Probing the Universe After Cosmological Recombination Through the Effect of Neutral Lithium on the Microwave Background Anisotropies

I show that neutral lithium couples strongly to the cosmic microwave background (CMB) through its 6708A resonant transition after it recombines at z~500. At observed wavelengths of <335 micron, the CMB anisotropies are significantly altered since the optical depth for resonant scattering by neutral lithium is substantial, of order 0.5. The scattering would suppress the original anisotropies, but will generate strong new anisotropies in the CMB temperature and polarization on sub-degree scales (l>100). Observations at different wavelengths in this spectral regime can probe different thin slices of the early universe. The anisotropy noise contributed by continuum radiation from foreground far--infrared sources could be taken out by subtracting maps at slightly different wavelengths. Detection of the above effects can be used to study structure at z<500 and to constrain the primordial abundance and recombination history of lithium.