It has been known for decades that the observed number of baryons in the local Universe falls about 30-40 per cent short1,2 of the total number of baryons predicted 3 by Big Bang nucleosynthesis, as inferred4,5 from density fluctuations of the cosmic microwave background and seen during the first 2-3 billion years of the Universe in the so-called 'Lyman α forest'6,7 (a dense series of intervening H I Lyman α absorption lines in the optical spectra of background quasars). A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm-hot intergalactic medium. However, it is difficult to detect them there because the largest by far constituent of this gas-hydrogen-is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios8,9. Indeed, despite large observational efforts, only a few marginal claims of detection have been made so far2,10. Here we report observations of two absorbers of highly ionized oxygen (O VII) in the high-signal-to-noise-ratio X-ray spectrum of a quasar at a redshift higher than 0.4. These absorbers show no variability over a two-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar's intrinsic outflow or the host galaxy's interstellar medium implausible. The O VII systems lie in regions characterized by large (four times larger than average 11 ) galaxy overdensities and their number (down to the sensitivity threshold of our data) agrees well with numerical simulation predictions for the long-sought warm-hot intergalactic medium. We conclude that the missing baryons have been found.
Warm-hot Baryons Comprise 5-10 Per Cent of Filaments in the Cosmic WebD Eckert et al. Nature 528 (7580), 105-7. PMID 26632589.Observations of the cosmic microwave background indicate that baryons account for 5 per cent of the Universe's total energy content. In the local Universe, the census of …
The Far-Ultraviolet Signature of the 'Missing' Baryons in the Local Group of GalaxiesF Nicastro et al. Nature 421 (6924), 719-21. PMID 12610618.
The number of baryons detected in the low-redshift (z < 1) Universe is far smaller than the number detected in corresponding volumes at higher redshifts. Simulations o…
A Cosmic Web Filament Revealed in Lyman-α Emission Around a Luminous High-Redshift QuasarS Cantalupo et al. Nature 506 (7486), 63-6. PMID 24463517.Simulations of structure formation in the Universe predict that galaxies are embedded in a 'cosmic web', where most baryons reside as rarefied and highly ionized gas. Thi …
The Mass of the Missing Baryons in the X-ray Forest of the Warm-Hot Intergalactic MediumF Nicastro et al. Nature 433 (7025), 495-8. PMID 15690033.Recent cosmological measurements indicate that baryons comprise about four per cent of the total mass-energy density of the Universe, which is in accord with the predicti …
Resolving the Structure of Ionized Helium in the Intergalactic Medium With the Far Ultraviolet Spectroscopic ExplorerGA Kriss et al. Science 293 (5532), 1112-6. PMID 11498584.The neutral hydrogen (H I) and ionized helium (He II) absorption in the spectra of quasars are unique probes of structure in the early universe. We present Far-Ultraviole …