A census of baryons in the Universe from localized fast radio bursts

Abstract

More than three-quarters of the baryonic content of the Universe resides in a highly diffuse state that is difficult to detect, with only a small fraction directly observed in galaxies and galaxy clusters^1,2. Censuses of the nearby Universe have used absorption line spectroscopy^3,4 to observe the ‘invisible’ baryons, but these measurements rely on large and uncertain corrections and are insensitive to most of the Universe’s volume and probably most of its mass. In particular, quasar spectroscopy is sensitive either to the very small amounts of hydrogen that exist in the atomic state, or to highly ionized and enriched gas^4,5,6 in denser regions near galaxies⁷. Other techniques to observe these invisible baryons also have limitations; Sunyaev–Zel’dovich analyses^8,9 can provide evidence from gas within filamentary structures, and studies of X-ray emission are most sensitive to gas near galaxy clusters^9,10. Here we report a measurement of the baryon content of the Universe using the dispersion of a sample of localized fast radio bursts; this technique determines the electron column density along each line of sight and accounts for every ionized baryon^{11,<a data-track="click" data-track-action="reference anchor" data-track-label="link"...}