Effects of primordial chemistry on the cosmic microwave background
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 and the molecule. The free-free process of yields a relative change in the CMB temperature of up to 2×10-11, 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 is significantly below a previous suggestion. 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.