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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 physical processes, still finding potentially measurable consequences for the  energy spectrum  and  temperature anisotropies  of the Cosmic Microwave Background (CMB). In this contribution we provide a short historical overview in connection with the cosmological recombination epoch and its connection to the CMB. Also we highlight some of the detailed physics that were studied over the past few years in the context of the cosmological recombination of  hydrogen  and  helium . 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 CosmicMicrowave Background (CMB) energy spectrum  from a perfect blackbody , which, in particular at decimeter wavelength and the Wien part of the CMB spectrum, may become observable in the near future. Despite the fact that the abundance of helium is rather small, it still contributes a sizeable amount of photons to the full recombination spectrum, leading to additional distinct spectral features. 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). 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  of the cosmological recombination spectrum may be utilized for this purpose. We also show that because hydrogen and helium recombine at very different epochs it is possible to address questions related to the  thermal history  of our Universe. In particular the cosmological recombination radiation may allow us to distinguish between Compton  y  ‐distortions that were created by energy release  before  or  after  the recombination of the Universe finished.  (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. 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, reaches the percent level. In particular He II → He I recombination occurs significantly faster because of the presence of a tiny fraction of neutral hydrogen at  z  ∼ 2400. Also recently it was demonstrated that in the case of H I Lyman  α  photons the  timedependence  of the emission process and the  asymmetry  between the emission and absorption profile cannot be ignored. However, it is indeed surprising how  inert  the cosmological recombination history is even at percent‐level accuracy. Observing the cosmological recombination spectrum should in principle allow us to directly check this conclusion, which until now is purely theoretical. Also it may allow to  reconstruct the ionization history  using observational data (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Signals from the epoch of cosmological recombination – Karl Schwarzschild Award Lecture 2008