The Cosmic Web of Baryons
  Only about 10% of the baryons in the universe lie in galaxies as stars or
cold gas, with the remainder predicted to exist as a dilute gaseous filamentary
network known as the Cosmic Web. Some of this gas is detected through UV
absorption line studies, but half of the gas remains undetected. Growth of
structure simulations suggest that these "missing" baryons were shock heated in
unvirialized cosmic filaments to temperatures of 10^5.5-10^7 K, and that the
gas is chemically enriched by galactic superwinds. Most of the gas in this
temperature regime can be detected only by X-ray observations through
absorption and emission from the He-like and H-line ions of C, N, and O. This
white paper shows that an X-ray telescope such as IXO can test the most central
predictions of the Cosmic Web: the distribution of gas mass with temperature;
the dynamics of the gas and its relationship to nearby galaxies; and the
topology of the Cosmic Web material.
