The Impact of an Extra Background of Relativistic Particles on the Cosmological Parameters derived from Microwave Background Anisotropies
R. Bowen, S. H. Hansen, A. Melchiorri, J. Silk, R. Trotta
TL;DR
This work investigates how relaxing the assumption of a fixed relativistic energy density $\omega_{rel}$ affects cosmological parameter inferences from CMB anisotropies. By treating $\omega_{rel}$ as a free parameter linked to $N_{eff}$, the authors show a strong degeneracy with $\omega_m$ and quantify how this degeneracy propagates to $z_{eq}$ and $n_s$, with current data unable to tightly constrain $N_{eff}$. Forecasts indicate that Planck's small-scale measurements can break the degeneracy and constrain $\Delta N_{eff}$ to about $0.24$, whereas MAP would yield much weaker constraints. These results highlight the importance of choosing parameterizations like $z_{eq}$ and the shift parameter ${\mathcal{R}}$ to obtain robust CMB-based inferences and demonstrate Planck's potential to tightly measure the relativistic energy density in the early universe.
Abstract
Recent estimates of cosmological parameters derived from Cosmic Microwave Background (CMB) anisotropies are based on the assumption that we know the precise amount of energy density in relativistic particles in the universe, $ω_{rel}$, at all times. There are, however, many possible mechanisms that can undermine this assumption. In this paper we investigate the effect that removing this assumption has on the determination of the various cosmological parameters. We obtain fairly general bounds on the redshift of equality, $z_{eq}= ω_{m}/ω_{rel}=3100_{-400}^{+600}$. We show that $ω_{rel}$ is nearly degenerate with the amount of energy in matter, $ω_m$, and that its inclusion in CMB parameter estimation also affects the present constraints on other parameters such as the curvature or the scalar spectral index of primordial fluctuations. This degeneracy has the effect of limiting the precision of parameter estimation from the MAP satellite, but it can be broken by measurements on smaller scales such as those provided by the Planck satellite mission.