Intuitive content of Loop Gravity--Rovelli's program

marcus

brief mention:
http://arxiv.org/abs/1305.3497
Varying electric charge in multi-scale spacetimes
Gianluca Calcagni, Joao Magueijo, David Rodríguez Fernández
...
28 pages.

http://arxiv.org/abs/1305.3509
Interpretation of quantal manifolds
Johan Noldus
(Submitted on 11 May 2013)
In quantum gravity, one looks for alternative structures to ... ordinary real manifolds. Here, we propose an alternative universal construction containing the latter as an equilibrium state under the action of the universal diffeomorphism group. Our theory contains many other previous proposals ... as special cases. However, the crucial point we make is that those have to be appreciated in the universal context developed here.
16 pages.

MTd2

http://arxiv.org/abs/1305.3313

Is Quantum Gravity a Super-Quantum Theory?

Lay Nam Chang, Zachary Lewis, Djordje Minic, Tatsu Takeuchi
(Submitted on 14 May 2013)
We argue that quantum gravity should be a super-quantum theory, that is, a theory whose non-local correlations are stronger than those of canonical quantum theory. As a super-quantum theory, quantum gravity should display distinct experimentally observable super-correlations of entangled stringy states.

atyy

http://arxiv.org/abs/1305.1045
A lattice non-perturbative definition of an SO(10) chiral gauge theory and its induced standard model
Xiao-Gang Wen
(Submitted on 5 May 2013)
The standard model is a chiral gauge theory where the gauge fields couple to the right-hand and the left-hand fermions differently. The standard model is defined perturbatively and describes all elementary particles (except gravitons) very well. However, for a long time, we do not know if we can have a non-perturbative definition of standard model as a Hamiltonian quantum mechanical theory. In this paper, we propose a way to give a modified standard model (with 48 two-component Weyl fermions) a non-perturbative definition by embeding the modified standard model into a SO(10) chiral gauge theory and then putting the SO(10) chiral gauge theory on a 3D spatial lattice with a continuous time. Such a non-perturbatively defined standard model is a Hamiltonian quantum theory with a finite-dimensional Hilbert space for a finite space volum. Using the defining connection between gauge anomalies and the symmetry-protected topological orders, we show that any chiral gauge theory can be non-perturbatively defined by putting it on a lattice in the same dimension, as long as the chiral gauge theory is free of all anomalies.

marcus

http://arxiv.org/abs/1305.4512
Loop quantum cosmology for nonminimally coupled scalar field
Michal Artymowski, Andrea Dapor, Tomasz Pawlowski
(Submitted on 20 May 2013)
We perform a LQC-quantization of the FRW cosmological model with nonminimally coupled scalar field. Making use of a canonical transformation, we recast the theory in the minimally coupled form (Einstein frame), for which standard LQC techniques can be applied to find the physical Hilbert space and the dynamics. We then focus on the semiclassical sector, obtaining a classical effective Hamiltonian, which can be used to study the dynamics. We show that the classical singularity is replaced by a "mexican hat"-shaped bounce, joining the contracting and expanding branches. The model accommodates Higgs-driven inflation, with more than enough e-folding for any physically meaningful initial condition.
3 pages, 2 figures. To appear in Proceedings of the 13th Marcel Grossmann Meeting (MG13), Stockholm, Sweden, 1-7 July 2012
[my comment: this was based on a revised version of http://arxiv.org/abs/1207.4353 which went on arxiv 20 May 2013 at the same time and gives a more complete presentation of the same material]

http://arxiv.org/abs/1305.4513
Quantum field theory on LQC Bianchi spacetimes
Andrea Dapor, Jerzy Lewandowski, Yaser Tavakoli
(Submitted on 20 May 2013)
Quantum theory of a scalar field is developed on the LQC Bianchi I space-time. By comparing the quantum field theory for a single mode on classical and quantum background geometries we find that an effective Bianchi I space-time emerges. We show that by disregarding the back-reaction no Lorentz-violation is present, despite the effective metric being different than the classical Bianchi I one.
3 pages. To appear in Proceedings of the 13th Marcel Grossmann Meeting (MG13), Stockholm, Sweden, 1-7 July 2012

http://arxiv.org/abs/1305.4516
Modified Friedmann equation and survey of solutions in effective Bianchi-I loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Submitted on 20 May 2013)
In this article, we study the equations driving the dynamics of a Bianchi-I universe described by holonomy corrected effective loop quantum cosmology. We derive the LQC-modified generalized Friedmann equation, which is used as a guide to find different types of solutions. It turns out that, in this framework, most solutions never reach the classical behavior.
8 pages, 9 figures.

http://arxiv.org/abs/1305.4526
Variables suitable for constructing quantum states for the Teleparallel Equivalent of General Relativity I
Andrzej Okolow
(Submitted on 20 May 2013)
In this paper we present the first part of an analysis aimed at introducing variables which are suitable for constructing a space of quantum states for the Teleparallel Equivalent of General Relativity via projective techniques - the space of quantum states is meant to be applied in a canonical quantization of the theory. We show that natural configuration variables on the phase space of the theory can be used to construct a space of quantum states which however possesses an undesired property. We introduce then a family of new variables such that some elements of the family can be applied to build a space of quantum states free of that property.
Comments: 33 pages, 5 figures.

brief mention:
http://arxiv.org/abs/1305.4524
The Black Hole Particle Accelerator as a Machine to make Baby Universes
A. J. S. Hamilton
(Submitted on 20 May 2013)
7 pages, 2 figures. Entered in the 2013 Essay Competition of the Gravity Research Foundation

MTd2

http://arxiv.org/abs/1305.4582

Causal Dynamical Triangulations without Preferred Foliation

S. Jordan, R. Loll
(Submitted on 20 May 2013)
We introduce a generalized version of the Causal Dynamical Triangulations (CDT) formulation of quantum gravity, in which the regularized, triangulated path integral histories retain their causal properties, but do not have a preferred proper-time foliation. An extensive numerical study of the associated nonperturbative path integral in 2+1 dimensions shows that it can nevertheless reproduce the emergence of an extended de Sitter universe on large scales, a key feature of CDT quantum gravity. This suggests that the preferred foliation normally used in CDT is not a crucial (albeit convenient) part of its background structure.

marcus

http://arxiv.org/abs/1305.4790
Quantum-gravity inspired collapse with far observable bounce
Cosimo Bambi, Daniele Malafarina, Leonardo Modesto
(Submitted on 21 May 2013)
We consider general relativistic homogeneous gravitational collapses for dust and radiation. We show that replacing the density profile with an effective density justified by some quantum gravity framework leads to the avoidance of the final singularity. The effective density acts on the collapsing cloud by introducing an isotropic pressure, which is negligible at the beginning of the collapse and becomes negative and dominant in the strong field regime. For sufficiently small sources, the horizon never forms. For larger objects, the horizon forms, it disappears when the matter density approaches a critical value and gravity becomes very weak (asymptotic freedom regime), it forms again after the bounce as a consequence of the decrease in the matter density, and it eventually disappears when the density becomes too low and the matter is radiated away. In both cases, the bounce stage regulated by quantum gravity effects turns out to be visible to distant observers. The possibility of detecting radiation coming from the high density region of a collapsing astrophysical object in which classically there would be the creation of a singularity could open a new window to experimentally test theories of quantum gravity.
10 pages, 6 figures

http://arxiv.org/abs/1305.4702
Exploring Torus Universes in Causal Dynamical Triangulations
T.G. Budd, R. Loll
(Submitted on 21 May 2013)
Motivated by the search for new observables in nonperturbative quantum gravity, we consider Causal Dynamical Triangulations (CDT) in 2+1 dimensions with the spatial topology of a torus. This system is of particular interest, because one can study not only the global scale factor, but also global shape variables in the presence of arbitrary quantum fluctuations of the geometry. Our initial investigation focusses on the dynamics of the scale factor and uncovers a qualitatively new behaviour, which leads us to investigate a novel type of boundary conditions for the path integral. Comparing large-scale features of the emergent quantum geometry in numerical simulations with a classical minisuperspace formulation, we find partial agreement. By measuring the correlation matrix of volume fluctuations we succeed in reconstructing the effective action for the scale factor directly from the simulation data. Apart from setting the stage for the analysis of shape dynamics on the torus, the new set-up highlights the role of nontrivial boundaries and topology.
30 pages, 12 figures

brief mention:
http://arxiv.org/abs/1305.4685
Machian Classical and Semiclassical Emergent Time
Edward Anderson
(Submitted on 21 May 2013)
Classical and semiclassical schemes are presented that are timeless at the primary level and recover time from Mach's "time is to be abstracted from change" principle at the emergent secondary level. The semiclassical scheme is a Machian variant of the Semiclassical Approach to the Problem of Time in Quantum Gravity. The classical scheme is Barbour's, cast here explicitly as the classical precursor of the Semiclassical Approach...
22 pages and 1 figure.