Quantum Gravity in the Southern Cone IV

Abstracts of Quantum Gravity in the Southern Cone IV

Abstract list is subject to change without prior notice. The current state is as of 09/28/2007.
Full list of abstracts & Posters list of abstracts.

Alfaro, Jorge

Monday, Oct 22 | 14:20 - 14:40

Quantum Gravity and Maximum Attainable Velocities in the Standard Model

A main difficulty in the quantization of the gravitational field is the lack of experiments that discriminate among the theories proposed to quantize gravity. Recently we showed that the Standard Model (SM) itself contains tiny Lorentz invariance violation (LIV) terms coming from QG. All terms depend on one arbitrary parameter a that set the scale of QG effects. In this talk we review the LIV for mesons nucleons and leptons and apply it to study several effects, including the GZK anomaly.

Anabalón, Andrés

Tuesday, Oct 23 | 16:20 - 16:40

The Universe as a topological defect

Four-dimensional Einstein's General Relativity is shown to arise from a gauge theory for the conformal group, SO(4,2). The theory is constructed from a topological dimensional reduction of the six-dimensional Euler density integrated over a manifold with a four-dimensional topological defect. The resulting action is a four-dimensional theory defined by a gauged Wess Zumino Witten term. An ansatz is found which reduces the full set of field equations to the ones of Einstein's General Relativity; the same ansatz when replaced in the action reduces the gauged WZW term to the Einstein-Hilbert action.

Ashtekar, Abhay

Monday, Oct 22 | 9:00 - 9:50

Quantum Extensions of Classically Singular Space-times

By now there are several examples in which effects of quantum geometry became important, dominate the Planck regime and resolve classical singularities. The resulting quantum space-times are typically significantly larger than the original classical space-times. In simple examples, the physics of these quantum extensions has shed considerable light on issues such as the quantum nature of the big-bang and information loss puzzle. I will present a few examples to illustrate this growing area.

Bañados, Máximo

Wednesday, Oct 25 | 11:10 - 12:00

Eddington Action, Dark Matter and Dark Energy

Motivated by a formulation of gravity near a state with zero metric, we consider general relativity coupled to Eddington's action. We show that this fluid behaves as dark energy at large scales and as dark matter at smaller scales.

Berenstein, David

Tuesday, Oct 23 | 10:20 - 11:10

Emergent geometry: the wave function of the universe in the AdS/CFT context

I will explain some recent developments regarding gravity on AdS via a dual conformal field theory description. In particular, I will show how the dynamics of the conformal field theory at strong coupling localize on a particular set of prefeerd configurations. If one quantizes these special configurations one can see that different choices of wave functions correspond to different spacetime geometries in the AdS space. I will also describe some simulations of these wave functions and prospects for computer simulations of quantum gravitational physics.

Bojowald, Martin

Thursday, Oct 25 | 9:00 - 9:50

The dynamics of loop quantum gravity and its effective descriptions

Loop quantum gravity suggests several characteristic properties for the dynamics of a quantum space-time, which may all have important physical implications. Recent developments have improved the understanding of effective equations applicable to background independent quantum gravity, which allows one to shed light on possible physical effects taking into account issues such as the physical inner product and anomaly-freedom. Physical applications include cosmology and the question of whether Lorentz-violating effects are implied by quantum gravity.

Cuervo, William

Tuesday, Oct 23 | 14:00 - 14:20

Connection formulation of classical closed inhomogeneous cosmologies

SU(2) compact principal bundles acted upon by a 2d-compact lie group are studied as models of closed inhomogeneoous cosmologies in the context of the Ashtekar-Barbero formulation of gravity.

Freidel, Laurent

Monday, Oct 22 | 10:20 - 11:10

Spin Foam models of 4d gravity

In this talk i will review the construction based on a spacetime approach of 4 dimensional quantum gravity amplitudes associated with loop quantum gravity, i-e spin foam models. I will present recent developpements in this field that resolve some of the key difficulties associated with previous models and allows us to incorporate the Immirzi parameter and shed new light on the relationship between the canonical and the spin foam framework.

Garat, Alcides

Wednesday, Oct 24 | 16:00 - 16:20

Tetrads in Geometrodynamics

A new tetrad is introduced within the framework of geometrodynamics for non-null electromagnetic fields. This tetrad diagonalizes the electromagnetic stress-energy tensor and allows for maximum simplification of the expression of the electromagnetic field. The Einstein-Maxwell equations will also be simplified. The tetrad that diagonalizes the stress-energy tensor will be written in terms of the standard extremal field, and two other arbitrary vector fields. It will be proved that these two extra vector fields are available freedom that we have in the construction of a general tetrad for non-null electromagnetic fields. The fact that in geometrodynamics Maxwell's equations have zero source terms, introduces the existence of two potential vector fields, not independent from each other, natural candidates for a particular and explicit choice or example of these two vector fields. In this particular example, an unexpected question will arise at this point. If our tetrad involves in its construction the potential vectors, how is the tetrad going to be affected by electromagnetic gauge transformations. The geometry of electromagnetic fields defines at every point in spacetime two orthogonal planes related to the symmetries of the stress-energy tensor. Gauge transformations, that leave invariant the electromagnetic field, will generate proper and improper tetrad Lorentz transformations on one of the planes. Gauge transformations, that leave invariant the dual of the electromagnetic field, will generate tetrad spatial rotations on the other plane. In this way we find for the Abelian case a relationship through group isomorphisms between the internal local gauge transformations and spacetime local tetrad transformations.

A Stringy Alternative to Inflation: The Cosmological Slingshot Scenario

We propose a cosmological model, alternative to the standard inflationary paradigm, where all problems that afflict standard non-inflationary cosmology are naturally solved. In this model, the Universe is a wandering brane moving, with non-zero angular momentum, in a warped throat on a Calabi-Yau space. It is assumed that mirage effects drive the cosmic evolution at early time. The result is a bouncing cosmology without cosmic singularity as experienced by an observer living on the brane. Density perturbations are calculated in our model and we find a slightly red spectral index, in compatibility with WMAP data.

Guendelman, Eduardo

Tuesday, Oct 23 | 14:20 - 14:40

Child Universe Production and its impact on Quantum Gravity

Inflationary cosmology proposes that in the early universe a false vacuum, that is a phase of matter with equation of state pressure = - energy density existed. This produces an exponential expansion of the early universe, solving many important cosmological problems, like the flatness problem, the horizon problem, etc. In my talk I will deal with the consideration of a bubble of false vacuum that could be formed in our present Universe. This leads to interesting questions: The false vacuum bubble has an energy density which is higher than the sourrounding space. Because of this, although the space inside the bubble can undergo an exponential expansion, the pressure difference with respect to the outside implies the bubble cannot displace the external space. What is obtained instead is the child universe solution, where a new universe is created which disconnects from the exterior region. These solutions contain a wormhole region and also a white hole singularity, a sort of local big bang singularity. Different scenarios that avoid the initial singularity will be discussed. These could be quantum, like tunneling from a particle like , classically stable configuration to other classical processes, but that circumvent relevant singularity theorems. The possibility of child universe production in particle accelerators will be discussed. Finally, based on simple models where creation of child universes can be achieved at arbitrarily low energy cost when transplankian energy densities are involved, I will argue that child universes production is unsupressed at transplanckian scales and the relevance of this for quantum gravity will be discussed.

Girotti, Horacio

Wednesday, Oct 24 | 15:00 - 15:20

On noncommutative quantum mechanics

This paper is dedicated to present model independent results for non-commutative quantum mechanics. Within the operator framework we first determine sufficient conditions to be fulfilled by the potential in order to secure the convergence of the Born series and, then, show that for this kind of potentials the theory is unitary. Afterwards, we focus on the functional quantization of noncommutative systems. The compatibility between the operator and the functional approaches for this kind of models is established. We also study the phase space path integral description of the dynamics of noncommutative systems when this integral is defined through the time slicing procedure. As known, this definition brings into play a new real parameter related with the arbitrariness of the point on the slice to be chosen for defining the path integral. Uniqueness of the functional formulation demands that the dependence on the just mentioned parameter should disappear when performing the limit of letting to zero the size of the time slice. This turns out to be the case but the proof is considerably more involved that in the commutative case. We exhibit in detail the interplay between the slice arbitrariness and the non-commutativity.

Maldacena, Juan M.

Tuesday, Oct 23 | 9:00 - 9:50

Anti-de-Sitter and field theory scattering amplitudes

We discuss a procedure for computing field theory scattering amplitudes using its gravity dual. One can compute scattering amplitudes in N=4 super Yang Mills at strong coupling by finding minimal surfaces in AdS. We will also discuss some aspects of Jet physics in conformal theories.

Mattingly, David

Monday, Oct 22 | 11:10 - 12:00

Current status of quantum gravity phenomenology

The search for small deviations from local Poincare invariance in the low energy physics of particles in vacuum is one of the few ways we might experimentally glimpse quantum gravity. If a quantum gravity theory violates Poincare symmetry at the Planck scale, we argue that given current constraints either new physics must also be present below 100 TeV or quantum gravity corrections at TeV scales do not obey the usual rules of field theory. Upcoming laboratory tests and the recent MAGIC and Auger results are sensitive to existing ideas with either of these possibilites and have already started to limit some severely. If these models are ruled out in the next few years, we show that after there is likely an experimental desert for searches of this kind.

Miskovic, Olivera

Wednesday, Oct 24 | 14:00 - 14:20

Regularization of Lovelock Gravity

We discuss two regularization procedures for gravity theory in asymptotically anti-de Sitter (AdS) spacetimes, in arbitrary dimension. We consider General Relativity and its natural extension to higher dimensions given by Lovelock gravity, which action is a linear combination of all dimensionally continued lower-dimensional Euler densities, so that it produces field equations with at most second-order derivatives in the metric.

A standard holographic renormalization corresponds to the Dirichlet counterterm method. In Einstein-Hilbert gravity, the procedure consists in addition of a Gibbons-Hawking term to the bulk action which then becomes stationary for Dirichlet boundary conditions on the boundary metric. The regularization of this action is performed by adding boundary terms which are local functionals of the boundary metric. In the Lovelock gravity case, a generalized Gibbons-Hawking term can be obtained following the Myers' procedure. This algorithm is proved to be equivalent to the Hamiltonian formulation for a radial foliation of spacetime. However, a closed expression for the Dirichlet counterterms does not exist for a generic Lovelock gravity.

In the alternative regularization method, the bulk gravity action is supplemented by a boundary term that explicitly depends on the extrinsic curvature, and whose explicit form is independent on the particular Lovelock theory considered. Such a non-Dirichlet action has an extremum based on boundary conditions imposed on the extrinsic curvature.

We compare these two regularization schemes in Fefferman-Graham coordinates adapted to asymptotically AdS spacetimes. We conclude that for a three-dimensional AdS gravity action, the extrinsic curvature-dependent boundary term that is one half the Gibbons-Hawking term, differs from the standard one (Gibbons-Hawking term plus Dirichlet counterterms) by a topological invariant; thus, these two regularization procedures are equivalent.

In case of Dimensionally Continued AdS Gravity (special case of Lovelock gravity that corresponds to Chern-Simons-AdS in odd and Born-Infeld-AdS in even dimensions), we perform the first explicit comparison between both regularization prescriptions in higher dimensions. We show that two regularization methods are consistent, since they are equivalent, up to a regularization scheme-dependent finite counterterms which do no modify the Weyl anomaly.

Nicolai, Hermann

Wednesday, Oct 24 | 9:00 - 9:50

E10 and K(E10): re-inventing M theory?

Recent work on symmetry structures of maximal supergravity theories has revealed a remarkable link between the dynamics of these theories and geodesic motion on the coset manifold E10/K(E10). In this talk I would like to review these developments at a somewhat introductory level, and argue that E10 has all the requisite properties to unify the various duality symmetries so far encountered in string and M theory.

Parisi, María Florencia

Monday, Oct 22 | 16:00 - 16:20

Light Propagation on Quantum Curved Spacetime and Back reaction effects

We discuss the effects Loop Quantum Gravity might have on the propagation of light and, in particular, the appearance of Lorentz invariance breaking terms, by studying the electromagnetic field equations on an arbitrary quantum curved background in the semiclassical approximation. The effective interaction hamiltonian for the Maxwell and gravitational fields is obtained and the corresponding field equations, which can be expressed as a modified wave equation for the Maxwell potential, are derived. We use these results to analyze electromagnetic wave propagation on a quantum Robertson-Walker space time and show that Lorentz Invariance is not preserved. The formalism developed can be applied to the case where back reaction effects on the metric due to the electromagnetic field are taken into account, leading to non covariant field equations.

Petrov, Albert

Monday, Oct 22 | 16:20 - 16:40

Lorentz violation in the linearized gravity

We study some physical consequences of the introduction of a Lorentz-violating modification term in the linearized gravity, which leads to modified dispersion relations for gravitational waves in the vacuum. We discuss two possible mechanisms for the induction of such a term in the Lagrangian. First, it is generated at the quantum level by a Lorentz-breaking coupling of the gravity field to a spinor field. Second, it appears as consequence of a particular modification of the Poisson algebra of the canonical variables, in the spirit of the so-called "noncommutative fields approach".

Porto, Rafael

Thursday, Oct 25 | 10:20 - 11:10

Riding with the King in a Relational Universe

In this talk we discuss the problem of time in generally covariant quantum theories within the Relational approach. In particular we focus on the different interpretations, fundamental limits on spacetime measurements, locality, unitarity vs decoherence, black hole information paradox, entanglement, quantum computers, the measurement problem and all that Jazz.
The ideas presented here are vastly resulting from (many) discussions/collaborations with Rodolfo Gambini during the last years, and this talk is dedicated to him on the occasion of his 60th birthday.

Pullin, Jorge

Thursday, Oct 25 | 10:20 - 11:10

Gambini Fest: "The physics of Rodolfo"

We will review some of Rodolfo's papers, particularly earlier ones, and their impact.

Reisenberger, Michael

Monday, Oct 22 | 14:40 - 15:00

Canonical gravity with free null initial data

Free (unconstrained) gravitational initial data variables are known for initial hypersurfaces consisting of two intersecting null hypersurfaces. Recently the Poisson bracket on functions of such data has been obtained. This opens the prospect of a constraint free canonical formulation of general relativity. The reasons for looking for such a formulation, and some of its features will be discussed.

Reyes, Carlos

Monday, Oct 22 | 16:40 - 17:00

Lorentz and P invariance in higher-derivative theories and loop quantum gravity

We show that the coupling of higher order time derivative terms to the Lagrangian may lead to an apparent violation of Lorentz and parity symmetry in an effective Hamiltonian which can be constructed perturbatively by reducing the phase space.
In particular, we exhibit a relation between the higher time derivative extension of the Chern-Pontryagin theory and the Lorentz violating terms contained in the modified electrodynamics of loop quantum gravity.
It is suggested that Lorentz invariance may not be necessarily broken in loop quantum gravity phenomenology. We give some extensions of previous granular models including fermions and focus on a possible canonical redefinition of gauge variables which bring the theory to the usual.

Rivelles, Víctor

Wednesday Oct, 24 | 10:20 - 11:10

The S-matrix of the Faddeev-Reshetikhin Model

The Faddeev-Reshetikhin model appears as a certain limit of strings in AdS⁵×S₅. We discuss the several techniques used to obtain its two particle S-matrix and possible generalizations. We find a family of PT symmetric Hamiltonians which are not hermitian but have a unitary S-matrix.

Rodrigues, Davi

Wednesday, Oct 24 | 14:20 - 14:40

Anisotropic Cosmological Constant and the CMB Quadrupole Anomaly

There are evidences that the cosmic microwave background (CMB) large-angle anomalies imply a departure of statistical isotropy and hence of the standard cosmological model. We propose a LCDM model extension whose dark energy component preserves its nondynamical character but wield anisotropic vacuum pressure. Exact solutions for the cosmological scale factors are presented, upper bounds for the deformation parameter are evaluated and its value is estimated considering the elliptical universe proposal to solve the quadrupole anomaly. This model can be constructed from a Bianchi I cosmology with cosmological constant from two different ways: i) a straightforward anisotropic modification of the vacuum pressure consistently with energy-momentum conservation; ii) a Poisson structure deformation between canonical momenta such that the dynamics remain invariant under scale factors rescalings.

arXiv:0708.1168v1 [astro-ph], submitted to PRD.

Spradlin, Marcus

Tuesday, Oct 23 | 11:10 - 12:00

AdS/CFT and Multiloop Gluon Amplitudes

Some recent investigations into the structure of the AdS/CFT correspondence rely on input from increasingly complicated technical calculations. Two related examples in planar N=4 super Yang-Mills theory include testing consequences of integrability and exploring iteration relations amongst multiloop gluon scattering amplitudes. I will review the latest developments in these areas and the methods used to carry out relevant calculations through four loops.

Swain, John

Wednesday, Oct 24 | 15:00 - 15:20

Nonperturbative Fractal Corrections to Propagators due to Quantum Gravity

The simple pole structure of the propagator for a charged point particle is transformed to a branch point by radiative corrections, exhibiting a nontrivial fractal scaling behaviour in the infrared. There is a physical interpretation of this non perturbative effect in terms of an effective nonlocality due to an infinite number of infinitely soft photon exchanges. There is also an interpretation in terms of the roughening of fractal quantum mechanical paths in proper time. We review these results and extend them to include gravity, which works to reduce the effective fluctuations, that is, gravity tends to soften the effects of electromagnetic quantum fluctuations.
Implications of this result are discussed, as well as its connection to the possible emergence of classical mechanics due to gravity in a quantum mechanical system. Suggestions are made that there may be non perturbative quantum gravity effects which are not greatly suppressed by powers of the Planck mass.

Taveras, Victor

Tuesday, Oct 23 | 15:00 - 15:20

Quantum Extension of Spacetime: CGHS Model and the Information Problem

We re-examine the CGHS model from a nonperturbative Hamiltonian perspective. We present a framework for describing the quantum geometry of spacetime. In this framework a pure quantum state on left-scri-minus evolves to a pure state on right-scri-plus, thus there is no information loss. A truncation to 1st order (mean field approximation) recovers the Hawking effect and there is still a singularity at this approximation even though the quantum geometry is perfectly fine.
A 2nd order truncation yields semiclassical equations including the backreaction. An asymptotic analysis of the metric near right-scri-plus yields the back-reaction corrected semiclassical metric. Summarily, although the truncated theories yield a consistent semiclassical picture, there is no information loss in the full quantum theory since the quantum spacetime is larger than the classically singular spacetime.

Toppan, Francesco

Tuesday, Oct 23 | 14:40 - 15:00

CS AdS₅ supergravity in a RS background

Chern-Simons AdS supergravity theories are gauge theories for the super-AdS group. These theories possess a fermionic symmetry which differs from standard supersymmetry. In this paper, we study five-dimensional Chern-Simons AdS supergravity in a Randall-Sundrum scenario with two Minkowski 3-branes. After making modifications to the D = 5 Chern-Simons AdS supergravity action and fermionic symmetry transformations, we obtain a Z_2-invariant total action S = \tilde{S}_bulk + S_brane and fermionic transformations \tilde{\delta}_\epsilon. While \tilde{\delta}_\epsilon \tilde{S}_bulk = 0, the fermionic symmetry is broken by S_brane. Our total action reduces to the original Randall-Sundrum model when \tilde{S}_bulk is restricted to its gravitational sector. We solve the Killing spinor equations for a bosonic configuration with vanishing su(N) and u(1) gauge fields.

Based on arXiv:0705.4082v1 [hep-th]

Urrutia, Luis

Monday, Oct 22 | 14:40 - 15:00

Lorentz Violating Electrodynamics Revisited

Starting with the Gambini-Pullin model we review some of the phenomenological approaches to incorporate Planck-scale corrections to electrodynamics, together with the stringent bounds upon the corresponding parameters imposed by current experiments and observations. A summary of the description of synchrotron radiation in the Myers-Pospelov model is also presented, together with a generalized construction of a modified non-local, birrefringent and dispersive electrodynamics incorporating dimension five and six operators.

Vergara, Jose David

Wednesday, Oct 24 | 14:40 - 15:00

Noncommutative spaces, the quantum of time, and Lorentz symmetry

We introduce three space-times that are discrete in time and compatible with the Lorentz symmetry.We show that these spaces are not commutative, with commutation relations similar to the relations of the Snyder and Yang spaces. Furthermore, using a reparametrized relativistic particle we obtain a realization of the Snyder type spaces and we construct an action for them.

Zanelli, Jorge

Thursday Oct 24 | 11:10 - 12:00

Chern-Simons Gravity and the Universe as a Topological Defect

Chern-Simons theories for the AdS gauge group in D=2n-1 dimensions are reviewed. They can be viewed as describing the boundary degrees of freedom for a 2n-dimensional topological field theory in a bounded region. If instead of being bounded, the region has a topological defect of co-dimension 2, the action describes a (2n-2)-dimensional gravitational effective theory. Starting with a topological action in 6D, the execise yields 4D Einstein gravity at the defect.