Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/astro-ph/0606506
Strong gravitational lens probability in TeVeS
Da-Ming Chen, HongSheng Zhao
5 pages, 4 figures, submitted to ApJL

"We calculate the strong lensing probability with the image-separation greater than a given value $\Delta\theta$ and the image flux ratio $q_r$ less than 10 in a TeVeS (tensor-vector-scalar) cosmology, which is a relativistic version of the MOND (modified Newtonian dynamics). The lensing galaxy is modeled by the Hernquist profile. We assume a flat cosmology with $\Omega_b=0.04$ and the simplest interpolating function $\mu(x)$
with $\mu(x)=x$ for $x<1$ and
$\mu(x)=1$ for $x>1$. Our calculations show that MOND exhibits a very high lensing efficiency compared with the CDM paradigm. In order to match the well defined sample of the combined radio Cosmic Lens All-Sky Survey (CLASS) and Jodrell Bank/Very Large Array Astrometric Survey (JVAS), the upper limit of the mass of galaxies should be $1.7\times 10^{11} M_\sun$. The galaxies produce much more large image-separation lenses in MOND regime than that in CDM regime, while at small image-separation both regimes match the observational data well."

I wish to highlight this because it is Modified Gravity phenomenology----testing the "Dark Matter" hypotheis against the TeVeS relativistic MOND.

I noticed the article initially because of a confusion of names, a different person (Hongbao Zhang) has reported from Beijing about some talks given this week by Carlo Rovelli.
Here is H-B Zhang's post on N.E.W.
http://www.math.columbia.edu/~woit/wordpress/?p=415#comment-12456
Here is H-B Zhang's blog
http://hongbaozhang.blog.edu.cn/user1/3890/
Carlo Rovelli is giving a series of talks about LQG scattering amplitudes at Beijing Normal coinciding with the international String 2006 conference.

In the following paper STUFF if a technical mathematical term
don't knock it might be interesting------comes from UC Riverside
http://arxiv.org/abs/math.QA/0601458
Categorified Algebra and Quantum Mechanics
Jeffrey Morton (University of California, Riverside)
67 pages, 25 figures
"Interest in combinatorial interpretations of mathematical entities stems from the convenience of the concrete models they provide. Finding a bijective proof of a seemingly obscure identity can reveal unsuspected significance to it. Finding a combinatorial model for some mathematical entity is a particular instance of the process called "categorification''. Examples include the interpretation of N as the Burnside rig of the category of finite sets with product and coproduct, and the interpretation of N[x] as the category of combinatorial species. This has interesting applications to quantum mechanics, and in particular the quantum harmonic oscillator, via Joyal's ``species'', a new generalization called ``stuff types'', and operators between these, which can be represented as rudimentary Feynman diagrams for the oscillator. In quantum mechanics, we want to represent states in an algebra over the complex numbers, and also want our Feynman diagrams to carry more structure than these ``stuff operators'' can do, and these turn out to be closely related. We will show how to construct a combinatorial model for the quantum harmonic oscillator in which the group of phases, U(1), plays a special role. We describe a general notion of "M-Stuff Types'' for any monoid M, and see that the case M=U(1) provides an interpretation of time evolution in the combinatorial setting, as well as other quantum mechanical features of the harmonic oscillator."

 

[marcus]

http://arxiv.org/abs/hep-th/0606232
Quantum Gravity and Higher Curvature Actions
Martin Bojowald, Aureliano Skirzewski
28 pages, based on a lecture course at the 42nd Karpacz Winter School of Theoretical Physics "Current Mathematical Topics in Gravitation and Cosmology,'' Ladek, Poland, February 6-11, 2006

"Effective equations are often useful to extract physical information from quantum theories without having to face all technical and conceptual difficulties. One can then describe aspects of the quantum system by equations of classical type, which correct the classical equations by modified coefficients and higher derivative terms. In gravity, for instance, one expects terms with higher powers of curvature. Such higher derivative formulations are discussed here with an emphasis on the role of degrees of freedom and on differences between Lagrangian and Hamiltonian treatments. A general scheme is then provided which allows one to compute effective equations perturbatively in a Hamiltonian formalism. Here, one can expand effective equations around any quantum state and not just a perturbative vacuum. This is particularly useful in situations of quantum gravity or cosmology where perturbations only around vacuum states would be too restrictive. The discussion also demonstrates the number of free parameters expected in effective equations, used to determine the physical situation being approximated, as well as the role of classical symmetries such as Lorentz transformation properties in effective equations. An appendix collects information on effective correction terms expected from loop quantum gravity and string theory."

===========
I didn't catch some of these when they first went on arxiv.

http://arxiv.org/abs/math.DG/0511710
Higher Gauge Theory
John C. Baez, Urs Schreiber
28 pages, 10 figures

"Just as gauge theory describes the parallel transport of point particles using connections on bundles, higher gauge theory describes the parallel transport of 1-dimensional objects (e.g. strings) using 2-connections on 2-bundles. A 2-bundle is a categorified version of a bundle: that is, one where the fiber is not a manifold but a category with a suitable smooth structure. Where gauge theory uses Lie groups and Lie algebras, higher gauge theory uses their categorified analogues: Lie 2-groups and Lie 2-algebras. We describe a theory of 2-connections on principal 2-bundles and explain how this is related to Breen and Messing's theory of connections on nonabelian gerbes. The distinctive feature of our theory is that a 2-connection allows parallel transport along paths and surfaces in a parametrization-independent way. In terms of Breen and Messing's framework, this requires that the "fake curvature" must vanish. In this paper we summarize the main results of our theory without proofs."

this is a pretty good paper, which clears up doubts about why (from viewpoint of someone to whom spin networks and spinfoam make sense) one should know a bit about twogroups and twoholonomy.
You have to listen when somebody says allows parallel transport along paths and surfaces in a parametrization-independent way-----parallel transport along SURFACES---and then comes up with a necessary condition for that which says that "something" has to equal zero. things that have to be zero for what you want to happen---in this case setting it equal to zero apparently gives something that looks like the BF equation, which I was always wondering where it came from. The paper seems to show that serious old fogeys with gravitas can be interested in twogroups---it is not only just fun, it is required.
Here is an earlier, much longer, related paper:

http://arxiv.org/abs/hep-th/0412325
Higher Gauge Theory: 2-Connections on 2-Bundles
John Baez, Urs Schreiber
73 pages, 4 figures
"Connections and curvings on gerbes are beginning to play a vital role in differential geometry and mathematical physics -- first abelian gerbes, and more recently nonabelian gerbes. These concepts can be elegantly understood using the concept of '2-bundle' recently introduced by Bartels. A 2-bundle is a generalization of a bundle in which the fibers are categories rather than sets. Here we introduce the concept of a '2-connection' on a principal 2-bundle. We describe principal 2-bundles with connection in terms of local data, and show that under certain conditions this reduces to the cocycle data for nonabelian gerbes with connection and curving subject to a certain constraint -- namely, the vanishing of the 'fake curvature', as defined by Breen and Messing. This constraint also turns out to guarantee the existence of '2-holonomies': that is, parallel transport over both curves and surfaces, fitting together to define a 2-functor from the `path 2-groupoid' of the base space to the structure 2-group. We give a general theory of 2-holonomies and show how they are related to ordinary parallel transport on the path space of the base manifold."http://arxiv.org/abs/hep-th/0511086
Calabi-Yau Manifolds and the Standard Model
John C. Baez
4 pages
"For any subgroup G of O(n), define a "G-manifold" to be an n-dimensional Riemannian manifold whose holonomy group is contained in G. Then a G-manifold where G is the Standard Model gauge group is precisely a Calabi-Yau manifold of 10 real dimensions whose tangent spaces split into orthogonal 4- and 6-dimensional subspaces, each preserved by the complex structure and parallel transport. In particular, the product of Calabi-Yau manifolds of dimensions 4 and 6 gives such a G-manifold. Moreover, any such G-manifold is naturally a spin manifold, and Dirac spinors on this manifold transform in the representation of G corresponding to one generation of Standard Model fermions and their antiparticles.

=================

http://arxiv.org/abs/gr-qc/0606104
Numerical evolutions of a black hole-neutron star system in full General Relativity
Frank Löffler, Luciano Rezzolla, Marcus Ansorg
16 pages, 12 figures

"We present the first simulations in full General Relativity of the head-on collision between a neutron star and a black hole of comparable mass. These simulations are performed through the solution of the Einstein equations combined with an accurate solution of the relativistic hydrodynamics equations via high-resolution shock-capturing techniques. The initial data is obtained by following the York-Lichnerowicz conformal decomposition with the assumption of time symmetry. Unlike other relativistic studies of such systems, no limitation is set for the mass ratio between the black hole and the neutron star, nor on the position of the black hole, whose apparent horizon is entirely contained within the computational domain. The latter extends over 400 M and is covered with six levels of fixed mesh refinement. Concentrating on a prototypical binary system with mass ratio ~6, we find that although a tidal disruption is evident the neutron star is accreted promptly and entirely into the black hole. While the collision is completed before ~300 M, the evolution is carried over up to ~1700 M, thus providing time for the extraction of the gravitational-wave signal produced and allowing for a first estimate of the radiative efficiency of processes of this type."

Basically, the question addressed is: "What kind of ripples does it cause when a black hole eats a neutron star?"

But they were considering the head-on case, and I don't think that makes as much ripples as the spiraling-in case. either way, though, somebody has to compute the gravitational waves that ripple off such events, in case the detectors ever get good enough to pick them up.

================
Recalling that Robert Leigh recently co-authored with Laurent Freidel about 4D Yang-Mills
and there were two papers posted which got a fair amount of attention, in case anyone wants to see a video about this, there is one now:
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/

in the left sidebar menu, scroll down and click where it says THEORY CANADA 2
and you get a menu of 40 presentations at the recent Theory Canada conference,
displayed 12 to a page. Robert Leigh is on page 3, right at the end (he is #36)

I was impressed and thought he gave a good 30 minute talk and even that he and Freidel are making real
progress toward understanding Y-M, and it helps to see the guy talking with his slides instead of only reading the paper.

but if you want the paper too, it is
http://arxiv.org/abs/hep-th/0604184
Towards a solution of pure Yang-Mills theory in 3+1 dimensions
Laurent Freidel, Robert G. Leigh, Djordje Minic
12 pages
"We discuss an analytic approach towards the solution of pure Yang-Mills theory in 3+1 dimensional spacetime. The approach is based on the use of local gauge invariant variables in the Schr\"odinger representation and the large N, planar limit. In particular, within this approach we point out unexpected parallels between pure Yang-Mills theory in 2+1 and 3+1 dimensions. The most important parallel shows up in the analysis of the ground state wave-functional especially in view of the numerical similarity of the existing large N lattice simulations of the spectra of 2+1 and 3+1 Yang Mills theories."

and the accompanying Freidel solo paper was
http://arxiv.org/abs/hep-th/0604185
On pure Yang-Mills theory in 3+1 dimensions: Hamiltonian, vacuum and gauge invariant variables
Laurent Freidel
35 pages
"In this work we discuss an analytic approach towards the solution of pure Yang-Mills theory in 3+1 dimensional spacetime which strongly suggests that the recent strategy already applied to pure Yang-Mills theory in 2+1 can be extended to 3+1 dimensions. We show that the local gauge invariant variables introduced by Bars gives a natural generalisation to any dimension of the formalism of Karabali and Nair which recently led to a new understanding of the physics of QCD in dimension 2+1. After discussing the kinematics of these variables, we compute the jacobian between the Yang-Mills and Bars variables and propose a regularization procedure which preserves a generalisation of holomorphic invariance. We discuss the construction of the QCD hamiltonian properly regularized and compute the behavior of the vacuum wave functional both at weak and strong coupling. We argue that this formalism allows the developpement of a strong coupling expansion in the continuum by computing the first local eigenstate of the kinetic part of Yang-Mills hamiltonian."

 

[Mike2]

So the spinfoam goes from one spin network to another in the fashion of a path integral. And each spin network is the eigenfunction of the QG Hamiltonion, blah, blah, blah.

OK. But by what process is the spin network actually measured? What "interaction" causes the superposition of spin networks to "colapse" to a particular eigenstate? I have to wonder if such a measurement process can exist at that level. And if not, then could it be that by definition spacetime must always be assumed to be in a superposition? And what would be the implications of that?

 

[marcus]

So the spinfoam goes from one spin network to another in the fashion of a path integral...

this thread is being used purely for bibliography, Mike, discussion is off topic. Please start a separate thread to ask questions like this--I'm sure folks will be happy to respond. I would not want to reply to your question here in this context (and hope no one else would either) because making a practice of that would clutter the link library and make it harder to use.

 

[marcus]

http://arxiv.org/abs/gr-qc/0606120
A possible experimental test of quantized gravity
P. J. Salzman, S. Carlip
9 pages, 2 figures

"While it is widely believed that gravity should ultimately be treated as a quantum theory, there remains a possibility that general relativity should not be quantized. If this is the case, the coupling of classical gravity to the expectation value of the quantum stress-energy tensor will naturally lead to nonlinearities in the Schrodinger equation. By numerically investigating time evolution in the nonrelativistic 'Schrodinger-Newton' approximation, we show that such nonlinearities may be observable in the next generation of molecular interferometry experiments."
===============
Wolram called attention to this newly posted paper by Floyd Stecker (NASA, Goddard Space Flight Center), an authority on high energy astronomy----cosmic rays, GRB.

http://arxiv.org/abs/astro-ph/0606641
Testing Relativity at High Energies Using Spaceborne Detectors
F. W. Stecker (NASA/GSFC)
13 pages, Text of invited talk at the "From Quantum to Cosmos: Fundamental Physics Studies from Space" meeting

"The Gamma-ray Large Area Space Telescope (GLAST) will measure the spectra of distant extragalactic sources of high energy gamma-rays. GLAST can look for energy dependent propagation effects from such sources as a signal of Lorentz invariance violation (LIV). Such sources should also exhibit high energy spectral cutoffs from pair production interactions with low energy photons. The properties of such cutoffs can also be used to test LIV. ..."
=========
http://arxiv.org/abs/gr-qc/0606121
Uniform discretizations: a quantization procedure for totally constrained systems including gravity
Miguel Campiglia, Cayetano Di Bartolo, Rodolfo Gambini, Jorge Pullin
4 pages

"We present a new method for the quantization of totally constrained systems including general relativity. The method consists in constructing discretized theories that have a well defined and controlled continuum limit. The discrete theories are constraint-free and can be readily quantized. This provides a framework where one can introduce a relational notion of time and that nevertheless approximates in a well defined fashion the theory of interest. The method is equivalent to the group averaging procedure for many systems where the latter makes sense and provides a generalization otherwise. In the continuum limit it can be shown to contain, under certain assumptions, the 'master constraint' of the 'Phoenix project'. It also provides a correspondence principle with the classical theory that does not require to consider the semiclassical limit."
==============
this seems very strange. hovering drops of superfluid helium ?
maybe they could be good for something, if not necessarily to detect gravity waves
I may have to erase this because it is so crazy.

http://arxiv.org/gr-qc/0606118
Proposed observations of gravity waves from the early Universe via "Millikan oil drops"
Raymond. Y. Chiao
10 pages, 2 figures, NASA "Quantum-to-Cosmos" conference proceedings to be published in IJMPD

"Pairs of Planck-mass drops of superfluid helium coated by electrons (i.e., 'Millikan oil drops'), when levitated in a superconducting magnetic trap, can be efficient quantum transducers between electromagnetic (EM) and gravitational (GR) radiation. This leads to the possibility of a Hertz-like experiment, in which EM waves are converted at the source into GR waves, and then back-converted at the receiver from GR waves back into EM waves. Detection of the gravity-wave analog of the cosmic microwave background using these drops can discriminate between various theories of the early Universe."

Maybe (maybe!) it is not so crazy after all. It seems that Raymond Chiao was awarded the Lamb Medal in 2006 or in any case delivered the Lamb Medal Lecture at a January 2006 Conference in Utah. (Remember the Lamb Shift?) He is at the new UC Merced campus jointly in the Schools of Natural Science and of Engineering. His Lamb Medal talk was also about these charged helium droplets (!)
http://arxiv.org/abs/quant-ph/0601193
The Interface between Quantum Mechanics and General Relativity
Raymond Y. Chiao
22 pages, 7 figures; Lamb medal lecture on January 5, 2006 at the Physics of Quantum Electronics Winter Colloquium at Snowbird, Utah; accepted for publication in J. Mod. Optics

"The generation, as well as the detection, of gravitational radiation by means of charged superfluids is considered. One example of such a 'charged superfluid' consists of a pair of Planck-mass-scale, ultracold 'Millikan oil drops,' each with a single electron on its surface, in which the oil of the drop is replaced by superfluid helium. When levitated in a magnetic trap, and subjected to microwave-frequency electromagnetic radiation, a pair of such "Millikan oil drops" separated by a microwave wavelength can become an efficient quantum transducer between quadrupolar electromagnetic and gravitational radiation. This leads to the possibility of a Hertz-like experiment, in which the source of microwave-frequency gravitational radiation consists of one pair of 'Millikan oil drops' driven by microwaves, and the receiver of such radiation consists of another pair of 'Millikan oil drops' in the far field driven by the gravitational radiation generated by the first pair. The second pair then back-converts the gravitational radiation into detectable microwaves. The enormous enhancement of the conversion efficiency for these quantum transducers over that for electrons arises from the fact that there exists macroscopic quantum phase coherence in these charged superfluid systems."

 

[marcus]

http://arxiv.org/abs/gr-qc/0607014
Particles as Wilson lines of gravitational field
L. Freidel, J. Kowalski--Glikman, A. Starodubtsev
19 pages

"Since the work of Mac-Dowell-Mansouri it is well known that gravity can be written as a gauge theory for the de Sitter group. In this paper we consider the coupling of this theory to the simplest gauge invariant observables that is, Wilson lines. The dynamics of these Wilson lines is shown to reproduce exactly the dynamics of relativistic particles coupled to gravity, the gauge charges carried by Wilson lines being the mass and spin of the particles. Insertion of Wilson lines breaks in a controlled manner the diffeomorphism symmetry of the theory and the gauge degree of freedom are transmuted to particles degree of freedom."

 

[marcus]

http://arxiv.org/abs/gr-qc/0607032
The group field theory approach to quantum gravity
Daniele Oriti
17 pages, 2 figures; expanded version of a contribution to "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", edited by D. Oriti, to be published by Cambridge University Press

"We give a very concise review of the group field theory formalism for non-perturbative quantum gravity, a higher dimensional generalisation of matrix models. We motivate it as a simplicial and local realisation of the idea of 3rd quantization of the gravitational field, equivalently of a quantum field theory of simplicial geometry, in which also the topology of space is fully dynamical. We highlight the basic structure of the formalism, and discuss briefly various models that are being studied, some recent results and the many open issues that future research should face. Finally, we point out the connections with other approaches to quantum gravity, such as loop quantum gravity, quantum Regge calculus and dynamical triangulations, and causal sets."

http://arxiv.org/abs/gr-qc/0607013
The emergence of background geometry from quantum fluctuations
J. Ambjorn, R. Janik, W. Westra, S. Zohren
12 pages

"We show how the quantization of two-dimensional gravity leads to an (Euclidean) quantum space-time where the average geometry is that of constant negative curvature and where the Hartle-Hawking boundary condition arises naturally."

 

[marcus]

http://arxiv.org/abs/gr-qc/0607039
Quantum Nature of the Big Bang: Improved dynamics
Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh
40 pages, 9 figures

"An improved Hamiltonian constraint operator is introduced in loop quantum cosmology. Quantum dynamics of the spatially flat, isotropic model with a massless scalar field is then studied in detail using analytical and numerical methods. The scalar field continues to serve as `emergent time', the big bang is again replaced by a quantum bounce, and quantum evolution remains deterministic across the deep Planck regime. However, while with the Hamiltonian constraint used so far in loop quantum cosmology the quantum bounce can occur even at low matter densities, with the new Hamiltonian constraint it occurs only at a Planck-scale density. Thus, the new quantum dynamics retains the attractive features of current evolutions in loop quantum cosmology but, at the same time, cures their main weakness."

Also posted today:
http://arxiv.org/abs/gr-qc/0607033
Variational Methods in Loop Quantum Cosmology
http://arxiv.org/abs/gr-qc/0607034
On the Green's function and iterative solutions of Loop Quantum Cosmology

More on Loop Quantum Cosmology:
http://arxiv.org/abs/gr-qc/0607044
Instabilities in numerical loop quantum cosmology
Jessica Rosen, Jae-Hun Jung, Gaurav Khanna
6 pages, 8 figures

"In this article we perform von Neumann analysis of the difference equations that arise as a result of loop quantum gravity being applied to models of cosmology and black holes. In particular, we study the numerical stability of Bianchi I LRS (symmetric and non-symmetric constraint) and Schwarzschild interior (symmetric constraint) models, and find that there exist domains over which there are instabilities, generically. We also present explicit evolutions of wave-packets in these models and clearly demonstrate the presence of these instabilities."

Khanna is in the Physics Department at Dartmouth. His papers go back a ways and he has co-authored with Bojowald. His two co-authors are in the Dartmouth Math Department. E.g. J-H Jung has 26 papers in arxiv grp_physics going back to 1998. Good for Dartmouth.

http://arxiv.org/abs/astro-ph/0607227
Why anthropic reasoning cannot predict Lambda
Glenn D. Starkman (Case Western Reserve University and Oxford U.), Roberto Trotta (Oxford University)
4 pages, 1 figure
"We revisit anthropic arguments purporting to explain the measured value of the cosmological constant. We argue that different ways of assigning probabilities to candidate universes lead to totally different anthropic predictions. As an explicit example, we show that weighting different universes by the total number of possible observations leads to an extremely small probability for observing a value of Lambda equal to or greater than what we now measure. We conclude that anthropic reasoning within the framework of probability as frequency is ill-defined and that it cannot be used to explain the value of Lambda, nor, likely, any other physical parameters."

 

[marcus]

cant evaluate, but anybody's new idea of a way to test QG, I want to know about it.

http://arxiv.org/abs/hep-ph/0607145
GRBs Neutrinos as a Tool to Explore Quantum Gravity induced Lorentz Violation
Uri Jacob, Tsvi Piran

"Lorentz Invariance Violation (LIV) arises in various quantum-gravity theories. As the typical energy for quantum gravity is the Planck mass, $$M_{pl}$$, LIV will, most likely, be manifested at very high energies that are not accessible on Earth in the foreseeable future. One has to turn to astronomical observations. Time of flight measurement from different astronomical sources set current limits on the energy scale of possible LIV to $$> 0.01 M_{pl}$$ (for n=1 models) and $$> 10^{-9} M_{pl}$$ (for n=2). According to current models Gamma-Ray Bursts (GRBs) are accompanied by bursts of high energy ($$\gsim 100$$TeV) neutrinos. At this energy range the background level of currently constructed neutrino detectors is so low that a detection of a single neutrino from the direction of a GRB months or even years after the burst would imply an association of the neutrino with the burst and will establish a measurement of a time of flight delay. Such time of flight measurements provide the best way to observe (or set limits) on LIV. Detection of a single GRB neutrino would open a new window on LIV and would improve current limits by many orders of magnitude."

and Tsvi Piran is a recognized specialist in figuring out how to test QG
he was invited to the February 2004 Polish winter symposium on testing QG
and gave a bunch of talks (at the time not so many people had thought to investigate
observational tests of QG) I can't tell if his ideas are good but other people pay attention.
Maybe synching neutrino detection with gammaray observation makes sense---leastwise we have the tools
to do it.

 

[marcus]

this thread is non-string QG bibliography, it gives links to recent non-string QG papers. usually I just keep track of postings on arxiv.org.
this time I am going to add the latest edition of John Baez "This Week's Finds".
the reason is I have a hunch that eventually we are all going to want to understand something about SO(4,1)

I think that particular Lie group is going to be the kitchen stool by which humans reach the high shelf, so it is apt to become familiar, the way the poincaré group and lorentz group are familiar.

Curiously enough, I don't know anything online that is a real kindergarten introduction to SO(4,1). Hey, don't knock kindergarten, it is probably the basis of western civilization or something We all at some point need a basic introduction to things!

So maybe someone can suggest a better ONLINE intuitive intro to DeSitter group?

Until then, the best I know is a few paragraphs in TWF 235, around page 5 out of 8 pages that talks about a lot of other stuff.

http://www.math.ucr.edu/home/baez/week235.html

 

[marcus]

http://arxiv.org/abs/astro-ph/0607380
Solving the Problem of Time in General Relativity and Cosmology with Phantoms and k -- Essence
Thomas Thiemann
38 pages

"We show that if the Lagrangean for a scalar field coupled to General Relativity only contains derivatives, then it is possible to completely deparametrise the theory. This means that
1.Physical observables, i.e. functions which Poisson commute with the spatial diffeomorphism and Hamiltonian constraints of General Relativity, can be easily constructed.
2. The physical time evolution of those observables is generated by a natural physical Hamiltonian which is (constrained to be) positive.

The mechanism by which this works is due to Brown and Kuchar. In order that the physical Hamiltonian is close to the Hamiltonian of the standard model and the one used in cosmology, the required Lagrangean must be that of a Dirac -- Born -- Infeld type. Such matter has been independently introduced previously by cosmologists in the context of k -- essence due to Armendariz-Picon, Mukhanov and Steinhardt in order to solve the cosmological coincidence (dark energy) problem. We arrive at it by totally unrelated physical considerations originating from quantum gravity. Our manifestly gauge invariant approach leads to important modifications of the interpretation and the the analytical appearance of the standard FRW equations of classical cosmology in the late universe. In particular, our concrete model implies that the universe should recollapse at late times on purely classical grounds."http://arxiv.org/abs/gr-qc/0607075
Approximating the physical inner product of Loop Quantum Cosmology
Benjamin Bahr, Thomas Thiemann
35 pages===================

Quantum Theory in Cosmology
The seminar series at Perimeter Institute has a talk given by Martin Bojowald on 28 July
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False
It is about Quantum Gravity in Cosmology and is particularly interesting because it discusses
the role QG plays in early structure formation in the universe---in the conversion of quantum fluctuations
into corresponding classcial ones.

========at FH's suggestion===========

http://arxiv.org/gr-qc/0607068
Dirac Quantization of Parametrized Field Theory
Madhavan Varadarajan
33 pages

"Parametrized field theory (PFT) is free field theory on flat spacetime in a diffeomorphism invariant disguise. It describes field evolution on arbitrary foliations of the flat spacetime instead of only the usual flat ones, by treating the `embedding variables' which describe the foliation as dynamical variables to be varied in the action in addition to the scalar field. A formal Dirac quantization turns the constraints of PFT into functional Schrodinger equations which describe evolution of quantum states from an arbitrary Cauchy slice to an infinitesimally nearby one.This formal Schrodinger picture- based quantization is unitarily equivalent to the standard Heisenberg picture based Fock quantization of the free scalar field if scalar field evolution along arbitrary foliations is unitarily implemented on the Fock space. Torre and Varadarajan (TV) showed that for generic foliations emanating from a flat initial slice in spacetimes of dimension greater than 2, evolution is not unitarily implemented, thus implying an obstruction to Dirac quantization.
We construct a Dirac quantization of PFT, unitarily equivalent to the standard Fock quantization, using techniques from Loop Quantum Gravity (LQG) which are powerful enough to supercede the no-go implications of the TV results. The key features of our quantization include an LQG type representation for the embedding variables, embedding dependent Fock spaces for the scalar field, an anomaly free representation of (a generalization of) the finite transformations generated by the constraints and group averaging techniques. The difference between 2 and higher dimensions is that in the latter, only finite gauge transformations are defined in the quantum theory, not the infinitesimal ones."

 

[marcus]

Mann and Popescu have co-authored with Freidel. This one sounds like it might connect with Freidel papers on the 3D case. Also with Higher Gauge Theory (HGT) discussed by John Baez----several of whose papers they cite.
http://arxiv.org/abs/gr-qc/0607076
Higher Gauge Theory and Gravity in (2+1) Dimensions
R.B. Mann, Eugeniu M. Popescu
22 pages
"Non-abelian higher gauge theory has recently emerged as a generalization of standard gauge theory to higher dimensional (2-dimensional in the present context) connection forms, and as such, it has been successfully applied to the non-abelian generalizations of the Yang-Mills theory and 2-form electrodynamics. (2+1)-dimensional gravity, on the other hand, has been a fertile testing ground for many concepts related to classical and quantum gravity, and it is therefore only natural to investigate whether we can find an application of higher gauge theory in this latter context. In the present paper we investigate the possibility of applying the formalism of higher gauge theory to gravity in (2+1) dimensions, and we show that a nontrivial model of (2+1)-dimensional gravity coupled to scalar and tensorial matter fields - the $$\Sigma\Phi EA$$ model - can be formulated both as a standard gauge theory and as a higher gauge theory. Since the model has a very rich structure - it admits as solutions black-hole BTZ-like geometries, particle-like geometries as well as Robertson-Friedman-Walker cosmological-like expanding geometries - this opens a wide perspective for higher gauge theory to be tested and understood in a relevant gravitational context. Additionally, it offers the possibility of studying gravity in (2+1) dimensions coupled to matter in an entirely new framework."

 

[selfAdjoint]

Kristina Giesl & Thomas Thiemann - AQG

Krisitna Giesel and Thomas Thiemann have submitted three papers on a new subject in QG: Algebraic Quantum Gravity. Here are the descriptions.

http://www.arxiv.org/abs/gr-qc/0607099
gr-qc/0607099
From: Kristina Giesel [view email]
Date: Sun, 23 Jul 2006 21:23:01 GMT (40kb)
Algebraic Quantum Gravity (AQG) I. Conceptual Setup
Authors: K. Giesel, T. Thiemann
Report-no: AEI-2006-058
"We introduce a new top down approach to canonical quantum gravity, called Algebraic Quantum Gravity (AQG):The quantum kinematics of AQG is determined by an abstract $*-$algebra generated by a countable set of elementary operators labelled by an algebraic graph. The quantum dynamics of AQG is governed by a single Master Constraint operator. While AQG is inspired by Loop Quantum Gravity (LQG), it differs drastically from it because in AQG there is fundamentally no topology or differential structure. A natural Hilbert space representation acquires the structure of an infinite tensor product (ITP) whose separable strong equivalence class Hilbert subspaces (sectors) are left invariant by the quantum dynamics. The missing information about the topology and differential structure of the spacetime manifold as well as about the background metric to be approximated is supplied by coherent states. Given such data, the corresponding coherent state defines a sector in the ITP which can be identified with a usual QFT on the given manifold and background. Thus, AQG contains QFT on all curved spacetimes at once, possibly has something to say about topology change and provides the contact with the familiar low energy physics. In particular, in two companion papers we develop semiclassical perturbation theory for AQG and LQG and thereby show that the theory admits a semiclassical limit whose infinitesimal gauge symmetry agrees with that of General Relativity. In AQG everything is computable with sufficient precision and no UV divergences arise due to the background independence of the undamental combinatorial structure. Hence, in contrast to lattice gauge theory on a background metric, no continuum limit has to be taken, there simply is no lattice regulator that must be sent to zero."

http://www.arxiv.org/abs/gr-qc/0607100
gr-qc/0607100
From: Kristina Giesel [view email]
Date: Sun, 23 Jul 2006 21:27:43 GMT (43kb)
Algebraic Quantum Gravity (AQG) II. Semiclassical Analysis
Authors: K. Giesel, T. Thiemann
Report-no: AEI-2006-59
"In the previous article a new combinatorial and thus purely algebraical approach to quantum gravity, called Algebraic Quantum Gravity (AQG), was introduced. In the framework of AQG existing semiclassical tools can be applied to operators that encode the dynamics of AQG such as the Master constraint operator. In this article we will analyse the semiclassical limit of the (extended) algebraic Master constraint operator and show that it reproduces the correct infinitesimal generators of General Relativity. Therefore the question whether General Relativity is included in the semiclassical sector of the theory, which is still an open problem in LQG, can be significantly improved in the framework of AQG. For the calculations we will substitute SU(2) by U(1)^3. That this substitution is justified will be demonstrated in the third article of this series "

http://www.arxiv.org/abs/gr-qc/0607101
gr-qc/0607101
From: Kristina Giesel [view email]
Date: Sun, 23 Jul 2006 21:30:29 GMT (27kb)
Algebraic Quantum Gravity (AQG) III. Semiclassical Perturbation Theory
Authors: K. Giesel, T. Thiemann
Report-no: AEI-2006-60
"In the two previous papers of this series we defined a new combinatorical approach to quantum gravity, Algebraic Quantum Gravity (AQG). We showed that AQG reproduces the correct infinitesimal dynamics in the semiclassical limit, provided one incorrectly substitutes the non -- Abelean group SU(2) by the Abelean group $U(1)^3$ in the calculations. The mere reason why that substitution was performed at all is that in the non -- Abelean case the volume operator, pivotal for the definition of the dynamics, is not diagonisable by analytical methods. This, in contrast to the Abelean case, so far prohibited semiclassical computations. In this paper we show why this unjustified substitution nevertheless reproduces the correct physical result: Namely, we introduce for the first time semiclassical perturbation theory within AQG (and LQG) which allows to compute expectation values of interesting operators such as the master constraint as a power series in $\hbar$ with error control. That is, in particular matrix elements of fractional powers of the volume operator can be computed with extremely high precision for sufficiently large power of $\hbar$ in the $\hbar$ expansion. With this new tool, the non -- Abelean calculation, although technically more involved, is then exactly analogous to the Abelean calculation, thus justifying the Abelean analysis in retrospect. The results of this paper turn AQG into a calculational discipline."

I can see this new area as being a magnet for young trained theoretical physicsts because it is both Algebraic (and algebraic is so cool!) and Perturbative (and hence at least potentially accessible to people with non-LQG training).

 

[marcus]

Good find, selfAdjoint! I was just about to post the Thiemann and Giesel work myself.

Here is another that arrived today

http://arxiv.org/abs/gr-qc/0607097
Semiclassical states for quantum cosmology
Viqar Husain, Oliver Winkler
13 pages
"In a metric variable based Hamiltonian quantization, we give a prescription for constructing semiclassical matter-geometry states for homogeneous and isotropic cosmological models. These "collective" states arise as infinite linear combinations of fundamental excitations in an unconventional "polymer" quantization. They satisfy a number of properties characteristic of semiclassicality, such as peaking on classical phase space configurations. We describe how these states can be used to determine quantum corrections to the classical evolution equations, and to compute the initial state of the universe by a backward time evolution."

keeping an eye on the ekpyrotic alternative:
http://arxiv.org/abs/hep-th/0607164
Cosmic Perturbations Through the Cyclic Ages

Mattingly may have an improved way to constrain the causal set approach to QG
http://arxiv.org/abs/astro-ph/0607485
Low energy bounds on Poincare violation in causal set theory
Nemanja Kaloper, David Mattingly
8 pages
"In the causal set approach to quantum gravity, Poincaré symmetry is modified by swerving in spacetime, induced by the random lattice discretization of the space-time structure. The broken translational symmetry at short distances is argued to lead to a residual diffusion in momentum space, whereby a particle can acquire energy and momentum by drift along its mass shell and a system in equilibrium can spontaneously heat up. We consider bounds on the rate of momentum space diffusion coming from astrophysical molecular clouds, nuclear stability and cosmological neutrino background. We find that the strongest limits come from relic neutrinos, which we estimate to constrain the momentum space diffusion constant by $k < 10^{-61} {\rm GeV}^3$ for neutrinos with masses $m_\nu > 0.01 {\rm eV}$, improving the previously quoted bounds by roughly 17 orders of magnitude."

 

[marcus]

On Thursday 3 August, a few days from now, Andy Randono will be giving a talk at Perimeter. Andy is a graduate student at University of Texas. He did his undergraduate work at Tufts. he has been studying the Kodama state---a QG state with connections to DeSitter space. here is the abstract of the talk:

Andy Randono
Generalizing the Kodama State
Thursday August 3, 2006, 1:30 AM
"The Kodama state is unique in being an exact solution to all the constraints of quantum gravity that also has a well defined semi-classical interpretation as a quantum version of a classical spacetime, namely de-Sitter spacetime. In addition, the exact form of the state is known in both the connection and spin network bases, thereby concretely realizing the beautiful connection between spin networks and functionals on the space of connections. However, the state is riddled with difficulties that we will track down to the complexification necessary in its construction. This suggests a generalization to real variables, which, we will argue, can be accomplished. This solves most of the standard problems associated with the state, but in addition opens up a large Hilbert class of states vastly generalizing the de-Sitter/Chern-Simons solution."

Perimeter Streamers website sometimes puts these seminar talks online. I hope they do this one. Andy also has a couple of papers on arxiv:

http://arxiv.org/abs/gr-qc/0504010
A Generalization of the Kodama State for Arbitrary Values of the Immirzi Parameter
Andrew Randono
16 pages
"The Kodama State for Lorentzian gravity presupposes a particular value for the Immirzi-parameter, namely $\beta=-i$. However, the derivation of black hole entropy in Loop Quantum Gravity suggests that the Immirzi parameter is a fixed value whose magnitude is on the order of unity but larger than one. Since the Kodama state has de-Sitter spacetime as its classical limit, to get the proper radiation temperature, the Kodama state should be extended to incorporate a more physical value for $\beta$. Thus, we present an extension of the Kodama state for arbitrary values of the Immirzi parameter, $\beta$, that reduces to the ordinary Chern-Simons state for the particular value $\beta=-i$. The state for real values of $\beta$ is free of several of the outstanding problems that cast doubts on the original Kodama state as a ground state for quantum general relativity. We show that for real values of $\beta$, the state is invariant under large gauge transformations, it is CPT invariant (but not CP invariant), and it is expected to be delta-function normalizable with respect to the kinematical inner product. To aid in the construction, we first present a general method for solving the Hamiltonian constraint for imaginary values of $\beta$ that allows one to use the simpler self-dual and anti-self-dual forms of the constraint as an intermediate step."

http://arxiv.org/abs/hep-th/0510001
A Note on Parity Violation and the Immirzi Parameter
Andrew Randono
7 pages
There has been considerable recent interest in the Immirzi parameter as a measure of parity violating effects in the classical theory of gravitation with fermion coupling. Most recently it was shown that the Immirzi parameter together with the non-minimal coupling constant of Dirac spinors provides the measure for parity violating spin-spin interaction terms in the effective field theory. For complex values of the Immirzi parameter, the resulting effective field theory yields complex values for the torsion, and a non-unitary effective field theory that blows up for the special cases $\gamma=\pm i$ where the gravitational kinetic term is the Ashtekar action. We show that by restricting ourselves to real values for the torsion, there is a natural set of choice for the non-minimal coupling constant that yields real and unitary effective field theory that does not blow up for the special cases $\gamma=\pm i$. We then show that these particular values for the non-minimal coupling coefficients most naturally follow from a non-minimal pseudo-kinetic term in the fermion Lagrangian."

Some pictures of Andy and friends hiking in the Colorado Rockies
http://www.ma.utexas.edu/users/stirling/0508colorado/0508colorado.html
The best one of Andy is scroll halfway down to where the guy snapping the pictures has captioned one:
"I think that Andy looks like a dork with his walking stick."
In fact he does not look dorky. He is dressed and equipped practically for high mountain hiking
and he looks cheerful, which some of the others don't.

It is amusing to think of Randono occupying the same physics department as Jacques (death-to-Loop-Gravity!) Distler. It shows a certain fortitude and independent spirit IMO.
=======================
http://arxiv.org/abs/gr-qc/0607130
Quantum Geometry and its Implications for Black Holes
Martin Bojowald
16 pages, Plenary talk at "Einstein's Legacy in the New Millenium,'' Puri, India, December 2005

"General relativity successfully describes space-times at scales that we can observe and probe today, but it cannot be complete as a consequence of singularity theorems. For a long time there have been indications that quantum gravity will provide a more complete, non-singular extension which, however, was difficult to verify in the absence of a quantum theory of gravity. By now there are several candidates which show essential hints as to what a quantum theory of gravity may look like. In particular, loop quantum gravity is a non-perturbative formulation which is background independent, two properties which are essential close to a classical singularity with strong fields and a degenerate metric. In cosmological and black hole settings one can indeed see explicitly how classical singularities are removed by quantum geometry: there is a well-defined evolution all the way down to, and across, the smallest scales. As for black holes, their horizon dynamics can be studied showing characteristic modifications to the classical behavior. Conceptual and physical issues can also be addressed in this context, providing lessons for quantum gravity in general. Here, we conclude with some comments on the uniqueness issue often linked to quantum gravity in some form or another."

 

[marcus]

http://arxiv.org/abs/gr-qc/0607136
Quantum Gowdy T³ model: A uniqueness result
Alejandro Corichi, Jeronimo Cortez, Guillermo A. Mena Marugan, Jose M. Velhinho
27 pages

"Modulo a homogeneous degree of freedom and a global constraint, the linearly polarised Gowdy T³ cosmologies are equivalent to a free scalar field propagating in a fixed nonstationary background. Recently, a new field parameterisation was proposed for the metric of the Gowdy spacetimes such that the associated scalar field evolves in a flat background in 1+1 dimensions with the spatial topology of S¹, although subject to a time dependent potential. Introducing a suitable Fock quantisation for this scalar field, a quantum theory was constructed for the Gowdy model in which the dynamics is implemented as a unitary transformation. A question that was left open is whether one might adopt a different, nonequivalent Fock representation by selecting a distinct complex structure. The present work proves that the chosen Fock quantisation is in fact unique (up to unitary equivalence) if one demands unitary implementation of the dynamics and invariance under the group of constant S¹translations. These translations are precisely those generated by the global constraint that remains on the Gowdy model. It is also shown that the proof of uniqueness in the choice of complex structure can be applied to more general field dynamics than that corresponding to the Gowdy cosmologies."

 

[marcus]

http://arxiv.org/abs/astro-ph/0608034
The Accelerating Universe and a Limiting Curvature Proposal
Damien A. Easson
21 pages, 7 figures

"We consider the hypothesis of a limiting minimal curvature in gravity as a way to construct a class of theories exhibiting late-time cosmic acceleration. Guided by the minimal curvature conjecture (MCC) we are naturally lead to a set of scalar tensor theories in which the scalar is non-minimally coupled to the matter Lagrangian. The model is compared to the Lambda Cold Dark Matter concordance model and to the observational data using the gold SNeIa sample of Riess et. al. (2004). We present a toy model designed to demonstrate the possibility that such a new, possibly fundamental, principle may be responsible for the recent period of cosmological acceleration."

Damien Easson is a young fellow in UK who has co-authored with Sean Carroll, Mark Trodden, Michael Turner, Robert Brandenburger. this is just a toy model illustrating a preliminary idea. It is an odd idea---though not too different from just admitting a bare CC in the Einstein equation. He proposes a permanent minimal warp as substitute for Dark Energy. IIRC there is a type of relativistic MOND which dispenses with Dark Energy. In any case, since DE is a puzzle, we might keep an eye out for variations of gravity which explain acceleration some other way. even if the idea seems peculiar and preliminary.
==================
http://arxiv.org/abs/gr-qc/0608018
The History and Present Status of Quantum Field Theory in Curved Spacetime
Robert M. Wald
15 pages, no figures; contribution to 7th International Conference on the History of General Relativity

"Quantum field theory in curved spacetime is a theory wherein matter is treated fully in accord with the principles of quantum field theory, but gravity is treated classically in accord with general relativity. It is not expected to be an exact theory of nature, but it should provide a good approximate description when the quantum effects of gravity itself do not play a dominant role. A major impetus to the theory was provided by Hawking's calculation of particle creation by black holes, showing that black holes radiate as perfect black bodies. During the past 30 years, considerable progress has been made in giving a mathematically rigorous formulation of quantum field theory in curved spacetime. Major issues of principle with regard to the formulation of the theory arise from the lack of Poincare symmetry and the absence of a preferred vacuum state or preferred notion of 'particles'. By the mid-1980's, it was understood how all of these difficulties could be overcome for free (i.e., non-self-interacting) quantum fields by formulating the theory via the algebraic approach and focusing attention on the local field observables rather than a notion of 'particles'. However, these ideas, by themselves, were not adequate for the formulation of interacting quantum field theory, even at a perturbative level, since standard renormalization prescriptions in Minkowski spacetime rely heavily on Poincare invariance and the existence of a Poincare invariant vacuum state. However, during the past decade, great progress has been made, mainly due to the importation into the theory of the methods of 'microlocal analysis'. This article will describe the historical development of the subject and describe some of the recent progress."

survey by a senior guy
================
http://arxiv.org/abs/quant-ph/0608035
Quantum measurements without sums
Bob Coecke, Dusko Pavlovic
36 pages and 46 pictures; earlier version circulated since November 2005 with as title 'Quantum Measurements as Coalgebras''. Invited paper to appear in: The Mathematics of Quantum Computation and Technology; Chen, Kauffman and Lomonaco (eds.); Taylor and Francis

"Sums play a prominent role in the formalisms of quantum mechanics, be it for mixing and superposing states, or for composing state spaces. Surprisingly, a conceptual analysis of quantum measurement seems to suggest that quantum mechanics can be done without direct sums, expressed entirely in terms of the tensor product. The corresponding axioms define classical spaces as objects that allow copying and deleting data. Indeed, the information exchange between the quantum and the classical worlds is essentially determined by their distinct capabilities to copy and delete data. The sums turn out to be an implicit implementation of this capabilities. Realizing it through explicit axioms not only dispenses with the unnecessary structural baggage, but also allows a simple and intuitive graphical calculus. In category-theoretic terms, classical data types are dagger-compact Frobenius algebras, and quantum spectra underlying quantum measurements are Eilenberg-Moore coalgebras induced by these Frobenius algebras."

---sample exerpt from page 2 of the article---
A recent research thread, initiated by Abramsky and the first author [2], aims at recasting the quantum mechanical formalism in categorical terms. The upshot of categorical semantics is that it displays concepts in a compositional and typed framework. In the case of quantum mechanics, it uncovers the quantum information-flows [6] which are hidden in the usual formalism. Moreover, while the investigations of quantum structures have so far been predominantly academic, categorical semantics open an alley towards a practical, low-overhead tool for the design and analysis of quantum informatic protocols, versatile enough to capture both quantitative and qualitative aspects of quantum information [2, 7, 10, 13, 31]. In fact, some otherwise complicated quantum informatic protocols become trivial exercises in this framework [8]. On the other hand, compared with the order-theoretic framework for quantum mechanics in terms of Birkhoff-von Neumann’s quantum logic [29], this categorical setting comes with logical derivations, topologically embodied into something as simple as “yanking a rope”. 2 Moreover, in terms of deductive machanism, it turns out to be some kind of “super-logic” as compared to the Birkhoff-von Neumann “non-logic”.
---endquote---

Baez was talking about stretching out a piece of wet spaghetti. curious propositions in quantum theory, seeming paradoxes, become trivial exercises as Coecke says. Baez was trying to get that idea across---basically one of the reasons why one might see categorical semantics infiltrate into physics.

I am guessing that this paper of Coecke et al will be mentioned in the #237 issue of This Weeks Finds.

 

[marcus]

http://arxiv.org/abs/gr-qc/0608074
A Modified Gravity and its Consequences for the Solar System, Astrophysics and Cosmology
J. W. Moffat
Comments: 15 pages. Talk given at the International Workshop "From Quantum to Cosmos: Fundamental Physics in Space", 22-24 May, 2006, Warrenton, Virginia, USA

"A relativistic modified gravity (MOG) theory leads to a self-consistent, stable gravity theory that can describe the solar system, galaxy and clusters of galaxies data and cosmology."

http://arxiv.org/abs/astro-ph/0608344
The rapid formation a large rotating disk galaxy three billion years after the Big Bang
R. Genzel, L. Tacconi, F. Eisenhauer, N. M. Forster Schreiber, A. Cimatti, E. Daddi, N. Bouche, R. Davies, M. D. Lehnert, D. Lutz, N. Nesvadba, A. Verma, R. Abuter, K. Shapiro, A. Sternberg, A. Renzini, X. Kong, N. Arimoto, M. Mignoli
Nature, accepted (Released Aug 17th)

"Over the past two decades observations and theoretical simulations have established a global frame-work of galaxy formation and evolution in the young Universe. Galaxies formed as baryonic gas cooled at the centres of collapsing dark matter halos. Mergers of halos led to the build up of galaxy mass. A major step forward in understanding these issues requires well resolved physical information on individual galaxies at high redshift. Here we report adaptive optics, spectroscopic observations of a representative luminous star forming galaxy when the Universe was only twenty percent of its age. The superior angular resolution of these data reveals the physical and dynamical properties of a high redshift galaxy in unprecedented detail. A large and massive rotating proto-disk is channelling gas towards a growing central stellar bulge hosting an accreting massive black hole."

http://arxiv.org/abs/astro-ph/0608345
Quark Matter in Neutron Stars: An aperçu
Prashanth Jaikumar, Sanjay Reddy, Andrew W. Steiner
15 pages, 1 figure. Invited review for Modern Physics Letters A

"The existence of deconfined quark matter in the superdense interior of neutron stars is a key question that has drawn considerable attention over the past few decades. Quark matter can comprise an arbitrary fraction of the star, from 0 for a pure neutron star to 1 for a pure quark star, depending on the equation of state of matter at high density. From an astrophysical viewpoint, these two extreme cases are generally expected to manifest different observational signatures. An intermediate fraction implies a hybrid star, where the interior consists of mixed or homogeneous phases of quark and nuclear matter, depending on surface and Coulomb energy costs, as well as other finite size and screening effects. In this brief review article, we discuss what we can deduce about quark matter in neutron stars in light of recent exciting developments in neutron star observations. We state the theoretical ideas underlying the equation of state of dense quark matter, including color superconducting quark matter. We also highlight recent advances stemming from re-examination of an old paradigm for the surface structure of quark stars and discuss possible evolutionary scenarios from neutron stars to quark stars, with emphasis on astrophysical observations."

http://arxiv.org/abs/math.CT/0608420
Lectures on n-Categories and Cohomology
John C. Baez, Michael Shulman
68 pages, 2 ps and 2 eps figures

"The goal of these talks was to explain how cohomology and other tools of algebraic topology are seen through the lens of n-category theory. Special topics include nonabelian cohomology, Postnikov towers, the theory of "n-stuff", and n-categories for n = -1 and -2. The talks were very informal, and so are these notes. A lengthy appendix clarifies certain puzzles and ventures into deeper waters such as higher topos theory. For readers who want more details, we include an annotated bibliography."

http://arxiv.org/abs/hep-th/0608120
Holography of Gravitational Action Functionals
A. Mukhopadhyay, T. Padmanabhan
17 pages

"Einstein-Hilbert (EH) action can be separated into a bulk and a surface term, with a specific ("holographic") relationship between the two, so that either can be used to extract information about the other. The surface term can also be interpreted as the entropy of the horizon in a wide class of spacetimes. Since EH action is likely to just the first term in the derivative expansion of an effective theory, it is interesting to ask whether these features continue to hold for more general gravitational actions. We provide a comprehensive analysis of lagrangians of the form L=Q_a^{bcd}R^a_{bcd}, in which Q_a^{bcd} is a tensor with the symmetries of the curvature tensor, made from metric and curvature tensor and satisfies the condition \nabla_cQ^{abcd}=0, and show that they share these features. The Lanczos-Lovelock lagrangians are a subset of these in which Q^{abcd} is a homogeneous function of the curvature tensor. They are all holographic, in a specific sense of the term, and -- in all these cases -- the surface term can be interpreted as the horizon entropy. The thermodynamics route to gravity, in which the field equations are interpreted as TdS=dE+pdV, seems to have greater degree of validity than the field equations of Einstein gravity itself. The results suggest that the holographic feature of EH action could also serve as a new symmetry principle in constraining the semiclassical corrections to Einstein gravity. The implications are discussed."

 

[marcus]

http://arxiv.org/abs/gr-qc/0608100
Large scale effective theory for cosmological bounces
Martin Bojowald
4 pages, 1 figure

"An exactly solvable bounce model in loop quantum cosmology is identified which serves as a perturbative basis for realistic bounce scenarios. Its bouncing solutions are derived analytically, demonstrating why recent numerical simulations robustly led to smooth bounces under the assumption of semiclassicality. Several effects, easily included in a perturbative analysis, can however change this smoothness. The effective theory is not only applicable to such situations where numerical techniques become highly involved but also allows one to discuss conceptual issues. For instance, consequences of the notoriously difficult physical inner product can be implemented at the effective level. This indicates that even physical predictions from full quantum gravity can be obtained from perturbative effective equations."

http://arxiv.org/abs/astro-ph/0608407
A direct empirical proof of the existence of dark matter
Douglas Clowe (1), Marusa Bradac (2), Anthony H. Gonzalez (3), Maxim Markevitch (4), Scott W. Randall (4), Christine Jones (4), Dennis Zaritsky (1) ((1) Steward Observatory, Tucson, (2) KIPAC, Stanford, (3) Department of Astronomy, Gainesville, (4) CfA, Cambridge)
Accepted for publication in ApJL
"We present new weak lensing observations of 1E0657-558 (z=0.296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated. By using both wide-field ground based images and HST/ACS images of the cluster cores, we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8-sigma significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen."

http://arxiv.org/abs/astro-ph/0608408
Strong and weak lensing united III: Measuring the mass distribution of the merging galaxy cluster 1E0657-56
Marusa Bradac (1,2), Douglas Clowe (3), Anthony H. Gonzalez (4), Phil Marshall (1), William Forman (5), Christine Jones (5), Maxim Markevitch (5), Scott Randall (5), Tim Schrabback (2), Dennis Zaritsky (3) ((1) KIPAC, Stanford, (2) AIfA, Bonn, (3) Steward Observatory, Tucson, (4) Department of Astronomy, Gainesville, (5) CfA, Cambridge)
Accepted for publication in ApJ; Version with full-resolution figures available at this URL:
http://www.slac.stanford.edu/~marusa/Work/bradac_strong_weak_III.pdf

"The galaxy cluster 1E0657-56 (z = 0.296) is remarkably well-suited for addressing outstanding issues in both galaxy evolution and fundamental physics. We present a reconstruction of the mass distribution from both strong and weak gravitational lensing data. Multi-color, high-resolution HST ACS images allow detection of many more arc candidates than were previously known, especially around the subcluster. Using the known redshift of one of the multiply imaged systems, we determine the remaining source redshifts using the predictive power of the strong lens model. Combining this information with shape measurements of "weakly" lensed sources, we derive a high-resolution, absolutely-calibrated mass map, using no assumptions regarding the physical properties of the underlying cluster potential. This map provides the best available quantification of the total mass of the central part of the cluster. We also confirm the result from Clowe et al. (2004,2006a)."

http://arxiv.org/abs/quant-ph/0605070
High Energy Quantum Teleportation Using Neutral Koans
Yu Shi
6 pages, 4 figures. Appearing in Physics Letters B

[sic]

for traveling Zen masters

http://arxiv.org/abs/astro-ph/0608417
Planetesimals To Brown Dwarfs: What is a Planet?
Gibor Basri, Michael E. Brown (Univ. of California, Berkeley and California Inst. of Technology)
23 pages
Annual Reviews of Earth and Planetary Science, 2006, v. 34, pp. 193-216
"The past 15 years have brought about a revolution in our understanding of our Solar System and other planetary systems. During this time, discoveries include the first Kuiper Belt Objects, the first brown dwarfs, and the first extra-solar planets. Although discoveries continue apace, they have called into question our previous perspectives on planets, both here and elsewhere. The result has been a debate about the meaning of the word ''planet'' itself. It became clear that scientists do not have a widely accepted or clear definition of what a planet is, and both scientists and the public are confused (and sometimes annoyed) by its use in various contexts. Because ''planet'' is a very widely used term, it seems worth the attempt to resolve this problem. In this essay, we try to cover all the issues that have come to the fore, and bring clarity (if not resolution) to the debate."

Mike Brown has good sense. I've heard him talk. I'd take his advice on terminology and science+culture issues.
Besides, he found Sedna.

http://arxiv.org/abs/physics/0608156
Threat Mitigation: The Asteroid Tugboat
Russell Schweickart (B612 Foundation), Clark Chapman, Dan Durda, (Southwest Research Institute), Piet Hut (Institute for Advanced Studies)
Submitted to NASA Workshop on Near-Earth Objects, Vail, Colorado, June 2006

"The Asteroid Tugboat (AT) is a fully controlled asteroid deflection concept using a robotic spacecraft powered by a high efficiency, electric propulsion system (ion or plasma) which docks with and attaches to the asteroid, conducts preliminary operations, and then thrusts continuously parallel to the asteroid velocity vector until the desired velocity change is achieved. Based on early warning, provided by ground tracking and orbit prediction, it would be deployed a decade or more prior to a potential impact. On completion of the initial rendezvous with the near-Earth object (NEO) the AT would first reduce the uncertainty in the orbit of the asteroid via Earth tracking of its radio transponder while it is station keeping with the asteroid. If on analysis of tracking data a deflection is required the AT would execute a reconnaissance phase collecting and processing information about the physical characteristics of the asteroid to support subsequent operations. The AT would then dock at the appropriate pole (i.e. on the spin axis), attach to the asteroid surface, and initiate a NEO reorientation maneuver. Following completion of the NEO reorientation the AT would initiate the deflection phase by thrusting continuously parallel to the asteroid velocity vector until the resultant target orbit is achieved. The orbit of the asteroid is continuously monitored throughout the deflection process and the end state is known in real time. If one assumes a nuclear-electric propulsion (NEP) system similar to that formerly under development in the recently canceled Prometheus Program, the AT would be capable of deflecting threatening NEOs up to 800 meters in diameter or more."

 

[marcus]

http://arxiv.org/abs/hep-th/0608175
Spacetime topology change and black hole information
Stephen D.H. Hsu
5 pages, 5 figures

"Topology change -- the creation of a disconnected baby universe -- due to black hole collapse may resolve the information loss paradox. Evolution from an early time Cauchy surface to a final surface which includes a slice of the disconnected region can be unitary and consistent with conventional quantum mechanics. We discuss the issue of cluster decomposition, showing that any violations thereof are likely to be unobservably small. Topology change is similar to the black hole remnant scenario and only requires assumptions about the behavior of quantum gravity in Planckian regimes. It does not require non-locality or any modification of low-energy physics."

Steve Hsu has 80 papers on arxiv, several co-authored with Tony Zee, or Brian Murray, or Roman Buniy
a smart and original thinker.
http://arxiv.org/find/grp_physics/1/au:+Hsu_S/0/1/0/all/0/1

"We have proposed a solution of the black hole information paradox which depends entirely on details of Planckian physics — no modifications of low-energy physics, such as non-locality, are required. The main assumptions are that the endpoint of interior black hole evolution is topology change and that the quantum gravitational dynamics of pinching off are strongly coupled. Thus, small perturbations to the initial state of a black hole lead to different internal state vectors describing the resulting baby universe, even if the semiclassical properties are only slightly changed. Under this assumption, any violation of cluster decomposition will be practically unobservable.

If our scenario is correct, there is no violation of causality or locality at the semiclassical black hole horizon, and no stable Planck mass remnant of black hole evaporation. Instead, much as Hawking first proposed, information is lost: to a baby universe, from which it may or may not someday emerge via tunneling. If the information emerges again, evolution within the parent universe is unitary. If information remains in the baby universe, the parent universe appears to evolve from a pure to mixed state, but the evolution of parent and baby together is unitary. There are no dire consequences, such as energy non-conservation."

This resolution of the BH info puzzle is not unfamiliar to LQG researchers, but it is hopeful and refreshing to see it surface like this in a different context---Hsu has been talking to people like Giddings and Strominger, rather than Martin Bojowald, for example.

http://arxiv.org/abs/astro-ph/0608535
Dark matter burners
I. V. Moskalenko (Stanford), L. Wai (SLAC)
4 pages, 3 figures

"We show that a star orbiting close enough to an adiabatically grown supermassive black hole can capture a large number of weakly interacting massive particles (WIMPs) during its lifetime. WIMP annihilation energy release in low- to medium-mass stars is comparable with or even exceeds the luminosity of such stars due to thermonuclear burning. The excessive energy release in the stellar core may result in an evolution scenario different from what is expected for a regular star. The model thus predicts the existence of unusual stars within the central parsec of galactic nuclei. If found, such stars would provide evidence for the existence of particle dark matter. The excess luminosity of such stars attributed to WIMP "burning" can be used to infer the local WIMP matter density. A white dwarf with a highly eccentric orbit around the central black hole may exhibit variations in brightness correlated with the orbital phase. On the other hand, white dwarfs shown to lack such orbital brightness variations can be used to provide constraints on WIMP matter density, WIMP-nucleus scattering and pair annihilation cross sections."

http://arxiv.org/abs/gr-qc/0608116
Causality and matter propagation in 3d spin foam quantum gravity
Daniele Oriti, Tamer Tlas
25 pages, 2 figures

"In this paper we tackle the issue of causality in quantum gravity, in the context of 3d spin foam models. We identify the correct procedure for implementing the causality/orientation dependence restriction that reduces the path integral for BF theory to that of quantum gravity in first order form. We construct explicitly the resulting causal spin foam model. We then add matter degrees of freedom to it and construct a causal spin foam model for 3d quantum gravity coupled to matter fields. Finally, we show that the corresponding spin foam amplitudes admit a natural approximation as the Feynman amplitudes of a non-commutative quantum field theory, with the appropriate Feynman propagators weighting the lines of propagation, and that this effective field theory reduces to usual QFT in flat space in the no-gravity limit."

http://arxiv.org/abs/gr-qc/0608117
The Shallow Waters of the Big-Bang
Pablo Laguna
5 pages, 3 figures

"Loop quantum cosmology homogeneous models with a massless scalar field show that the big-bang singularity can be replaced by a big quantum bounce. To gain further insight on the nature of this bounce, we study the semi-discrete loop quantum gravity Hamiltonian constraint equation from the point of view of numerical analysis. We show that the bounce is closely related to the method for the temporal update of the system and demonstrate that, in particular, explicit time-updates in general yield bounces. These bounces can be understood as spurious reflections in finite difference discretizations of wave equations in nonuniform grids or, equivalently, as spurious reflections found when solving wave equations with varying coefficients, such as the shallow water equations. We present an implicit time-update devoid of bounces and show back-in-time, deterministic evolutions that reach and partially jump over the big-bang singularity."

http://arxiv.org/abs/astro-ph/0608576
Status and recent results of the Pierre Auger Observatory
Serguei Vorobiov (for the Pierre Auger Collaboration)
Talk given at the International Cosmic Ray Workshop "Tien-Shan 2006", held from 25 to 27 August 2006 near Almaty, Kazakhstan; 8 pages, 4 figures
"The Pierre Auger Observatory aims to determine the nature and origin of the ultra-high energy cosmic rays (UHECR). The Auger hybrid detector combines fluorescence observations of extended air showers, initiated in the atmosphere by these most energetic particles, with measures of the shower front at the ground level by its large array of Cherenkov water tanks. This allows to improve considerably the precision on reconstructed primary cosmic ray parameters, and to make important cross-calibrations between two techniques at these energies, unreachable with accelerator experiments. The Southern Auger site in Argentina is close to completion. The first measure of the primary cosmic rays energy spectrum, the anisotropy search results, and the limit on the photon fraction in the UHECR are discussed."

 

[marcus]

http://arxiv.org/abs/gr-qc/0608121
Constraining Torsion with Gravity Probe B
Yi Mao (MIT), Max Tegmark (MIT), Alan Guth (MIT), Serkan Cabi (MIT)
26 pages, 6 figures, 5 tables

"It is well-entrenched folklore that torsion gravity theories predict observationally negligible torsion in the solar system, since torsion (if it exists) couples only to the intrinsic spin of elementary particles, not to rotational angular momentum. We argue that this assumption has a logical loophole which can and should be tested experimentally. We give an explicit counterexample where a rotating body generates a torsion field in Weitzenbock spacetime with a Hayashi-Shirafuji Lagrangian. More generally, in the spirit of action=reaction, if a rotating mass like a planet can generate torsion, then a gyroscope should also feel torsion.
Using symmetry arguments, we show that to lowest order, the torsion field around a uniformly rotating spherical mass is determined by seven dimensionless parameters. These parameters effectively generalize the PPN formalism and provide a concrete framework for further testing GR. We construct a parametrized Lagrangian that includes both standard torsion-free GR and Hayashi-Shirafuji maximal torsion gravity as special cases. We demonstrate that classic solar system tests rule out the latter and constrain two observable parameters. We show that Gravity Probe B (GPB) is an ideal experiment for further constraining torsion theories, and work out the most general torsion-induced precession of its gyroscope in terms of our torsion parameters."

http://arxiv.org/abs/gr-qc/0608124
Loop Quantum Gravity Modification of the Compton Effect
Kourosh Nozari, S. Davood Sadatian
11 pages, 2 figures

"Modified dispersion relations(MDRs) as a manifestation of Lorentz invariance violation, have been appeared in alternative approaches to quantum gravity problem. Loop quantum gravity is one of these approaches which evidently requires modification of dispersion relations. These MDRs will affect the usual formulation of the Compton effect. The purpose of this letter is to incorporate the effects of loop quantum gravity MDRs on the formulation of Compton scattering. Using limitations imposed on MDRs parameters from Ultra High Energy Cosmic Rays(UHECR), we estimate the quantum gravity-induced wavelength shift of scattered photons in a typical Compton process. Possible experimental detection of this wavelength shift will provide strong support for underlying quantum gravity proposal."

I need to take a critical look at this, or get someone else to comment, before I decide what to think. It would be very nice if they have discovered an additional way to test LQG. I am not familiar with previous work by the lead author K. Nozari.

http://arxiv.org/abs/hep-th/0608210
Loop Quantum Gravity: An Inside View
Thomas Thiemann
58 pages

"This is a (relatively) non -- technical summary of the status of the quantum dynamics in Loop Quantum Gravity (LQG). We explain in detail the historical evolution of the subject and why the results obtained so far are non -- trivial. The present text can be viewed in part as a response to an article by Nicolai, Peeters and Zamaklar [hep-th/0501114]. We also explain why certain no go conclusions drawn from a mathematically correct calculation in a recent paper by Helling et al [hep-th/0409182] are physically incorrect."

Good old Thomas Thiemann! Glad he has responded to the Nicolai et al and Helling et al papers!http://arxiv.org/abs/astro-ph/0608602
Can Cosmic Structure form without Dark Matter?
Scott Dodelson, Michele Liguori
5 pages, 3 figures

"One of the prime pieces of evidence for dark matter is the observation of large overdense regions in the universe. Since we know from the cosmic microwave background that the regions that contained the most baryons when the universe was ~400,000 years old were overdense by only one part in ten thousand, perturbations had to have grown since then by a factor greater than $(1+z_*)\simeq 1180 where z_*$ is the epoch of recombination. This enhanced growth does not happen in general relativity, so dark matter is needed in the standard theory. We show here that enhanced growth can occur in alternatives to general relativity, in particular in Bekenstein's relativistic version of MOdified Newtonian Dynamics (MOND). The vector field introduced in that theory for a completely different reason plays a key role in generating the instability that produces large cosmic structures today."

Looks like Scott Dodelson isn't sure that TeVes is dead after all. And we shouldn't forget this earlier paper:

http://arxiv.org/abs/astro-ph/0606216
Can MOND take a bullet? Analytical comparisons of three versions of MOND beyond spherical symmetry
Garry W. Angus, Benoit Famaey, HongSheng Zhao
14 pages, 9 figures, accepted for publication in MNRAS

"A proper test of Modified Newtonian Dynamics (MOND) in systems of non-trivial geometries depends on modelling subtle differences in several versions of its postulated theories. This is especially important for lensing and dynamics of barely virialised galaxy clusters with typical gravity of scale $\sim a_0 \sim 1\AA{\rm s}^{-2}$. The original MOND formula, the classical single field modification of the Poisson equation, and the multi-field general relativistic theory of Bekenstein (TeVeS) all lead to different predictions as we stray from spherical symmetry. In this paper, we study a class of analytical MONDian models for a system with a semi-Hernquist baryonic profile. After presenting the analytical distribution function of the baryons in spherical limits, we develop orbits and gravitational lensing of the models in non-spherical geometries. In particular, we can generate a multi-centred baryonic system with a weak lensing signal resembling that of the merging galaxy cluster 1E 0657-56 with a bullet-like light distribution. We finally present analytical scale-free highly non-spherical models to show the subtle differences between the single field classical MOND theory and the multi-field TeVeS theory."

check out Figure 7 on page 7. they illustrate that TeVeS can make a DOUBLECENTER lensing map like the one observed by Maxim Markevitch et al----they get TeVes to produce a two-lens lensing map like goggles or spectacles rather like what the CHANDRA people showed us.

http://arxiv.org/abs/gr-qc/0608131
Group Integral Techniques for the Spinfoam Graviton Propagator
Etera R. Livine, Simone Speziale
16 pages

"We consider the proposal of gr-qc/0508124 for the extraction of the graviton propagator from the spinfoam formalism. We propose a new ansatz for the boundary state, using which we can write the propagator as an integral over SU(2). The perturbative expansion in the Planck length can be recast into the saddle point expansion of this integral. We compute the leading order and recover the behavior expected from low--energy physics. In particular, we prove that the degenerate spinfoam configurations are suppressed."

http://arxiv.org/abs/gr-qc/0608135
Towards a Covariant Loop Quantum Gravity
Etera R. Livine
13 pages, review, draft chapter for the book "Approaches to quantum gravity", being prepared by Daniele Oriti for Cambridge University Press, comments welcome

"We review the canonical analysis of the Palatini action without going to the time gauge as in the standard derivation of Loop Quantum Gravity. This allows to keep track of the Lorentz gauge symmetry and leads to a theory of Covariant Loop Quantum Gravity. This new formulation does not suffer from the Immirzi ambiguity, it has a continuous area spectrum and uses spin networks for the Lorentz group. Finally, its dynamics can easily be related to Barrett-Crane like spin foam models."

http://arxiv.org/abs/quant-ph/0608243
Relational physics with real rods and clocks and the measurement problem of quantum mechanics
Rodolfo Gambini, Jorge Pullin
19 pages

"The use of real clocks and measuring rods in quantum mechanics implies a natural loss of unitarity in the description of the theory. We briefly review this point and then discuss the implications it has for the measurement problem in quantum mechanics. The intrinsic loss of coherence allows to circumvent some of the usual objections to the measurement process as due to environmental decoherence."

 

[marcus]

http://arxiv.org/abs/hep-th/0608221
A Lorentzian version of the non-commutative geometry of the standard model of particle physics
John W. Barrett
14 pages

"A formulation of the non-commutative geometry for the standard model of particle physics with a Lorentzian signature metric is presented. The elimination of the fermion doubling in the Lorentzian case is achieved by a modification of Connes' internal space geometry so that it has signature 6 (mod 8) rather than 0. The fermionic part of the Connes-Chamseddine spectral action can be formulated, and it is shown that it allows an extension with right-handed neutrinos and the correct mass terms for the see-saw mechanism of neutrino mass generation."

http://arxiv.org/abs/hep-th/0608226
Noncommutative Geometry and the standard model with neutrino mixing
Alain Connes
17 pages

"We show that allowing the metric dimension of a space to be independent of its KO-dimension and turning the finite noncommutative geometry F-- whose product with classical 4-dimensional space-time gives the standard model coupled with gravity--into a space of KO-dimension 6 by changing the grading on the antiparticle sector into its opposite, allows to solve three problems of the previous noncommutative geometry interpretation of the standard model of particle physics:
The finite geometry F is no longer put in "by hand" but a conceptual understanding of its structure and a classification of its metrics is given.
The fermion doubling problem in the fermionic part of the action is resolved.
The spectral action of our joint work with Chamseddine now automatically generates the full standard model coupled with gravity with neutrino mixing and see-saw mechanism for neutrino masses. The predictions of the Weinberg angle and the Higgs scattering parameter at unification scale are the same as in our joint work but we also find a mass relation (to be imposed at unification scale)."
=======================

those are the two main things: the Barrett and the Connes papers
but since I can still edit I will tack on some other

http://arxiv.org/abs/astro-ph/0609060
First results from the Pierre Auger Observatory
R. C. Shellard
10 pages, 16 figures, Brazilian National Meeting on Particle and Fields 2005

"We review in these notes the status of the construction of the Pierre Auger Observatory and present the first Physics results, based on the data collected during the first year and a half of operation. These results are preliminary, once the work to understand the systematics of the detectors are still underway. We discuss the cosmic ray spectrum above 3 EeV, based on the measurement done using the Surface Detector and the Fluorescence Detector, both, components of the observatory. We discuss, as well, the search for anisotropy near the Galactic Center and the limit on the photon fraction at the highest energies."

here is from the summary:
"... The Pierre Auger Observatory is still under construction but has already the largest integrated exposure to high energy cosmic rays. The combination of fluorescence and the surface detector measurements allow for the reconstruction of the shower geometry and its energy with much greater quality than what could be achieved with either detector standing alone. Each of the detectors have different systematics, allowing for valuable information for cross-checking the results from each of them. The observatory should be finished by mid 2007, accumulating by then a much larger exposure than what was used for the preliminary results presented here. That will allow for the search of anisotropies in the southern sky, as well as the test of the predicted GZK suppression."

 

[marcus]

http://arxiv.org/abs/gr-qc/0609012
Why Does Gravity Ignore the Vacuum Energy?
T. Padmanabhan
Invited Contribution to the IJMPD Special Issue on Dark Matter and Dark Energy edited by D.Ahluwalia and D. Grumiller. Appendix clarifies several conceptual and pedgogical aspects of surface term in Hilbert action

"The equations of motion for matter fields are invariant under the shift of the matter lagrangian by a constant. Such a shift changes the energy momentum tensor of matter by T^a_b --> T^a_b +\rho \delta^a_b. In the conventional approach, gravity breaks this symmetry and the gravitational field equations are not invariant under such a shift of the energy momentum tensor. I argue that until this symmetry is restored, one cannot obtain a satisfactory solution to the cosmological constant problem. I describe an alternative perspective to gravity in which the gravitational field equations are [G_{ab} -\kappa T_{ab}] n^an^b =0 for all null vectors n^a. This is obviously invariant under the change T^a_b --> T^a_b +\rho \delta^a_b and restores the symmetry under shifting the matter lagrangian by a constant. These equations are equivalent to G_{ab} = \kappa T_{ab} + Cg_{ab} where C is now an integration constant so that the role of the cosmological constant is very different in this approach. The cosmological constant now arises as an integration constant, somewhat like the mass M in the Schwarzschild metric, the value of which can be chosen depending on the physical context. These equations can be obtained from a variational principle which uses the null surfaces of spacetime as local Rindler horizons and can be given a thermodynamic interpretation. This approach turns out to be quite general and can encompass even the higher order corrections to Einstein's gravity and suggests a principle to determine the form of these corrections in a systematic manner."

In the past there has been considerable interest in the papers of T. Padmanabhan. I can't evaluate this one on its merits, but include it on account of the author. Likewise with the next, a Lisa Randall paper:http://arxiv.org/abs/hep-ph/0607158
Gravitational Waves from Warped Spacetime
Lisa Randall, Geraldine Servant
18 pages, 15 figures

"We argue that the RSI model can provide a strong signature in gravitational waves. This signal is a relic stochastic background generated during the cosmological phase transition from an AdS-Schwarschild phase to the RS1 geometry that should occur at a temperature in the TeV range. We estimate the amplitude of the signal in terms of the parameters of the potential stabilizing the radion and show that over much of the parameter region in which the phase transition completes, a signal should be detectable at the planned space interferometer, LISA."

 

[marcus]

http://arxiv.org/abs/gr-qc/0609027
A Measurement of Newton's Gravitational Constant
St. Schlamminger, E. Holzschuh, W. Kündig, F. Nolting, R.E. Pixley, J. Schurr, U. Straumann
26 pages, 20 figures, Accepted for publication by Phys. Rev. D

"A precision measurement of the gravitational constant G has been made using a beam balance. Special attention has been given to determining the calibration, the effect of a possible nonlinearity of the balance and the zero-point variation of the balance. The equipment, the measurements and the analysis are described in detail. The value obtained for G is $6.674252(109)(54) 10^{-11} m^3 kg^{-1} s^{-2}$. The relative statistical and systematic uncertainties of this result are $16.3 10^{-6}$ and $8.1 10^{-6}$, respectively."

the following is briefly noted in part for the simple reason that I haven't seen a paper by Larry Krauss for some time,
previous one was early 2004

http://arxiv.org/abs/gr-qc/0609024
Observation of Incipient Black Holes and the Information Loss Problem
Tanmay Vachaspati, Dejan Stojkovic, Lawrence M. Krauss
13 pages; 8 figures

 

[marcus]

http://arxiv.org/abs/gr-qc/0609034
Loop quantum cosmology and inhomogeneities
Martin Bojowald
25 pages, 1 figure
IGPG-06/8-1

"Inhomogeneities are introduced in loop quantum cosmology using regular lattice states, with a kinematical arena similar to that in homogeneous models considered earlier. The framework is intended to encapsulate crucial features of background independent quantizations in a setting accessible to explicit calculations of perturbations on a cosmological background. It is used here only for qualitative insights but can be extended with further more detailed input. One can thus see how several parameters occurring in homogeneous models appear from an inhomogeneous point of view. Their physical roles in several cases then become much clearer, often making previously unnatural choices of values look more natural by providing alternative physical roles. This also illustrates general properties of symmetry reduction at the quantum level and the roles played by inhomogeneities. Moreover, the constructions suggest a picture for gravitons and other metric modes as collective excitations in a discrete theory, and lead to the possibility of quantum gravity corrections in large universes."

exerpt from Conclusions section:
"Models of loop quantum gravity are being investigated actively regarding their phenomenological properties, and perturbative inhomogeneities are currently being included. This brings us closer to reliable computations of potentially observable properties such as those of structure formation. It is then important to check all intrinsic details of such models and see how faithfully they incorporate features of the full theory. As we discussed, qualitative effects are realized in homogeneous as well as inhomogeneous lattice models in the same way. We introduced inhomogeneous lattice constructions in a way which allows for a relation to a homogeneous background. The relation to isotropic models is then clear, which provides a new step toward relating isotropic models to the full theory. Although the lattice construction is analogous to that of isotropic models, quantitative aspects can change which has a bearing on which ranges of parameters one considers as natural or unnatural. It also plays a role for which correction terms will be dominant in different regimes which is the most important aspect for phenomenological investigations."http://arxiv.org/abs/gr-qc/0609032
Exploring the diffeomorphism invariant Hilbert space of a scalar field
Hanno Sahlmann
19 pages
ITP-UU-06/35, SPIN-06/31

"As a toy model for the implementation of the diffeomorphism constraint, the interpretation of the resulting states, and the treatment of ordering ambiguities in loop quantum gravity, we consider the Hilbert space of spatially diffeomorphism invariant states for a scalar field. We give a very explicit formula for the scalar product on this space, and discuss its structure.
Then we turn to the quantization of a certain class of diffeomorphism invariant quantities on that space, and discuss in detail the ordering issues involved. On a technical level these issues bear some similarity to those encountered in full loop quantum gravity."http://arxiv.org/abs/gr-qc/0609029
Loop Quantum Cosmology in Bianchi I Models: Analytical Investigation
Dah-Wei Chiou
51 pages

"The comprehensive formulation for loop quantum cosmology in the spatially flat, isotropic model was recently constructed. In this paper, the methods are extended to the anisotropic Bianchi I model. Both the precursor and the improved strategies are applied and the expected results are established: i) the scalar field again serves as an internal clock and is treated as emergent time; ii) the total Hamiltonian constraint is derived by imposing the fundamental discreteness and gives the evolution as a difference equation; and iii) the physical Hilbert space, Dirac observables and semi-classical states are constructed rigorously. It is also shown that the states in the kinematical Hilbert space associated with the singularity and planar collapse are decoupled in the difference evolution equation, indicating that the big bounce may take place when any of the length scales undergoes the vanishing behavior and thereby supporting the long-held assertion: the physics below the Planck regime is unattainable. The investigation affirms the robustness of the framework used in the isotropic model by enlarging its domain of validity and provides foundations to conduct the detailed numerical analysis."

Dah-Wei is currently at the UC Berkeley physics department and also at Ashtekar's insitute at Penn State. This work is closely connected to recent Penn State papers studying the quantum bounce (replacing bang and hole singularities).
Dah-Wei has 6 papers on arxiv going back to 2003, all that time at Berkeley/LBL. So he could be a Berkeley PhD student now moving on to Penn State postdoc. something like that. this is his first LQG paper.
http://arxiv.org/find/grp_physics/1/au:+Chiou_Dah_Wei/0/1/0/all/0/1

http://arxiv.org/abs/gr-qc/0609037
Semi-classical States in Homogeneous Loop Quantum Cosmology
Huahai Tan, Yongge Ma
13 pages, submitted to CQG

"Semi-classical states in homogeneous loop quantum cosmology (LQC) are constructed by two different ways. In the first approach, we firstly construct an exponentiated annihilation operator. Then a kind of semi-classical (coherent) state is obtained by solving the eigen-equation of that operator. Moreover, we use these coherent states to analyze the semi-classical limit of the quantum dynamics. It turns out that the Hamiltonian constraint operator employed currently in homogeneous LQC has correct classical limit with respect to the coherent states. In the second approach, the other kind of semi-classical state is derived from the mathematical construction of coherent states for compact Lie groups due to Hall."

Yong-ge Ma is at Beijing Normal. Author of several LQG papers already.
========

string overview, for general audience (off topic here, but might come in handy):
http://arxiv.org/abs/physics/0609062
String Theory and Einstein's Dream
Ashoke Sen
11 figures

 

[marcus]

http://arxiv.org/abs/gr-qc/0609040
Fermions in three-dimensional spinfoam quantum gravity
Winston Fairbairn (CPT)
40 pages, 3 figures

"We study the coupling of massive fermions to the quantum mechanical dynamics of spacetime emerging from the spinfoam approach in three dimensions. We first recall the classical theory before constructing a spinfoam model of quantum gravity coupled to spinors. The technique used is based on a finite expansion in inverse fermion masses leading to the computation of the vacuum to vacuum transition amplitude of the theory. The path integral is derived as a sum over closed fermionic loops wrapping around the spinfoam. The effects of quantum torsion are realized as a modification of the intertwining operators assigned to the edges of the two-complex, in accordance with loop quantum gravity. The creation of non-trivial curvature is modeled by a modification of the pure gravity vertex amplitudes. The appendix contains a review of the geometrical and algebraic structures underlying the classical coupling of fermions to three dimensional gravity."

we know Fairbairn from a paper co-authored with Rovelli a couple of years ago. this is his first solo paper that I know of.

this next one I cannot evaluate but thought certain other PF members might like to know about
Khovanov is in the math department at Columbia
Lot of pictures reminiscent of Sundance preon model, possibly also some recent J.B. papers.
http://arxiv.org/abs/math.QA/0609335
Braid cobordisms, triangulated categories, and flag varieties
Mikhail Khovanov, Richard Thomas
89 pages, 21 figures

"We argue that various braid group actions on triangulated categories should be extended to projective actions of the category of braid cobordisms and illustrate how this works in examples. We also construct an action of both the affine braid group and the braid cobordism category on the derived category of coherent sheaves on the cotangent bundle to the full flag variety."

==============
the Alain Connes Teheran interview
http://www.ipm.ac.ir/IPM/news/connes-interview.pdf

 

[Dcase]

braid cobordisms ~ knot theory ~ game theory

1 - The language of "braid cobordisms" suggests a possible relationship to knot theory, from my perspective.

'Braid cobordisms, triangulated categories, and flag varieties'
Mikhail Khovanov, Richard Thomas
89 pages, 21 figures

http://arxiv.org/abs/math.QA/0609335

2 - In 3D braids appear to be helices. Some game theorists think that saddle points [found in helicoids] are equivalent to Nash Equilibria from Mathematical Game Theory.

http://ggierz.ucr.edu/Math121/Winter06/LectureNotes/09SaddlePointsNashEqui.pdf#search=%22saddle%20points%20Nash%20Equilibrium%22

3 - Is it possible that two such diverse mathematical representations of objects might somehow be unifiable? [through Nash Equilibrium and Nash embedding theorems]

Are differential geometries essentially manifestations of energy interactions of energy games [attractor v Disipator / braid v unbraid]?

 

[marcus]

Dcase this is a bibliography thread. It is not for discussion.
If you want to discuss the Khovanov paper, please do not do it here (if people discussed things it would clutter the already loaded thread.
Anyone wanting to discuss Khovanov paper please continue discussion elsewhere.

For your convenience I have started a discussion thread for you here
https://www.physicsforums.com/showthread.php?p=1083810#post1083810
=============================
****************************************

The Fall 2006 issue of the APS newsletter Matters of Gravity is out.
http://arxiv.org/abs/gr-qc/0609045

A new editor, David Garfinkle, has taken over from Jorge Pullin.

=============================

an interesting article bearing on the Smolin CNS conjecture appeared yesterday

the CNS conjecture involves a prediction is that no modification in the measured values of standard dimensionless parameters would cause a monotone increase in the observed abundance of black holes.

this prediction was made in 1993 or 1994 and is still standing. The conjecture could be discredited by reliably determining the mass of a neutron star to be above a certain value, and in various other ways, but so far this has not happened.

The CNS conjecture could also be discredited by evidence that the universe is spatially infinite. It predicts instead that the universe is spatially closed---as in the so-called "nearly flat" picture, with slight overall positive curvature---Omega > 1.

http://arxiv.org/abs/gr-qc/0609045
Was the universe open or closed before inflation?
Eduard Masso, Subhendra Mohanty, Gabriel Zsembinszki
5 pages, 3 figures
UAB-FT-609

"If the spatial curvature of the universe at the beginning of inflation is negative, there is an enhancement of the temperature anisotropy of the Cosmic Background Radiation at large angles. On the other hand if at the start of inflation the universe was closed with curvature there will be a suppression of temperature anisotropy at the scale of the present horizon. The observation of a low quadrupole anisotropy by WMAP suggests that the universe was closed with (Omega-1) of order unity at the time when the perturbation scales of the size of our present horizon were exiting the inflationary horizon."

Eduard Masso is at Barcelona---his 47 arxiv papers go back to 1993.
http://arxiv.org/find/grp_physics/1/au:+Masso_E/0/1/0/all/0/1

Subhendra Mohanty is at Ahmedabad PRL---his estimated 60-some arxiv papers go back to 1992.
http://arxiv.org/find/grp_physics/1/au:+Mohanty_S/0/1/0/all/0/1

they did a computer simulation of how CMB anisotropies arose with various angular sizes assuming various initial conditions
and found that a natural explanation for the shape of the power spectrum at large angles is to assume spatial finiteness.

If anyone would like to discuss it, the link and abstract are posted on a suitable discussion thread here:
https://www.physicsforums.com/showthread.php?p=1083650#post1083650

 

[marcus]

http://arxiv.org/abs/quant-ph/0609109
Could quantum mechanics be an approximation to another theory?
Lee Smolin
10 pages

"We consider the hypothesis that quantum mechanics is an approximation to another, cosmological theory, accurate only for the description of subsystems of the universe. Quantum theory is then to be derived from the cosmological theory by averaging over variables which are not internal to the subsystem, which may be considered non-local hidden variables. We find conditions for arriving at quantum mechanics through such a procedure. The key lesson is that the effect of the coupling to the external degrees of freedom introduces noise into the evolution of the system degrees of freedom, while preserving a notion of averaged conserved energy and time reversal invariance.
These conditions imply that the effective description of the subsystem is Nelson's stochastic formulation of quantum theory. We show that Nelson's formulation is not, by itself, a classical stochastic theory as the conserved averaged energy is not a linear function of the probability density. We also investigate an argument of Wallstrom posed against the equivalence of Nelson's stochastic mechanics and quantum mechanics and show that, at least for a simple case, it is in error."

http://arxiv.org/abs/astro-ph/0609417
Tests of general relativity from timing the double pulsar
M. Kramer, I.H. Stairs, R.N. Manchester, M.A. McLaughlin, A.G. Lyne, R.D. Ferdman, M. Burgay, D.R. Lorimer, A. Possenti, N. D'Amico, J.M. Sarkissian, G.B. Hobbs, J.E. Reynolds, P.C.C. Freire, F. Camilo
Appeared in Science Express, Sept. 14, 2006. Includes supporting material

"The double pulsar system, PSR J0737-3039A/B, is unique in that both neutron stars are detectable as radio pulsars. This, combined with significantly higher mean orbital velocities and accelerations when compared to other binary pulsars, suggested that the system would become the best available testbed for general relativity and alternative theories of gravity in the strong-field regime. Here we report on precision timing observations taken over the 2.5 years since its discovery and present four independent strong-field tests of general relativity. Use of the theory-independent mass ratio of the two stars makes these tests uniquely different from earlier studies. By measuring relativistic corrections to the Keplerian description of the orbital motion, we find that the ``post-Keplerian'' parameter s agrees with the value predicted by Einstein's theory of general relativity within an uncertainty of 0.05%, the most precise test yet obtained. We also show that the transverse velocity of the system's center of mass is extremely small. Combined with the system's location near the Sun, this result suggests that future tests of gravitational theories with the double pulsar will supersede the best current Solar-system tests. It also implies that the second-born pulsar may have formed differently to the usually assumed core-collapse of a helium star."

http://arxiv.org/abs/astro-ph/0609416
The Formation of the Double Pulsar PSR J0737-3039A/B
I. H. Stairs, S. E. Thorsett, R. J. Dewey, M. Kramer, C. A. McPhee
To appear in MNRAS Letters
"Recent timing observations of the double pulsar J0737-3039A/B have shown that its transverse velocity is extremely low, only 10 km/s, and nearly in the Plane of the Galaxy. With this new information, we rigorously re-examine the history and formation of this system, determining estimates of the pre-supernova companion mass, supernova kick and misalignment angle between the pre- and post-supernova orbital planes. We find that the progenitor to the recently formed `B' pulsar was probably less than 2 MSun, lending credence to suggestions that this object may not have formed in a normal supernova involving the collapse of an iron core. At the same time, the supernova kick was likely non-zero. A comparison to the history of the double-neutron-star binary B1534+12 suggests a range of possible parameters for the progenitors of these systems, which should be taken into account in future binary population syntheses and in predictions of the rate and spatial distribution of short gamma-ray burst events."

 

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http://arxiv.org/abs/gr-qc/0609057
Hamiltonian cosmological perturbation theory with loop quantum gravity corrections
Martin Bojowald, Hector H. Hernández, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski
24 pages, 1 figure
IGPG--06/9--7

"Cosmological perturbation equations are derived systematically in a canonical scheme based on Ashtekar variables. A comparison with the covariant derivation and various subtleties in the calculation and choice of gauges are pointed out. Nevertheless, the treatment is more systematic when correction terms of canonical quantum gravity are to be included. This is done throughout the paper for one characteristic modification expected from loop quantum gravity."

a replacement by the final published version:
http://arxiv.org/abs/gr-qc/0504147
Uniqueness of diffeomorphism invariant states on holonomy-flux algebras
Jerzy Lewandowski, Andrzej Okolow, Hanno Sahlmann, Thomas Thiemann
38 pages, one figure. v2: Minor changes, final version, as published in CMP
AEI-2005-093, CGPG-04/5-3
Comm. Math. Phys., Vol. 267 No. 3 (2006), 703-733

"Loop quantum gravity is an approach to quantum gravity that starts from the Hamiltonian formulation in terms of a connection and its canonical conjugate. Quantization proceeds in the spirit of Dirac: First one defines an algebra of basic kinematical observables and represents it through operators on a suitable Hilbert space. In a second step, one implements the constraints. The main result of the paper concerns the representation theory of the kinematical algebra: We show that there is only one cyclic representation invariant under spatial diffeomorphisms.
While this result is particularly important for loop quantum gravity, we are rather general: The precise definition of the abstract *-algebra of the basic kinematical observables we give could be used for any theory in which the configuration variable is a connection with a compact structure group. The variables are constructed from the holonomy map and from the fluxes of the momentum conjugate to the connection. The uniqueness result is relevant for any such theory invariant under spatial diffeomorphisms or being a part of a diffeomorphism invariant theory."

 

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thanks to arivero for prompting me to add this to the bibliography thread

http://arxiv.org/abs/hep-th/0609140
Noncommutative geometry, topology and the standard model vacuum
R. A. D. Martins
24 pages

"As a ramification of a motivational discussion for previous joint work, in which equations of motion for the finite spectral action of the Standard Model were derived, we provide a new analysis of the results of the calculations herein, switching from the perspective of Spectral triple to that of Fredholm module and thus from the analogy with Riemannian geometry to the pre-metrical structure of the Noncommutative geometry. Using a suggested Noncommutative version of Morse theory together with algebraic K-theory to analyse the vacuum solutions, the first two summands of the algebra for the finite triple of the Standard Model arise up to Morita equivalence. We also demonstrate a new vacuum solution whose features are compatible with the physical mass matrix."

Arivero notes that, as mentioned in the abstract, this developed out of joint work with John Barrett.

 

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http://arxiv.org/abs/gr-qc/0609108
Prolegomena to any future Quantum Gravity
John Stachel
26 pages

"I shall discuss some 'conditions of possibility' of a quantum theory of gravity, stressing the need for solutions to some of fundamental problems confronting any attempt to apply some method of quantization to the field equations of general relativity."

Stachel is a prominent expert in the history and philosophy of physics---and himself a physicist. There could be some interest in what he finds by taking a look at the basic conceptual circumstances and philosophical underpinning that would apply to any QG theory. I can't tell if this paper is likely to be helpful or not.
============

Carlo Rovelli is giving a seminar today (Tuesday 26 September) that will be available online:

http://relativity.phys.lsu.edu/ilqgs/

The slides are already posted
http://relativity.phys.lsu.edu/ilqgs/rovelli092606.pdf

The title of the talk is
Graviton propagator from loop quantum gravity

One good thing about this seminar series is that people from half a dozen different places in the world can ASK QUESTIONS.
Last week at the regular (Tuesday) time, Ashtekar gave one of these seminars. He was talking from Penn State and he got questions from various places and at one point he got a question that he wanted to refer to Carlo Rovelli, who was also "participating" in the seminar from Marseille and who was listening to the talk. and Rovelli answered---it came across very clear (his audio quality was even better than the seminar-average, maybe he had a better mike) so it was like having people from several QG centers present in the same room.

Another good thing is the policy to make the slides for the talk available a day or more in advance of the talk, so you can read the slides ahead of time and understand the talk better

The last slide of todays talk gives 3 conclusions

1. Low energy limit:
(One component of) the graviton propagator (or the Newton law) appears to be correct, to first order in lambda.
2. Barrett-Crane vertex:
...The BC vertex works.
3. Scattering amplitudes:
A technique to compute n-point functions within a background independent formalism exists.

These seminars are always supposed to occur at 9AM Central Time, which is 10 AM eastern and 6 AM pacific.
But I can never keep track of "daylight saving time" which makes an hour difference.
I think it means that they happen at 1:00 PM Greenwich.

In his talk, Rovelli says to be sure to hear next week's talk which will be by Simone Speziale on Spinfoam graviton propagator

 

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http://arxiv.org/abs/hep-ph/0609274
The Standard Model: Alchemy and Astrology
Joseph D. Lykken
12 pages, 0 figures, review talk from "Physics at LHC", Krakow, 3-8 July 2006
Report-no: FERMILAB-CONF-06-347-T

"An brief unconventional review of Standard Model physics, containing no plots."http://arxiv.org/abs/gr-qc/0609122
Black hole entropy quantization
Alejandro Corichi, Jacobo Diaz-Polo, Enrique Fernandez-Borja
4 pages, 4 figures

"Ever since the pioneer works of Bekenstein and Hawking, black hole entropy has been known to have a quantum origin. Furthermore, it has long been argued by Bekenstein that entropy should be quantized in discrete (equidistant) steps given its identification with horizon area in (semi-)classical general relativity and the properties of area as an adiabatic invariant. This lead to the suggestion that black hole area should also be quantized in equidistant steps to account for the discrete black hole entropy. Here we shall show that loop quantum gravity, in which area is not quantized in equidistant steps can nevertheless be consistent with Bekenstein's equidistant entropy proposal in a subtle way. For that we perform a detailed analysis of the number of microstates compatible with a given area and show that an observed oscillatory behavior in the entropy-area relation, when properly interpreted yields an entropy that has discrete, equidistant values that are consistent with the Bekenstein framework."http://arxiv.org/abs/hep-th/0609177
Analytic Results in 2D Causal Dynamical Triangulations: A Review
Stefan Zohren
66 pages, 17 figures. Based on the author's thesis for the Master of Science in Theoretical Physics, supervised by R. Loll and co-supervised by J. Ambjorn, J. Jersak, July 2005

"We describe the motivation behind the recent formulation of a nonperturbative path integral for Lorentzian quantum gravity defined through Causal Dynamical Triangulations (CDT). In the case of two dimensions the model is analytically solvable, leading to a genuine continuum theory of quantum gravity whose ground state describes a two-dimensional "universe" completely governed by quantum fluctuations. One observes that two-dimensional Lorentzian and Euclidean quantum gravity are distinct. In the second part of the review we address the question of how to incorporate a sum over space-time topologies in the gravitational path integral. It is shown that, provided suitable causality restrictions are imposed on the path integral histories, there exists a well-defined nonperturbative gravitational path integral including an explicit sum over topologies in the setting of CDT. A complete analytical solution of the quantum continuum dynamics is obtained uniquely by means of a double scaling limit. We show that in the continuum limit there is a finite density of infinitesimal wormholes. Remarkably, the presence of wormholes leads to a decrease in the effective cosmological constant, reminiscent of the suppression mechanism considered by Coleman and others in the context of a Euclidean path integral formulation of four-dimensional quantum gravity in the continuum. In the last part of the review universality and certain generalizations of the original model are discussed, providing additional evidence that CDT define a genuine continuum theory of two-dimensional Lorentzian quantum gravity."

http://arxiv.org/abs/gr-qc/0609129
Rainbow universe
Yi Ling
5 pages

"The formalism of rainbow gravity is studied in a cosmological setting. We consider the very early universe which is radiation dominated. A novel treatment in our paper is to look for an ``averaged'' cosmological metric probed by radiation particles themselves. Taking their cosmological evolution into account, we derive the modified Friedmann-Robertson-Walker(FRW) equations which is a generalization of the solution presented by Magueijo and Smolin. Based on this phenomenological cosmological model we argue that the spacetime curvature has an upper bound such that the cosmological singularity is absent. These modified $FRW$ equations can be treated as effective equations in the semi-classical framework of quantum gravity and its analogy with the one recently proposed in loop quantum cosmology is also discussed."

http://arxiv.org/abs/gr-qc/0609130
The kinematics of particles moving in rainbow spacetime
Yi Ling, Song He, Hongbao Zhang
5 pages

"The kinematics of particles moving in rainbow spacetime is studied in this paper. In particular the geodesics of a massive particle in rainbow flat spacetime is obtained when the semi-classical effect of its own energy on the background is taken into account. We show that in general the trajectory of a freely falling particle remains unchanged which is still a straight line as in the flat spacetime. The implication to the Unruh effect in rainbow flat spacetime is also discussed."

http://arxiv.org/abs/hep-th/0607228
Renormalization Group Running of Newton's G: The Static Isotropic Case
Herbert W. Hamber, Ruth M. Williams
61 pages, 3 figures
CERN-PH-TH/2006-145

"Corrections are computed to the classical static isotropic solution of general relativity, arising from non-perturbative quantum gravity effects. A slow rise of the effective gravitational coupling with distance is shown to involve a genuinely non-perturbative scale, closely connected with the gravitational vacuum condensate, and thereby, it is argued, related to the observed effective cosmological constant. Several analogies between the proposed vacuum condensate picture of quantum gravitation, and non-perturbative aspects of vacuum condensation in strongly coupled non-abelian gauge theories are developed. In contrast to phenomenological approaches, the underlying functional integral formulation of the theory severely constrains possible scenarios for the renormalization group evolution of couplings. The expected running of Newton's constant $G$ is compared to known vacuum polarization induced effects in QED and QCD. The general analysis is then extended to a set of covariant non-local effective field equations, intended to incorporate the full scale dependence of $G$, and examined in the case of the static isotropic metric. The existence of vacuum solutions to the effective field equations in general severely restricts the possible values of the scaling exponent $\nu$."

 

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http://arxiv.org/abs/gr-qc/0610018
The Asymptotic Safety Scenario in Quantum Gravity -- An Introduction
M. Niedermaier
72 pages, 1 figure. Introduction to the subject based on several talks and conference contributions. An elaboration on the evidence (c) and (d) (see abstract) can be found in a forthcoming `Living Reviews in Relativity' article with M. Reuter

"The asymptotic safety scenario in quantum gravity is reviewed, according to which a renormalizable quantum theory of the gravitational field is feasible which reconciles asymptotically safe couplings with unitarity.All presently known evidence is surveyed: (a) from the 2+\epsilon expansion,(b) from renormalizable higher derivative gravity theories and a `large N' expansion in the number of matter fields, (c) from the 2-Killing vector reduction, and (d) from truncated flow equations for the effective average action. Special emphasis is given to the role of perturbation theory as a guide to `asymptotic safety'. Further it is argued that as a consequence of the scenario the selfinteractions appear two-dimensional in the extreme ultraviolet. Two appendices discuss the distinct roles of the ultraviolet renormalization in perturbation theory and in the flow equation formalism."

Niedermeyer is a collaborator of Martin Reuter.

The next paper (orig. 2004) was significantly expanded and improved for resubmission January 2006. Because it did not show up as a new paper then, I did not notice it. But actually it is highly relevant current work.

http://arxiv.org/abs/math-ph/0407006
Representations of the Weyl Algebra in Quantum Geometry
Christian Fleischhack
67 pages, 1 figure. Changes v1 to v2: 1) Due to inconsistencies (paths vs. edges), detected by Garth Warner in v1: old Sect. 3.2 replaced by new Sects. 3.2 and 3.3, begin of Sect. 3.1 extended; proof of Lemma 3.27, Sect. 3.6, Acknowledgements updated; overall results, however, not affected; 2) comparison with paper by Lewandowski, Okolow, Sahlmann, Thiemann added (pp. 63--65); 3) Footnote 2 and Refs. [12], [21], [23] added; Footnote 9 corrected; 4) Begin of Sect. 6.3 slightly modified

"The Weyl algebra A of continuous functions and exponentiated fluxes, introduced by Ashtekar, Lewandowski and others, in quantum geometry is studied. It is shown that, in the piecewise analytic category, every regular representation of A having a cyclic and diffeomorphism invariant vector, is already unitarily equivalent to the fundamental representation. Additional assumptions concern the dimension of the underlying analytic manifold (at least three), the finite wide triangulizability of surfaces in it to be used for the fluxes and the naturality of the action of diffeomorphisms -- but neither any domain properties of the represented Weyl operators nor the requirement that the diffeomorphisms act by pull-backs. For this, the general behaviour of C*-algebras generated by continuous functions and pull-backs of homeomorphisms, as well as the properties of stratified analytic diffeomorphisms are studied. Additionally, the paper includes also a short and direct proof of the irreducibility of A."

There is an interesting CHART which compares Fleischhack's result with the "LOST" result of Lewandowski, Okolow, Sahlmann, and Theimann. They wrote up their results around the same time, Fleischhack maybe even a little earlier. (!) The similarities and slight differences are interesting. BOTH use stratified diffeomorphisms. It looks like we may need to add those to our familiar language of terminology.

http://arxiv.org/abs/gr-qc/0610017
Physical Diffeomorphisms in Loop Quantum Gravity
Tim Koslowski
25 pages, 2 figures
"We investigate the action of diffeomorphisms in the context of Hamiltonian Gravity. By considering how the diffeomorphism-invariant Hilbert space of Loop Quantum Gravity should be constructed, we formulate a physical principle by demanding, that the gauge-invariant Hilbert space is a completion of gauge- (i.e. diffeomorphism-)orbits of the classical (configuration) variables, explaining which extensions of the group of diffeomorphisms must be implemented in the quantum theory. It turns out, that these are at least a subgroup of the stratified analytic diffeomorphisms. Factoring these stratified diffeomorphisms out, we obtain that the orbits of graphs under this group are just labelled by their knot classes, which in turn form a countable set. Thus, using a physical argument, we construct a separable Hilbert space for diffeomorphism invariant Loop Quantum Gravity, that has a spin-knot basis, which is labelled by a countable set consisting of the combination of knot-classes and spin quantum numbers. It is important to notice, that this set of diffeomorphism leaves the set of piecewise analytic edges invariant, which ensures, that one can construct flux-operators and the associated Weyl-operators. A note on the implications for the treatment of the Gauss- and the Hamilton-constraint of Loop Quantum Gravity concludes our discussion."

I gather Koslowski is maybe 26, teaching astronomy at Würzburg, and just getting started in QG. No previous papers. He gave a Loop Quantum Cosmology talk at a conference last year. He was a participant at Loops '05, but did not give a paper. I hadnt heard of him before now. This first paper is outstanding quality (in my humble estimation). Besides English, Koslowski appears to be literate in latin and greek, and to have contributed to Wikipedia. associate of martin bojowald. watch this guy. oh yeah, stratified diffeomorphisms looks like they might be a real good idea to know about.
http://www.astro.uni-wuerzburg.de/~koslowski/

I think Vilenkin has failed to refute the conjecture called Cosmological Natural Selection (CNS) but there are several interesting things coming from this next paper:

http://arxiv.org/abs/hep-th/0610051
On cosmic natural selection
Alexander Vilenkin
4 pages

"The rate of black hole formation can be increased by increasing the value of the cosmological constant. This falsifies Smolin's conjecture that the values of all constants of nature are adjusted to maximize black hole production."

Instead of falsifying the CNS as Smolin would state it, Vilenkin seems to me to have modified the conjecture so it can be about black holes formed by quantum fluctuation rather than by the gravitational collaps of matter in the form of stars. Smolin's writings about the CNS have not been about black holes formed by quantum fluctuation---a somewhat hypothetical sort of object. As a reminder one form of CNS can be stated as the following challenge.

CONJECTURE: You cannot show a direction in which to change fundamental dimensionless constants such that the abundance of black holes formed by stellar collapse would initially increase. If it dips first and THEN rises, that doesn't count. (i.e. we are at a local max.)

 

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http://arxiv.org/abs/gr-qc/0610023
Uniform discretizations: a new approach for the quantization of totally constrained systems
Miguel Campiglia, Cayetano Di Bartolo, Rodolfo Gambini, Jorge Pullin
17 pages

"We discuss in detail the uniform discretization approach to the quantization of totally constrained theories. This approach allows to construct the continuum theory of interest as a well defined, controlled, limit of well behaved discrete theories. We work out several finite dimensional examples that exhibit behaviors expected to be of importance in the quantization of gravity. We also work out the case of BF theory. At the time of quantization, one can take two points of view. The technique can be used to define, upon taking the continuum limit, the space of physical states of the continuum constrained theory of interest. In particular we show in models that it agrees with the group averaging procedure when the latter exists. The technique can also be used to compute, at the discrete level, conditional probabilities and the introduction of a relational time. Upon taking the continuum limit one can show that one reproduces results obtained by the use of evolving constants, and therefore recover all physical predictions of the continuum theory. This second point of view can also be used as a paradigm to deal with cases where the continuum limit does not exist. There one would have discrete theories that at least at certain scales reproduce the semiclassical properties of the theory of interest. In this way the approach can be viewed as a generalization of the Dirac quantization procedure that can handle situations where the latter fails."


http://arxiv.org/abs/hep-th/0610064
Scale Dependent Metric and Minimal Length in QEG
Martin Reuter, Jan-Markus Schwindt
10 pages, 1 figure, Talk given by M.R. at IRGAC 2006, Barcelona, Spain, July 11-15, 2006

"The possibility of a minimal physical length in quantum gravity is discussed within the asymptotic safety approach. Using a specific mathematical model for length measurements ("COM microscope") it is shown that the spacetimes of Quantum Einstein Gravity (QEG) based upon a special class of renormalization group trajectories are "fuzzy" in the sense that there is a minimal coordinate separation below which two points cannot be resolved."

http://arxiv.org/abs/gr-qc/0610026
Nieh-Yan Invariant and Fermions in Ashtekar-Barbero-Immirzi Formalism
Simone Mercuri
3 pages, proceedings of the XI Marcel Grossmann meeting on Relativistic Astrophysics, July 23-29, 2006, Berlin

"In order to consistently introduce an interaction between gravity and fermions in the Ashtekar-Barbero-Immirzi formalism a non-minimal term is necessary. The non-minimal term together with the Holst modification to the Hilbert-Palatini action reconstruct the Nieh-Yan invariant. The Immirzi parameter does not affect the classical dynamics, which is described by the Einstein-Cartan effective action. "