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| missing baryon problem | |
|---|---|
| Name | Missing baryon problem |
| Field | Cosmology |
| Discovered | 1990s |
| Status | Ongoing research |
missing baryon problem The missing baryon problem describes the discrepancy between the abundance of baryonic matter predicted by observations of the Cosmic Microwave Background and Big Bang nucleosynthesis and the lower amount directly observed in stars, gas, and galaxies. First highlighted by comparisons between Wilkinson Microwave Anisotropy Probe results and surveys such as the Sloan Digital Sky Survey and measurements of primordial element abundances from studies of Henry Norris Russell-era spectroscopy, it motivates searches for diffuse baryons in the low-redshift universe. Resolving this problem bears on interpretations of results from missions like Planck (spacecraft), measurements by the Hubble Space Telescope, and large-scale structure results from projects including Dark Energy Survey and 2dF Galaxy Redshift Survey.
The problem arises because baryon density inferred from anisotropies measured by Planck (spacecraft) and WMAP and primordial deuterium measured in quasar absorption lines linked to work by Geoffrey Burbidge and Margaret Burbidge implies Ω_b that exceeds the sum of baryons seen in inventories of stars cataloged by Two Micron All-Sky Survey, cold gas mapped by Arecibo Observatory surveys, and hot gas in clusters studied with Chandra X-ray Observatory. Early synthesis by researchers affiliated with institutions such as Harvard University, Institute for Advanced Study, and Max Planck Society framed the missing component as a key question for frameworks developed at institutions like California Institute of Technology and Princeton University.
Observational lines include discrepancies noted between baryon density from Big Bang nucleosynthesis constraints using spectra from Keck Observatory, abundance patterns analyzed by teams at University of Cambridge and University of Oxford, and baryon census efforts drawing on data from the Sloan Digital Sky Survey, Galaxy Evolution Explorer, and ROSAT. Measurements of the baryon fraction in galaxy clusters by groups using Chandra X-ray Observatory, XMM-Newton, and the Suzaku (satellite) show cluster baryon fractions consistent with cosmological Ω_b but leave diffuse components beyond virial radii unaccounted for, a tension investigated by researchers at European Space Agency and NASA. Observations of the intergalactic medium through quasar sightlines with instruments on Hubble Space Telescope and ground-based spectrographs on Very Large Telescope and Subaru Telescope reveal the low-redshift Lyman-alpha forest and metal-line absorbers but do not fully close the budget, prompting searches informed by surveys like COS-Halos and eBOSS.
Leading proposals posit baryons reside in diffuse phases such as the warm–hot intergalactic medium (WHIM) predicted by shock heating during structure formation in models developed at Los Alamos National Laboratory and Lawrence Berkeley National Laboratory. Candidate reservoirs include warm-hot gas traced by O VI and Ne VIII absorption identified by teams at Johns Hopkins University and University of Maryland, circumgalactic media around galaxies surveyed in programs affiliated with Yale University and University of Chicago, and faint X-ray–emitting filaments connecting clusters studied in contexts involving Coma Cluster and Perseus Cluster. Alternative ideas invoke feedback processes driven by Active galactic nucleus jets from sources such as Messier 87 and star-formation–driven winds seen in galaxies like M82, with theoretical frameworks advanced by groups at Stanford University and Massachusetts Institute of Technology.
Detection strategies combine ultraviolet spectroscopy of quasar sightlines using the Cosmic Origins Spectrograph on Hubble Space Telescope and ground-based echelle spectrographs on Keck Observatory and Very Large Telescope with X-ray absorption and emission searches using Chandra X-ray Observatory, XMM-Newton, and the planned Athena (spacecraft). Sunyaev–Zel'dovich effect measurements using instruments on Atacama Cosmology Telescope, South Pole Telescope, and Planck (spacecraft) probe hot, diffuse baryons through microwave background distortions, while fast radio burst (FRB) dispersion measures obtained by arrays like CHIME and Australian Square Kilometre Array Pathfinder provide integrated electron-column constraints, pursued by teams at National Radio Astronomy Observatory and University of Toronto.
Hydrodynamical cosmological simulations such as Illustris, EAGLE, and Magneticum developed by collaborations spanning Max Planck Society, ETH Zurich, and Leiden University predict that a large fraction of baryons should exist in the WHIM. These simulations incorporate subgrid feedback prescriptions calibrated by comparisons to observations from Hubble Space Telescope, Chandra X-ray Observatory, and the Sloan Digital Sky Survey and use codes like AREPO and GADGET developed by researchers at Heidelberg University and Max Planck Institute for Astrophysics. Discrepancies among simulation suites about the spatial distribution and ionization state of baryons motivate cross-validation by consortia including groups at Princeton University and University College London.
If a significant baryon fraction resides in diffuse WHIM or circumgalactic reservoirs, this affects interpretations of baryon acoustic oscillation measurements used by Baryon Oscillation Spectroscopic Survey and Dark Energy Survey and informs models of galaxy formation by teams at California Institute of Technology and Carnegie Institution for Science. The baryon partition impacts feedback prescriptions in semi-analytic models promoted at University of Cambridge and influences mass estimates of structures used in tests involving Lambda-CDM parameters constrained by Planck (spacecraft) and WMAP. Resolving the missing baryons also bears on chemical enrichment histories traced by observations of Lyman-alpha forest and metal absorbers analyzed by researchers affiliated with University of California, Berkeley.
Key open questions include the detailed temperature, density, and metallicity distribution of the WHIM, the role of feedback from Active galactic nucleus and supernovae in redistributing baryons, and the completeness of current absorption- and emission-based searches conducted by observatories like Hubble Space Telescope and Chandra X-ray Observatory. Future facilities such as Athena (spacecraft), proposed missions like Lynx (space telescope), upgraded radio arrays including Square Kilometre Array, and expanded FRB surveys at CHIME and MeerKAT are expected to tighten constraints. International collaborations among institutions such as NASA, European Space Agency, Max Planck Society, and university consortia will continue coordinated observational and simulation efforts to close the baryon census.