How well do we understand cosmological recombination?

Abstract

The major theoretical limitation for extracting cosmological parameters from the CMB sky lies in the precision with which we can calculate the cosmological recombination process. Uncertainty in the details of hydrogen and helium recombination could effectively increase the errors or bias the values of the cosmological parameters derived from the Planck satellite, for example. Here we modify the cosmological recombination code RECFAST by introducing one more parameter to reproduce the recent numerical results for the speed-up of the helium recombination. Together with the existing hydrogen fudge factor, we vary these two parameters to account for the remaining dominant uncertainties in cosmological recombination. By using the CosmoMC code with Planck forecast data, we find that we need to determine the parameters to better than ten per cent for He I and one per cent for H, in order to obtain negligible effects on the cosmological parameters. For helium recombination, if the existing studies have calculated the ionization fraction to the 0.1 per cent level by properly including the relevant physical processes, then we already have numerical calculations which are accurate enough for Planck. For hydrogen, setting the fudge factor to speed up low redshift recombination by 14 per cent appears to be sufficient for Planck. However, more work still needs to be done to carry out comprehensive numerical calculations of all the relevant effects for hydrogen, as well as to check for effects which couple hydrogen and helium recombinaton through the radiation field.

6 pages, 6 figures; Paper revised according to the reviewer's suggestions. An updated RECFAST (version 1.4) available at http://www.astro.ubc.ca/people/scott/recfast.html

References in corpus (6)

• Wilkinson Microwave Anisotropy Probe (WMAP) Three Year Results: Implications for Cosmology
• Cosmological hydrogen recombination: populations of the high level sub-states
• The Cosmic Microwave Background and the Ionization History of the Universe
• The effect of forbidden transitions on cosmological hydrogen and helium recombination
• Two-Photon 2s<->1s Transitions during Recombination of Hydrogen in the Universe
• Cosmological hydrogen recombination: Lyn line feedback and continuum escape

Cited by in corpus (7)

• Reionization - A probe for the stellar population and the physics of the early universe
• Two-photon transitions in primordial hydrogen recombination
• The influence of magnetic fields on the thermodynamics of primordial star formation
• Time-Dependent Corrections to the Ly-alpha Escape Probability During Cosmological Hydrogen Recombination
• Uncertainties in Theoretical HeI Emissivities: HII Regions, Primordial Abundance, and Cosmological Recombination
• WMAP 5-year constraints on time variation of $\alpha$ and $m_e$ in a detailed recombination scenario
• Probing the last scattering surface through the recent and future CMB observations