Signals From the Epoch of Cosmological Recombination

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 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 Cosmic Microwave 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. 1(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 hi...

A Wideband Resistive Beam-Splitter Screen

“…In this Recombination Epoch the trickle of bound electrons down atomic states adds recombination lines to the radiation content. This forest of recombination lines is expected to be detectable today as a ripple in the spectrum of the cosmic radio background [1,2]. Subsequently, following the formation of the first collapsed objects-which may be massive Population III stars or ultradwarf galaxies-the neutral recombined hydrogen gas is irradiated by first light.…”

Two-photon transitions in primordial hydrogen recombination

“…In the past several years, there has been a resurgence of interest in the recombination problem, driven primarily by the new CMB experiments [9,11] and also the possibility of measuring the spectral distortion [12][13][14][15][16][17][18]. Several groups have added additional physics to the Seager et al…”

“…A number of effects that are not considered here such as the very high-n levels, Ly diffusion and recoil, and feedback from the helium spectral distortion [16,18,25,29,[36][37][38], may also be important if subpercent accuracy is desired. Rather, the purpose of this paper is to clear up the physical picture of the two-photon transitions and estimate their effect on recombination, as a first step toward a treatment that will ultimately include these other processes as well.…”

Primordial helium recombination. I. Feedback, line transfer, and continuum opacity

“…There are several reasons why it is now timely to revisit the cosmological recombination [37][38][39]. Recent work [40 -51] has led to suspicion that some pieces of the recombination problem, such as matter temperature evolution [42,45], two-photon transitions from high-lying states [41,47,49], the effect of He I intercombination lines [41,43], departures of the l sublevels of hydrogen from their statistical population ratios [52], and stimulated twophoton transitions [40,44] are not completely understood or were absent in past work.…”