Lines in the cosmic microwave background spectrum from the epoch of cosmological helium recombination

TL;DR

This paper calculates helium-related features in the cosmic recombination spectrum, proposing these signals can reveal primordial helium abundance and test early Universe physics through spectral observations.

Contribution

It introduces detailed calculations of helium bound-bound transitions and their spectral signatures, including effects of neutral hydrogen on helium recombination dynamics.

Findings

Distinct spectral features from helium recombination identified

Neutral hydrogen accelerates helium recombination, producing narrow spectral features

Some helium transitions appear in absorption, offering observational signatures

Abstract

The main goal of this work is to calculate the contributions to the cosmological recombination spectrum due to bound-bound transitions of helium. We show that due to the presence of helium in the early Universe unique features appear in the total cosmological recombination spectrum. These may provide a unique observational possibility to determine the relative abundance of primordial helium, well before the formation of first stars. We include the effect of the tiny fraction of neutral hydrogen atoms on the dynamics of HeII -> HeI recombination at redshifts $z\sim 2500$. As discussed recently, this process significantly accelerates HeII -> HeI recombination, resulting in rather narrow and distinct features in the associated recombination spectrum. In addition this process induces some emission within the hydrogen Lyman-$\alpha$ line, before the actual epoch of hydrogen recombination round $z\sim 1100-1500$. We also show that some of the fine structure transitions of neutral helium appear in absorption, again leaving unique traces in the Cosmic Microwave Background blackbody spectrum, which may allow to confirm our understanding of the early Universe and detailed atomic physics.