  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 \rho, only about 0.5% of energy density
is lost to the black hole or, equivalently, to the `dark radiation'
(\Omega_{d,N} \simeq 0.005 at nucleosynthesis). The preceding, unconventional
\rho^2 period can produce up to 5% dark radiation (\Omega_{d,N} <\sim 0.05).
The AdS/CFT correspondence provides an equivalent description of late RSII
cosmology. We show how the AdS/CFT formulation can reproduce the \rho^2
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 \rho^2 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 AdS_5 and string-motivated AdS_5\timesS^5 definitions differ
in their cosmological implications.
