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RE: Primordial Helium Abundance
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Title: Primordial Helium Abundance from CMB: a constraint from recent observations and a forecast
Authors: Kazuhide Ichikawa, Toyokazu Sekiguchi, Tomo Takahashi
(Version v2)

We studied a constraint on the primordial helium abundance Y_p from current and future observations of CMB. Using the currently available data from WMAP, ACBAR, CBI and BOOMERANG, we obtained the constraint as Y_p = 0.25^{+0.10}_{-0.07} at 68% C.L. We also provide a forecast for the Planck experiment using the Markov chain Monte Carlo approach. In addition to forecasting the constraint on Y_p, we investigate how assumptions for Y_p affect constraints on the other cosmological parameters.

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RE: Primordial Helium Recombination
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Title: Primordial helium recombination III: Thomson scattering, isotope shifts, and cumulative results
Authors: Eric R. Switzer, Christopher M. Hirata

Upcoming precision measurements of the temperature anisotropy of the cosmic microwave background (CMB) at high multipoles will need to be complemented by a more complete understanding of recombination, which determines the damping of anisotropies on these scales. This is the third in a series of papers describing an accurate theory of HeI and HeII recombination. Here we describe the effect of Thomson scattering, the ^3He isotope shift, the contribution of rare decays, collisional processes, and peculiar motion. These effects are found to be negligible: Thomson and ^3He scattering modify the free electron fraction x_e at the level of several \times 10^{-4}. The uncertainty in the 2^3P^o-1^1S rate is significant, and for conservative estimates gives uncertainties in x_e of order 10^{-3}. We describe several convergence tests for the atomic level code and its inputs, derive an overall C_\ell error budget, and relate shifts in x_e(z) to the changes in C_\ell, which are at the level of 0.5% at \ell =3000. Finally, we summarize the main corrections developed thus far. The remaining uncertainty from known effects is \sim 0.3% in x_e.

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Title: Primordial helium recombination II: two-photon processes
Authors: Christopher M. Hirata, Eric R. Switzer

Interpretation of precision measurements of the cosmic microwave background (CMB) will require a detailed understanding of the recombination era, which determines such quantities as the acoustic oscillation scale and the Silk damping scale. This paper is the second in a series devoted to the subject of helium recombination, with a focus on two-photon processes in He I. The standard treatment of these processes includes only the spontaneous two-photon decay from the 2^1S level. We extend this treatment by including five additional effects, some of which have been suggested in recent papers but whose impact on He I recombination has not been fully quantified. These are: (i) stimulated two-photon decays; (ii) two-photon absorption of redshifted HeI line radiation; (iii) two-photon decays from highly excited levels in HeI (n^1S and n^1D, with n>=3); (iv) Raman scattering; and (v) the finite width of the 2^1P^o resonance. We find that effect (iii) is highly suppressed when one takes into account destructive interference between different intermediate states contributing to the two-photon decay amplitude. Overall, these effects are found to be insignificant: they modify the recombination history at the level of several parts in 10^4.

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RE: Primordial Helium Abundance
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Title: Primordial helium recombination I: feedback, line transfer, and continuum opacity
Authors: Eric R. Switzer, Christopher M. Hirata

Precision measurements of the cosmic microwave background temperature anisotropy on scales \ell > 500 will be available in the near future. Successful interpretation of these data is dependent on a detailed understanding of the damping tail and cosmological recombination of both hydrogen and helium. This paper and two companion papers are devoted to a precise calculation of helium recombination. We discuss several aspects of the standard recombination picture, and then include feedback, radiative transfer in HeI lines with partial redistribution, and continuum opacity from HI photoionisation. In agreement with past calculations, we find that HeII recombination proceeds in Saha equilibrium, whereas HeI recombination is delayed relative to Saha due to the low rates connecting excited states of HeI to the ground state. However, we find that at z<2200 the continuum absorption by the rapidly increasing HI population becomes effective at destroying photons in the HeI 2^1P^o-1^1S line, causing HeI recombination to finish around z\approx 1800, much earlier than previously estimated.

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Title: Revised Primordial Helium Abundance Based on New Atomic Data
Authors: M. Peimbert (IA-UNAM), V. Luridiana (IAA, CSIC), A. Peimbert (IA-UNAM)

We have derived a primordial helium abundance of Yp = 0.2474 0.0028, based on new atomic physics computations of the recombination coefficients of He I and of the collisional excitation of the H I Balmer lines together with observations and photoionisation models of metal-poor extragalactic H II regions. The new atomic data increase our previous determination of Yp by 0.0083, a very significant amount. By combining our Yp result with the predictions made by the standard Big Bang nucleosynthesis model, we find a baryon-to-photon ratio, \eta, in excellent agreement both with the \eta value derived by the primordial deuterium abundance value observed in damped Lyman-alpha systems and with the one obtained from the WMAP observations.

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Title: On the Primordial Helium Abundance and the DeltaY/DeltaO Ratio
Authors: M. Peimbert (IA-Unam), V. Luridiana (IAA, Csic), A. Peimbert (IA-Unam), L. Carigi (IA-Unam)

We present a review on the determination of the primordial helium abundance, Yp, based on the study of hydrogen and helium recombination lines in extragalactic H II regions. We also discuss the observational determinations of the increase of helium to the increase of oxygen by mass Delta Y/Delta O, and compare them with predictions based on models of galactic chemical evolution.

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