Departamento de Gravitación y Teoría de Campos

                          Seminarios 2013

  

                          Instituto de Ciencias Nucleares, UNAM
[ ICN-UNAM]

Jueves 19 de septiembre, a las 17:00 hrs en la Sala de Seminarios del ICN

Roberto Sussman (ICN, UNAM)

The notion of "Gravitational Entropy" in General Relativity

I examine in this talk a theoretical proposal (by Clifton, Ellis and Tavakol) to define a "Gravitational Entropy" in General Relativity, as an independent concept from the entropy of the sources of the gravitational field (thermal fluids or black holes). This proposal is based on an "effective" construction based on the Weyl tensor and reproduces the Hawking-Beckenstein area formula when applied to black holes. By applying this proposal to LTB and Szekeres dust models with zero cosmological constant, I find that the gravitational entropy production is positive if there exists a negative correlation between fluctuations of the density and Hubble expansion. This negative correlation holds for generic expanding and collapsing models. It also reveals that inhomogeneity renders the expansion/collapse evolution as an irreversible process. Another proposal to define a "Gravitational Entropy", based on Information Theory (Hosoya and Buchert), yields nearly the same results for LTB and Szekeres models. This suggests that the concept of a Gravitational Entropy is theoretically robust, as two very different and unrelated proposals yield the same conditions for entropy production. Qualitative arguments suggest that a nonzero cosmological constant and/or dark energy type sources do not affect these results, though "ghost" type source would likely be ruled out.