New observational limits on dark radiation in brane-world cosmology

TL;DR

This paper refines observational limits on dark radiation in brane-world cosmology using recent light-element abundance data and CMB measurements, constraining its contribution to a narrow range around zero.

Contribution

It provides the most stringent observational constraints to date on dark radiation in brane-world models by combining BBN and CMB data.

Findings

Dark radiation is constrained to between -12.1% and +6.2% of background energy density by BBN.

Combined BBN and CMB data restrict dark radiation to between -6.0% and +6.2%.

The ratio of dark radiation to total relativistic energy density is consistent with zero.

Abstract

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.