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100th anniversary of the birth of Ya.B. Zeldovich

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

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Astro Space Centre, Lebedev Physical Institute, Russian Academy of Sciences, ul. Profsoyuznaya 84/32, Moscow, 117997, Russian Federation

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 due to the release of photons during this epoch. These additional photons form a cosmological recombination spectrum imposed on thermal CMB spectrum. The recombination dynamics of hydrogen is controlled by two processes — the two-photon decay $\rm 2s\to 1s$ and the L$_{\alpha}$ photon escape due to multiple scattering in an expanding medium — of which the first process is dominant. About 57% of all hydrogen atoms in the Universe at $z \lesssim 1400$ recombined through two-photon decay channel. Because the ratio of CMB photon and baryon number densities is extremely large, the additional photons make only a small fraction of the total amount, so that 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 maximum CMB. In the Wien range, observations will be strongly hampered — and indeed made impossible — by the infrared and submillimeter cosmic background. It is not possible yet — given the current level of instrumentation — to measure small ($10^{-8}$) distortions 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, it being desirable that the contribution of various cosmic backgrounds and noises be minimal there (for example, near the Galactic pole). It is also essential that the signal sought for should be nonpolarized in order that it could be separated from signals from other sources.