Abstract

Context: 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. Aims: We seek a detailed understanding of the formation of molecules in the postrecombination universe and their interactions with the CMB. We present a detailed and updated chemical network and an overview of the interactions of molecules with the CMB. Methods: We calculate the evolution of primordial chemistry in a homogeneous universe and determine the optical depth due to line absorption, photoionization and photodissociation, and estimate the resulting changes in the CMB temperature and its power spectrum. Corrections for stimulated and spontaneous emission are taken into account. Results: The most promising results are obtained for the negative hydrogen ion H$^{-}$ and the HeH$^{+}$ molecule. The free-free process of H$^{-}$ yields a relative change in the CMB temperature of up to 2{\times}10$^{-11}$, 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 H$^{-}$ is significantly below a previous suggestion. HeH$^{+}$ efficiently scatters CMB photons and smears out primordial fluctuations, leading to a change in the power spectrum of the order 10$^{-8}$. Conclusions: We demonstrate that primordial chemistry does not alter the CMB during the dark ages of the universe at the significance level of current CMB experiments. We determine and quantify the essential effects that may contribute to changes in the CMB and leave an imprint from the dark ages, thus cons tituting a potential probe of the early universe.