Baryon accountants balance the books

Just under 5% of the cosmos is composed of the protons and neutrons—baryons, in particle-physics lingo—that make up the atoms of everyday life. Some of that baryonic matter is visible in galaxies, but according to theory, most of it is hiding in the extended halos of galaxies or on the filamentary intergalactic cosmic web that permeates the universe.

The intergalactic matter is difficult to see because it has largely been ionized by the violent processes that created the web and the galaxies that lie at its nodes. Until a decade ago, astronomers simply did not have the technology to measure the absorption signatures of the ionized atoms, which absorb in the far UV and beyond. Finally, in 2008, Charles Danforth and Michael Shull (both at the University of Colorado Boulder) observed the absorption of far-UV quasar light by neutral hydrogen and several ionic species in the intergalactic medium (see Physics Today, July 2008, page 12 ). When the deduced quantity of matter was added to the baryon content that had been previously accounted for, the result was still 30–60% short of the value predicted by the standard cosmological model. Presumably the remaining baryons were in hotter parts of the intergalactic web and would absorb in the x-ray band.

The missing baryons have now been found, by a collaboration led by Fabrizio Nicastro (National Institute for Astrophysics, Italy) and including Danforth, Shull, and many others. The team analyzed 18 days of spectroscopic data taken over a two-year period by an instrument aboard XMM-Newton, as the satellite was pointed at the luminous x-ray-emitting galaxy 1ES 1553+113. Researchers searched for valleys resulting from x-ray absorption by the oxygen ion O⁺⁶, which traces baryonic matter as a whole, and were rewarded with twin dips; as manifested by their different redshifts, each corresponds to a different cloud of plasma. Detailed modeling of the x-ray absorption data established that the newly observed baryonic matter is hot and plentiful enough to put the total baryon content of the universe in line with prediction of the standard cosmological picture. Moreover, both clouds are located in neighborhoods with a higher than normal density of galaxies, as one would expect for denizens of the web’s filaments. (F. Nicastro et al., Nature 558, 406, 2018 .)