Astronomy Colloquium / Spring 2008

April 30

Prof. Rashid Sunyaev

Max Planck Institute for Astrophysics

"The Richness and Beauty of the Physics of Cosmological Recombination"

The physical ingredients to describe the epoch of cosmological recombination are amazingly simple and well-understood. This fact allows us to take into account a very large variety of processes, still finding potentially measurable consequences. In this talk we highlight some of the detailed physics that were recently studied in connection with cosmological hydrogen and helium recombination. The impact of these considerations is two-fold: (i) the associated release of photons during this epoch leads to interesting and unique deviations of the Cosmic Microwave Background (CMB) energy spectrum from a perfect blackbody, which, in particular at cm and decimeter wavelength, may become observable in the near future. Observing the spectral distortions from the epochs of hydrogen and helium recombination, in principle would provide an additional way to determine some of the key parameters of the Universe (e.g. the specific entropy, the CMB monopole temperature and the pre-stellar abundance of helium. (II) This spectral features will be strongly amplified or change their shape in the case of the early energy release in the Universe due to, for example, particle decay or annihilation. (iiI) with the advent of high precision CMB data, a very accurate theoretical understanding of the ionization history of the Universe becomes necessary for the interpretation of the CMB temperature and polarization anisotropies. Here we show that the uncertainty in the ionization history due to several processes, which until now were not taken in to account in the standard recombination code RECFAST, exceed the level of 0.1% to 0.5%. However, it is indeed surprising how inert the cosmological recombination history is even at percent-level accuracy.