Constraining nonstandard recombination: A worked example

Abstract

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 $\ensuremath{\alpha}$ compared to its present value. In addition to $\ensuremath{\alpha}$ 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 accommodated 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 ${\ensuremath{\Omega}}_{b}{h}^{2}$ 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 are 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 ${z}_{*}=900, {\ensuremath{\Omega}}_{b}{h}^{2}=0.024, {\ensuremath{\Omega}}_{m}{h}^{2}=0.14, {H}_{0}=49$ and $n=1.02.$ Compatible with present data at 95% confidence level $780<{z}_{*}<1150, 0.018<{\ensuremath{\Omega}}_{b}{h}^{2}<0.036, 0.07<{\ensuremath{\Omega}}_{m}{h}^{2}<0.3$ and $0.9<n<1.2.$