§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)]