Limits on Brane-World and Particle Dark Radiation from Big Bang
  Nucleosynthesis and the CMB
  The term dark radiation is used both to describe a noninteracting neutrino
species and as a correction to the Friedmann Equation in the simplest
five-dimensional RS-II brane-world cosmology. In this paper we consider the
constraints on both meanings of 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 (BBN) 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 the BBN constraint reduces the allowed range
for both types of dark radiation at 10 Mev to between $-12.1\%$ and $+6.2\%$ of
the {\bf total} background energy density at 10 Mev. Combining this result with
fits to the CMB power spectrum, produces different results for particle vs.
brane-world dark radiation. In the brane-world, 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 there remains a very slight preference for a positive (rather
than negative) contribution.
