Missing baryon problem

The missing baryon problem was a problem related to the fact that the observed amount of baryonic matter did not match theoretical predictions. The density of baryons can be constrained according to Big Bang nucleosynthesis and the cosmic microwave background. Observations by the Planck spacecraft in 2015, yielded a theoretical value for baryonic matter of 4.85% of the contents of the Universe.[1] However, directly adding up all the known baryonic matter produces a baryonic density slightly less than half of this.[2] The missing baryon problem is distinct from the dark matter problem, which is mainly non-baryonic in nature.[3]

Not to be confused with the dark matter problem or the baryon asymmetry problem.

The missing baryons are believed to be located in the warm–hot intergalactic medium (WHIM) (hot intergalactic gas), with recent observations providing strong support.[4][5]

Theoretical predictions

The density of baryonic matter can be obtained indirectly from two independent methods.

The CMB constraint ()[1] is much more precise than the BBN constraint (),[9][10] but the two are in agreement.

Observations

The density of baryonic matter can be obtained directly by summing up all the known baryonic matter. This is highly nontrivial, since although luminous matter such as stars and galaxies are easily summed, baryonic matter can also exist in highly non-luminous form, such as black holes, planets, and highly diffuse interstellar gas. Nonetheless it can still be done, using techniques such as:

  • Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars. The resulting spectra can be used to infer the mass between the star and the observer (us).[11]
  • Gravitational microlensing. If a planet or other dark object moves between the observer and a faraway source, the image of the source is distorted. The mass of the dark object can be inferred based on the amount of distortion.

Prior to 2017, the result came to about 70% of the theoretical predictions.[12]

In 2020 astrophysicists reported the first direct X-ray emissions measurement of baryonic matter of cosmic web filaments, strengthening empirical support for the recent solution to the problem.[13][5]

Resolution

The missing baryon problem was proclaimed to be solved in 2017 when two groups of scientists who were working independently managed to find the missing baryons in intergalactic matter. The missing baryons had been postulated to exist as hot strands between galaxy pairs. Since the strands are diffuse and they are not hot enough to emit x-rays, they are difficult to detect. The groups used the Sunyaev–Zeldovich effect to measure the density of the strands. If baryons are present there, then some amount of energy should be lost when light from the cosmic microwave background scatters off of them. These show up as very dim patches in the CMB. The patches are too dim to see directly, but when overlaid with the visible galaxy distribution, become detectable. The density of the strands comes up to about 30% of the baryonic density, the exact amount needed to solve the problem.[4][14][15][16]

References
  1. Ade, P.A.R.; et al. (2016). "Planck 2015 results. XIII. Cosmological parameters". Astron. Astrophys. 594: A13. arXiv:1502.01589. Bibcode:2016A&A...594A..13P. doi:10.1051/0004-6361/201525830. S2CID 119262962.
  2. Henry C. Ferguson. ""The Case of the "Missing Baryons""".
  3. See Lambda-CDM model. Baryons make up only ~5% of the universe, while dark matter makes up 26.8%.
  4. "Half the universe's missing matter has just been finally found". New Scientist. Retrieved 2017-10-12.
  5. Nicastro, F.; Kaastra, J.; Krongold, Y.; Borgani, S.; Branchini, E.; Cen, R.; Dadina, M.; Danforth, C. W.; Elvis, M.; Fiore, F.; Gupta, A.; Mathur, S.; Mayya, D.; Paerels, F.; Piro, L.; Rosa-Gonzalez, D.; Schaye, J.; Shull, J. M.; Torres-Zafra, J.; Wijers, N.; Zappacosta, L. (2018). "Observations of the missing baryons in the warm–hot intergalactic medium". Nature. 558 (7710): 406–409. arXiv:1806.08395. Bibcode:2018Natur.558..406N. doi:10.1038/s41586-018-0204-1. PMID 29925969. S2CID 49347964. Available under CC BY 4.0.
  6. Achim Weiss, "Big Bang Nucleosynthesis: Cooking up the first light elements Archived 2013-02-06 at the Wayback Machine" in: Einstein Online Vol. 2 (2006), 1017
  7. Raine, D.; Thomas, T. (2001). An Introduction to the Science of Cosmology. IOP Publishing. p. 30. ISBN 978-0-7503-0405-4.
  8. Canetti, L.; Drewes, M.; Shaposhnikov, M. (2012). "Matter and Antimatter in the Universe". New J. Phys. 14 (9): 095012. arXiv:1204.4186. Bibcode:2012NJPh...14i5012C. doi:10.1088/1367-2630/14/9/095012. S2CID 119233888.
  9. Mike Anderson. "Missing Baryons" (PDF).
  10. Fields, Brian D; Molaro, Paolo; Sarkar, Subir (2014). "Big-Bang Nucleosynthesis". Chinese Physics C. 38 (9): 339–344. arXiv:1412.1408. Bibcode:2014ChPhC..38i0001O. doi:10.1088/1674-1137/38/9/090001.
  11. See Lyman-alpha forest.
  12. Shull, J. Michael; Smith, Britton D; Danforth, Charles W (2012). "The Baryon Census in a Multiphase Intergalactic Medium: 30% of the Baryons May Still be Missing". The Astrophysical Journal. 759 (1): 23. arXiv:1112.2706. Bibcode:2012ApJ...759...23S. doi:10.1088/0004-637X/759/1/23. S2CID 119295243.
  13. "Has the hidden matter of the universe been discovered?". phys.org. Retrieved 8 December 2020.
  14. Tanimura, Hideki; Hinshaw, Gary; McCarthy, Ian G; Ludovic Van Waerbeke; Ma, Yin-Zhe; Mead, Alexander; Hojjati, Alireza; Tröster, Tilman (2017). "A Search for Warm/Hot Gas Filaments Between Pairs of SDSS Luminous Red Galaxies". Monthly Notices of the Royal Astronomical Society. 483: 223–234. arXiv:1709.05024. Bibcode:2018MNRAS.tmp.2970T. doi:10.1093/mnras/sty3118. S2CID 119440127.
  15. Anna de Graaff; Cai, Yan-Chuan; Heymans, Catherine; Peacock, John A (2019). "Missing baryons in the cosmic web revealed by the Sunyaev-Zel'dovich effect". Astronomy & Astrophysics. A48: 624. arXiv:1709.10378. doi:10.1051/0004-6361/201935159. S2CID 119262891.
  16. Nicastro, F.; Kaastra, J.; Krongold, Y.; Borgani, S.; Branchini, E.; Cen, R.; Dadina, M.; Danforth, C. W.; Elvis, M. (June 2018). "Observations of the missing baryons in the warm–hot intergalactic medium". Nature. 558 (7710): 406–409. arXiv:1806.08395. Bibcode:2018Natur.558..406N. doi:10.1038/s41586-018-0204-1. ISSN 0028-0836. PMID 29925969. S2CID 49347964.
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