Marco Bruni                                  GGI Florence, October 2006
§Isotropy and singularities:
In GR, the observed isotropic universe cannot emerge out of generic initial conditions
Penrose conjecture: initial state isotropic=low gravitational entropy (Weyl tensor)
Inflation great for perturbations, but does need fairly homogeneous isotropic initial conditions to start (low energy anyway)
In the brane scenario, the effective energy momentum tensor gives r    terms in the equations, making the singularity isotropic, as shown by many for homogeneus models, and by a perturbative analysis [Dunsby, Goheer, MB, Coley, PRD 69, 101303 (2004); Goheer, Dunsby, Coley, MB, PRD 70, 123517 (2004)]
w>1 needed in ekpyrotic/cyclic and pre-big bang for isotropic singularity [Erickson, Wesley, Stenhardt, Turok, PRD 69, 063514 (2004)]


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