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The value that we get of Omegab, gal ~ 0.003 in galaxies is only about one-tenth of the value of the baryonic density of the universe Omegab ~ 0.03 derived from measurements of light element abundances ([Coc et al. 2004]) and from the shape of the angular power spectrum of the cosmic microwave background [Spergel et al. 2003]. This opens up the important question: if they are not in galaxies, where do 90% of the baryons in the Universe reside?

Hydrodynamic simulations ([Cen & Ostriker 1999]) suggest that most of these baryons are in a warm/hot intergalactic medium (WHIM), that is highly ionised. This medium is very difficult to detect, but there might be indirect evidence for its existence, from observations of OVII X-ray absorption along the lines of sight to high redshift quasars (e.g. [Fang et al. 2003]). Directly detecting the WHIM and will be a major area of observational study over the next few years.