CMB spectral distortions during the recombination of the primeval plasma in the early Universe

Virtually all physical processes occurring during hydrogen and helium recombination $(900 < z < 7000)$ are currently well understood. The theoretical work of the last decade on this topic provides a comprehensive picture of recombination and related processes. Of particular observational interest is the fact that the CMB spectrum experiences a unique distortion from the blackbody spectrum due to the release of photons during this epoch. These additional photons form a cosmological recombination spectrum imposed on the thermal CMB spectrum. The recombination dynamics of hydrogen are controlled by two processes — the two-photon decay $2{\rm s}\to 1{\rm s}$ and the ${\rm L}_\alpha $ photon escape due to multiple scattering in an expanding medium — of which the first is dominant. About $57\%$ of all hydrogen atoms in the Universe at $z \lesssim 1400$ recombined through the two-photon decay channel. Because the ratio of the CMB photon and baryon number densities is extremely large, the additional photons make up only a small fraction ($10^{-8}{-}10^{-9}$) of the total number, and hence their distorting effect of the CMB spectrum is small. Of most promise for future observations are relative distortions in the Rayleigh–Jeans range of the CMB spectrum (the decimeter range). For example, at $300$ MHz, relative intensity distortions of the order of $10^{-8}{-}10^{-9}$ are expected. The Balmer and Paschen hydrogen series fall into the range of a CMB maximum. In the Wien range, observations are greatly hampered — and indeed made impossible — by the infrared and submillimeter cosmic background. Given the current level of instrumentation, it is not yet possible to measure small distortions $(<10^{-8})$ near the maximum. Some researchers believe, however, that an accuracy of $\sim 10$ nK can soon be achieved. Because the CMB spectrum does not depend on the direction, any region of the sky can be chosen for observation, preferable so as to minimize the contribution of various cosmic backgrounds and noises (for example, near a galactic pole). It is also essential that the sought signal be nonpolarized in order that it can be separated from signals from other sources.