1. Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, Massachusetts 02138, USA
2. Astronomy Department, The Ohio State University, Columbus, Ohio 43210, USA
3. University of California-Berkeley, Berkeley, California, 94720 USA
4. Istituto de Astronomia, UNAM, Mexico City, DF 04510, Mexico
5. Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

Correspondence to: Fabrizio Nicastro¹ Correspondence and requests for materials should be addressed to F.N. (Email: fnicastro@cfa.harvard.edu).

Abstract

Recent cosmological measurements indicate that baryons comprise about four per cent of the total mass-energy density of the Universe^{1, 2}, which is in accord with the predictions arising from studies of the production of the lightest elements³. It is also in agreement with the actual number of baryons detected at early times (redshifts z > 2)^{4, 5}. Close to our own epoch (z < 2), however, the number of baryons detected add up to just over half ( 55 per cent) of the number seen at z > 2 (refs 6–11), meaning that about 45 per cent are 'missing'. Here we report a determination of the mass-density of a previously undetected population of baryons, in the warm–hot phase of the intergalactic medium. We show that this mass density is consistent, within the uncertainties, with the mass density of the missing baryons.

1. Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, Massachusetts 02138, USA
2. Astronomy Department, The Ohio State University, Columbus, Ohio 43210, USA
3. University of California-Berkeley, Berkeley, California, 94720 USA
4. Istituto de Astronomia, UNAM, Mexico City, DF 04510, Mexico
5. Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

Correspondence to: Fabrizio Nicastro¹ Correspondence and requests for materials should be addressed to F.N. (Email: fnicastro@cfa.harvard.edu).

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