Intuitive content of Loop Gravity--Rovelli's program

marcus

http://arxiv.org/abs/1208.1463
Loop quantum gravity as an effective theory
Martin Bojowald
(Submitted on 7 Aug 2012)
As a canonical and generally covariant gauge theory, loop quantum gravity requires special techniques to derive effective actions or equations. If the proper constructions are taken into account, the theory, in spite of considerable ambiguities at the dynamical level, allows for a meaningful phenomenology to be developed, by which it becomes falsifiable. The traditional problems plaguing canonical quantum-gravity theories, such as the anomaly issue or the problem of time, can be overcome or are irrelevant at the effective level, resulting in consistent means of physical evaluations. This contribution presents aspects of canonical equations and related notions of (deformed) space-time structures and discusses implications in loop quantum gravity, such as signature change at high density from holonomy corrections, and falsifiability thanks to inverse-triad corrections.
30 pages, lecture series at Sixth International School on Field Theory and Gravitation 2012 (Petropolis, Brazil)

http://arxiv.org/abs/1208.1502
A cosmological solution of Regge calculus
Adrian P. Gentle
(Submitted on 7 Aug 2012)
We revisit the Regge calculus model of the Kasner cosmology first considered by S. Lewis. One of the most highly symmetric applications of lattice gravity in the literature, Lewis' discrete model closely matched the degrees of freedom of the Kasner cosmology. As such, it was surprising that Lewis was unable to obtain the full set of Kasner-Einstein equations in the continuum limit. Indeed, an averaging procedure was required to ensure that the lattice equations were even consistent with the exact solution in this limit. We correct Lewis' calculations and show that the resulting Regge model converges quickly to the full set of Kasner-Einstein equations in the limit of very fine discretization. Numerical solutions to the discrete and continuous-time lattice equations are also considered.
12 pages, 3 figures

http://arxiv.org/abs/1208.1375
Structural Aspects Of Gravitational Dynamics And The Emergent Perspective Of Gravity
T. Padmanabhan
(Submitted on 7 Aug 2012)
I describe several conceptual aspects of a particular paradigm which treats the field equations of gravity as emergent. These aspects are related to the features of classical gravitational theories which defy explanation within the conventional perspective. The alternative interpretation throws light on these features and could provide better insights into possible description of quantum structure of spacetime. This review complements the discussion in arXiv:1207.0505, which describes space itself as emergent in the cosmological context.
29 pages. Updated version of talks given at: (a) Petropolis, Brazil, 2012 (b) Institute of Astrophysics, Paris, 2012 and (c) International Centre for Theoretical Sciences, Bangalore, 2012

brief mention:
http://arxiv.org/abs/1208.1428
Perturbative algebraic quantum field theory
Klaus Fredenhagen, Katarzyna Rejzner
(Submitted on 7 Aug 2012)
These notes are based on the course given by Klaus Fredenhagen at the Les Houches Winter School in Mathematical Physics (January 29 - February 3, 2012) and the course "QFT for mathematicians" given by Katarzyna Rejzner in Hamburg for the Research Training Group 1670 (February 6 -11, 2012). Both courses were meant as an introduction to modern approach to perturbative quantum field theory and are aimed both at mathematicians and physicists.
41 pages, 1 figure

marcus

http://arxiv.org/abs/1208.1514
Combinatorial Dark Energy
Aaron Trout
(Submitted on 8 Aug 2012)
In this paper, we give a conceptual explanation of dark energy as a small negative residual scalar curvature present even in empty spacetime. This curvature ultimately results from postulating a discrete spacetime geometry, very closely related to that used in the dynamical triangulations approach to quantum gravity. In this model, there are no states which have total scalar curvature exactly zero. Moreover, numerical evidence in dimension three suggests that, at a fixed volume, the number of discrete-spacetime microstates strongly increases with decreasing curvature. Because of the resulting entropic force, any dynamics which push empty spacetime strongly toward zero scalar curvature would instead produce typically observed states with a small negative curvature. This provides a natural explanation for the empirically observed small positive value for the cosmological constant (Lambda is about 10^(-121) in Planck units.) In fact, we derive the very rough estimate Lambda=6x10^(-118) from a simple model containing only the two (highly-degenerate) quantum states with total scalar-curvature closest to zero.
20 pages, 2 tables

marcus

http://arxiv.org/abs/1208.2228
Bohr-Sommerfeld Quantization of Space
Eugenio Bianchi, Hal M. Haggard
(Submitted on 10 Aug 2012)
We introduce semiclassical methods into the study of the volume spectrum in loop gravity. The classical system behind a 4-valent spinnetwork node is a Euclidean tetrahedron. We investigate the tetrahedral volume dynamics on phase space and apply Bohr-Sommerfeld quantization to find the volume spectrum. The analysis shows a remarkable quantitative agreement with the volume spectrum computed in loop gravity. Moreover, it provides new geometrical insights into the degeneracy of this spectrum and the maximum and minimum eigenvalues of the volume on intertwiner space.
32 pages, 10 figures

http://arxiv.org/abs/1208.2038
Fixed-Functionals of three-dimensional Quantum Einstein Gravity
Maximilian Demmel, Frank Saueressig, Omar Zanusso
(Submitted on 9 Aug 2012)
We study the non-perturbative renormalization group flow of f(R)-gravity in three-dimensional Asymptotically Safe Quantum Einstein Gravity. Within the conformally reduced approximation, we derive an exact partial differential equation governing the RG-scale dependence of the function f(R). This equation is shown to possess two isolated and one continuous one-parameter family of scale-independent, regular solutions which constitute the natural generalization of RG fixed points to the realm of infinite-dimensional theory spaces. All solutions are bounded from below and give rise to positive definite kinetic terms. Moreover, they admit either one or two UV-relevant deformations, indicating that the corresponding UV-critical hypersurfaces remain finite dimensional despite the inclusion of an infinite number of coupling constants. The impact of our findings on the gravitational Asymptotic Safety program and its connection to new massive gravity is briefly discussed.
34 pages, 14 figures

of possible interest:
http://arxiv.org/abs/1208.2168
Emergent gravity in two dimensions
D. Sexty, C. Wetterich
(Submitted on 10 Aug 2012)
We explore models with emergent gravity and metric by means of numerical simulations. A particular type of two-dimensional non-linear sigma-model is regularized and discretized on a quadratic lattice. It is characterized by lattice diffeomorphism invariance which ensures in the continuum limit the symmetry of general coordinate transformations. We observe a collective order parameter with properties of a metric, showing Minkowski or euclidean signature. The correlation functions of the metric reveal an interesting long-distance behavior with power-like decay. This universal critical behavior occurs without tuning of parameters and thus constitutes an example of "self-tuned criticality" for this type of sigma-models. We also find a non-vanishing expectation value of a "zweibein" related to the "internal" degrees of freedom of the scalar field, again with long-range correlations. The metric is well described as a composite of the zweibein. A scalar condensate breaks euclidean rotation symmetry.
22 pages, 17 figures

marcus

of possible general interest, though not QG-related:
http://arxiv.org/abs/1208.2660
A Numerical Simulation of Chern-Simons Inflation
Annie Preston, David Garrison, Stephon Alexander
(Submitted on 13 Aug 2012)
In this work, we present results of numerical simulations of the Chern-Simons Inflation Model proposed by Alexander, Marciano and Spergel. According to this model, inflation begins with a fermion condensate interacting with a gauge field. Crucial to the success of this mechanism is the assumption that the Chern-Simons interaction would drive energy from the initial random spectrum into a narrow band of frequencies at superhorizon scales. In this work we numerically confirm this expectation. These gauge fields, when combined with the Friedmann equations, were broken into a system of hyperbolic equations and numerically simulated with a novel relativistic MHD code. We show that the amplification of horizon sized gauge fields produces the conditions to cause cosmological inflation and that the onset of inflation are robust against certain fine tunings in the initial conditions.
10 pages, 2 figures

http://arxiv.org/abs/1208.2611
Space time and the passage of time
George F. R. Ellis, Rituparno Goswami
(Submitted on 13 Aug 2012)
This paper examines the various arguments that have been put forward suggesting either that time does not exist, or that it exists but its flow is not real. I argue that (i) time both exists and flows; (ii) an Evolving Block Universe (`EBU') model of spacetime adequately captures this feature, emphasizing the key differences between the past, present, and future; (iii) the associated surfaces of constant time are uniquely geometrically and physically determined in any realistic spacetime model based in General Relativity Theory; (iv) such a model is needed in order to capture the essential aspects of what is happening in circumstances where initial data does not uniquely determine the evolution of spacetime structure because quantum uncertainty plays a key role in that development. Assuming that the functioning of the mind is based in the physical brain, evidence from the way that the mind apprehends the flow of time is prefers this evolving time model over those where there is no flow of time.
25 pages, 2 figures. For Springer Handbook of Spacetime.

MTd2

http://arxiv.org/abs/1208.3186

A Reasonable Ab Initio Cosmological Constant Without Holography

Aaron D. Trout
(Submitted on 15 Aug 2012)
We give a well-motivated explanation for the origin of dark energy, claiming that it arises from a small residual negative scalar-curvature present even in empty spacetime. The vacuum has this residual curvature because spacetime is fundamentally discrete and there are more ways for a discrete geometry to have negative curvature than positive. We explicitly compute this effect in the well-known {\em dynamical triangulations} (DT) model for quantum gravity and the predicted cosmological constant $\Lambda$ agrees with observation.
We begin by almost completely characterizing the DT-model's vacuum energies in dimension three. Remarkably, the energy gap between states comes in increments of [\Delta\mathcal{A} =\frac{\ell}{8\mathcal{V}}] in natural units, where $\ell$ is the "Planck length" in the model and $\mathcal{V}$ is the volume of the universe. Then, using only vacua in the $N$ energy levels nearest zero, where $N$ is the universe's radius in units of $\ell$, we apply our model to the current co-moving spatial volume to get $|\Lambda| \approx 10^{-123}$.
This result comes with a rigorous proof and does not depend on any holographic principle or carefully tuned parameters. Our only unknown is the relative entropy of the low-energy states, which sets the sign of $\Lambda$. Numerical evidence strongly suggests that spacetime entropy in the DT-model is a decreasing function of scalar-curvature, so the model also predicts the correct sign for $\Lambda$.

marcus

http://arxiv.org/abs/1208.3388
Holonomy Spin Foam Models: Definition and Coarse Graining
Benjamin Bahr, Bianca Dittrich, Frank Hellmann, Wojciech Kaminski
(Submitted on 16 Aug 2012)
We propose a new holonomy formulation for spin foams, which naturally extends the theory space of lattice gauge theories. This allows current spin foam models to be defined on arbitrary two-complexes as well as to generalize current spin foam models to arbitrary, in particular finite groups. The similarity with standard lattice gauge theories allows to apply standard coarse graining methods, which for finite groups can now be easily considered numerically. We will summarize other holonomy and spin network formulations of spin foams and group field theories and explain how the different representations arise through variable transformations in the partition function. A companion paper will provide a description of boundary Hilbert spaces as well as a canonical dynamic encoded in transfer operators.
36 pages, 12 figures

http://arxiv.org/abs/1208.3335
Quantum Hall Effect and Black Hole Entropy in Loop Quantum Gravity
Deepak Vaid
(Submitted on 16 Aug 2012)
In LQG, black hole horizons are described by 2+1 dimensional boundaries of a bulk 3+1 dimensional spacetime. The horizon is endowed with area by lines of gravitational flux which pierce the surface. As is well known, counting of the possible states associated with a given set of punctures allows us to recover the famous Bekenstein-Hawking area law according to which the entropy of a black hole is proportional to the area of the associated horizon S_BH ∝ A_Hor. It is also known that the dynamics of the horizon degrees of freedom is described by the Chern-Simons action of a su(2) (or u(1) after a certain gauge fixing) valued gauge field A_μⁱ. Recent numerical work which performs the state-counting for punctures, from first-principles, reveals a step-like structure in the entropy-area relation. We argue that both the presence of the Chern-Simons action and the step-like structure in the entropy-area curve are indicative of the fact that the effective theory which describes the dynamics of punctures on the horizon is that of the Quantum Hall Effect.
24 pages, 7 figures; comments welcome

brief mention, not QG but conceivably of interest:
http://arxiv.org/abs/1208.3373
The cosmology of the Fab-Four
Edmund J. Copeland, Antonio Padilla, Paul M. Saffin
(Submitted on 16 Aug 2012)
We have recently proposed a novel self tuning mechanism to alleviate the famous cosmological constant problem, based on the general scalar tensor theory proposed by Horndeski. The self-tuning model ends up consisting of four geometric terms in the action, with each term containing a free potential function of the scalar field; the four together being labeled as the Fab-Four. ...
22 pages, 6 figures

marcus

general interest:
http://arxiv.org/abs/1208.3662
Astrophysical and cosmological probes of dark matter
Matts Roos
(Submitted on 17 Aug 2012)
Dark matter has been introduced to explain mass deficits noted at different astronomical scales, in galaxies, groups of galaxies, clusters, superclusters and even across the full horizon. Dark matter makes itself felt only through its gravitational effects. This review summarizes phenomenologically all the astrophysical and cosmological probes that have been used to give evidence for its existence.
39 pages, 24 figures. Accepted by J. of Modern Physics and will be released as Special Issue in September, 2012

http://arxiv.org/abs/1208.3841
Constraints on Chronologies
Alfred Shapere, Frank Wilczek
(Submitted on 19 Aug 2012)
The time ordering of two spacelike separated events is arbitrary, when all inertial frames are taken into account, but for three or more events it is not generally so. We determine the structure of possible time orderings, or chronologies, for multiple events in any number of dimensions, analytically and exhaustively for three events in four space-time dimensions, algorithmically in other cases. We also formulate an alternative criterion, based on convexity, for determining the allowed chronologies of a set of events. We show how the metric of a Lorentz invariant spacetime can be partially reconstructed from a knowledge of the chronologies it supports. Finally, we propose a different but related criterion for allowed chronologies in curved spacetimes.
22 pages, 4 figures

brief mention:
http://arxiv.org/abs/1208.3703
Quantum Geometry and Interferometry
Craig Hogan
(Submitted on 17 Aug 2012)
10 pages

tom.stoer

http://arxiv.org/abs/1112.1961
Spin Foams and Canonical Quantization
Authors: Sergei Alexandrov, Marc Geiller, Karim Noui
(Submitted on 8 Dec 2011 (v1), last revised 19 Aug 2012 (this version, v3))
Abstract: This review is devoted to the analysis of the mutual consistency of the spin foam and canonical loop quantizations in three and four spacetime dimensions. In the three-dimensional context, where the two approaches are in good agreement, we show how the canonical quantization \`a la Witten of Riemannian gravity with a positive cosmological constant is related to the Turaev-Viro spin foam model, and how the Ponzano-Regge amplitudes are related to the physical scalar product of Riemannian loop quantum gravity without cosmological constant. In the four-dimensional case, we recall a Lorentz-covariant formulation of loop quantum gravity using projected spin networks, compare it with the new spin foam models, and identify interesting relations and their pitfalls. Finally, we discuss the properties which a spin foam model is expected to possess in order to be consistent with the canonical quantization, and suggest a new model illustrating these results.

John86

something different.

http://fqxi.org/data/essay-contest-f...essay201_1.pdf
A chicken-and-egg problem: Which came first, the quantum state or spacetime? by Torsten Asselmeyer-Maluga
Essay Abstract
In this essay I will discuss the question: Is spacetime quantized, as in quantum geometry, or is it possible to derive the quantization procedure from the structure of spacetime? All proposals of quantum gravity try to quantize spacetime or derive it as an emergent phenomenon. In this essay, all major approaches are analyzed to find an alternative to a discrete structure on spacetime or to the emergence of spacetime. Here I will present the idea that spacetime defines the quantum state by using new developments in the differential topology of 3- and 4-manifolds. In particular the plethora of exotic smoothness structures in dimension 4 could be the corner stone of quantum gravity.

marcus

http://arxiv.org/abs/1208.5023
Asymptotic safety, hypergeometric functions, and the Higgs mass in spectral action models
Christopher Estrada, Matilde Marcolli
(Submitted on 24 Aug 2012)
We study the renormalization group flow for the Higgs self coupling in the presence of gravitational correction terms. We show that the resulting equation is equivalent to a singular linear ODE, which has explicit solutions in terms of hypergeometric functions. We discuss the implications of this model with gravitational corrections on the Higgs mass estimates in particle physics models based on the spectral action functional.
25 pages

possible interest, briefly mentioned:
http://arxiv.org/abs/1208.5038
Free fermi and bose fields in TQFT and GBF
Robert Oeckl (UNAM)
(Submitted on 24 Aug 2012)
We present a rigorous and functorial quantization scheme for linear fermionic and bosonic field theory targeting the topological quantum field theory (TQFT) that is part of the general boundary formulation (GBF). Motivated by geometric quantization, we generalize a previous axiomatic characterization of classical linear bosonic field theory to include the fermionic case. We proceed to describe the quantization scheme, combining a Fock space quantization for state spaces with the Feynman path integral for amplitudes. We show rigorously that the resulting quantum theory satisfies the axioms of the TQFT, in a version generalized to include fermionic theories. In the bosonic case we show the equivalence to a previously developed holomorphic quantization scheme. Remarkably, it turns out that consistency in the fermionic case requires state spaces to be Krein spaces rather than Hilbert spaces. This is also supported by arguments from geometric quantization and by the explicit example of the Dirac field theory. Contrary to intuition from standard quantum theory, we show that this is compatible with a consistent probability interpretation in the GBF. Another surprise in the fermionic case is the emergence of an algebraic notion of time, already in the classical theory, but inherited by the quantum theory. As in earlier work we need to impose an integrability condition in the bosonic case for all TQFT axioms to hold, due to the gluing anomaly. In contrast, we are able to renormalize this gluing anomaly in the fermionic case.
59 pages

marcus

not Loop-and-allied QG but possibly of general interest:
http://arxiv.org/abs/1208.5481
Gamma Ray Signals from Dark Matter: Concepts, Status and Prospects
Torsten Bringmann, Christoph Weniger
(Submitted on 27 Aug 2012)
Weakly interacting massive particles (WIMPs) remain a prime candidate for the cosmological dark matter (DM), even in the absence of current collider signals that would unambiguously point to new physics below the TeV scale. The self-annihilation of these particles in astronomical targets may leave observable imprints in cosmic rays of various kinds. In this review, we focus on gamma rays which we argue to play a pronounced role among the various possible messengers. We discuss the most promising spectral and spatial signatures to look for, give an update on the current state of gamma-ray searches for DM and an outlook concerning future prospects. We also assess in some detail the implications of a potential signal identification for particle DM models as well as for our understanding of structure formation. Special emphasis is put on the possible evidence for a 130 GeV line-like signal that was recently identified in the data of the Fermi gamma-ray space telescope.
43 pages, 6 figures, 2 tables; invited contribution to special issue `The next decade in Dark Matter and Dark Energy' in 'Physics of the Dark Universe'.

http://arxiv.org/abs/1208.5715
The Top 10⁵⁰⁰ Reasons Not to Believe in the Landscape
T. Banks
(Submitted on 28 Aug 2012)
The String Landscape is a fantasy. We actually have a plausible landscape of minimally supersymmetric AdS₄ solutions of supergravity modified by an exponential superpotential. None of these solutions is accessible to world sheet perturbation theory. If they exist as models of quantum gravity, they are defined by conformal field theories, and each is an independent quantum system, which makes no transitions to any of the others. This landscape has nothing to do with CDL tunneling or eternal inflation.
A proper understanding of CDL transitions in QFT on a fixed background dS space, shows that the EI picture of this system is not justified within the approximation of low energy effective field theory. The cutoff independent physics, defined by the Euclidean functional integral over the 4-sphere admits only a finite number of instantons. Plausible extensions of these ideas to a quantum theory of gravity obeying the holographic principle explain all of the actual facts about CDL transitions in dS space, and lead to a picture radically different from eternal inflation.
Theories of Eternal Inflation (EI) have to rely too heavily on the anthropic principle to be consistent with experiment. Given the vast array of effective low energy field theories that could be produced by the conventional picture of the string landscape one is forced to conclude that the most numerous anthropically allowed theories will disagree with experiment violently.
38 pages

marcus

An up-to-date formulation of Loop, and Loop BH developments can be found in Rovelli's July 2012 Stockholm slides:
http://www.cpt.univ-mrs.fr/~rovelli/...lmSpinFoam.pdf
Covariant Loop Quantum Gravity: Recent developments and open problems.

http://www.cpt.univ-mrs.fr/~rovelli/...kholmTermo.pdf
Horizon Entropy and LQG

http://arxiv.org/abs/1208.5874
A possibility to solve the problems with quantizing gravity
S. Hossenfelder
(Submitted on 29 Aug 2012)
It is generally believed that quantum gravity is necessary to resolve the known tensions between general relativity and the quantum field theories of the standard model. Since perturbatively quantized gravity is non-renormalizable, the problem how to unify all interactions in a common framework has been open since the 1930s. Here, I propose a possibility to circumvent the known problems with quantizing gravity, as well as the known problems with leaving it unquantized: By changing the prescription for second quantization, a perturbative quantization of gravity is sufficient as an effective theory because matter becomes classical before the perturbative expansion breaks down. This is achieved by considering the vanishing commutator between a field and its conjugated momentum as a symmetry that is broken at low temperatures, and by this generates the quantum phase that we currently live in, while at high temperatures Planck's constant goes to zero.
4 pages, 1 figure

John86

two interesting FQXI essays

http://fqxi.org/community/forum/topic/1442
Against Spacetime by Giovanni Amelino-Camelia
The notion of ``location" physics really needs is exclusively the one of ``detection at a given detector" and the time for each such detection is most primitively assessed as the readout of some specific material clock. The redundant abstraction of a macroscopic spacetime organizing all our particle detections is unproblematic and extremely useful in the classical-mechanics regime. But I here observe that in some of the contexts where quantum mechanics is most significant, such as quantum tunneling through a barrier, the spacetime abstraction proves to be cumbersome. And I argue that in quantum-gravity research we might limit our opportunities for discovery if we insist on the availability of a spacetime picture.

http://fqxi.org/community/forum/topic/1443
What if Natural Numbers Are Not Constant? by Jerzy Krol
Mathematics, via model theory, gives us the possibility that natural numbers could be understood as varying objects. We analyze this from the point of view of physics were standard models of natural and real numbers are not always absolute or fixed. The extended equivalence principle appears covering the changes of the numbers. As the consequence strange exotic geometry emerges with which a kind of gravity is assigned. Taking such perspective, from the foundations of mathematics, sheds completely new light on the nature and construction of a theory of quantum gravity.

marcus

To get to the discussion of Hossenfelder's essay when it is submitted to FQXI, go to http://fqxi.org/community/forum/category/31418
and choose alphabetical ordering by author's surname, and scroll down to H. that's one way anyway. It apparently has not been turned in yet.

http://arxiv.org/abs/1208.6217
A complete hybrid quantization in inhomogeneous cosmology
Mikel Fernández-Méndez, Guillermo A. Mena Marugán, Javier Olmedo
(Submitted on 30 Aug 2012)
A complete quantization of a homogeneous and isotropic spacetime with closed spatial sections coupled to a massive scalar field is provided, within the framework of Loop Quantum Cosmology. We identify solutions with their initial data on the minimum volume section, and from this we construct the physical Hilbert space. Moreover, a perturbative study allows us to introduce small inhomogeneities. After gauge fixing, the inhomogeneous part of the system is reduced to a linear field theory. We then adopt a standard Fock representation to quantize these degrees of freedom. For the considered case of compact spatial topology, the requirements of: i) invariance under the spatial isometries, and ii) unitary implementation of the quantum dynamics, pick up a unique Fock representation and a particular set of canonical fields (up to unitary equivalence).
6 pages

http://arxiv.org/abs/arXiv:1208.5456
Numerical loop quantum cosmology: an overview
Parampreet Singh
(Submitted on 27 Aug 2012)
A brief review of various numerical techniques used in loop quantum cosmology and results is presented. These include the way extensive numerical simulations shed insights on the resolution of classical singularities, resulting in the key prediction of the bounce at the Planck scale in different models, and the numerical methods used to analyze the properties of the quantum difference operator and the von Neumann stability issues. Using the quantization of a massless scalar field in an isotropic spacetime as a template, an attempt is made to highlight the complementarity of different methods to gain understanding of the new physics emerging from the quantum theory. Open directions which need to be explored with more refined numerical methods are discussed.
33 Pages, 4 figures. Invited contribution to appear in a special issue of Classical and Quantum Gravity devoted to numerical methods

marcus

http://arxiv.org/abs/1209.0065
General relativistic statistical mechanics
Carlo Rovelli
(Submitted on 1 Sep 2012)
Understanding thermodynamics and statistical mechanics in the full general relativistic context is an open problem. I give tentative definitions of equilibrium state, mean values, mean geometry, entropy and temperature, which reduce to the conventional ones in the non-relativistic limit, but remain valid for a general covariant theory. The formalism extends to quantum theory. The construction builds on the idea of thermal time, on a notion of locality for this time, and on the distinction between global and local temperature. The last is the temperature measured by a local thermometer, and is given by kT = h dτ/ds, with k the Boltzmann constant, h the Planck constant, ds proper time and dτ the equilibrium thermal time.
9 pages. A tentative second step in the thermal time direction, 10 years after the paper with Connes. The aim is the full thermodynamics of gravity. The language of the paper is a bit technical: look at the Appendix first

http://arxiv.org/abs/1209.0396
Lorentz-covariant Hamiltonian analysis of BF gravity with the Immirzi parameter
Mariano Celada, Merced Montesinos
(Submitted on 3 Sep 2012)
We perform the Lorentz-covariant Hamiltonian analysis of two Lagrangian action principles that describe general relativity as a constrained BF theory and that include the Immirzi parameter. The relation between these two Lagrangian actions has been already studied through a map among the fields involved. The main difference between these is the way the Immirzi parameter is included, since in one of them the Immirzi parameter is included explicitly in the BF terms, whereas in the other (the CMPR action) it is in the constraint on the B fields. In this work we continue the analysis of their relationship but at the Hamiltonian level. Particularly, we are interested in seeing how the above difference appears in the constraint structure of both action principles. We find that they both possess the same number of first-class and second-class constraints and satisfy a very similar (off-shell) Poisson-bracket algebra on account of the type of canonical variables employed. The two algebras can be transformed into each other by making a suitable change of variables
16 pages

marcus

http://arxiv.org/abs/1209.0473
Observational effects from quantum cosmology
Gianluca Calcagni
(Submitted on 3 Sep 2012)
The status of quantum cosmologies as testable models of the early universe is assessed in the context of inflation. While traditional Wheeler-DeWitt quantization is unable to produce sizable effects in the cosmic microwave background, the more recent loop quantum cosmology can generate potentially detectable departures from the standard cosmic spectrum. Thus, present observations constrain the parameter space of the model, which could be made falsifiable by near-future experiments.
14 pages, 3 figures. Invited review article also containing original material

brief mention:
http://arxiv.org/abs/1209.0480
Beyond H₀ and q₀: Cosmology is no longer just two numbers
Abraham R. Neben, Michael S. Turner
(Submitted on 3 Sep 2012)
For decades, H₀ and q₀ were the quest of cosmology, as they promised to characterize our "world model" in a model-independent way. Using simulated data, we show that q₀ cannot be both accurately and precisely determined using distance indicators. While H₀ can be both accurately and precisely determined, to avoid a small bias in its direct measurements (of order -5 %) we demonstrate that H₀/Ω_M (assuming flatness and w=-1) is a better choice of two parameters, even if our world model is not precisely Lambda CDM. We illustrate with the analysis of the Constitution set of supernovae and indirectly infer q₀ = -0.57 +/- 0.04. Finally, we show that it may be possible to directly determine q₀ using the time dependence of redshifts, a method far less susceptible to the biases that plague measurements using distance indicators.
8 pages, 9 figures

John86

Three new FQXI essays

http://fqxi.org/community/forum/topic/1495
http://fqxi.org/data/essay-contest-f...ductionism.pdf
Reductionist Doubts by Julian Barbour
According to reductionism, every complex phenomenon can and should be explained in terms of the simplest possible entities and mechanisms. The parts determine the whole. This approach has been an outstanding success in science, but this essay will point out ways in which it could nevertheless be giving us wrong ideas and holding back progress. For example, it may be impossible to understand key features of the universe such as its pervasive arrow of time and remarkably high degree of isotropy and homogeneity unless we study it holistically -- as a true whole. A satisfactory interpretation of quantum mechanics is also likely to be profoundly holistic, involving the entire universe. The phenomenon of entanglement already hints at such a possibility.

http://fqxi.org/community/forum/topic/1504
http://fqxi.org/data/essay-contest-f...r_fqxi2012.pdf
Not on but of. by Olaf Dreyer
In physics we encounter particles in one of two ways. Either as fundamental constituents of the theory or as emergent excitations. These two ways differ by how the particle relates to the background. It either sits \emph{on} the background, or it is an excitation \emph{of} the background. We argue that by choosing the former to construct our fundamental theories we have made a costly mistake. Instead we should think of particles as excitations of a background. We show that this point of view sheds new light on the cosmological constant problem and even leads to observable consequences by giving a natural explanation for the appearance of MOND-like behavior. In this context it also becomes clear why there are numerical coincidences between the MOND acceleration parameter $a_0$, the cosmological constant $\Lambda$ and the Hubble parameter $H_0$.

http://fqxi.org/community/forum/topic/1506
http://fqxi.org/data/essay-contest-f...ano_FQXi_1.pdf
Quantum-Informational Principles for Physics by Giacomo Mauro D'Ariano
t is time to to take a pause of reflection on the general foundations of physics, re-examining the solidity of the most basic principles, as the relativity and the equivalence principles that are currently under dispute for violations at the Planck scale. A constructive criticism engages us in seeking new general principles, which reduce to the old ones as approximations holding in the physical domain already explored. At the very basis of physics are epistemological and operational rules for the same formulability of the physical law and for the computability of its theoretical predictions, rules that give rise to new solid principles. These rules lead us to a quantum-information theoretic formulation, hinging on a logical identification of the experimental protocol with the quantum algorithm