Cosmological perturbations and the reionization epoch

We investigate the dependence of the epoch of reionization on the properties of cosmological perturbations, in the context of cosmologies permitted by WMAP. We compute the redshift of reionization using a simple model based on the Press–Schechter approximation. For a power-law initial spectrum we estimate that reionization is likely to occur at a redshift zreion = 17+10 −7 , consistent with the WMAP determination based on the temperature-polarization cross power spectrum. We estimate the delay in reionization if there is a negative running of the spectral index, as weakly indicated by WMAP. We then investigate the dependence of the reionization redshift on the nature of the initial perturbations. We consider chi-squared probability distribution functions with various degrees of freedom, motivated both by non-standard inflationary scenarios and by defect models. We find that in these models reionization is likely to occur much earlier, and to be a slower process, than in the case of initial Gaussian fluctuations. We also consider a hybrid model in which cosmic strings make an important contribution to the seed fluctuations on scales relevant for reionization. We find that in order for that model to agree with the latest WMAP results, the string contribution to the matter power spectrum on the standard 8 h−1 Mpc scale is likely to be at most at the level of 1 per cent, which imposes tight constraints on the value of the string mass per unit length.