Observation of resonant scattering from CMB thermal and angular power spectrum

Resonant scattering of the cosmic microwave background (CMB) photons by primordial molecules, such as HD, LiH and H2D+, has been considered as a possible method to search for the first collapsed objects and constrain the chemistry of the early universe for almost three decades. The hope had been to detect a spectral change in the CMB intensity at the line frequencies where the signal amplitude depends on the peculiar motion of the scattering cloud. We show that it will be very difficult to detect the signal of scattering through such spectral survey not only because the signal is small, but also because very low density values are needed before line emission will start to dominate in the telescope beam. A more promising approach would be to look for distortions caused by scattering in the primordial CMB temperature anisotropies, and upcoming CMB experiments like Planck will already be able to constrain the ionization and enrichment history of the universe by this method.