Metals at the surface of last scatter

TL;DR

This paper investigates whether primordial metals could influence CMB observations and if current or upcoming measurements can constrain their abundance, concluding that Planck data won't improve existing limits on primordial metallicity.

Contribution

The study analyzes the impact of primordial metals on CMB recombination and shows that Planck's measurements are unlikely to tighten existing metallicity constraints.

Findings

Primordial metals affect CMB via three mechanisms.

Detectable effects require metal abundance above a few hundredths to tenths of solar.

Planck cannot improve constraints beyond current Lyman-alpha forest and halo star limits.

Abstract

Standard big-bang nucleosynthesis (BBN) predicts only a trace abundance of lithium and no heavier elements, but some alternatives predict a nonzero primordial metallicity. Here we explore whether CMB measurements may set useful constraints to the primordial metallicity and/or whether the standard CMB calculations are robust, within the tolerance of forthcoming CMB maps, to the possibility of primordial metals. Metals would affect the recombination history (and thus CMB power spectra) in three ways: (1) Lyman-alpha photons can be removed (and recombination thus accelerated) by photoionizing metals. (2) The Bowen resonance-fluorescence mechanism may degrade Lyman-beta photons and thus enhance the Lyman-beta escape probability and speed up recombination. (3) Metals could affect the low-redshift tail of the CMB visibility function by providing additional free electrons. The last two of these provide the strongest CMB signal. However, the effects are detectable in the Planck satellite only if the primordial metal abundance is at least a few hundredths of solar for (2) and a few tenths of solar for (3). We thus conclude that Planck will not be able to improve upon current constraints to primordial metallicity, at the level of a thousandth of solar, from the Lyman-alpha forest and ultra-metal-poor halo stars, and that the CMB power-spectrum predictions for Planck suffer no uncertainty arising from the possibility that there may be primordial metals.