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We investigate the effect of non-evaporating primordial black holes (PBHs) onthe ionization and thermal history of the universe. X-rays emitted by gasaccretion onto PBHs modify the cosmic recombination history, producingmeasurable effects on the spectrum and anisotropies of the Cosmic MicrowaveBackground (CMB). Using the third-year WMAP data and FIRAS data we improveexisting upper limits on the abundance of PBHs with masses >0.1 Msun by severalorders of magnitude. Fitting WMAP3 data with cosmological models that do notallow for non-standard recombination histories, as produced by PBHs or otherearly energy sources, may lead to an underestimate of the best-fit values ofthe amplitude of linear density fluctuations (sigma_8) and the scalar spectralindex (n_s). Cosmological parameter estimates are affected because models withPBHs allow for larger values of the Thomson scattering optical depth, whosecorrelation with other parameters may not be correctly taken into account whenPBHs are ignored. Values of tau_e=0.2, n_s=1 and sigma_8=0.9 are allowed at 95%CF. This result that may relieve recent tension between WMAP3 data and clustersdata on the value of sigma_8. PBHs may increase the primordial molecularhydrogen abundance by up to two orders of magnitude, this promoting cooling andstar formation. The suppression of galaxy formation due to X-ray heating isnegligible for models consistent with the CMB data. Thus, the formation rate ofthe first galaxies and stars would be enhanced by a population of PBHs.

Effect of Primordial Black Holes on the Cosmic Microwave Background and Cosmological Parameter Estimates