How well do we understand cosmological recombination?

TL;DR

This paper assesses the accuracy needed in modeling cosmological recombination to reliably extract parameters from CMB data, highlighting current uncertainties and their potential impact on cosmological inferences.

Contribution

It introduces a modified recombination code with adjustable parameters to quantify uncertainties and evaluates the precision required for Planck data analysis.

Findings

Helium recombination must be known to better than 10% for negligible impact.

Hydrogen recombination accuracy needs to be within 1% for reliable parameter estimation.

Current numerical calculations are sufficiently accurate for helium, but hydrogen requires further refinement.

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.