Talk detail

Participant	Mercati, Flavio
Institution	Perimeter Institute for Theoretical Physics - 31 Caroline St N - Waterloo - Ontario - Canada

Session	ES2	Accepted	Yes	Order	7	Time	16:50	20'
Talk	Oral abstract	Title	On the Typicality of Universes
Coauthors	Julian Barbour, Tim Koslowski
Abstract	The universal validity of the second law of thermodynamics is widely attributed to a finely tuned initial condition of the universe. This creates a problem: why is the universe atypical? I argue that the problem is an artefact created by inappropriately apply the concept of entropy - which applies to subsystems - to the whole universe. Studying the Newtonian N-body model of a spatially closed universe (on which my PRL paper 113.181101) was based), I introduce a Gibbons-Hawking-Stewart measure on the space of solutions, which is finite and natural, and overcomes the limitations of current attempts at defining such a measure. This allows me to define the notion of `Entaxy', measuring how typical a state of the universe is, and to develop a theory of the typicality of solutions. It suggests that our universe is typical in that we find ourselves in an expanding universe in which the second law of thermodynamics holds in subsystems stabilized by nongravitational forces and all arrows of time point in the same direction.
Pdf file

Session	AT3	Accepted	Yes	Order	4	Time	15:45	15'
Talk	Oral abstract	Title	Thin shell collapse in CMC/maximal slicing and shape dynamics
Coauthors	Henrique Gomes, Tim Koslowski, Andrea Napoletano
Abstract	I study the gravitational collapse of (massive or null) thin shells of dust in the ADM Hamiltonian formalism, in a particular foliation, namely, Constant-Mean-extrinsic-Curvature in the case of a spatially compact universe, and its analogue in the case of asymptotically flat space: maximal slicing. I obtain exact solutions to Einstein's equations at the nonlinear level, which take fully into account the backreaction of matter on geometry. My result is interesting because, on top of providing an exact solution of GR, it also represents a solution of the newly discovered theory of Shape Dynamics. This theory is classically equivalent to General Relativity but highlights a different (dual) symmetry to refoliation invariance: spatial Weyl invariance. For this reason Shape Dynamics is expected to differ at the quantum level from the standard covariant quantization schemes of GR, and suggests that the fundamental degrees of freedom of GR are spatial conformal and diffeorphism invariant, not 4D diffeo invariant.
Pdf file

Session	QG2	Accepted	Yes	Order	4	Time	16:30	30'
Talk	Oral abstract	Title	Gravitational collapse in Shape Dynamics
Coauthors	Henrique Gomes, Tim Koslowski, Andrea Napoletano
Abstract	I study the gravitational collapse of (massive or null) thin shells of dust in the ADM Hamiltonian formalism, in a particular foliation, namely, Constant-Mean-extrinsic-Curvature in the case of a spatially compact universe, and its analogue in the case of asymptotically flat space: maximal slicing. I obtain exact solutions to Einstein's equations at the nonlinear level, which take fully into account the backreaction of matter on geometry. My result is interesting because, on top of providing an exact solution of GR, it also represents a solution of the newly discovered theory of Shape Dynamics. This theory is classically equivalent to General Relativity but highlights a different (dual) symmetry to refoliation invariance: spatial Weyl invariance. For this reason Shape Dynamics is expected to differ at the quantum level from the standard covariant quantization schemes of GR, and suggests that the fundamental degrees of freedom of GR are spatial conformal and diffeorphism invariant, not 4D diffeo invariant.
Pdf file