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Aug3-11, 07:12 PM
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PF Gold
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On the structure of a background independent quantum theory: Hamilton function, transition amplitudes, classical limit and continuous limit
Carlo Rovelli
(Submitted on 3 Aug 2011)
The Hamilton function is a powerful tool for studying the classical limit of quantum systems, which remains meaningful in background-independent systems. In quantum gravity, it clarifies the physical interpretation of the transitions amplitudes and their truncations.
7 pages
A critical look at strings
Carlo Rovelli
(Submitted on 3 Aug 2011)
This is an invited contribution to the Special Issue of "Foundations of Physics" titled "Forty Years Of String Theory: Reflecting On the Foundations". I have been asked to assess string theory as an outsider, and to compare it with the theory, methods, and expectations in my own field.
7 pages

Gerard 't Hooft is the chief editor of that special issue "Forty Years of String Theory". Presumably he is the one who invited Rovelli to provide an outside assessment of the program, in comparison with the Loop program.
Clocks and Relationalism in the Thermal Time Hypothesis
Nicolas C. Menicucci, S. Jay Olson, Gerard J. Milburn
(Submitted on 3 Aug 2011)
The Thermal Time Hypotheis (TTH) has been proposed as a general method for identifying a time variable from within background-free theories which do not come equipped with a pre-defined clock variable. Here, we explore some implications of the TTH in an entirely relational context by constructing a protocol for the creation of "thermal clocks" from components of a large but finite quantum mechanical system. The protocol applies locally, in the sense that we do not attempt to construct a single clock describing the evolution of the entire system, but instead we construct clocks which describe the evolution of each subsystem of interest. We find that a consistency condition required for the evolution of our clocks is operationally equivalent to the general relativistic Tolman-Ehrenfest relation for thermal equilibrium in a static gravitational field but without the assumption of gravity or a metric field of any kind.
11 pages, 3 figures
Loop Quantum Cosmology: A Status Report
Abhay Ashtekar, Parampreet Singh
(Submitted on 3 Aug 2011)
The goal of this article is to provide an overview of the current state of the art in loop quantum cosmology for three sets of audiences: young researchers interested in entering this area; the quantum gravity community in general; and, cosmologists who wish to apply loop quantum cosmology to probe modifications in the standard paradigm of the early universe. An effort has been made to streamline the material so that each of these communities can read only the sections they are most interested in, without a loss of continuity.
136 pages, 15 figures
The black hole information paradox and relative locality
Lee Smolin
(Submitted on 3 Aug 2011)
We argue that the recently proposed principle of relative locality offers a new way to resolve the black hole information puzzle.
11 pages, one figure
Chirality of tensor perturbations for complex values of the Immirzi parameter
Laura Bethke, Joao Magueijo
(Submitted on 3 Aug 2011)
In this paper we generalise previous work on tensor perturbations in a de Sitter background in terms of Ashtekar variables to cover all complex values of the Immirzi parameter gamma (previous work was restricted to imaginary gamma). Particular attention is paid to the case of real gamma. Following the same approach as in the imaginary case, we can obtain physical graviton states by invoking reality and torsion free conditions. The Hamiltonian in terms of graviton states has the same form whether gamma has a real part or not; however changes occur for the vacuum energy and fluctuations. Specifically, we observe a gamma dependent chiral asymmetry in the vacuum fluctuations only if gamma has an imaginary part. Ordering prescriptions also change this asymmetry. We thus present a measurable result for CMB polarisation experiments that could shed light on the workings of quantum gravity.
6 pages, 1 figure
Prescriptions in Loop Quantum Cosmology: A comparative analysis
Guillermo A. Mena Marugan, Javier Olmedo, Tomasz Pawlowski
(Submitted on 3 Aug 2011)
Various prescriptions proposed in the literature to attain the polymeric quantization of a homogeneous and isotropic flat spacetime coupled to a massless scalar field are carefully analyzed in order to discuss their differences. A detailed numerical analysis confirms that, for states which are not deep in the quantum realm, the expectation values and dispersions of some natural observables of interest in cosmology are qualitatively the same for all the considered prescriptions. On the contrary, the amplitude of the wave functions of those states differs considerably at the bounce epoch for these prescriptions. This difference cannot be absorbed by a change of representation. Finally, the prescriptions with simpler superselection sectors are clearly more efficient from the numerical point of view.
18 pages, 6 figures
Thermodynamics of Ideal Gas in Doubly Special Relativity
Nitin Chandra, Sandeep Chatterjee
(Submitted on 30 Jul 2011)
We study thermodynamics of an ideal gas in Doubly Special Relativity. New type of special functions (which we call Incomplete Modified Bessel functions) emerge. We obtain a series solution for the partition function and derive thermodynamic quantities. We observe that DSR thermodynamics is non-perturbative in the SR and massless limits. A stiffer equation of state is found.
11 pages, 6 figures

Brief mention:
On the Role of Space-Time Foam in Breaking Supersymmetry via the Barbero-Immirzi Parameter
John Ellis, Nick E. Mavromatos
(Submitted on 3 Aug 2011)
We discuss how: (i) a dilaton/axion superfield can play the role of a Barbero-Immirzi field in four-dimensional conformal quantum supergravity theories, (ii) a fermionic component of such a dilaton/axion superfield may play the role of a Goldstino in the low-energy effective action obtained from a superstring theory with F-type global supersymmetry breaking, (iii) this global supersymmetry breaking is communicated to the gravitational sector via the supergravity coupling of the Goldstino, and (iv) such a scenario may be realized explicitly in a D-foam model with D-particle defects fluctuating stochastically.
:24 pages, 2 figures