  The maximal symmetry, or Perfect Cosmological Principle(PCP), that prevents
AdS type spaces from degenerating into anti-inflationary collapse is argued to
be unphysical. For example, the simple requirement that brane-bulk models
should be the result of having evolved from even more energetic string
phenomena picks out a preferred time direction.
  We question whether quantum cosmological reasoning can be applied in any
meaningful way to obtain, what are essentially, classical constructs . An
alternative scheme is to more readily accept the PCP and allow the branes to
also become eternal. A perpetually expanding and contracting brane model could
be driven by the presence of charged black holes in the AdS bulk, that
effectively violates the weak-energy condition as singularities are approached.
This can be contrasted with the so-called Ekpyrotic universe which also closely
accepts the PCP. This being broken only by occasional collisions between
branes, that can then simulate a big bang cosmology.
