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 _ { 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 .