## [astro-ph/0501672] Dynamics of the Primordial Hydrogen and Helium (HeI) Recombination in the Universe

 Authors: V.K.Dubrovich, S.I.Grachev Abstract: The dependences on z of fractional number densities of H+ and He+ ions are calculated with a proper allowance for two-photon decays of upper levels of hydrogen and parahelium and radiative transfer in intercombination line 2 3P_1 --> 1 1S_0 of helium. It is shown that for hydrogen this gives corrections for a degree of ionization in no more than a few percents but for helium this leads to a significant acceleration of recombination compared to the recent papers by Seager et al. (1999; 2000) where these effects were ignored. [PDF]  [PS]  [BibTex]  [Bookmark]

Discussion related to specific recent arXiv papers
Antony Lewis
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### [astro-ph/0501672] Dynamics of the Primordial Hydrogen and H

In this paper the authors claim to do a more detailed treatment of the recombination history of the universe, in particular including effects not accounted for in the RECFAST code used by CMBFAST/CAMB/etc.

The authors kindly sent me their modified recfast code. Here's a comparison of the ionization fraction as a function of redshift

(blue is recfast, red is the new result). In terms of a comparison on CMB $C_l$ this implies the following difference between the two results

Obviously if the new code is more accurate, this is a very important correction!

I would be very interested to hear comments on this paper.

Roberto Trotta
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### [astro-ph/0501672] Dynamics of the Primordial Hydrogen and H

Comparing the new plot of the ionization history with Fig. 9 of the paper by Seager et al astro-ph/9912182, which introduced the corrections to the Saha recombination history, it seems that taking the new effects into account gives an HeII to HeI recombination which is actually quite similar to the original Saha result.

As for the impact of this correction on cosmological parameter extraction, I would guess that the effect should largely be independent of the cosmology (?), giving thus a systematic, almost constant change to the power spectra. Therefore I would not expect a major impact on most posterior contraints, settings aside perhaps the baryon abundance (because of $\omega_b$ - He correlations) and neutrino properties, especially the neutrino families (see astro-ph/0310198).

Clearly, the changes are potentially important for the determination of the He mass fraction from the CMB, since the He effect on the power spectrum is of the same order as the corrections plotted by Antony.

Antony Lewis
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### Re: [astro-ph/0501672] Dynamics of the Primordial Hydrogen a

Here's the $C_l$ comparison plot of standard Peebles recombination against RECFAST (red) and the new result (blue)

The systematic change in power on small scales between Peebles/new result and RECFAST would probably bias parameters a bit (n_s etc).

Antony Lewis
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### Re: [astro-ph/0501672] Dynamics of the Primordial Hydrogen a

Out of interest I did a Planck parameter forecast at WMAP3 fiducial values. This shows the effect of using the Dubrovich and Grachev evolution (black) compared to RECFAST (red).

(the fiducial model was generated with RECFAST). Obviously very important!

The effect on WMAP3 results is slightly smaller than the effect of secondaries (see astro-ph/0603753) and likelihood approximations (see http://cosmocoffee.info/viewtopic.php?t=503).