Randall–Sundrum II cosmology, AdS/CFT, and the bulk black hole Original Research Article

We analyse the cosmology of a brane world model where a single brane carrying the standard model fields forms the boundary of a 5-dimensional AdS bulk (the Randall–Sundrum II scenario). We focus on the thermal radiation of bulk gravitons, the formation of the bulk black hole, and the holographic AdS/CFT definition of the RSII theory. Our detailed calculation of bulk radiation reduces previous estimates to a phenomenologically acceptable, although potentially visible level. In late cosmology, in which the Friedmann equation depends linearly on the energy density ρ, only about 1% of energy density is lost to the black hole or, equivalently, to the ‘dark radiation’ (Ωd,N≃0.01 at nucleosynthesis). The preceding, unconventional ρ2 period can produce up to 10% dark radiation (Ωd,N≲0.1). The AdS/CFT correspondence provides an equivalent description of late RSII cosmology. We show how the AdS/CFT formulation can reproduce the ρ2 correction to the standard treatment at low matter density. However, the 4-dimensional effective theory of CFT + gravity breaks down due to higher curvature terms for energy densities where ρ2 behaviour in the Friedmann equation is usually predicted. We emphasize that, in going beyond this energy density, the microscopic formulation of the theory becomes essential. For example, the pure AdS5 and string-motivated AdS5×S5 definitions differ in their cosmological implications.