ABSTRACT

Cosmological recombination at redshift z~1500 produces ~5 photons per recombining hydrogen atom, thereby slightly increasing the entropy of the Universe. In the spectral bands of a few GHz and at the frequencies ~350 and ~1700 GHz distinct features appear in the spectrum of CMB due to highly redshifted recombinational lines of hydrogen, including H-alpha and Ly-alpha. In the frequency range from ~1-3 GHz the predicted pattern is absolutely unique and has no foregrounds similar in the spectral variability. The amplitude of this variability is of the order of 100 nK and its detection requires the scanning over a broad frequency band, while extremely successful and informative studies of CMB angular fluctuations requires scanning on the angular coordinate. Existing and planned CMB experiment investigating the power spectrum of CMB angular fluctuations are achieving sensitivities on the level of 10 nK.

The challenge to detect recombinational lines coming to us from redshift 1400 should give direct evidence of the physical processes at very important stage of the evolution of the Universe, details of the shape of the last scattering surface, and opens new ways to measure the temperature of the CMB monopole and entropy of the Universe. Obviously, the detection of these lines will lead to crucial tests of non-standard cosmologies, predicting tiny energy release of UV photons during the time of recombination with the energy output on the level of 10^-9 of the CMB energy density.

The lines under discussion are formed in the unique physical conditions of slowly expanding extremely low density radiation dominated plasma with negligible role of electronic collisions and dominance of stimulated recombination and bound-bound transitions.