Randall-Sundrum II Cosmology, AdS/CFT, and the Bulk Black Hole

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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 0.5% of energy density is lost to the black hole or, equivalently, to the `dark radiation' (Ωd,ₙ ≃ 0.005 at nucleosynthesis). The preceding, unconventional ρ² period can produce up to 5% dark radiation (Ωd,ₙ < 0.05). 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 AdS₅⁵ definitions differ in their cosmological implications.