Does brane cosmology have realistic principles?

Abstract

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.