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2.1. Baryons

The inventory of cosmic baryons is readily compiled. Stars and their remnants, and gas in galaxies, contribute no more than 1% of the critical density (i.e. they give Omegab < 0.01). However several percent more could be contributed by diffuse material pervading intergalactic space: warm gas (with kT appeq 0.1 keV) in groups of galaxies and loose clusters, and cooler gas pervading intergalactic space that manifests itself via the `picket fence' absorption lines in quasar spectra. (Rich clusters are rare, so their conspicuous gas content, at several KeV, is not directly significant for the total inventory, despite its importance as a probe)

These baryon estimates are concordant with those inferred by matching the He and D abundances at the birth of galaxies with the predicted outcome of nucleosynthesis in the big bang, which is sensitive to the primordial baryon/photon ratio, and thus to Omegab. The observational estimates have firmed up, with improved measurements of deuterium in high-z absorbing clouds. The best fit occurs for Omegab appeq 0.02h-2 where h is the Hubble constant in units of 100 km s-1 Mpc-1. Observations favour h appeq 0.7.

Omegab is now pinned down by a variety of argument to be 0.04 - 0.05. This corresponds to only ~ 0.3 baryons per cubic metre, a value so low that it leaves little scope for dark baryons. (It is therefore unsurprising that the MACHO/OGLE searches should have found that compact objects do not make a substantial contribution to the total mass of our own galactic halo.)

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