Delayed Recombination and Standard Rulers

TL;DR

This paper investigates how delayed recombination, due to additional radiation sources, affects the accuracy of dark energy measurements from BAO surveys, finding the impact small but potentially biasing results if ignored.

Contribution

It quantifies the effect of non-standard recombination on dark energy parameter estimation and shows that including an extra parameter mitigates bias without degrading precision.

Findings

Delayed recombination can bias dark energy estimates if ignored.

Adding a parameter for recombination deviations prevents bias.

Impact on dark energy parameters is small but non-negligible.

Abstract

Measurements of Baryonic Acoustic Oscillations in galaxy surveys have been recognized as a powerful tool for constraining dark energy. However, this method relies on the knowledge of the size of the acoustic horizon at recombination derived from Cosmic Microwave Background Anisotropy measurements. This estimate is typically derived assuming a standard recombination scheme; additional radiation sources can delay recombination altering the cosmic ionization history and the cosmological inferences drawn from CMB and BAO data. In this paper we quantify the effect of delayed recombination on the determination of dark energy parameters from future BAO surveys such as BOSS and WFMOS. We find the impact to be small but still not negligible. In particular, if recombination is non-standard (to a level still allowed by CMB data), but this is ignored, future surveys may incorrectly suggest the presence of a redshift dependent dark energy component. On the other hand, in the case of delayed recombination, adding to the analysis one extra parameter describing deviations from standard recombination, does not significantly degrade the error-bars on dark energy parameters and yields unbiased estimates.