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We use our most recent training set for the  rico  code to estimate the impact of recombination uncertainties on the posterior probability distributions which will be obtained from future cosmic microwave background experiments, and in particular the  Planck  satellite. Using a Monte Carlo Markov Chain (MCMC) analysis to sample the posterior distribution of the cosmological parameters, we find that  Planck  will have biases of  −0.7, −0.3  and  −0.4σ  for   n  S , Ω b  h  2   and  log(10 10  A  S ) , respectively, in the minimal six‐parameter Λ cold dark matter model, if the description of the recombination history given by  rico  is not used. The remaining parameters (e.g. τ or  Ω dm  h  2  ) are not significantly affected. We also show that the cosmology dependence of the corrections to the recombination history modelled with  rico  has a negligible impact on the posterior distributions obtained for the case of the  Planck  satellite. In practice, this implies that the inclusion of additional corrections to existing recombination codes can be achieved using simple cosmology‐independent ‘fudge functions’.  Finally, we also investigated the impact of some recent improvements in the treatment of hydrogen recombination which are still not included in the current version of our training set for  rico , by assuming that the cosmology dependence of those corrections can be neglected. In summary, with our current understanding of the complete recombination process, the expected biases in the cosmological parameters inferred from  Planck  might be as large as  −2.3, −1.7  and  −1σ  for   n  S , Ω b  h  2   and  log(10 10  A  S ) , respectively, if all those corrections are not taken into account. We note that although the list of physical processes that could be of importance for  Planck  seems to be nearly complete, still some effort has to be put into the validation of the results obtained by the different groups.  The new  rico  training set as well as the fudge functions used for this paper are publicly available on the  rico  webpage.

Estimating the impact of recombination uncertainties on the cosmological parameter constraints from cosmic microwave background experiments