Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1311.7565
Time evolution as refining, coarse graining and entangling
Bianca Dittrich, Sebastian Steinhaus
(Submitted on 29 Nov 2013)
We argue that refining, coarse graining and entangling operators can be obtained from time evolution operators. This applies in particular to geometric theories, such as spin foams. We point out that this provides a construction principle for the physical vacuum in quantum gravity theories and more generally allows to construct a (cylindrically) consistent continuum limit of the theory.
33 pages, 9 figures

http://arxiv.org/abs/1311.7498
Noncommutative Space-time from Quantized Twistors
Jerzy Lukierski, Mariusz Woronowicz
(Submitted on 29 Nov 2013)
We consider the relativistic phase space coordinates (x_μ,p_μ) as composite, described by functions of the primary pair of twistor coordinates. It appears that if twistor coordinates are canonicaly quantized the composite space-time coordinates are becoming noncommutative. We obtain deformed Heisenberg algebra which in order to be closed should be enlarged by the Pauli-Lubanski four-vector components. We further comment on star-product quantization of derived algebraic structures which permit to introduce spin-extended deformed Heisenberg algebra.
Comments: 7 pages; talk given at the Conference in Honour of 90-th Birthday of Freeman Dyson at Nanyang Technical University, Singapore,26-29 August 2013; to be published in Int.Journ.Mod.Phys.A

 

[John86]

http://arxiv.org/abs/1311.7619
Casimir forces on atoms in optical cavities
Alvaro M. Alhambra, Achim Kempf, Eduardo Martin-Martinez
(Submitted on 29 Nov 2013)
Casimir-type forces, such as those between two neutral conducting plates, or a sphere, atom or molecule and a plate have been widely studied and are becoming of increasing significance, for example, in nanotechnology. A key challenge is to better understand, from a fundamental microscopic approach, why the Casimir force is in some circumstances attractive and in others repulsive. Here, we study the Casimir-Polder forces experienced by small quantum systems such as atoms or molecules in an optical cavity. In a 1+1 dimensional setting, we model the small quantum systems as 2-level 'atoms', i.e., as qubits, and we model the electromagnetic field components as scalar fields with Dirichlet or Neumann boundary conditions. In this setting we determine the conditions for the Casimir force being attractive or repulsive for individual atoms. We also study the microscopic-macroscopic transition, finding that as the number of atoms in the cavity is increased, the atoms start to affect the Casimir force exerted on the cavity walls similarly to a dielectric medium.

http://arxiv.org/abs/1311.7146
Spacetime Entanglement Entropy in 1+1 Dimensions
Mehdi Saravani, Rafael D. Sorkin, Yasaman K. Yazdi
(Submitted on 27 Nov 2013)
arXiv:1205.2953 defines an entropy for a gaussian scalar field ϕ in an arbitrary region of either a causal set or a continuous spacetime, given only the correlator ⟨ϕ(x)ϕ(y)⟩ within the region. As a first application, we compute numerically the entanglement entropy in two cases where the asymptotic form is known or suspected from conformal field theory, finding excellent agreement when the required ultraviolet cutoff is implemented as a truncation on spacetime mode-sums. We also show how the symmetry of entanglement entropy reflects the fact that RS and SR share the same eigenvalues, R and S being arbitrary matrices.

 

[John86]

Not related but interesting.

http://arxiv.org/abs/1312.0580
Quantization of Emergent Gravity
Hyun Seok Yang
(Submitted on 2 Dec 2013)
Emergent gravity is based on a novel form of the equivalence principle known as the Darboux theorem or the Moser lemma in symplectic geometry stating that the electromagnetic force can always be eliminated by a local coordinate transformation as far as spacetime admits a symplectic structure, in other words, a microscopic spacetime becomes noncommutative (NC). If gravity emerges from U(1) gauge theory on NC spacetime, this picture of emergent gravity suggests a completely new quantization scheme where quantum gravity is defined by quantizing spacetime itself, leading to a dynamical NC spacetime. Therefore the quantization of emergent gravity is radically different from the conventional approach trying to quantize a phase space of metric fields. This approach for quantum gravity allows a background independent formulation where spacetime as well as matter fields is equally emergent from a universal vacuum of quantum gravity.

http://arxiv.org/abs/1312.0405
Ray tracing Einstein-Æther black holes: Universal versus Killing horizons
Bethan Cropp, Stefano Liberati, Arif Mohd, Matt Visser
(Submitted on 2 Dec 2013)
Violating Lorentz-invariance, and so implicitly permitting some form of superluminal communication, necessarily alters the notion of a black hole. Nevertheless, in both Einstein-{\AE}ther gravity, and Ho\v{r}ava-Lifshitz gravity, there is still a causally disconnected region in black-hole solutions; now being bounded by a "Universal horizon", which traps excitations of arbitrarily high velocities. To better understand the nature of these black holes, and their Universal horizons, we study ray trajectories in these spacetimes. We find evidence that Hawking radiation is associated with the Universal horizon, while the "lingering" of ray trajectories near the Killing horizon hints at reprocessing there. In doing this we solve an apparent discrepancy between the surface gravity of the Universal horizon and the associated temperature derived by tunneling method. These results advance the understanding of these exotic horizons, and provide hints for a full understanding of black-hole thermodynamics in Lorentz-violating theories.

 

[marcus]

http://arxiv.org/abs/arXiv:1312.0905
Quantum group spin nets: refinement limit and relation to spin foams
Bianca Dittrich, Mercedes Martin-Benito, Sebastian Steinhaus
(Submitted on 3 Dec 2013)
So far spin foam models are hardly understood beyond a few of their basic building blocks. To make progress on this question, we define analogue spin foam models, so called spin nets, for quantum groups SU(2)_k and examine their effective continuum dynamics via tensor network renormalization. In the refinement limit of this coarse graining procedure, we find a vast non-trivial fixed point structure beyond the degenerate and the BF phase. In comparison to previous work, we use fixed point intertwiners, inspired by Reisenberger's construction principle [1] and the recent work [2], as the initial parametrization. In this new parametrization fine tuning is not required in order to flow to these new fixed points. Encouragingly, each fixed point has an associated extended phase, which allows for the study of phase transitions in the future. Finally we also present an interpretation of spin nets in terms of melonic spin foams. The coarse graining flow of spin nets can thus be interpreted as describing the effective coupling between two spin foam vertices or space time atoms.
30+5 pages, many figures

general interest/prominent authors:
http://arxiv.org/abs/1312.0613
The Habitable Epoch of the Early Universe
Abraham Loeb (Harvard)
(Submitted on 2 Dec 2013)
In the redshift range 100<(1+z)<110, the cosmic microwave background (CMB) had a temperature of 273-300K (0-30 degrees Celsius), allowing early rocky planets (if any existed) to have liquid water chemistry on their surface and be habitable, irrespective of their distance from a star. In the standard LCDM cosmology, the first star-forming halos within our Hubble volume started collapsing at these redshifts, allowing the chemistry of life to possibly begin when the Universe was merely 15 million years old. The possibility of life starting when the average matter density was a million times bigger than it is today argues against the anthropic explanation for the low value of the cosmological constant.
6 pages, submitted to Astrobiology

http://arxiv.org/abs/1312.0612
Holographic thermal field theory on curved spacetimes
Donald Marolf, Mukund Rangamani, Toby Wiseman
(Submitted on 2 Dec 2013)
The AdS/CFT correspondence relates certain strongly coupled CFTs with large effective central charge ceff to semi-classical gravitational theories with AdS asymptotics. We describe recent progress in understanding gravity duals for CFTs on non-trivial spacetimes at finite temperature, both in and out of equilibrium. Such gravity methods provide powerful new tools to access the physics of these strongly coupled theories, which often differs qualitatively from that found at weak coupling.
Our discussion begins with basic aspects of AdS/CFT and progresses through thermal CFTs on the Einstein Static Universe and on periodically identified Minkowski spacetime. In the latter context we focus on states describing so-called plasma balls, which become stable at large ceff. We then proceed to out-of-equilibrium situations associated with dynamical bulk black holes. In particular, the non-compact nature of these bulk black holes allows stationary solutions with non-Killing horizons that describe time-independent flows of CFT plasma. As final a topic we consider CFTs on black hole spacetimes. This discussion provides insight into how the CFT transports heat between general heat sources and sinks of finite size. In certain phases the coupling to small sources can be strongly suppressed, resulting in negligible heat transport despite the presence of a deconfined plasma with sizeable thermal conductivity. We also present a new result, explaining how this so-called droplet behaviour is related to confinement via a change of conformal frame.
77 pages. 9 figures

http://arxiv.org/abs/1312.0739
Sailing through the big crunch-big bang transition
Itzhak Bars, Paul Steinhardt, Neil Turok
(Submitted on 3 Dec 2013)
In a recent series of papers, we have shown that theories with scalar fields coupled to gravity (e.g., the standard model) can be lifted to a Weyl-invariant equivalent theory in which it is possible to unambiguously trace the classical cosmological evolution through the transition from big crunch to big bang. The key was identifying a sufficient number of finite, Weyl-invariant conserved quantities to uniquely match the fundamental cosmological degrees of freedom across the transition. In so doing we had to account for the well-known fact that many Weyl-invariant quantities diverge at the crunch and bang. Recently, some authors rediscovered a few of these divergences and concluded based on their existence alone that the theories cannot be geodesically complete. In this note, we show that this conclusion is invalid. Using conserved quantities we explicitly construct the complete set of geodesics and show that they pass continuously through the big crunch-big bang transition.
3 pages, 1 figure

 

[John86]

Nice article at Sabine Hossenfelder's "Backreaction" blog it's about the CDT program.

http://backreaction.blogspot.nl/

 

[John86]

http://arxiv.org/abs/1312.1229
Hamiltonian formalism for integer-valued variables and integer time steps and a possible application in quantum physics
Gerard 't Hooft
(Submitted on 4 Dec 2013)
Most classical mechanical systems are based on dynamical variables whose values are real numbers. Energy conservation is then guaranteed if the dynamical equations are phrased in terms of a Hamiltonian function, which then leads to differential equations in the time variable. If these real dynamical variables are instead replaced by integers, and also the time variable is restricted to integers, it appears to be hard to enforce energy conservation unless one can also derive a Hamiltonian formalism for that case. We here show how the Hamiltonian formalism works here, and how it may yield the usual Hamilton equations in the continuum limit. The question was motivated by the author's investigations of special quantum systems that allow for a deterministic interpretation. The 'discrete Hamiltonian formalism' appears to shed new light on these approaches.

 

[marcus]

http://arxiv.org/abs/1312.1538
Gravitational Energy, Local Holography and Non-equilibrium Thermodynamics
Laurent Freidel
(Submitted on 5 Dec 2013)
We study the properties of gravitational system in finite regions bounded by gravitational screens. We present the detail construction of the total energy of such regions and of the energy and momentum balance equations due to the flow of matter and gravitational radiation through the screen. We establish that the gravitational screen possesses analogs of surface tension, internal energy and viscous stress tensor, while the conservations are analogs of non-equilibrium balance equations for a viscous system. This gives a precise correspondence between gravity in finite regions and non-equilibrium thermodynamics.
41 pages, 3 figures

brief mention---possible interest:
I keep seeing more papers about so called "f(T)" gravity aka "teleparallel". The torsion T plays an important role. My hunch is that I should start including at least some of the f(T) teleparallel research in our bibliography.
http://arxiv.org/abs/1303.3144
On the Gravitational Energy-Momentum Vector in f(T) Theories
S. C. Ulhoa, E. P. Spaniol
(Submitted on 13 Mar 2013 (v1), last revised 4 Dec 2013)
This work is devoted to analyze the energy of the Universe in the context of f(T) theories. Such theories are the analogous counterpart of the well known f(R) theories that, however, uses torsion instead of curvature. We obtain a general expression for the gravitational energy-momentum vector in this framework. Using the hypothesis of the isotropy of spacetime, we find the energy of the Universe and compare it with the energy obtained in the realm of teleparallelism equivalent to general relativity (TEGR).
9 pages
http://arxiv.org/abs/1312.1367
On Gravitational Entropy of de Sitter Universe
S. C. Ulhoa, E. P. Spaniol
(Submitted on 4 Dec 2013)
The paper deals with the calculation of the gravitational entropy in the context of teleparallel gravity for de Sitter space-time. In such a theory it is possible to define gravitational energy and pressure, thus we use those expressions to construct the gravitational entropy. We interpret the cosmological constant as the temperature and write the first law of thermodynamics. In the limit Λ≪1 we find that the entropy is proportional to volume and ΔS≥0.
11 pages

 

[marcus]

http://arxiv.org/abs/1312.1696
BTZ Black Hole Entropy and the Turaev-Viro model
Marc Geiller, Karim Noui
(Submitted on 5 Dec 2013)
We show the explicit agreement between the derivation of the Bekenstein-Hawking entropy of a Euclidean BTZ black hole from the point of view of spin foam models and canonical quantization. This is done by considering a graph observable (corresponding to the black hole horizon) in the Turaev-Viro state sum model, and then analytically continuing the resulting partition function to negative values of the cosmological constant.
22+4 pages, 3 figures

 

[marcus]

http://arxiv.org/abs/1312.2385
Magnetic Bianchi type II string cosmological model in loop quantum cosmology
Victor Rikhvitsky, Bijan Saha, Mihai Visinescu
(Submitted on 9 Dec 2013)
The loop quantum cosmology of the Bianchi type II string cosmological model in the presence of a homogeneous magnetic field is studied. We present the effective equations which provide modifications to the classical equations of motion due to quantum effects. The numerical simulations confirm that the big bang singularity is resolved by quantum gravity effects.
11 pages, 9 figures

==quote from introduction==
More recently, it has been shown that the big bang singularity is also resolved for anisotropic
Bianchi type I (BI) [1], II (BII) [2] and IX [3]. Usually the matter source that was considered is
a massless scalar field that plays the role of internal time. The investigations have been extended
to more complicated models including a perfect fluid, magnetic fields [4], cosmological strings
[5]. It is remarkable the fact that in all these studies it was observed that the bounce prediction is
robust.
The purpose of this paper is to investigate the dynamics of a BII string cosmological model
in the presence of a magnetic field in the framework of LQC. We show that a bounce occurs in a collapsing magnetized BII string cosmological model, thus extending the known cases of singularity resolution.
==endquote==
At first the LQC bounce was shown in the simple homogeneous isotropic case with a massless scalar field serving as matter. To test the robustness of this prediction, researchers have been examining different anisotropic versions (e.g. Bianchi types) with different sorts of matter. The authors of this paper had an earlier article which I accidentally overlooked when it came out. It was published in Astrophysics and Space Science 339, 371-377 (2012) and investigated the LQC bounce (with magnetized cosmic string as matter) in the Bianchi type I case.

http://inspirehep.net/record/945923?ln=en
Bianchi type-I string cosmological model in the presence of a magnetic field: classical versus loop quantum cosmology approaches
Victor Rikhvitsky, Bijan Saha, Mihai Visinescu
(Submitted on 15 Nov 2011)
A Bianchi type-I cosmological model in the presence of a magnetic flux along a cosmological string is considered. The first objective of this study is to investigate Einstein equations using a tractable assumption usually accepted in the literature. Quantum effects of the present cosmological model are examined in the framework of loop quantum cosmology. Finally we draw a parallel between the classical and quantum approaches.
14 pages, 8 figures

 

[MTd2]

http://arxiv.org/abs/1312.2831

A gauge theoretic approach to Einstein 4-manifolds

Joel Fine, Kirill Krasnov, Dmitri Panov
(Submitted on 10 Dec 2013)
This article investigates a new gauge theoretic approach to Einstein's equations in dimension 4. Whilst aspects of the formalism are already explained in various places in the mathematics and physics literature, our first goal is to give a single coherent account of the theory in purely mathematical language. We then explain why the new approach may have important mathematical applications: the possibility of using the calculus of variations to find Einstein 4-manifolds, as well as links to symplectic topology. We also carry out some of the technical groundwork to attack these problems.

 

[marcus]

http://arxiv.org/abs/1312.3253
General Relativity from a Thermodynamic Perspective
T. Padmanabhan
(Submitted on 11 Dec 2013)
Several recent results suggest that gravity is an emergent phenomenon with its field equations having the same status as, say, the equations of fluid dynamics. I describe several additional results, supporting this paradigm and connecting the gravitational dynamics in a bulk region of space with a thermodynamic description in the boundary of that region:

(1) The Noether charge contained in a bulk region, associated with a specific time evolution vector field, has a direct thermodynamic interpretation as the gravitational heat content of the boundary surface.

(2) This result, in turn, shows that all static spacetimes maintain holographic equipartition; in these spacetimes, the number of degrees of freedom in the boundary is equal to the number of degrees of freedom in the bulk.

(3) In a general, evolving spacetime, the rate of change of gravitational momentum is related to the difference between the number of bulk and boundary degrees of freedom. It is this departure from the holographic equipartition which drives the time evolution of the spacetime.

(4) When the equations of motion hold, the (naturally defined) total energy of the gravity plus matter within a bulk region, will be equal to the boundary heat content.

(5) After motivating the need for an alternate description of gravity (if we have to solve the cosmological constant problem), I describe a thermodynamic variational principle based on null surfaces to achieve this goal. The concept of gravitational heat density of the null surfaces arises naturally from the Noether charge associated with the null congruence. The null surface variational principle, in fact, extremises the total heat content of the matter plus gravity system. Several variations on this theme and implications are described. [Abridged]
53 pages

The more complete, detailed, and elaborate form of the abstract is on page 1 of the PDF:
http://arxiv.org/pdf/1312.3253v1.pdf
What appears here is an abridged form. It could be advisable to read BOTH versions of the abstract.

http://arxiv.org/abs/1312.3220
Multisymplectic effective General Boundary Field Theory
Mona Arjang, José A. Zapata
(Submitted on 11 Dec 2013)
The transfer matrix in lattice field theory connects the covariant and the initial data frameworks; in spin foam models, it can be written as a composition of elementary cellular amplitudes/propagators. We present a framework for discrete spacetime classical field theory in which solutions to the field equations over elementary spacetime cells may be amalgamated if they satisfy simple gluing conditions matching the composition rules of cellular amplitudes in spin foam models. Furthermore, the formalism is endowed with a multisymplectic structure responsible for local conservation laws.
Some models within our framework are effective theories modeling a system at a given scale. Our framework allows us to study coarse graining and the continuum limit.

http://arxiv.org/abs/1312.3313
Planck Data Reconsidered
David Spergel, Raphael Flauger, Renee Hlozek
(Submitted on 11 Dec 2013)
The tension between the best fit parameters derived by the Planck team and a number of other astronomical measurements suggests either systematics in the astronomical measurements, systematics in the Planck data, the need for new physics, or a combination thereof. We re-analyze the Planck data and find that the 217GHz×217GHz detector set spectrum used in the Planck analysis is responsible for some of this tension. We use a map-based foreground cleaning procedure, relying on a combination of 353 GHz and 545 GHz maps to reduce residual foregrounds in the intermediate frequency maps used for cosmological inference. For our baseline data analysis, which uses 47% of the sky and makes use of both 353 and 545 GHz data for foreground cleaning, we find the ΛCDM cosmological parameters Ω_ch²=0.1169±0.0025, n_s=0.9671±0.0069, H₀=68.0±1.1kms⁻¹Mpc⁻¹, Ω_bh²=0.02197±0.00027, ln10¹⁰A_s=3.080±0.025, and τ=0.089±0.013. While in broad agreement with the results reported by the Planck team, these revised parameters imply a universe with a lower matter density of Ω_m=0.302±0.015, and parameter values generally more consistent with pre-Planck CMB analyses and astronomical observations. We compare our cleaning procedure with the foreground modeling used by the Planck team and find good agreement. The difference in parameters between our analysis and that of the Planck team is mostly due to our use of cross-spectra from the publicly available survey maps instead of their use of the detector set cross-spectra which include pixels only observed in one of the surveys. We show evidence suggesting residual systematics in the detector set spectra used in the Planck likelihood code, which is substantially reduced for our spectra.
Comments: 15 pages

 

[marcus]

http://arxiv.org/abs/1312.3595
Hawking radiation from a spherical loop quantum gravity black hole
Rodolfo Gambini, Jorge Pullin
(Submitted on 12 Dec 2013)
We introduce quantum field theory on quantum space-times techniques to characterize the quantum vacua as a first step towards studying black hole evaporation in spherical symmetry in loop quantum gravity and compute the Hawking radiation. We use as quantum space time the recently introduced exact solution of the quantum Einstein equations in vacuum with spherical symmetry and consider a spherically symmetric test scalar field propagating on it. The use of loop quantum gravity techniques in the background space-time naturally regularizes the matter content, solving one of the main obstacles to back reaction calculations in more traditional treatments. The discreteness of area leads to modifications of the quantum vacua, eliminating the trans-Planckian modes close to the horizon, which in turn eliminates all singularities from physical quantities, like the expectation value of the stress energy tensor. Apart from this, the Boulware, Hartle--Hawking and Unruh vacua differ little from the treatment on a classical space-time. The asymptotic modes near scri are reproduced very well. We show that the Hawking radiation can be computed, leading to an expression similar to the conventional one but with a high frequency cutoff. Since many of the conclusions concern asymptotic behavior, where the spherical mode of the field behaves in a similar way as higher multipole modes do, the results can be readily generalized to non spherically symmetric fields.
13 pages
==quote Conclusions==
We have studied the quantization of a scalar field on a quantum space time that approximates well the geometry of a Schwarzschild black hole. The treatment reproduces the results of quantum field theory on a classical space-time well, with some interesting differences. The presence of a discrete structure for the space-time eliminates the divergences associated with the Boulware and Unruh vacua arising from the trans-Planckian modes and only slight modifications for the Hartle–Hawking vacuum. All the different vacua’s modes change considerably on the horizons where all the singularities present in the usual analysis disappear.
We have carried out the analysis for a given spin network, but it is valid and can be extended without significant changes (except the one we will mention next) for generic refinements of the given spin network that include more vertices such that k_n grows monotonically with n. …
...The cutoff the type of discreteness here considered introduces is similar in nature to the one considered by [12] and leads to a similar calculation of the Hawking radiation, which does not suffer significant modifications with respect to the continuum, at least for large black holes and typical frequencies.
Summarizing, we have shown that the midisuperspace formulation of loop quantum gravity with spherical symmetry is able to reproduce many features of standard analysis of quantum vacua in black hole space-times in the limit in which one considers a quantum test field living on a quantum space time. The discreteness of the quantum space-time has implications for some of the vacua even in regions of low curvature, in particular eliminating singularities. ...We have only taken the first steps towards computing Hawking radiation in loop quantum gravity. A more complete treatment, including superpositions of the quantum spin network states and a more complete discussion of the properties of the Green’s functions of the theory, in particular their Lorentz invariance will be pursued in further publications.
==endquote==

 

[marcus]

http://arxiv.org/abs/1312.3657
Structural aspects of loop quantum gravity and loop quantum cosmology from an algebraic perspective
Alexander Stottmeister, Thomas Thiemann
(Submitted on 12 Dec 2013)
We comment on structural properties of the algebras A_LQG/LQC underlying loop quantum gravity and loop quantum cosmology, especially the representation theory, relating the appearance of the (dynamically induced) superselection structure (θ-sectors) in loop quantum cosmology to recently proposed representations with non-degenerate background geometries in loop quantum gravity with Abelian structure group. To this end, we review and employ the concept of extending a given (observable) algebra with possibly non-trivial centre to a (charged) field algebra with (global) gauge group.We also interpret the results in terms of the geometry of the structure group G. Furthermore, we analyze the Koslowski-Sahlmann representations with non-degenerate background in the case of a non-Abelian structure group. We find that these representations can be interpreted from two different, though related, points view: Either, the standard algebras of loop quantum gravity need to be extended by a (possibly) central term, or the elementary flux vector fields need to acquire a shift related to the (classical) background to make these representations well-defined. Both perspectives are linked by the fact that the background shift is not an automorphism of the algebras, but rather an affine transformation. Finally, we show how similar algebraic mechanisms, which are used to explain the breaking of chiral symmetry and the occurrence of θ-vacua in quantum field theory, extend to loop quantum gravity. Thus, opening a path for the discussion of these questions in loop quantum gravity.
45 pages

http://arxiv.org/abs/1312.3674
Scalar Field Theory in Curved Momentum Space
Laurent Freidel, Trevor Rempel
(Submitted on 12 Dec 2013)
We derive an action for scalar quantum field theory with cubic interaction in the context of relative locality. Beginning with the generating functional for standard φ³--theory and the corresponding Feynman rules we modify them to account for the non--trivial geometry of momentum space. These modified rules are then used to reconstruct the generating functional and extract the action for the theory. A method for performing a covariant Fourier transform is then developed and applied to the action. We find that the transformed fields depend implicitly on a fixed point in momentum space with fields based at different points being related by a non-local transformation. The interaction term in the action is also non--local, but the kinetic term can be made local by choosing the base point to be the origin of momentum space.
26 pages, 6 figures

 

[marcus]

http://arxiv.org/abs/1312.4173
The microlocal spectrum condition, initial value formulations and background independence
Alexander Stottmeister, Thomas Thiemann
(Submitted on 15 Dec 2013)
We analyze the implications of the microlocal spectrum/Hadamard condition for states in a (linear) quantum field theory on a globally hyperbolic spacetime M in the context of a (distributional) initial value formulation. More specifically, we work in a 3+1-split M≅ℝ×Σ and give a bound, independent of the spacetime metric, on the wave front sets of the initial data for a quasi-free Hadamard state in the quantum field theory defined by a normally hyperbolic differential operator P acting in a vector bundle π: E→M. This aims at a possible way to apply the concept of Hadamard states within approaches to quantum field theory/gravity relying on a Hamiltonian formulation, potentially without a (classical) background metric g.
24 pages

general interest:
http://arxiv.org/abs/1312.4100
The Planck and LHC results and particle physics
Fedor Bezrukov
(Submitted on 15 Dec 2013)
I will discuss the recent LHC and Planck results, which are completely compatible with the Standard Model of particle physics, and the standard cosmological model (ΛCDM), respectively. It turns out that the extension of the Standard Model is, of course, required, but can be very minimal. I will discuss also what future measurements may be important to test this approach.
7 pages, talk on the EPS-HEP 2013 prepared for conference proceedings

http://arxiv.org/abs/1312.4057
Aristotle's physics
Carlo Rovelli
(Submitted on 14 Dec 2013)
I show that Aristotelian physics is a correct approximation of Newtonian physics in its appropriate domain, in the same precise sense in which Newton theory is an approximation of Einstein's theory. Aristotelian physics lasted long not because it became dogma, but because it is a very good theory.
9 pages

 

[marcus]

http://arxiv.org/abs/1312.4591
Motion of a mirror under infinitely fluctuating quantum vacuum stress
Qingdi Wang, William G. Unruh
(Submitted on 16 Dec 2013)
The actual value of the quantum vacuum energy density is generally regarded as irrelevant in non-gravitational physics. However, this paper gives a non-gravitational system where this value does have physical significance. The system is a mirror with an internal degree of freedom which interacts with a scalar field. We find that the force exerted on the mirror by the field vacuum undergoes wild fluctuations with a magnitude proportional to the value of the vacuum energy density, which is mathematically infinite. This infinite fluctuating force gives infinite instantaneous acceleration of the mirror. We show that this infinite fluctuating force and infinite instantaneous acceleration make sense because they will not result in infinite fluctuation of the mirror's position. On the contrary, the mirror's fluctuating motion will be confined in a small region due to two special properties of the quantum vacuum: (1) the vacuum friction which resists the mirror's motion and (2) the strong anti-correlation of vacuum fluctuations which constantly changes the direction of the mirror's infinite instantaneous acceleration and thus cancels the effect of infinities to make the fluctuation of the mirror's position finite.
15 pages, 6 figures

 

[marcus]

not QG but possibly of general interest:
http://arxiv.org/abs/1312.4947
Tight bonds between sterile neutrinos and dark matter
Torsten Bringmann, Jasper Hasenkamp, Joern Kersten
(Submitted on 17 Dec 2013)
Despite the astonishing success of standard ΛCDM cosmology, there is mounting evidence for a tension with observations at small and intermediate scales. We introduce a simple model where both cold dark matter (DM) and sterile neutrinos are charged under a new U(1)X gauge interaction. The resulting DM self-interactions resolve the tension with the observed abundances and internal density structures of dwarf galaxies. At the same time, the sterile neutrinos can account for both the small hot DM component favored by cosmological observations and the neutrino anomalies found in short-baseline experiments.
7 pages, 3 figures

 

[atyy]

http://arxiv.org/abs/1312.5396
Action and Vertices in the Worldine Formalism
Laurent Freidel, Trevor Rempel
(Submitted on 19 Dec 2013)
Utilizing the worldline formalism we study the effects of demanding local interactions on the corresponding vertex factor. We begin by reviewing the familiar case of a relativistic particle in Minkowksi space, showing that localization gives rise to the standard conservation of momentum at each vertex. A generalization to curved geometry is then studied and a notion of covariant Fourier transform is introduced to aid in the analysis. The vertex factor is found to coincide with the one derived for flat spacetime. Next, we apply this formalism to a loop immersed in a gravitational field, demonstrating that the loop momenta is determined entirely by the external momenta. Finally, we postulate that the semi--classical effects of quantum gravity on the Feynman path integral can be accounted for by a modification to the vertex factor which de-localizes the vertex. We study one particular Lorentz invariant de-localization which, remarkably, has no effect on conservation of vertex momenta.

http://arxiv.org/abs/1312.5512
An introduction to spherically symmetric loop quantum gravity black holes
Rodolfo Gambini, Jorge Pullin
(Submitted on 19 Dec 2013)
We review recent developments in the treatment of spherically symmetric black holes in loop quantum gravity. In particular, we discuss an exact solution to the quantum constraints that represents a black hole and is free of singularities. We show that new observables that are not present in the classical theory arise in the quantum theory. We also discuss Hawking radiation by considering the quantization of a scalar field on the quantum spacetime.

http://arxiv.org/abs/1312.5646
Ising Model from Intertwiners
Bianca Dittrich, Jeff Hnybida
(Submitted on 19 Dec 2013)
Spin networks appear in a number of areas, for instance in lattice gauge theories and in quantum gravity. They describe the contraction of intertwiners according to the underlying network. We show that a certain generating function of intertwiner contractions leads to the partition function of the 2d Ising model. This implies that the intertwiner model possesses a second order phase transition, thus leading to a continuum limit with propagating degrees of freedom.

 

[John86]

http://arxiv.org/abs/1312.5373
Amplification, Redundancy, and the Quantum Chernoff Information
Michael Zwolak, C. Jess Riedel, Wojciech H. Zurek
(Submitted on 18 Dec 2013)
Amplification was regarded, since the early days of quantum theory, as a mysterious ingredient that endows quantum microstates with macroscopic consequences, key to the "collapse of the wavepacket", and a way to avoid embarrassing problems exemplified by Schr\"odinger's cat. Such a bridge between the quantum microworld and the classical world of our experience was postulated ad hoc in the Copenhagen Interpretation. Quantum Darwinism views amplification as replication, in many copies, of the information about quantum states. We show that such amplification is a natural consequence of a broad class of models of decoherence, including the photon environment we use to obtain most of our information. This leads to objective reality via the presence of robust and widely accessible records of selected quantum states. The resulting redundancy (the number of copies deposited in the environment) follows from the Quantum Chernoff Information that quantifies the information transmitted by a typical elementary subsystem of the environment

 

[marcus]

http://arxiv.org/abs/1312.6425
The Last 50 Years of General Relativity and Gravitation: From GR3 to GR20 Warsaw Conferences
Abhay Ashtekar
(Submitted on 22 Dec 2013)
This article has a dual purpose: i) to provide a flavor of the scientific highlights of the landmark conference, GR3, held in July 1962 at Jablonna, near Warsaw; and, ii) to present a bird's eye view of the tremendous advances that have occurred over the half century that separates GR3 and GR20, which was again held in Warsaw in July 2013.
15 pages, one figure

http://arxiv.org/abs/1312.6322
Time in Fundamental Physics
Abhay Ashtekar
(Submitted on 21 Dec 2013)
The first three sections of this article contain a broad brush summary of the profound changes in the notion of time in fundamental physics that were brought about by three revolutions: the foundations of mechanics distilled by Newton in his Principia, the discovery of special relativity by Einstein and its reformulation by Minkowski, and, finally, the fusion of geometry and gravity in Einstein's general relativity. The fourth section discusses two aspects of yet another deep revision that waits in the wings as we attempt to unify general relativity with quantum physics.
10 pages

possible side interest:
http://arxiv.org/abs/1312.6449
Quantum mechanics, matter waves, and moving clocks
Holger Mueller
(Submitted on 23 Dec 2013)
This paper is divided into three parts. In the first (section 1), we demonstrate that all of quantum mechanics can be derived from the fundamental property that the propagation of a matter wave packet is described by the same gravitational and kinematic time dilation that applies to a clock. We will do so in several steps, first deriving the Schroedinger equation for a nonrelativistic particle without spin in a weak gravitational potential, and eventually the Dirac equation in curved space-time describing the propagation of a relativistic particle with spin in strong gravity.
In the second part (sections 2-4), we present interesting consequences of the above quantum mechanics: that it is possible to use wave packets as a reference for a clock, to test general relativity, and to realize a mass standard based on a proposed redefinition of the international system of units, wherein the Planck constant would be assigned a fixed value. The clock achieved an absolute accuracy of 4 parts per billion (ppb). The experiment yields the fine structure constant α=7.297352589(15)×10⁻³ with 2.0 ppb accuracy. We present improvements that have reduced the leading systematic error about 8-fold and improved the statistical uncertainty to 0.33 ppb in 6 hours of integration time, referred to α.
In the third part (sections 5-7), we present possible future experiments with atom interferometry: A gravitational Aharonov-Bohm experiment and its application as a measurement of Newton's gravitational constant, antimatter interferometry, interferometry with charged particles, and interferometry in space.
We will give a review of previously published material when appropriate, but will focus on new aspects that haven't been published before.
74 pages. To be published in the proceedings of the International School of Physics "Enrico Fermi" 2013, Course 188 - Atom Interferometry

http://arxiv.org/abs/1312.6268
Quantum speed limits and optimal Hamiltonians for driven systems in mixed states
Ole Andersson, Hoshang Heydari
(Submitted on 21 Dec 2013)
Inequalities of Mandelstam-Tamm and Margolus-Levitin type provide lower bounds on the time it takes for a quantum system to evolve between two states. Knowledge of such bounds, called quantum speed limits, is of utmost importance in virtually all areas of physics, where determination of the minimum time required for a quantum process is of interest...
14 pages, 2 figures

 

[MTd2]

Well, this is very unrelated, but given the overall interest in numerology on this subforum, I think many people will enjoy this paper:

http://arxiv.org/abs/1312.6523

A magic pyramid of supergravities

A. Anastasiou, L. Borsten, M. J. Duff, L. J. Hughes, S. Nagy
(Submitted on 23 Dec 2013)
By formulating N = 1, 2, 4, 8, D = 3, Yang-Mills with a single Lagrangian and single set of transformation rules, but with fields valued respectively in R,C,H,O, it was recently shown that tensoring left and right multiplets yields a Freudenthal-Rosenfeld-Tits magic square of D = 3 supergravities. This was subsequently tied in with the more familiar R,C,H,O description of spacetime to give a unified division-algebraic description of extended super Yang-Mills in D = 3, 4, 6, 10. Here, these constructions are brought together resulting in a magic pyramid of supergravities. The base of the pyramid in D = 3 is the known 4x4 magic square, while the higher levels are comprised of a 3x3 square in D = 4, a 2x2 square in D = 6 and Type II supergravity at the apex in D = 10. The corresponding U-duality groups are given by a new algebraic structure, the magic pyramid formula, which may be regarded as being defined over three division algebras, one for spacetime and each of the left/right Yang-Mills multiplets. We also construct a conformal magic pyramid by tensoring conformal supermultiplets in D = 3, 4, 6. The missing entry in D = 10 is suggestive of an exotic theory with G/H duality structure F4(4)/Sp(3) x Sp(1).

 

[marcus]

http://arxiv.org/abs/1312.6835
Thermodynamics of the polymeric quantized Schwarzschild black hole
M. A. Gorji, K. Nozari, B. Vakili
(Submitted on 24 Dec 2013)
Polymer representation of quantum mechanics is an effective approach to loop quantum gravity. In this paper we develop statistical mechanics in the polymer framework. While to obtain energies of microstates one needs usually to solve the polymer-modified Schrödinger equation, we have not adopted this strategy here since it is not an easy task due to the complicated form of the Schrödinger equation in the polymer picture. Instead, we formulate the ensemble theory in polymer framework in a semi-classical regime through deformed density of states. We show that our results are in good agreement with those arising from quantum mechanical considerations. Applying this method to thermodynamics of quantum Schwarzschild black hole, we obtain corrections to the Bekenstein-Hawking entropy due to loop quantum gravity effects.
12 pages
Subjects:

 

[marcus]

http://arxiv.org/abs/1312.7273
On How Neutrino Protects the Axion
Gia Dvali, Sarah Folkerts, Andre Franca
(Submitted on 27 Dec 2013)
We show how the neutrino can sacrifice itself to quantum gravity and save the axion solution to the strong-CP problem. This mechanism puts an upper bound on the lightest neutrino mass.
9 pages
[my comment on the paper's significance is to quote its conclusion section]
==excerpt==
In this work we have put forward yet another example of the highly profound connection between particle physics and non-perturbative quantum gravity. The main players in this connection are the axion and the neutrino.
Quantum gravity is believed to violate global symmetries, and among other things, ruin the axion solution of the strong-CP problem. ...By identifying the source of the danger, we were able to see the possible protection mechanism against it. This mechanism is built-in in the Standard Model in form of light neutrinos. ...This mechanism gives a phenomenological bound on the neutrino mass. The precise measurement of this mass would reveal a bound on non-perturbative gravity scale.
==endquote==

in case anyone is following GFT and Rivasseau's tensor track program:
http://arxiv.org/abs/1312.7226
The Multiscale Loop Vertex Expansion
Razvan Gurau, Vincent Rivasseau
(Submitted on 27 Dec 2013)
The loop vertex expansion (LVE) is a constructive technique which uses only canonical combinatorial tools and no space-time dependent lattices. It works for quantum field theories without renormalization. Renormalization requires scale analysis. In this paper we provide an enlarged formalism which we call the multiscale loop vertex expansion (MLVE). We test it on what is probably the simplest quantum field theory which requires some kind of renormalization, namely a combinatorial model of the vector type with quartic interaction and a propagator which mimicks the power counting of ϕ⁴₂. An ordinary LVE would fail to treat even this simplest superrenormalizable model, but we show how to perform the ultraviolet limit and prove its analyticity in the Borel summability domain of the model with the MLVE.
20 pages

 

[marcus]

http://arxiv.org/abs/arXiv:1312.7747
Lorentz Spin-Foam with Non Unitary Representations by use of Holomorphic Peter-Weyl Theorem
Leonid Perlov
(Submitted on 30 Dec 2013)
We use the non-unitary spinor representations of SL(2,C) and the recently proved Holomorphic Peter-Weyl theorem to define the Hilbert space based on the holomorphic spin-networks, the non-unitary spin-foam, solve the simplicity constraints and calculate the vertex amplitude. The diagonal simplicity constraint provides two solutions. The first solution: Immirzi γ=i with the irreducible representations (j₁,j₂) projected to (0,j) and the second solution: Immirzi γ=−i and the irreducible non-unitary representations projected to (j,0). The off-diagonal constraint selects only the first of these two solutions. The solution is interesting in two aspects: a) it turns to be a topological BF model. b) Immirzi parameter γ=i corresponds to Ashtekar's self-dual connection of the complexified algebra sl(2,C)⊗C. The transition amplitude is finite and very similar to BF Euclidean model. We discuss the inner product Lorentz invariance and the viability of the non-unitary representations in the Loop Quantum Gravity (LQG) models. We also discuss the unitary versus non-unitary evolution as a consequence of the Schrödinger equation in classical quantum mechanics versus Wheeler-DeWitt equation in GR. We address the problem of a quantum measurement in the end.
14 pages

possibly of general interest:
http://arxiv.org/abs/1312.7454
Adaptive Coarse Graining, Environment, Strong Decoherence, and Quasiclassical Realms
Murray Gell-Mann, James B. Hartle
(Submitted on 28 Dec 2013)
Three ideas are introduced that when brought together characterize the realistic quasiclassical realms of our quantum universe as particular kinds of sets of alternative coarse-grained histories defined by quasiclassical variables: (1) Branch dependent adaptive coarse grainings that can be close to maximally refined and can simplify calculation. (2) Narrative coarse grainings that describe how features of the universe change over time and allow the construction of an environment. (3) A notion of strong decoherence that characterizes realistic mechanisms of decoherence.
11 pages
[Comment: a classical spacetime is used, so this is something of a "toy model" paper. One of a series by Gell-Mann and Hartle over the years, addressing the (somewhat philosophical) problems of applying QM to the cosmos as a whole.]

 

[John86]

http://arxiv.org/abs/1312.7797
Quantum indeterminacy in local measurement of cosmic expansion
Craig J. Hogan
(Submitted on 30 Dec 2013)
For a system of two small bodies in an expanding universe, bounds on mass and separation are estimated, from standard gravity and quantum mechanics, such that both their gravity and the process of quantum measurement affect their motion less than the cosmic expansion does. It is shown that such a direct local measurement of cosmic expansion or acceleration at rate H is only possible, even in principle, in a region of size greater than H −3/5 in Planck units, or about 60 meters in the current universe, a new scale that defines a boundary between quantum and classical expansion. A generalization to spatially extended linear density perturbations shows the same scale. Matching vacuum energy or directional information in localized field states to gravity on this system length scale yields a particle mass scale of H 3/10 , or about 7 GeV today. Possible connections of cosmic acceleration with the QCD vacuum are discussed.

http://arxiv.org/abs/1312.7798
Directional Entanglement of Quantum Fields with Quantum Geometry
Craig J. Hogan
(Submitted on 30 Dec 2013)
Using transversely localized solutions of the relativistic wave equation, the path of a massless particle with wavelength λ that travels a distance z is shown to have a wave function with indeterminacy in direction given by the diffraction scale, ⟨Δθ 2 ⟩>2 √ λ/πz . It is conjectured that the spatial structure of quantum field states is influenced by quantum directional indeterminacy of geometry set by the Planck length, l P . Entanglement of field and geometry states is described in the small angle approximation. The entanglement has almost no effect on local measurements, microscopic particle interactions, or measurements of propagating states that depend only on longitudinal coordinates, but significantly alters field states in systems larger than ≈λ 2 /l P that depend on transverse coordinates or direction. It reduces the information content of fields in large systems, consistent with holographic bounds from gravitation theory, and may lead to quantum-geometrical directional fluctuations of massive bodies detectable with interferometers. Possible connections are discussed with field vacuum energy, black hole information, and inflationary fluctuations.

http://arxiv.org/abs/1312.7767
Quantization and fixed points of non-integrable Weyl theory
Carlo Pagani, Roberto Percacci
(Submitted on 30 Dec 2013)
We consider a simple but generic model of gravity where Weyl--invariance is realized thanks to the presence of a gauge field for dilatations. We quantize the theory by suitably defining renormalization group flows that describe the integration of successive momentum shells, in such a way that Weyl--invariance is maintained in the flow. When the gauge fields are massless the theory has, in addition to Weyl invariance, an abelian gauge symmetry. According to the definition of the cutoff, the flow can break or preserve this extended symmetry. We discuss the fixed points of these flows.

http://arxiv.org/abs/1312.7842
Twistors and antipodes in de Sitter space
Yasha Neiman
(Submitted on 30 Dec 2013)
We develop the basics of twistor theory in de Sitter space, up to the Penrose transform for free massless fields. We treat de Sitter space as fundamental, as one does for Minkowski space in conventional introductions to twistor theory. This involves viewing twistors as spinors of the de Sitter group SO(4,1). When attached to a spacetime point, such a twistor can be reinterpreted as a local SO(3,1) Dirac spinor. Our approach highlights the antipodal map in de Sitter space, which gives rise to doublings in the standard relations between twistors and spacetime. In particular, one can generate a field with both handedness signs from a single twistor function. Such fields naturally live on antipodally-identified de Sitter space dS_4/Z_2, which has been put forward as the ideal laboratory for quantum gravity with positive cosmological constant.

http://arxiv.org/abs/1312.7856
Gravitation from Entanglement in Holographic CFTs
Thomas Faulkner, Monica Guica, Thomas Hartman, Robert C. Myers, Mark Van Raamsdonk(Submitted on 30 Dec 2013)
Entanglement entropy obeys a 'first law', an exact quantum generalization of the ordinary first law of thermodynamics. In any CFT with a semiclassical holographic dual, this first law has an interpretation in the dual gravitational theory as a constraint on the spacetimes dual to CFT states. For small perturbations around the CFT vacuum state, we show that the set of such constraints for all ball-shaped spatial regions in the CFT is exactly equivalent to the requirement that the dual geometry satisfy the gravitational equations of motion, linearized about pure AdS. For theories with entanglement entropy computed by the Ryu-Takayanagi formula S=A/(4G N ) , we obtain the linearized Einstein equations. For theories in which the vacuum entanglement entropy for a ball is computed by more general Wald functionals, we obtain the linearized equations for the associated higher-curvature theories. Using the first law, we also derive the holographic dictionary for the stress tensor, given the holographic formula for entanglement entropy. This method provides a simple alternative to holographic renormalization for computing the stress tensor expectation value in arbitrary higher derivative gravitational theories.

http://arxiv.org/abs/1312.7878
Into the Amplituhedron
Nima Arkani-Hamed, Jaroslav Trnka
(Submitted on 30 Dec 2013)
We initiate an exploration of the physics and geometry of the amplituhedron, starting with the simplest case of the integrand for four-particle scattering in planar N=4 SYM. We show how the textbook structure of the unitarity double-cut follows from the positive geometry. We also use the geometry to expose the behavior of the multicollinear limit, providing a direct motivation for studying the logarithm of the amplitude. In addition to computing the two and three-loop integrands, we explore various lower-dimensional faces of the amplituhedron, thereby computing non-trivial cuts of the integrand to all loop orders.

http://arxiv.org/abs/1312.7828
First order gravity: Actions, topological terms and boundaries
Alejandro Corichi, Irais Rubalcava, Tatjana Vukasinac
(Submitted on 30 Dec 2013)
We consider first order gravity in four dimensions. This means that the fundamental variables are a tetrad e and a SO(3,1) connection ω . We study the most general action principle compatible with diffeomorphism invariance. This implies, in particular, considering besides the standard Einstein-Hilbert term, other terms that either do not change the equations of motion, or are topological in nature. Having a well defined action principle also implies adding additional boundary terms, whose detailed form may depend on the particular boundary conditions at hand. We consider spacetimes that include a boundary at infinity, satisfying asymptotically flat boundary conditions and/or an internal boundary satisfying isolated horizons boundary conditions. For our analysis we employ the covariant Hamiltonian formalism where the phase space Γ is given by solutions to the equation of motion. For each of the possible terms contributing to the action we study the well posedness of the action, its finiteness, the contribution to the symplectic structure, and the Hamiltonian and Noether charges. While some of the results are not new, we have several results that are novel and have not appeared elsewhere. Furthermore, we point out and clarify some issues that have not been clearly understood in the literature. The aim of the paper is to present a comprehensive and self-contained treatment of the subject, so the style is somewhat pedagogical.

 

[marcus]

http://arxiv.org/abs/1401.0327
Quantum volume and length fluctuations in a midi-superspace model of Minkowski space
Jeremy Adelman, Franz Hinterleitner, Seth Major
(Submitted on 1 Jan 2014)
In a 1+1 dimensional midi-superspace model for gravitational plane waves, Minkowski space-time flatness is imposed with constraints derived from null Killing vectors. Solutions to a straightforward regularization of these constraints have diverging length and volume expectation values. Physically acceptable solutions in the kinematic Hilbert space are obtained with constraints weakened by multiplying with a power of the the volume operator, or by a similar modification of the Hamiltonian constraint. The solutions of the modified constraint have finite expectation values of geometric quantities. Further, the expectation value of the original constraint vanishes, but its moment is non-vanishing. As the power of the volume grows, while the moments of volume and length decrease. In this way possible kinematic states for flat space, with fluctuations, arise. As a consequence of the regularization of operators in quantum geometry, the quantum uncertainty relations between geometric quantities such as length and its conjugate momentum do not reflect naive expectations from the classical Poisson bracket relations.
41 pages
==quote==
I. INTRODUCTION
Loop quantum gravity (LQG) quantizes the spatial geometry by introducing “atoms of spatial geometry” in form of quanta of volume, area, length, and angle [1–6]. Unlike the Minkowski vacuum in quantum field theories of different kinds of matter, the quantum model of flat space appears to be, not a “no particle” state but rather, a highly excited state with a macroscopically homogeneous distribution of excited quanta of geometry. In this paper we explore…
==endquote==

Sabine resolves the "firewall" problem:
http://arxiv.org/abs/1401.0288
Disentangling the Black Hole Vacuum
S. Hossenfelder
(Submitted on 1 Jan 2014)
We study the question whether disentanglement of Hawking radiation can be achieved with any local operation. We assume that the operation we look for is unitary and can be described by a Bogoliubov transformation. This allows to formulate requirements on the operation of disentanglement. We then show that these requirements can be fulfilled by a timelike boundary condition in the near-horizon area and that the local observer does not notice the presence of the boundary and does not encounter a firewall.
16 pages, 2 figures
[my comment: Acknowledgements section thanks Raffael Bousso, Steve Giddings, Steve Hsu, Ted Jacobson, Don Marolf and Joe Polchinski for feedback and other assistance.]

 

[marcus]

http://arxiv.org/abs/1401.0931
Hamiltonian constraint in Euclidean LQG revisited: First hints of off-shell Closure
Alok Laddha
(Submitted on 5 Jan 2014)
We initiate the hunt for a definition of Hamiltonian constraint in Euclidean Loop Quantum Gravity (LQG) which faithfully represents quantum Dirac algebra. Borrowing key ideas from previous works on Hamiltonian constraint in LQG and several toy models, we present some evidence that there exists such a continuum Hamiltonian constraint operator which is well defined on a suitable generalization of the Lewandowski-Marolf Habitat and is anomaly free off-shell.
68 pages, 6 figures

 

[marcus]

http://arxiv.org/abs/1401.0931
Hamiltonian constraint in Euclidean LQG revisited: First hints of off-shell Closure
Alok Laddha
(Submitted on 5 Jan 2014)
We initiate the hunt for a definition of Hamiltonian constraint in Euclidean Loop Quantum Gravity (LQG) which faithfully represents quantum Dirac algebra. Borrowing key ideas from previous works on Hamiltonian constraint in LQG and several toy models, we present some evidence that there exists such a continuum Hamiltonian constraint operator which is well defined on a suitable generalization of the Lewandowski-Marolf Habitat and is anomaly free off-shell.
68 pages, 6 figures

briefly noted, possibly of general interest:
http://arxiv.org/abs/1401.1146
Is the Universe Simpler than LCDM?
Matthew G. Walker, Abraham Loeb
(Submitted on 6 Jan 2014)
In the standard cosmological model, the Universe consists mainly of two invisible substances: vacuum energy with mass density rho_v = Lambda /(8 pi G), and cold dark matter (CDM). This model has the virtue of simplicity, enabling straightforward calculation of the formation and evolution of cosmic structure against the backdrop of the Hubble flow. Here we discuss apparent discrepancies with observations on small galactic scales, which LCDM must attribute to complexity in the baryon physics of galaxy formation. Yet galaxies exhibit structural scaling relations that evoke simplicity, presenting a clear target for formation models. We use published data to examine the relationship between dynamical components of such relations. Tracers of gravitational potentials dominated by dark matter show a correlation between orbital size, R, and velocity, V, that can be expressed most simply as a characteristic acceleration, a_DM ~ 1 km²/s²/pc ~ 3 x 10^-9 cm/s² ~ 0.2c(G rho_v)^1/2. It remains to be seen whether LCDM predicts such behavior.
22 pages, 1 figure, submitted to Contemporary Physics

http://arxiv.org/abs/1401.0774
Quantum Limit on Stability of Clocks in a Gravitational Field
Supurna Sinha, Joseph Samuel
(Submitted on 4 Jan 2014)
Accurate and stable clocks are of importance both to fundamental physics and for applications in astronomy, metrology and global positioning systems. In a recent technological breakthrough, researchers at NIST have been able to achieve a stability of 1 part in 10¹⁸ using an Ytterbium clock. This naturally raises the question of whether there are fundamental limits to the stability of clocks. In this paper we point out that gravity and quantum mechanics set a fundamental limit on the stability of clocks. This limit comes from a combination of the uncertainty relation, the gravitational redshift and the relativistic time dilation effect. For example, a single ion hydrogen maser clock in a terrestrial gravitational field cannot achieve a stability better than one part in 10²². This observation has implications for laboratory experiments involving both gravity and quantum theory. Our result implies that quantum interference effects can be destroyed by a sufficiently strong gravitational field, which would be of interest to theorists attempting to combine gravity and quantum theory.
10 pages, 2 figures, 1 table

 

[marcus]

possibly of general interest:
http://arxiv.org/abs/1401.1523
Firewalls and flat mirrors: An alternative to the AMPS experiment which evades the Harlow-Hayden obstacle
Jonathan Oppenheim, William G. Unruh
(Submitted on 7 Jan 2014)
If quantum gravity does not lead to a breakdown of predictability, then Almheiri, Marolf, Polchinski and Sully (AMPS) have argued that an observer falling into a black hole can perform an experiment which verifies a violation of entanglement monogamy - that late time Hawking radiation is maximally entangled with early time Hawking radiation and also with infalling radiation - something impossible in quantum field theory. However, as pointed out by Hayden and Harlow, this experiment is infeasible, as the time required to perform the experiment is almost certainly longer than the lifetime of the black hole. Here we propose an alternative firewall experiment which could actually be performed within the black hole's lifetime. The alternative experiment involves forming an entangled black hole in which the unscrambling of information is precomputed on a quantum memory prior to the creation of the black hole and without acting on the matter which forms the black hole or emerges from it. This would allow an observer near a black hole to signal faster than light. As another application of our precomputing strategy, we show how one can produce entangled black holes which acts like "flat mirrors'', in the sense that information comes out almost instantly (as in the Hayden-Preskill scenario), but also emerge unscrambled, so that it can actually be observed instantly as well. Finally, we prove that a black hole in thermal equilibrium with its own radiation, is uncorrelated with this radiation.
19 pages, 3 figures

 

[marcus]

http://arxiv.org/abs/1401.2057
Generally covariant formulation of Relative Locality in curved spacetime
Francesco Cianfrani, Jerzy Kowalski-Glikman, Giacomo Rosati
(Submitted on 9 Jan 2014)
We construct a theory of particles moving in curved both momentum space and spacetime, being a generalization of Relative Locality. We find that in order to construct such theory, with desired symmetries, including the general coordinate invariance, we have to use non local position variables. It turns out that free particles move on geodesics and momentum dependent translations of Relative Locality are replaced with momentum dependent geodesic deviations.
14 pages

http://arxiv.org/abs/1401.2026
Quantum fields in curved spacetime
Stefan Hollands, Robert M. Wald
(Submitted on 9 Jan 2014)
We review the theory of quantum fields propagating in an arbitrary, classical, globally hyperbolic spacetime. Our review emphasizes the conceptual issues arising in the formulation of the theory and presents known results in a mathematically precise way. Particular attention is paid to the distributional nature of quantum fields, to their local and covariant character, and to microlocal spectrum conditions satisfied by physically reasonable states. We review the Unruh and Hawking effects for free fields, as well as the behavior of free fields in deSitter spacetime and FLRW spacetimes with an exponential phase of expansion. We review how nonlinear observables of a free field, such as the stress-energy tensor, are defined, as well as time-ordered-products. The "renormalization ambiguities" involved in the definition of time-ordered products are fully characterized. Interacting fields are then perturbatively constructed. Our main focus is on the theory of a scalar field, but a brief discussion of gauge fields is included. We conclude with a brief discussion of a possible approach towards a nonperturbative formulation of quantum field theory in curved spacetime and some remarks on the formulation of quantum gravity.
52 pages, 7 figures, invited contribution to "100 Years of General Relativity" monograph series

http://arxiv.org/abs/1401.1838
Comments On Torsion and MacDowell-Mansouri gravity
J. C. López-Domínguez, J. E. Rosales-Quintero, M. Sabido
(Submitted on 8 Jan 2014)
We construct a generalization for the MacDowell-Mansouri formulation of gravity. New parameters are introduced into the action to include the non-dynamical Holst term, independently from the topological Nieh-Yan class. Finally, we consider the new parameters as fields and analyze the solutions coming from their equations of motion. The new fields introduce torsional contributions to the theory that modify Einstein's equations.
15 pages

 

[marcus]

http://arxiv.org/abs/1401.3054
The Entropy of BTZ Black Hole from Loop Quantum Gravity
Jingbo Wang
(Submitted on 14 Jan 2014)
In this paper, we calculated the entropy of the BTZ black hole in the framework of loop quantum gravity. We got the result that the horizon degrees of freedom can be described by the 2D SO(1,1) punctured BF theory. Finally we got the area law for the entropy of BTZ black hole.
12 page