Brane classical and quantum cosmology from an effective action

Abstract

Motivated by the Randall-Sundrum braneworld scenario, we discuss the classical and quantum dynamics of a $(d+1)$-dimensional boundary wall between a pair of $(d+2)$-dimensional topological Schwarzschild-AdS black holes. We assume there are quite general---but not completely arbitrary---matter fields living on the boundary ``brane universe,'' and that its geometry is that of a Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) model. The effective action governing the model in the minisuperspace approximation is derived. We find that the presence of black hole horizons in the bulk gives rise to a complex action for certain classically allowed brane configurations, but that the imaginary contribution plays no role in the equations of motion. Classical and instanton brane trajectories are examined in general and for special cases, and we find a subset of configuration space that is not allowed at the classical or semiclassical level; this subset corresponds to spacelike branes carrying tachyonic matter. The Hamiltonization and Dirac quantization of the model is then performed for the general case; the latter involves the manipulation of the Hamiltonian constraint before it is transformed into an operator that annihilates physical state vectors. The ensuing covariant Wheeler-DeWitt equation is examined at the semiclassical level, and we consider the possible localization of the brane universe's wave function away from the cosmological singularity. This is easier to achieve for branes with low density and/or spherical spatial sections.