The impact of an extra background of relativistic particles on the cosmological parameters derived from the cosmic microwave background

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, ωrel, 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, zeq = ωm>m/ωrel = 3 100+600-400. We show that ωrel is nearly degenerate with the amount of energy in matter, ωm, 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.