On the baryon content of the Universe

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Abstract
It is often stated that there are insufficient baryons in galaxies and clusters to satisfy the constraints imposed by primordial nucleosynthesis. Here a recent analysis of the baryon content of the Universe by Persic & Salucci is reconsidered, with the aim of estimating the uncertainties induced in the results by uncertainties in the galaxy population parameters used. The estimates of Ωb turn out to be dependent upon poorly constrained input parameters, in particular the luminosity-function slope parameter, α, and the mass-to-light ratio index, η. It is found that with different, but allowed, choices of input parameters – which admit of large numbers of low-luminosity galaxies – a value of the baryon density consistent with standard primordial nucleosynthesis |$0.040\le\Omega_\text{b}h^2_{50}\le0.064$| can be recovered. Thus it is not necessarily the case that the majority of baryons in the Universe are in unobservable forms, although it remains true that most are currently unobserved.

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