The Imprint of Lithium Recombination on the Microwave Background Anisotropies

Following Loeb (2001), we explore the imprint of the resonant 6708Å line opacity of neutral lithium on the temperature and polarization anisotropies of the cosmic microwave background (CMB) at observed wavelengths of 250–350μm (0.9–1.2 THz). We show that if lithium recombines in the redshift range of z = 400–500 as expected, then the standard CMB anisotropies would be significantly modified in this wavelength band. The modified polarization signal could be comparable to the expected polarization anisotropies of the far–infrared background on sub–degree angular scales (l ∼> 100). Detection of the predicted signal can be used to infer the primordial abundance of lithium, and to probe structure in the Universe at z ∼ 500. Subject headings: Cosmology: theory — cosmic microwave background