Ehud Behar

The baryon content of the universe is tightly constrained by big-bang nucleosynthesis and the cosmic microwave background. However, in the local universe only about 10% of the baryons are found in galaxies. It is natural to look for the remaining baryons in the intergalactic medium (IGM), but only ~50% are found there. According to cosmological simulations, the remaining baryons are in a diffuse warm-hot phase of the IGM that is extremely difficult to observe.
X-ray absorption of γ-ray burst (GRB) afterglows is prevalent in Swift/XRT spectra, yet poorly understood; its behavior is significantly different from well-localized Lyman-α absorption systems, and provides a unique tool to probe the diffuse IGM on cosmological distances. In particular, it features a strong dependence on redshift that Lyman-α absorbers do not. I will discuss the interpretation of GRB X-ray afterglow absorption in the context of the diffuse IGM and confront it with new cosmological simulations.