The Richness and Beauty of the Physics of Cosmological Recombination

TL;DR

This paper discusses the detailed physics of cosmological hydrogen recombination, highlighting how spectral distortions in the CMB can reveal key universe parameters and improve our understanding of the recombination process.

Contribution

It emphasizes the importance of detailed recombination physics for interpreting CMB data and proposes using spectral distortions to measure universe parameters directly.

Findings

Recombination spectral distortions could be observable in the near future.

Spectral measurements can determine universe parameters independently of cosmic variance.

Accurate recombination models are essential for interpreting high-precision CMB data.

Abstract

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 contribution we highlight some of the detailed physics that were recently studied in connection with cosmological hydrogen 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 decimeter wavelength, may become observable in the near future. Observing these distortions, 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), not suffering from limitations set by cosmic variance. Also it permits us to confront our detailed understanding of the recombination process with direct observational evidence. In this contribution we illustrate how the theoretical spectral template for the cosmological recombination spectrum may be utilized for this purpose. (ii) with the advent of high precision CMB data, e.g. as will be available using the Planck Surveyor or Cmbpol, a very accurate theoretical understanding of the ionization history of the Universe becomes necessary for the interpretation of the CMB temperature and polarization anisotropies. (abridged)