Paper detail

Have the missing cosmic baryons been found?

The angular power spectrum and polarization of the cosmic microwave background radiation (CMB), the relative abundances of primordial hydrogen, deuterium and helium isotopes, and the large-scale structure of the universe all indicate that 4.5% of the current mass density of the universe consists of baryons. However, only a small fraction of these baryons can be accounted for in stars and gas inside galaxies, galaxy groups and galaxy clusters, and in spectral-line absorbing gas in the intergalactic medium (IGM). Too hot to show up in Lyman-absorption, too cool to cause detectable spectral distortions of the cosmic microwave background radiation, and too diffused to emit detectable X-rays, about 90% of the cosmic baryons remain missing in the local universe (redshift z~0). Here, we report on prevalent, isotropic, source independent, and fairly uniform soft X-ray absorption along the lines of sight to high-z gamma-ray bursts (GRBs) and quasars. It has the magnitude, redshift and energy dependence that are expected from a hot IGM that contains the missing cosmological baryons and has a mean metallicity similar to that in the intracluster medium (ICM) of galaxy clusters.