Quantum Gravity in the Southern Cone IV

Rafael A. Porto, Department of Physics, University of California, Santa Barbara, CA 93106
rporto-at-physics.ucsb.edu

The fourth installment of the, by now traditional, `Quantum Gravity in the Southern Cone' meeting took place in Punta del Este, Uruguay, as it did eleven years ago when this series first launched. The purpose of this workshop was twofold. Firstly as was expected, it allowed scientists from all over the world to gather together, present new ideas and discuss the many different approaches to a quantum theory of gravity. Secondly, it provided an opportunity to celebrate the physics of Rodolfo Gambini on the occasion of his 60th birthday (although he is 61 by now).

The workshop had over 100 participants, including plenary talks by Abhay
Ashtekar, David Berenstein, Martin Bojowald, Laurent Freidel, Juan Maldacena,
David Mattingly, Herman Nicolai, Rafael Porto and Marcus Spradlin. There were
afternoon sessions with talks by Jorge Alfaro, Andres Anabalon, William
Cuervo, Alcides Garat, Horacio Girotti, Eduardo Guendelman, Olivera Miskovic,
Maria Parisi, Albert Petrov, Michael Reisenberger, Carlos Reyes, Davi
Rodrigues, Jose Vergara, Victor Taveras, Francesco Toppan and Luis Urrutia.
Also Jorge Pullin reviewed Rodolfo's early work and its impact on a talk
entitled `The Physics of Rodolfo'. In addition several participants
contributed posters. The program and links to the talks can be found
at
http://qgsciv.fisica.edu.uy

The plenary talks covered different areas although we had three talks on the
ADS/CFT (Anti-De Sitter/ Conformal Field Theory) correspondence and another
three with a Loop Quantum Gravity (LQG) taste. Juan Maldacena gave an
introduction to the ADS/CFT correspondence, the new developments and attempts
to understand strongly coupled theories, such as QCD, as well as gluon
scattering amplitudes. Marcus Spradlin elaborated on multiloop gluon
amplitudes. Maldacena made two important remarks about ADS/CFT, namely
Lorentz Invariance (LI) is preserved by quantization (in the sense that the
dual QFT preserves conformal symmetry) and also on the issue of background
independence, where it was pointed out that there is a sum over all possible
geometries with ADS boundary conditions. One may argue on the necessity of a DS/CFT correspondence, or trying to implement flat boundary conditions. This however seems far from understood. During the question session the issue of whether ADS/CFT applies to QCD was brought up. Maldacena emphasized that the duality provides a sort of Ising Model as in Condensed Matter physics. One may wonder then whether we can do any better, and whether Super Yang-Mills theory and QCD are in the same `universality class'.
Aiming in the opposite direction to the ADS/CFT conjecture, David Berenstein talked about emergent geometry and the wave function of the universe. His talk centered on how to perform a strong coupling expansion by reducing the problem to spherically symmetric configurations in a matrix model of commuting hermitian matrices.

In the LQG corner, Abhay Ashtekar spoke about how quantum geometry extends
its life past the classical singularities of GR. Ashtekar concentrated on the Big Bang, discussed Loop Quantum Cosmology (LQC), and in particular the FRW model. He also introduced the idea of ``New Quantum Mechanics" , and that the
holonomies are the right objects to be quantized rather than the connection. Given the so called `Bohr compactification', LQC allows for inequivalent representations (bypassing the Von Neumann theorem), and therefore avoidance of singularities unlike in the Wheeler-DeWitt approach. In LQC (relational) evolution is deterministic across the classical big bang. The main open issue in LQC is whether it can be systematically derived from full LQG due to the `gauge fixing' imposed along the way. Martin Bojowald covered LQC as well. Bojowald also talked about the effective description, where expectation values are obtained in a systematic fashion by solving a set of effective equations in the semiclassical regime, though accounting for `quantum corrections', and also its application to cosmology. Also on the LQG side Laurent Freidel discussed spin foam models for 4d gravity. Freidel talked about the topological BF theory and its quantization, how to implement the simplicity constraints of gravity, and how the resulting spin foam model resembles the kinematical states (spin networks) in LQG. Yet is there to understand the link with the dynamics.

Among the plenary speakers we also had Hermann Nicolai talking about `Re-inventing M theory'. Nicolai talked about , and how it incorporates many relations between maximal supergravity theories. Victor Rivelles talked about the S-matrix of the Faddeev-Reshetikhin model and its connection with the ADS/CFT correspondence. Jorge Zanelli, who spoke about Chern-Simons gravity and the Universe as a topological defect, and Maximo Banados on GR with as the `ground state', where diffeomorphisms act as isometries. Banados elaborated on a would-be theory of gravity which incorporates the `no metric' condition and argued this theory could provide candidates for dark energy and dark matter. David Mattingly talked about violations of LI (LIV) and QG phenomenology. Experimental constraints on LIV are very tight. Also, even if one argues that violations come from higher dimensional operators, operators can be generated by radiative corrections. That introduces fine tunning, which one may argue could be avoided if CPT is unbroken, and SUSY is implemented, though broken at the TeV scale. From a QG point of view, LIV are highly suppressed, and that represents a strong constraint on the theories.

Summarizing, the meeting was very fruitful and interactive, with many discussions and exchange of ideas taking place. The next meeting will take place in Brasil.