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 \HM and the \HeHII molecule. The free-free process of \HM 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 \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.

16 pages, 15 figures, accepted for publication at A&A. Discussion section updated

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• The formation of massive primordial stars in the presence of moderate UV backgrounds
• Dark stars: Implications and constraints from cosmic reionization and extragalactic background radiation
• Interplay of gas and ice during cloud evolution
• Effects of turbulence and rotation on protostar formation as a precursor to seed black holes
• Isotopic and vibrational-level dependence of H${}_2$ dissociation by electron impact
• The first molecules in the intergalactic medium and halos of the Dark Ages and Cosmic Dawn
• A computed line list for the H2D+ molecular ion
• Non-thermal photons and direct photodissociation of H2, HD and HeH+ in the chemistry of the primordial Universe
• Distortion of the CMB spectrum by the first molecules of the Dark Ages
• Global Signals of the First Molecules from the Dark Ages in the Presence of Primordial Magnetic Fields