New observational limits on dark radiation in brane-world cosmology
  A dark radiation term arises as a correction to the energy momentum tensor in
the simplest five-dimensional RS-II brane-world cosmology. In this paper we
revisit the constraints on dark radiation based upon the newest results for
light-element nuclear reaction rates, observed light-element abundances and the
power spectrum of the Cosmic Microwave Background (CMB). Adding dark radiation
during big bang nucleosynthesis alters the Friedmann expansion rate causing the
nuclear reactions to freeze out at a different temperature. This changes the
final light element abundances at the end of BBN. Its influence on the CMB is
to change the effective expansion rate at the surface of last scattering. We
find that our adopted BBN constraints reduce the allowed range for dark
radiation to between $-12.1\%$ and $+6.2\%$ of the ambient background energy
density. Combining this result with fits to the CMB power spectrum, the range
decreases to $-6.0\%$ to $+6.2\%$. Thus, we find, that the ratio of dark
radiation to the background total relativistic mass energy density $\rho_{\rm
DR}/\rho$ is consistent with zero although in the BBN analysis there could be a
slight preference for a negative contribution. However, the BBN constraint
depends strongly upon the adopted primordial helium abundance.
