Effects of primordial chemistry on the cosmic microwave background

TL;DR

This paper investigates how primordial chemistry during the dark ages could subtly influence the cosmic microwave background through various molecular interactions, potentially detectable with future precise measurements.

Contribution

It provides an updated chemical network and detailed estimates of molecular effects on the CMB, highlighting the significance of ext{H}^- and ext{HeH}^+ in these interactions.

Findings

Free-free process of ext{H}^- causes a temperature change of up to 2×10^{-11}.

ext{HeH}^+ scattering leads to a power spectrum change of about 10^{-8}.

Results are below previous estimates but suggest future experiments could detect these signals.

Abstract

Previous works have demonstrated that the generation of secondary CMB anisotropies due to the molecular optical depth is likely too small to be observed. In this paper, we examine additional ways in which primordial chemistry and the dark ages might influence the CMB. We present a detailed and updated chemical network and give an overview of the interactions of molecules with the CMB. We consider the optical depth due to line absorption, photoionization, photodissociation and free-free processes, and estimate the resulting changes in the CMB temperature and its power spectrum. The most promising results are obtained for the negative hydrogen ion \HM and the \HeHII molecule. The free-free process of \HM yields a relative change in the CMB temperature of up to $2\times10^{-11}$, and leads to a frequency-dependent change in the power spectrum of the order $10^{-7}$ at 30 GHz. With a change of the order $10^{-10}$ in the power spectrum, our result for the bound-free process of \HM is significantly below a previous suggestion. \HeHII efficiently scatters CMB photons and smears out primordial fluctuations, leading to a change in the power spectrum of the order $10^{-8}$. Improvements in the accuracy of future CMB experiments may thus help to constrain and finally detect these interesting signals from the dark ages of the universe.