§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)]