now. See post #987 for a paper by Ward, another participant.

 

[marcus]

http://arxiv.org/abs/0910.1278
Loop quantum cosmology of Bianchi type II models
Abhay Ashtekar, Edward Wilson-Ewing
26 pages
(Submitted on 7 Oct 2009)
"The improved dynamics of loop quantum cosmology is extended to include the Bianchi type II model. Because these space-times admit both anisotropies and non-zero spatial curvature, certain technical difficulties arise over and above those encountered in the analysis of the (anisotropic but spatially flat) Bianchi type I space-times, and of the (spatially curved but isotropic) k=+/-1 models. We address these and show that the big-bang singularity is resolved in the same precise sense as in the recent analysis of the Bianchi I model. Bianchi II space-times are of special interest to quantum cosmology because of the expected behavior of the gravitational field near generic space-like singularities in classical general relativity."

 

[marcus]

http://arxiv.org/abs/0910.1542
2-Group Representations for Spin Foams
Aristide Baratin, Derek K. Wise
8 pages; to appear in proceedings of the XXV Max Born Symposium: "The Planck Scale", Wroclaw, Poland
(Submitted on 8 Oct 2009)
"Just as 3d state sum models, including 3d quantum gravity, can be built using categories of group representations, '2-categories of 2-group representations' may provide interesting state sum models for 4d quantum topology, if not quantum gravity. Here we focus on the 'Euclidean 2-group', built from the rotation group SO(4) and its action on the group of translations of 4d Euclidean space. We explain its infinite-dimensional unitary representations, and construct a model based on the resulting representation 2-category. This model, with clear geometric content and explicit 'metric data' on triangulation edges, shows up naturally in an attempt to write the amplitudes of ordinary quantum field theory in a background independent way."

 

[MTd2]

http://arxiv.org/abs/0910.2125

Lifetime of flying particles in canonical Doubly Special Relativity

S. Mignemi
(Submitted on 12 Oct 2009)
We discuss the corrections to the lifetime of unstable elementary particles in some models of doubly special relativity. We assume that the speed of light is invariant and that the position coordinates transform in such a way to ensure the invariance of the deformed symplectic structure of phase space.

 

[marcus]

http://arxiv.org/abs/0910.2117
Can causal dynamical triangulations probe factor-ordering issues?
R L Maitra
12 pages. Presented at the XLIX Cracow School of Theoretical Physics, "Non-perturbative Gravity and Quantum Chromodynamics," Zakopane, Poland, May 31 - June 10, 2009
(Submitted on 12 Oct 2009)
"The causal dynamical triangulations (CDT) program has for the first time allowed for path-integral computation of correlation functions in full general relativity without symmetry reductions and taking into account Lorentzian signature. One of the most exciting recent results in CDT is the strong agreement of these computations with (minisuperspace) path integral calculations in quantum cosmology. Herein I will describe my current project to compute minisuperspace (Friedman-Robertson-Walker) path integrals with a range of different measures corresponding to various factor orderings of the Friedman-Robertson-Walker Hamiltonian. The aim is to compare with CDT results and ask whether CDT can shed light on factor-ordering ambiguities in quantum cosmology models."

Since Rachel Lash Maitra (née Rachel Lash) is a comparatively new postdoc on Loll's team, I'll give background. MS Yale. PhD Yale 2007. As undergrad she majored in Math Physics and graduated first in her class, summa cum laude. I think her research direction as shown here is interesting and potentially valuable. A notable fact is that she is a professional-grade writer. You can see this by reading the excellent clear prose style of the paper and also her CV shows that she can support herself by moonlighting as a technical writing instructor while also doing physics. It's an asset. A research line like CDT needs to grow. The way a startup line of research grows is by attracting exceptional people like this and getting them on board. It's a hopeful sign that Loll's group appears to be doing this.
http://www1.phys.uu.nl/wwwitf/Foto's/Rachel.JPG
http://www.facebook.com/people/Rachel-Lash-Maitra/649117033
http://staff.science.uva.nl/%7Erlash/academic_resume_amsterdam.pdf
http://trostomaten.blogspot.com/ (surprise blog with remarkable nature photos shows yet another side)

 

[MTd2]

http://arxiv.org/abs/0910.2425

The 6j-symbol: Recursion, Correlations and Asymptotics

Maite Dupuis, Etera R. Livine
(Submitted on 13 Oct 2009)
We study the asymptotic expansion of the 6j-symbol using the Schulten-Gordon recursion relations. We focus on the particular case of the isosceles tetrahedron and we provide explicit formulas for up to the third order corrections beyond the leading order. Moreover, in the framework of spinfoam models for 3d quantum gravity, we show how these recursion relations can be used to derive Ward-Takahashi-like identities between the expectation values of graviton-like spinfoam correlations.

 

[MTd2]

http://arxiv.org/abs/0910.2498

Causal set as a discretized phase spacetime

Roman Sverdlov
(Submitted on 13 Oct 2009)
Re-interpretting causal set as a phase spacetime as opposed to the ordinary spacetime allows to restore locality. It also gives one hope to perform analytic calculations of QFT on a causal set.

http://arxiv.org/abs/0910.2574

Matter from Space

Domenico Giulini
(Submitted on 14 Oct 2009)

General Relativity offers the possibility to model attributes of matter, like mass, momentum, angular momentum, spin, chirality etc. from pure space, endowed only with a single field that represents its Riemannian geometry. I review this picture of `Geometrodynamics' and comment on various developments after Einstein.

 

[John86]

http://arxiv.org/abs/0910.2499
Reduction and Emergence in Bose-Einstein Condensates
Authors: Richard Healey
(Submitted on 14 Oct 2009)
Abstract: A closer look at some proposed Gedanken-experiments on BECs promises to shed light on several aspects of reduction and emergence in physics. These include the relations between classical descriptions and different quantum treatments of macroscopic systems, and the emergence of new properties and even new objects as a result of spontaneous symmetry breaking.

 

[marcus]

http://arxiv.org/abs/0910.2936
Quantum geometry and quantum dynamics at the Planck scale
Martin Bojowald
10 pages, 2 figures, Proceedings of "The Planck Scale"; XXV Max Born Symposium
(Submitted on 15 Oct 2009)
"Canonical quantum gravity provides insights into the quantum dynamics as well as quantum geometry of space-time by its implications for constraints. Loop quantum gravity in particular requires specific corrections due to its quantization procedure, which also results in a discrete picture of space. The corresponding changes compared to the classical behavior can most easily be analyzed in isotropic models, but perturbations around them are more involved. For one type of corrections, consistent equations have been found which shed light on the underlying space-time structure at the Planck scale: not just quantum dynamics but also the concept of space-time manifolds changes in quantum gravity. Effective line elements provide indications for possible relationships to other frameworks, such as non-commutative geometry."

http://arxiv.org/abs/0910.2892
Fully LQC-corrected propagation of gravitational waves during slow-roll inflation
J. Grain, T. Cailleteau, A. Barrau, A. Gorecki
9 pages
(Submitted on 15 Oct 2009)
"The cosmological primordial power spectrum is known to be one of the most promising observable to probe quantum gravity effects. In this article, we investigate how the tensor power spectrum is modified by Loop Quantum Gravity corrections. The two most important quantum terms, holonomy and inverse volume, are explicitly taken into account in a unified framework. The equation of propagation of gravitational waves is derived and solved for one set of parameters."

 

[marcus]

http://arxiv.org/abs/0910.3200
A Momentous Arrow of Time
Martin Bojowald
23 pages, 3 figures, Chapter contributed to "The Arrow of Time" Ed. L. Mersini-Houghton and R. Vaas (Springer-Verlag)
(Submitted on 16 Oct 2009)
"Quantum cosmology offers a unique stage to address questions of time related to its underlying (and perhaps truly quantum dynamical) meaning as well as its origin. Some of these issues can be analyzed with a general scheme of quantum cosmology, others are best seen in loop quantum cosmology. The latter's status is still incomplete, and so no full scenario has yet emerged. Nevertheless, using properties that have a potential of pervading more complicated and realistic models, a vague picture shall be sketched here. It suggests the possibility of deriving a beginning within a beginningless theory, by applying cosmic forgetfulness to an early history of the universe."

 

[MTd2]

This Week’s Finds in Mathematical Physics (Week 281)

http://golem.ph.utexas.edu/category/2009/10/this_weeks_finds_in_mathematic_42.html

[...]Finally, hear about strings in 4d BF theory, and spin foam models based on the representation theory of 2-groups.