Recent cosmic microwave background observations and the ionization historyof the universe - art. no. 083001

Interest in nonstandard recombination scenarios has been spurred by recent cosmic microwave background (CMB) results from BOOMERANG and MAXIMA, which show an unexpectedly low second acoustic peak, resulting in a best-fit bary on density that is 50% larger than the prediction of big-bang nucleosynthes is (BBN). This apparent discrepancy can be avoided if the universe has a no nstandard ionization history in which the recombination of hydrogen is sign ificantly delayed relative to the standard model. While future CMB observat ions may eliminate this discrepancy, it is useful to develop a general fram ework for analyzing nonstandard ionization histories. We develop such a fra mework, examining nonstandard models in which the hydrogen binding energy E -b and the overall expression for the time rate of change of the ionized fr action of electrons are multiplied by arbitrary factors. This set of models includes a number of previously proposed models as special cases. We find a wide range of models with delayed recombination that are able to fit the CMB data with a baryon density in accordance with BBN, but there are even a llowed models with earlier recombination than in the standard model. A gene ric prediction of these models is that the third acoustic CMB peak should b e very low relative to what is found in the standard model. This is the cas e even for the models with earlier recombination than in the standard model , because here the third peak is lowered by an increased diffusion damping at recombination relative to the standard model. Interestingly, the specifi c height of the third peak depends sensitively on the model parameters, so that future CMB measurements will be able to distinguish between different nonstandard recombination scenarios.