Missing baryon problem

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In cosmology, the missing baryon problem is an observed discrepancy between the amount of baryonic matter detected from shortly after the Big Bang and from more recent epochs. Observations of the cosmic microwave background and Big Bang nucleosynthesis studies have set constraints on the abundance of baryons in the early universe, finding that baryonic matter accounts for approximately 4.8% of the energy contents of the Universe. [1] [2] At the same time, a census of baryons in the recent observable universe has found that observed baryonic matter accounts for less than half of that amount. [3] [4] This discrepancy is commonly known as the missing baryon problem. The missing baryon problem is different from the dark matter problem, which is non-baryonic in nature. [5]

Contents

Early universe measurements

The abundance of baryonic matter in the early universe can be obtained indirectly from two independent methods:

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

Late universe 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 a range of techniques:

Generated image of the cosmic web which contains warm-hot regions where the missing baryons have been detected. Map of the Cosmic Web Generated from Slime Mould Algorithm.jpg
Generated image of the cosmic web which contains warm-hot regions where the missing baryons have been detected.

Prior to 2017, baryons were found to be distributed 10% inside galaxies, 50–60% in the circum-galactic medium, and 30–40% unaccounted, therefore accounting for about 70% of theoretical predictions. [4]

Large scale galaxy surveys in the 2000s revealed a baryon deficit. This led theorists to reexamine their models and predict that gas must flow between galaxies and galaxy clusters.

Warm-hot intergalactic medium

The Lambda-CDM model of the big bang predicts that matter between galaxies in the universe is distributed in web-like formations with a low density (1–10 particles per cubic meter) known as the Warm-hot intergalactic medium (WHIM). Cosmological hydrodynamical simulations from theory predict that a fraction of the missing baryons are located in galactic haloes at temperatures of 106 K [12] and the (WHIM) at temperatures of 105–107 K, with recent observations providing strong support. [13] [14] The WHIM is composed of three states: [15]

The warm phase of the WHIM had been previously detected and composes around 15% of the baryon content. [16] [17] The WHIM is mostly composed of ionized hydrogen. This creates difficulty for astronomers trying to detect baryons in the WHIM. It is easier to detect the WHIM through highly ionized oxygen such as OVI and OVII absorption. [18] [19] [20] [21]

Universe composition

The distribution of known baryons in the universe. BaryonPiev5.png
The distribution of known baryons in the universe.

The census of known baryons in the universe tallied to around 60% of total baryons until the resolution of the missing baryon problem. This is in distinction from composition of the entire universe which includes dark energy and dark matter of which baryonic matter composes only 5%. [19] Around 7% of baryons exists in stars and galaxies, while most of it exists around galaxies or galaxy clusters. The Lyman-alpha forest contains around 28% of the baryons. [17] The warm phase of the WHIM was detected by soft X-ray absorption in 2012 to establish 15% of total baryon content. [4] [22] The intracluster medium (ICM) accounts for around 4% of total baryon content. It is composed of mostly ionized hydrogen and is about 10% of a galaxy cluster's total mass; the rest being dark matter. The ICM is low density with around 10−3 particles per cm3. The circum-galactic medium (CGM) was confirmed in 2003 by Chandra and Xmm-Newton . The CGM is a large sphere surrounding galaxies with a radius > 70 - 200 kpc. [17] The CGM accounts for 5% of total baryons in the universe. [14]

Detection methods

There are three main methods of detecting the WHIM where the missing baryons lie: the Sunyaev-Zel'dovich effect, Lyman-alpha emission lines, and metal absorption lines.

Sunyaev-Zel'dovich effect

The thermal Sunyaev-Zel'dovich (tSZ) effect occurs when photons from the CMB inverse Compton scatter off ionized gas. For detecting baryons, the ionized gas from the WHIM is scattered by the CMB photons. The y-parameter quantifies the strength of the tSZ effect and is defined as:

, [23]

where is the Boltzmann constant, is the Thompson cross-section, is electron number density, is the electron rest mass energy, and is the temperature. Finding the y-parameter and overlaying that with a map of cosmic filament from millions of galaxies allows astronomers to find the weak signal from the WHIM. The y-parameter signal from a galaxy pair is overlaid on a model for galaxy halos. The signals are subtracted to reveal a signal between the two galaxies. [23] This resulting signal is the filament. To ensure the signal is not coming from any other source, astronomers generate a control simulation which they use to compare and are able to determine that source must be from the WHIM. [24]

Lyman-Alpha emission

The Lyman-alpha (Lyα) emission lines are detected from ionized hydrogen in cosmic filament. A source, such as a quasar, ionizes hydrogen in the cosmic filament leaving detectable dips in the absorption lines. [25]

Metal absorption lines

Highly ionized oxygen like O+6, O+7, and O+8 absorption lines in the soft X-rays at energies of 0.6–0.8 keV. The column density of these lines can be derived:

,

where is the abundance of a particular oxygen ion, is Hubble's constant, is the critical density . [9]

Claimed resolutions

In general, the missing baryon problem is a major unsolved problem in physics. Various scientists have proposed explanations, but none have received acceptance as adequately addressing the issue.

One claim of a solution was published in 2017 when two groups of scientists said they found evidence for the location of missing baryons in intergalactic matter. The missing baryons had been postulated to exist as hot strands between galaxy pairs in the Warm-hot intergalactic medium (WHIM). Since the strands are diffuse and they are not hot enough to emit x-rays, they are difficult to detect. The groups used the thermal Sunyaev–Zeldovich effect to measure the density of the strands in the local universe. 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, which the groups said was the exact amount needed to solve the problem. [13] [26] [23] [16] Even if granted to be accurate, these works only describe the distribution of baryons between nearby galaxies and do not provide a complete picture of cosmic gas in the late universe.

A 2021 article postulated that approximately 50% of all baryonic matter is outside dark matter haloes, filling the space between galaxies, and that this would explain the missing baryons not accounted for in the 2017 paper. [27]

Late 2010s and early 2020s

In the late 2010s and early 2020s, several groups observed the intergalactic medium and circum-galactic medium to obtain more measurements and observations of baryons to support the leading observations. Baryons have more or less been found, so groups are working to detect them to a higher level of significance. Methods used include soft X-ray, OVI, OVII, and OVIII absorption. [14]

In 2019, a group led by Orsolya E. Kovács detected OVII absorption in the X-ray spectrum of 17 stacked quasars, corresponding to WHIM in filaments of overdensity around 5–9 times the average cosmological density at the epochs of the individual quasars. [12] In 2020 astrophysicists reported the first direct X-ray emissions measurement of baryonic matter of cosmic web filaments. [25] [14] Both results are consistent with WHIM accounting for the missing baryons. [12] [25]

Related Research Articles

<span class="mw-page-title-main">Big Bang</span> Physical theory

The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. The notion of an expanding universe was first scientifically originated by physicist Alexander Friedmann in 1922 with the mathematical derivation of the Friedmann equations.

<span class="mw-page-title-main">Cosmic microwave background</span> Trace radiation from the early universe

The cosmic microwave background, or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dark. However, a sufficiently sensitive radio telescope detects a faint background glow that is almost uniform and is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum. The accidental discovery of the CMB in 1965 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s.

<span class="mw-page-title-main">Dark matter</span> Concept in cosmology

In astronomy, dark matter is a hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be observed. Such effects occur in the context of formation and evolution of galaxies, gravitational lensing, the observable universe's current structure, mass position in galactic collisions, the motion of galaxies within galaxy clusters, and cosmic microwave background anisotropies.

<span class="mw-page-title-main">Galaxy groups and clusters</span> Largest known gravitationally bound object in universe; aggregation of galaxies

Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large-scale structure of the Universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain ten to thousands of individual galaxies. The clusters themselves are often associated with larger, non-gravitationally bound, groups called superclusters.

<span class="mw-page-title-main">Sunyaev–Zeldovich effect</span> Spectral distortion of cosmic microwave background in galaxy clusters

The Sunyaev–Zeldovich effect is the spectral distortion of the cosmic microwave background (CMB) through inverse Compton scattering by high-energy electrons in galaxy clusters, in which the low-energy CMB photons receive an average energy boost during collision with the high-energy cluster electrons. Observed distortions of the cosmic microwave background spectrum are used to detect the disturbance of density in the universe. Using the Sunyaev–Zeldovich effect, dense clusters of galaxies have been observed.

<span class="mw-page-title-main">Reionization</span> Process that caused matter to reionize early in the history of the Universe

In the fields of Big Bang theory and cosmology, reionization is the process that caused electrically neutral atoms in the universe to reionize after the lapse of the "dark ages".

<span class="mw-page-title-main">Lambda-CDM model</span> Model of Big Bang cosmology

The Lambda-CDM, Lambda cold dark matter, or ΛCDM model is a mathematical model of the Big Bang theory with three major components:

  1. a cosmological constant, denoted by lambda (Λ), associated with dark energy
  2. the postulated cold dark matter, denoted by CDM
  3. ordinary matter
<span class="mw-page-title-main">Structure formation</span> Formation of galaxies, galaxy clusters and larger structures from small early density fluctuations

In physical cosmology, structure formation describes the creation of galaxies, galaxy clusters, and larger structures starting from small fluctuations in mass density resulting from processes that created matter. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.8 billion years ago. However, looking at the night sky today, structures on all scales can be seen, from stars and planets to galaxies. On even larger scales, galaxy clusters and sheet-like structures of galaxies are separated by enormous voids containing few galaxies. Structure formation models gravitational instability of small ripples in mass density to predict these shapes, confirming the consistency of the physical model.

<span class="mw-page-title-main">Baryon asymmetry</span> Imbalance of matter and antimatter in the observable universe

In physical cosmology, the baryon asymmetry problem, also known as the matter asymmetry problem or the matter–antimatter asymmetry problem, is the observed imbalance in baryonic matter and antibaryonic matter in the observable universe. Neither the standard model of particle physics nor the theory of general relativity provides a known explanation for why this should be so, and it is a natural assumption that the universe is neutral with all conserved charges. The Big Bang should have produced equal amounts of matter and antimatter. Since this does not seem to have been the case, it is likely some physical laws must have acted differently or did not exist for matter and/or antimatter. Several competing hypotheses exist to explain the imbalance of matter and antimatter that resulted in baryogenesis. However, there is as of yet no consensus theory to explain the phenomenon, which has been described as "one of the great mysteries in physics".

<span class="mw-page-title-main">South Pole Telescope</span> Telescope at the South Pole

The South Pole Telescope (SPT) is a 10-metre (390 in) diameter telescope located at the Amundsen–Scott South Pole Station, Antarctica. The telescope is designed for observations in the microwave, millimeter-wave, and submillimeter-wave regions of the electromagnetic spectrum, with the particular design goal of measuring the faint, diffuse emission from the cosmic microwave background (CMB). Key results include a wide and deep survey of discovering hundreds of clusters of galaxies using the Sunyaev–Zel'dovich effect, a sensitive 5 arcminute CMB power spectrum survey, and the first detection of B-mode polarized CMB.

In astronomy, the intracluster medium (ICM) is the superheated plasma that permeates a galaxy cluster. The gas consists mainly of ionized hydrogen and helium and accounts for most of the baryonic material in galaxy clusters. The ICM is heated to temperatures on the order of 10 to 100 megakelvins, emitting strong X-ray radiation.

<span class="mw-page-title-main">Diffusion damping</span> Physical process in cosmology

In modern cosmological theory, diffusion damping, also called photon diffusion damping, is a physical process which reduced density inequalities (anisotropies) in the early universe, making the universe itself and the cosmic microwave background radiation (CMB) more uniform. Around 300,000 years after the Big Bang, during the epoch of recombination, diffusing photons travelled from hot regions of space to cold ones, equalising the temperatures of these regions. This effect is responsible, along with baryon acoustic oscillations, the Doppler effect, and the effects of gravity on electromagnetic radiation, for the eventual formation of galaxies and galaxy clusters, these being the dominant large scale structures which are observed in the universe. It is a damping by diffusion, not of diffusion.

<span class="mw-page-title-main">Chronology of the universe</span> History and future of the universe

The chronology of the universe describes the history and future of the universe according to Big Bang cosmology.

<span class="mw-page-title-main">Warm–hot intergalactic medium</span> Plasma that cosmologists believe exists between galaxies

The warm–hot intergalactic medium (WHIM) is the sparse, warm-to-hot (105 to 107 K) plasma that cosmologists believe to exist in the spaces between galaxies and to contain 40–50% of the baryonic 'normal matter' in the universe at the current epoch. The WHIM can be described as a web of hot, diffuse gas stretching between galaxies, and consists of plasma, as well as atoms and molecules, in contrast to dark matter. The WHIM is a proposed solution to the missing baryon problem, where the observed amount of baryonic matter does not match theoretical predictions from cosmology.

<span class="mw-page-title-main">Baryon acoustic oscillations</span> Fluctuations in the density of the normal matter of the universe

In cosmology, baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter of the universe, caused by acoustic density waves in the primordial plasma of the early universe. In the same way that supernovae provide a "standard candle" for astronomical observations, BAO matter clustering provides a "standard ruler" for length scale in cosmology. The length of this standard ruler is given by the maximum distance the acoustic waves could travel in the primordial plasma before the plasma cooled to the point where it became neutral atoms, which stopped the expansion of the plasma density waves, "freezing" them into place. The length of this standard ruler can be measured by looking at the large scale structure of matter using astronomical surveys. BAO measurements help cosmologists understand more about the nature of dark energy by constraining cosmological parameters.

<span class="mw-page-title-main">Recombination (cosmology)</span> Epoch c. 370,000 years after the Big Bang

In cosmology, recombination refers to the epoch during which charged electrons and protons first became bound to form electrically neutral hydrogen atoms. Recombination occurred about 378000 years after the Big Bang. The word "recombination" is misleading, since the Big Bang theory does not posit that protons and electrons had been combined before, but the name exists for historical reasons since it was named before the Big Bang hypothesis became the primary theory of the birth of the universe.

Abell 222 is a galaxy cluster in the constellation of Cetus. It holds thousands of galaxies together. It is located at a distance of 2.4 billion light-years from Earth.

High-velocity clouds (HVCs) are large collections of gas found throughout the galactic halo of the Milky Way. Their bulk motions in the local standard of rest have velocities which are measured in excess of 70–90 km s−1. These clouds of gas can be massive in size, some on the order of millions of times the mass of the Sun, and cover large portions of the sky. They have been observed in the Milky Way's halo and within other nearby galaxies.

In cosmology, intensity mapping is an observational technique for surveying the large-scale structure of the universe by using the integrated radio emission from unresolved gas clouds.

<span class="mw-page-title-main">Cosmic microwave background spectral distortions</span> Fluctuations in the energy spectrum of the microwave background

CMB spectral distortions are tiny departures of the average cosmic microwave background (CMB) frequency spectrum from the predictions given by a perfect black body. They can be produced by a number of standard and non-standard processes occurring at the early stages of cosmic history, and therefore allow us to probe the standard picture of cosmology. Importantly, the CMB frequency spectrum and its distortions should not be confused with the CMB anisotropy power spectrum, which relates to spatial fluctuations of the CMB temperature in different directions of the sky.

References

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  2. ↑ Cooke, Ryan J.; Pettini, Max; Steidel, Charles C. (2018-03-12). "One Percent Determination of the Primordial Deuterium Abundance". The Astrophysical Journal. 855 (2): 102. arXiv: 1710.11129 . Bibcode:2018ApJ...855..102C. doi: 10.3847/1538-4357/aaab53 . ISSN   1538-4357. S2CID   56367851.
  3. ↑ Henry C. Ferguson. ""The Case of the "Missing Baryons""".
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  5. ↑ See Lambda-CDM model. Baryons make up only ~5% of the universe, while dark matter makes up 26.8%.
  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.
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  13. 1 2 "Half the universe's missing matter has just been finally found". New Scientist. Retrieved 2017-10-12.
  14. 1 2 3 4 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. CC-BY icon.svg Available under CC BY 4.0.
  15. ↑ Nicastro, F (2018). "Confirming the detection of two WHIM systems along the line of sight to 1ES 1553+113". Frascati Physics Series. 66. arXiv: 1811.03498 .
  16. 1 2 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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  18. ↑ We think we've found the missing matter in the universe - Ask a Spaceman!, 27 May 2020, retrieved 2021-04-30
  19. 1 2 Missing Baryonic Mass in the Universe... Found!, 17 July 2018, retrieved 2021-04-30
  20. ↑ We May Have Just Found the Universe's Missing Baryonic Matter, 29 June 2018, retrieved 2021-04-30
  21. ↑ Furlanetto, S. R., Phillips, L. A., & Kamionkowski, M. (2005). "Highly-Ionized Oxygen Absorbers in the Intergalactic Medium". Monthly Notices of the Royal Astronomical Society. 359 (1): 295–307. arXiv: astro-ph/0412351 . Bibcode:2005MNRAS.359..295F. doi: 10.1111/j.1365-2966.2005.08885.x . S2CID   14207531.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  22. ↑ Nicastro, F., Elvis, M., Krongold, Y., Mathur, S., Gupta, A., Danforth, C., Barcons, X., Borgani, S., Branchini, E., & Chen, R. (June 1, 2013). "Chandra view of the warm-hot intergalactic medium toward absorption-line detections and identifications" (PDF). The Astrophysical Journal. 769. doi:10.1088/0004-637X/769/2/90. hdl:10261/109273. S2CID   119259004.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  23. 1 2 3 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.
  24. ↑ Hinkle, Jason (2020-02-07). "Where Are All the Baryons?". Astrobites. Retrieved 2021-05-01.
  25. 1 2 3 Half the universe was missing... until now, 31 July 2020, retrieved 2021-04-30
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  27. ↑ Chaves-Montero, Jonás; Hernández-Monteagudo, Carlos; Angulo, Raúl E; Emberson, J D (2021-03-25). "Measuring the evolution of intergalactic gas from z = 0 to 5 using the kinematic Sunyaev–Zel'dovich effect". Monthly Notices of the Royal Astronomical Society. 503 (2): 1798–1814. arXiv: 1911.10690 . doi: 10.1093/mnras/staa3782 . ISSN   0035-8711.
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