Constraining non standard recombination: a worked example

We fit the BOOMERANG, MAXIMA and COBE/DMR measurements of the cosmic microwave background anisotropy in spatially flat cosmological models where departures from standard recombination of the primeval plasma are parametrized through a change in the fine structure constant α compared to its present value. In addition to α we vary the baryon and dark matter densities, the spectral index of scalar fluctuations, and the Hubble constant. Within the class of models considered, the lack of a prominent second acoustic peak in the measured spectrum can be accomodated either by a relatively large baryon density, by a tilt towards the red in the spectrum of density fluctuations, or by a delay in the time at which neutral hydrogen formed. The ratio between the second and first peak decreases by around 25% either if the baryon density Ωbh 2 is increased or the spectral index n decreased by a comparable amount, or if neutral hydrogen formed at a redshift z∗ about 15% smaller than its standard value. We find that the present data is best fitted by a delay in recombination, with a lower baryon density than the best fit if recombination is standard. Our best fit model has z∗ = 900, Ωbh 2 = 0.024, Ωmh 2 = 0.14, H0 = 49 and n = 1.02. Compatible with present data at 95% confidence level 780 < z∗ < 1150, 0.018 < Ωbh 2 < 0.036, 0.07 < Ωmh 2 < 0.3 and 0.9 < n < 1.2. Facultad de Ciencias Astronómicas y Geofísicas