Dark matter

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Not to be confused with antimatter, dark energy, dark fluid, or dark flow. For other uses, see Dark Matter (disambiguation)
Dark matter is invisible. Based on the effect of gravitational lensing, a ring of dark matter has been inferred in this image of a galaxy cluster (CL0024+17) and has been represented in blue.[1]

Dark matter is a kind of matter that hypothetically accounts for most of the universe's mass. Its existence and properties are inferred from its gravitational effects on visible matter, on radiation and on the universe's large-scale structure. Dark matter has not been directly observed. Dark matter is transparent to electromagnetic radiation (light, cosmic rays, etc.) and/or is so dense and small that it fails absorb or emit enough radiation to appear via imaging technology.

According to the Planck mission team, based on the standard model of cosmology, the total mass–energy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.[2][3] Thus, dark matter constitutes 84.5%[note 1] of total mass, while dark energy plus dark matter constitute 95.1% of total mass–energy content.[4][5][6]

Astrophysicists employed the dark matter hypothesis to account for discrepancies between the mass of astronomical objects measured by their gravitational effects and as calculated from stars, gas and dust that visible. Their gravitational effects suggest that their masses are much greater than the observable matter survey suggests.

History[edit]

The first to suggest using stellar velocities to infer the presence of dark matter was Dutch astronomer Jacobus Kapteyn in 1922.[7][8]

Fellow Dutchman and radio astronomy pioneer Jan Oort hypothesized the existence of dark matter, in 1932.[9][10][8] Oort was studying stellar motions in the local galactic neighborhood and found that the mass in the galactic plane must be greater than what was observed, but this measurement was later determined to be erroneous.[11]

In 1933, Swiss astrophysicist Fritz Zwicky, who studied galactic clusters while working at the California Institute of Technology, made a similar inference.[12][13][14] Zwicky applied the virial theorem to the Coma cluster and obtained evidence of unseen mass that he called dunkle Materie 'dark matter'. Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and number of galaxies. He estimated that the cluster had about 400 times more mass than was visually observable. The gravity effect of the visible galaxies was far too small for such fast orbits, thus mass must be hidden from view. Based on these conclusions, Zwicky inferred that some unseen matter provided the mass and associated gravitation attraction to hold the cluster together. This was the first formal inference about the existence of dark matter.[15] Zwicky's estimates were off by more than an order of magnitude.[16] The same calculation today shows a smaller fraction, using greater values for luminous mass. However, Zwicky did correctly infer that the bulk of the matter was dark.[15]

The first robust indications that the mass to light ratio was anything other than unity came from measurements of galaxy rotation curves. In 1939, Horace W. Babcock reported the rotation curve for the Andromeda nebula, which suggested that the mass-to-luminosity ratio increases radially.[17] He attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral and not to missing matter.

Vera Rubin and Kent Ford in the 1960s–1970s were the first to postulate "dark matter" based upon robust evidence, using galaxy rotation curves.[18][19] Rubin worked with a new spectrograph to measure the velocity curve of edge-on spiral galaxies with greater accuracy.[19] This result was independently confirmed in 1978.[20] An influential paper presented Rubin's results in 1980.[21] Rubin found that most galaxies must contain about six times as much dark as visible mass.

A stream of further observations indicated its presence, including gravitational lensing of background objects by galaxy clusters, the temperature distribution of hot gas in galaxies and clusters and the pattern of anisotropies in the cosmic microwave background. According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle.[22][23] The search for this particle, by a variety of means, is one of the major efforts in particle physics.[24]

Cosmic background radiation[edit]

The NASA Cosmic Background Explorer (COBE) found the CMB spectrum to be a blackbody spectrum with a temperature of 2.726 K. In 1992, COBE detected CMB fluctuations (anisotropies) at a level of about one part in 105.[25]

In the following decade, CMB anisotropies were investigated by ground-based and balloon experiments. Their primary goal was to measure the angular scale of the first acoustic peak of the anisotropies' power spectrum, for which COBE had insufficient resolution. 2000–2001 experiments, most notably BOOMERanG[26] found the universe to be almost spatially flat by measuring the anisotropies' typical angular size.

During the 1990s, the first peak was measured with increasing sensitivity and by 2000 the BOOMERanG experiment reported that the highest power fluctuations occur at scales of approximately one degree. These measurements were able to rule out cosmic strings as the leading theory of cosmic structure formation, and suggested cosmic inflation was the correct theory.

Ground-based interferometers provided fluctuation measurements with higher accuracy, including the Very Small Array, the Degree Angular Scale Interferometer (DASI) and the Cosmic Background Imager (CBI). DASI first detected the CMB polarization,[27][28] and CBI provided the first E-mode polarization spectrum with compelling evidence that it is out of phase with the T-mode spectrum.[29] COBE's successor, the Wilkinson Microwave Anisotropy Probe (WMAP) provided the most detailed measurements of (large-scale) anisotropies in the CMB in 2009. ESA's Planck spacecraft returned more detailed results in 2012-2014.[30]

WMAP's measurements played the key role in establishing the Standard Model of Cosmology, namely the Lambda-CDM model, which posits a dark energy-dominated flat universe, supplemented by dark matter and atoms with density fluctuations seeded by a Gaussian, adiabatic, nearly scale invariant process. Its basic properties are determined by matter density, atom density, the universe's age (or equivalently, the Hubble constant), the initial fluctuation amplitude and their scale dependence.

In cosmology, the CMB is explained as relic radiation from shortly after the Big Bang. The CMB's anisotropies are explained as the result of acoustic oscillations in the photon-baryon plasma (prior to the CMB's emission after photons decoupled from baryons 379,000 years after the Big Bang) whose restoring force is gravity.[31]

Overview[edit]

Overview[edit]

Estimated distribution of matter and energy , today (top) and when the CMB was released (bottom)

Dark matter is invoked to explain gravitational effects on visible matter and gravitational lensing of background radiation, and to account for discrepancies between calculations of the mass of galaxies, clusters of galaxies and the entire universe made through dynamical and general relativistic means, and calculations based on the mass of the visible "luminous" matter these objects contain: stars and the gas and dust of the interstellar and intergalactic medium.[32]

The most widely accepted explanation for these phenomena is that dark matter exists and that it is most probably[22] composed of weakly interacting massive particles (WIMPs) that interact only through gravity and the weak force.

Evidence is insufficient to confirm or disconfirm either dark matter or various alternative explanations, such as modified gravitational laws and quantum gravitational laws.[33]

Many experiments to detect proposed dark matter particles through non-gravitational means are under way.[24]

One alternative theory suggests the existence of a “Hidden Valley”, a parallel world made of dark matter having little in common with ordinary matter[34] and that interacts with the visible universe only through gravity.[35][36]

According to observations of structures larger than star systems, as well as Big Bang cosmology interpreted under the Friedmann equations and the Friedmann–Lemaître–Robertson–Walker metric, dark matter accounts for 26.8% of the mass-energy content of the observable universe. In comparison, ordinary (baryonic) matter accounts for only 4.9% of the mass-energy content of the observable universe, with the remainder attributable to dark energy.[37] From these figures, ordinary matter accounts for 31.7% of its mass-energy content and 84.5% of the matter is dark.

Dark matter plays a central role in state-of-the-art modeling of cosmic structure formation and galaxy formation and evolution and has measurable effects on the anisotropies observed in the cosmic microwave background (CMB). All these lines of evidence suggest that galaxies, clusters of galaxies and the universe as a whole contain far more matter than that which is visible with electromagnetic radiation.[35]

Although the existence of dark matter is generally accepted, alternative theories, such as MOND and TeVeS try to account for the observations without invoking additional matter. However, these theories cannot account for the properties of galaxy clusters.[38]

Baryonic and nonbaryonic dark matter[edit]

Fermi-LAT observations of dwarf galaxies provide new insights on dark matter.

For many decades, astronomers assumed that dark matter was made of the same baryons (protons and neutrons) as all other known matter, but in a form that had been overlooked because it was non-luminous. It could not be in the form of a diffuse gas or dust, as that would be apparent when backlit by visible stars, but searches were organized for other astronomical bodies emitting little or no electromagnetic radiation such as massive compact halo objects.

Evidence eventually accumulated that baryonic dark matter could only account for a small fraction of the missing mass. The study of nucleosynthesis in the Big Bang gives an upper bound on the amount of baryonic matter,[39] which indicates that the vast majority of dark matter cannot be baryons and thus does not form atoms. It also cannot interact with ordinary matter via electromagnetic forces; in particular, dark matter particles do not carry electric charge.

Three separate lines of evidence suggest the majority of dark matter is not made of baryons:

  • The theory of Big Bang nucleosynthesis predicts the observed abundance of the chemical elements[40] and that baryonic matter accounts for around 4–5 percent of the universe's critical density. In contrast, evidence from large-scale structure and other observations indicates that the total matter density is about 30% of the critical density.
  • Large astronomical searches for gravitational microlensing, including the MACHO, EROS and OGLE projects, have shown that only a small fraction of the dark matter in the Milky Way can be present in dark compact objects; the excluded range covers objects above half the Earth's mass up to 30 solar masses, excluding nearly all the plausible candidates.
  • Detailed analysis of the small irregularities (anisotropies) in the cosmic microwave background observed by WMAP and Planck shows that around five-sixths of the total matter is in a form that does not interact significantly with ordinary matter or photons except through gravitational effects.

Candidates for nonbaryonic dark matter are hypothetical particles such as axions, or supersymmetric particles; neutrinos can only form a small fraction of the dark matter, due to limits from large-scale structure and high-redshift galaxies.[41] Unlike baryonic dark matter, nonbaryonic dark matter did not contribute to the formation of the elements in the early universe ("Big Bang nucleosynthesis")[22] and so its presence is revealed only via its gravitational effects. In addition, if the particles of which it is composed are supersymmetric, they can undergo annihilation interactions with themselves, possibly resulting in observable by-products such as gamma rays and neutrinos ("indirect detection").[41]

Nonbaryonic dark matter is classified in terms of the mass of the particle(s) that is assumed to make it up, and/or the typical velocity dispersion of those particles (since more massive particles move more slowly).

The three prominent hypotheses on nonbaryonic dark matter are cold dark matter (CDM), warm dark matter (WDM), and hot dark matter (HDM); some combination of these is also possible. The most widely discussed models for nonbaryonic dark matter are based on cold dark matter. The corresponding particle is most commonly assumed to be a weakly interacting massive particle (WIMP). Hot dark matter may include (massive) neutrinos, but observations imply that only a small fraction of dark matter can be hot. Cold dark matter leads to a "bottom-up" formation of structure while hot dark matter would result in a "top-down" formation scenario; since the late 1990s, the latter has been ruled out by observations of high-redshift galaxies such as the Hubble Ultra-Deep Field.[24]

Observational evidence[edit]

This artist’s impression shows the expected distribution of dark matter in the Milky Way galaxy as a blue halo of material surrounding the galaxy.[42]

Much of the evidence comes from the motions of galaxies.[43] Many of these appear to be fairly uniform, so by the virial theorem, the total kinetic energy should be half the galaxies' total gravitational binding energy. Observationally, the total kinetic energy is much greater. In particular, assuming the gravitational mass is due to only visible matter, stars far from the center of galaxies have much higher velocities than predicted by the virial theorem. Galactic rotation curves, which illustrate the velocity of rotation versus the distance from the galactic center, show the "excess" velocity. Dark matter is the most straightforward way of accounting for this discrepancy.

The distribution of dark matter in galaxies required to explain the motion of the observed matter suggests the presence of a roughly spherically symmetric, centrally concentrated halo of dark matter with the visible matter concentrated in a central disc.

Low surface brightness dwarf galaxies are important sources of information for studying dark matter. They have an uncommonly low ratio of visible to dark matter, and have few bright stars at the center that would otherwise impair observations of the rotation curve of outlying stars.

Gravitational lensing observations of galaxy clusters allow direct estimates of the gravitational mass based on its effect on light coming from background galaxies, since large collections of matter (dark or otherwise) gravitationally deflect light. In clusters such as Abell 1689, lensing observations confirm the presence of considerably more mass than is indicated by the clusters' light. In the Bullet Cluster, lensing observations show that much of the lensing mass is separated from the X-ray-emitting baryonic mass. In July 2012, lensing observations were used to identify a "filament" of dark matter between two clusters of galaxies, as cosmological simulations predicted.[44]

Galaxy rotation curves[edit]

Main article: Galaxy rotation curve
Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). Dark matter can explain the 'flat' appearance of the velocity curve out to a large radius

A galaxy rotation curve is a plot of the orbital velocities (i.e., the speeds) of visible stars or gas in that galaxy versus their radial distance from that galaxy's center. The rotational/orbital speeds of galaxies/stars does not decline with distance, unlike other orbital systems such as stars/planets and planets/moons that also have most of their mass at the centre. In the latter cases, this reflects the mass distributions within those systems. The mass observations for galaxies based on the light that they emit are far too low to explain the velocity observations.

The dark matter hypothesis supplies the missing mass, resolving the anomaly.[17]

A universal rotation curve can be expressed as the sum of an exponential distribution of visible matter that tapers to zero with distance from the center, and a spherical dark matter halo with a flat core of radius r0 and density ρ0 = 4.5 × 10−2(r0/kpc)−2/3 Mpc−3.[45]

Low-surface-brightness (LSB) galaxies have a much larger visible mass deficit than others. This property simplifies the disentanglement of the dark and visible matter contributions to the rotation curves.[24]

Rotation curves for some elliptical galaxies do display low velocities for outlying stars (tracked for example by the motion of embedded planetary nebulae). A dark-matter compliant hypothesis proposes that some stars may have been torn by tidal forces from disk-galaxy mergers from their original galaxies during the first close passage and put on outgoing trajectories, explaining the low velocities of the remaining stars even in the presence of a halo.[46][24]

Velocity dispersions of galaxies[edit]

Velocity dispersion estimates of elliptical galaxies[47] With some exceptions measurements of ellipticals generally indicate a relatively high dark matter content.

Diffuse interstellar gas measurements of galactic edges indicate missing ordinary matter beyond the visible boundary, but that galaxies are virialized (i.e., gravitationally bound and orbiting each other with velocities that correspond to predicted orbital velocities of general relativity) up to ten times their visible radii.[48] This has the effect of pushing up the dark matter as a fraction of the total matter from 50% as measured by Rubin to the now accepted value of nearly 95%.

Dark matter seems to be a small component or absent in some places. Globular clusters show little evidence of dark matter,[49] except that their orbital interactions with galaxies do support galactic dark matter.[citation needed] Star velocity profiles seemed to indicate a concentration of dark matter in the disk of the Milky Way. It now appears, however, that the high concentration of baryonic matter in the disk (especially in the interstellar medium) can account for this motion. Galaxy mass and light profiles appear to not match. The typical model for dark matter galaxies is a smooth, spherical distribution in virialized halos. This avoids small-scale (stellar) dynamical effects. A 2006 study explained the warp the Milky Way's disk by the interaction of the Large and Small Magellanic Clouds and the 20 fold increase in predicted mass from dark matter.[50]

In 2005, astronomers claimed to have discovered a galaxy made almost entirely of dark matter, 50 million light years away in the Virgo Cluster, which was named VIRGOHI21.[51] Unusually, VIRGOHI21 does not appear to contain visible stars: it was discovered with radio frequency observations of hydrogen. Based on rotation profiles, the scientists estimate that this object contains approximately 1000 times more dark matter than hydrogen and has a mass of about 1/10 that of the Milky Way. The Milky Way is estimated to have roughly 10 times as much dark matter as ordinary matter. Models of the Big Bang and structure formation suggested that such dark galaxies should be very common,[citation needed] but VIRGOHI21 was the first to be detected.

The velocity profiles of some galaxies such as NGC 3379 indicate an absence of dark matter.[52]

Galaxy clusters and gravitational lensing[edit]

Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689 indicates the presence of dark matter—enlarge the image to see the lensing arcs.

Galactic clusters also lack sufficient luminous matter to explain the measured orbital velocities of galaxies within them. Galaxy cluster masses have been estimated in three independent ways:

  • Radial velocity scatter of the galaxies within clusters
  • X-rays emitted by hot gas. Gas temperature and density can be estimated from the X-ray energy and flux; assuming pressure and gravity balance determines the cluster's mass profile. Chandra X-ray Observatory experiments use this technique to independently determine cluster mass. These observations generally indicate that baryonic mass is approximately 12–15 percent, in reasonable agreement with the Planck spacecraft cosmic average of 15.5–16 percent.[53]
  • Gravitational lensing (usually on more distant galaxies) predicts masses without relying on observations of dynamics (e.g., velocity). Multiple Hubble projects used this method to measure cluster masses.

Generally these methods find missing luminous matter.

Gravity acts as a lens to bend the light from a more distant source (such as a quasar) around a massive object (such as a cluster of galaxies) lying between the source and the observer in accordance with general relativity.

Strong lensing is the observed distortion of background galaxies into arcs when their light passes through such a gravitational lens. It has been observed around a few distant clusters including Abell 1689.[54] By measuring the distortion geometry, the mass of the intervening cluster can be obtained. In the dozens of cases where this has been done, the mass-to-light ratios obtained correspond to the dynamical dark matter measurements of clusters.[55]

Weak gravitational lensing investigates minute distortions of galaxies, using statistical analyses from vast galaxy surveys. By examining the apparent shear deformation of the adjacent background galaxies, astrophysicists can characterize the mean distribution of dark matter. The mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements.[56]

Galactic cluster Abell 2029 comprises thousands of galaxies enveloped in a cloud of hot gas and dark matter equivalent to more than 1014 M. At the center of this cluster is an enormous elliptical galaxy likely formed from many smaller galaxies.[57]

The Bullet Cluster: HST image with overlays. The total projected mass distribution reconstructed from strong and weak gravitational lensing is shown in blue, while the X-ray emitting hot gas observed with Chandra is shown in red.

The most direct observational evidence comes from the Bullet Cluster. In most regions dark and visible matter are found together,[58] due to their gravitational attraction. In the Bullet Cluster however, the two matter types split apart. This was apparently caused by a collision between two smaller clusters. Electromagnetic interactions among passing gas particles would then have caused the luminous matter to slow and settle near the point of impact. Because dark matter does not interact electromagnetically, it did not slow and continued past the center.

X-ray observations show that much of the luminous matter (in the form of 107–108 Kelvin[59] gas or plasma) is concentrated in the cluster's center. Weak gravitational lensing observations show that much of the missing mass would reside outside the central region. Unlike galactic rotation curves, this evidence is independent of the details of Newtonian gravity, directly supporting dark matter.[59]

Dark matter's observed behavior constrains whether and how much it scatters off other dark matter particles, quantified as its self-interaction cross section. If dark matter has no pressure, it can be described as a perfect fluid that has no damping.[60] The distribution of mass in galaxy clusters has been used to argue both for[61][62] and against[63] the significance of self-interaction.

An ongoing survey using the Subaru telescope uses weak lensing to analyze background light, bent by dark matter, to determine how the shape of the lens (how dark matter is distributed in the foreground). The survey studies galaxies more than a billion light-years distant, across an area greater than a thousand square degrees (about one fortieth of the entire sky).[64][65]

Cosmic microwave background[edit]

The cosmic microwave background by WMAP

Angular CMB fluctuations provide evidence for dark matter. The typical angular scales of CMB oscillations, measured as the power spectrum of the CMB anisotropies, reveal the different effects of baryonic and dark matter. Ordinary matter interacts strongly via radiation whereas dark matter particles (WIMPs) do not; both affect the oscillations by way of their gravity, so the two forms of matter have different effects.

The spectrum shows a large first peak and smaller successive peaks.[30] The first peak tells mostly about the density of baryonic matter, while the third peak relates mostly to the density of dark matter, measuring the density of matter and the density of atoms.[clarification needed]

Sky surveys and baryon acoustic oscillations[edit]

The early universe's acoustic oscillations affected visible matter by way of Baryon Acoustic Oscillation (BAO) clustering, in a way that can be measured with sky surveys such as the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey.[66] These measurements are consistent CMB metrics derived from the WMAP spacecraft and further constrain the Lambda CDM model and dark matter. Note that CMB data and the BAO adopt different distance scales.[31]

Type Ia supernova distance measurements[edit]

Main article: Type Ia supernova

Type Ia supernovae can be used as "standard candles" to measure extragalactic distances, and extensive data sets of these supernovae can be used to constrain cosmological models.[67] They constrain the dark energy density ΩΛ = ~0.713 for a flat, Lambda CDM universe and the parameter w for a quintessence model. The results are roughly consistent with those derived from the WMAP observations and further constrain the Lambda CDM model and (indirectly) dark matter.[31]

Lyman-alpha forest[edit]

Main article: Lyman-alpha forest

In astronomical spectroscopy, the Lyman-alpha forest is the sum of the absorption lines arising from the Lyman-alpha transition of neutral hydrogen in the spectra of distant galaxies and quasars. Lyman-alpha forest observations can also constrain cosmological models.[68] These constraints agree with those obtained from WMAP data.

Structure formation[edit]

Main article: Structure formation
3D map of the large-scale distribution of dark matter, reconstructed from measurements of weak gravitational lensing with the Hubble Space Telescope.[69]

Dark matter corresponds directly to measurements of the parameters associated with Friedmann cosmology solutions to general relativity. In particular, measurements of the cosmic microwave background anisotropies correspond to a cosmology where most matter interacts with photons more weakly than the forces that couple light to baryonic matter. Likewise, a significant amount of dark matter is necessary to explain the universe's large-scale structure.

Observations suggest that structure formation proceeds hierarchically, with the smallest structures collapsing first, followed by galaxies and then clusters of galaxies. As the structures collapse in the evolving universe, they begin to "light up" as baryonic matter heats up through gravitational contraction and approaches hydrostatic pressure balance.

Originally, baryonic matter had too high a temperature and pressure left over from the Big Bang to collapse and form smaller structures, such as stars, via the Jeans instability. Dark matter acts as a compactor, allowing the creation of structure that otherwise could not form. This model corresponds with statistical surveys of the visible structure and precisely to CMB predictions.

This model of structure formation requires something like cold dark matter to succeed. Computer simulations of billions of dark matter particles[70] confirmed that the cold dark matter model of structure formation is consistent with the structures observed through galaxy surveys, such as the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey, as well as observations of the Lyman-alpha forest. These studies were crucial in constructing the Lambda-CDM model that measures cosmological parameters, including the relative proportions of baryons and dark matter. The recent discovery of the structure of Laniakea, a 500 million light year structure is currently the limit to structural formation. Laniakea is not gravitationally bound and is projected to be torn apart by dark energy.[71]

Several points of tension separate observation and simulations. Observations have turned up 90-99% fewer small galaxies than permitted by dark matter-based predictions.[72][73] In addition, simulations predict dark matter distributions with a dense cusp near galactic centers, but the observed halos are smoother than predicted.

History of the search for its composition[edit]

Question dropshade.png Unsolved problem in physics:
What is dark matter? How is it generated? Is it related to supersymmetry?
(more unsolved problems in physics)

Although dark matter had historically been inferred from astronomical observations, its composition long remained speculative. Early theories of dark matter concentrated on hidden heavy normal objects (such as black holes, neutron stars, faint old white dwarfs, and brown dwarfs) as possible candidates, collectively known as massive compact halo objects or MACHOs. Astronomical surveys for gravitational microlensing, including the MACHO, EROS and OGLE projects, along with Hubble telescope searches for ultra-faint stars, found too few MACHOs.[74][75][76] Observations found too few cases to support speculations about hard-to-detect baryonic matter such as MACHOs and some forms of gas.[77][78][79]

Data from other lines of evidence, including galaxy rotation curves, gravitational lensing, structure formation, the fraction of baryons in clusters and cluster abundance combined with independent evidence for baryon density, indicated that 85–90% of the mass does not interact with the electromagnetic force. This "nonbaryonic dark matter" is evident through its gravitational effect. This matter is made of elementary particles other than the usual electrons, protons, neutrons and known neutrinos. The most commonly proposed particles were weakly interacting massive particles, including neutralinos, axions, or sterile neutrinos, along with many others.

Dark matter candidates can be divided into cold, warm and hot categories.[80] These categories refer to velocity rather than temperature, indicating how far they moved due to random motions in the early universe, before they slowed due to expansion – this is an important distance called the "free streaming length" (FSL). Primordial density fluctuations smaller than this length get washed out as particles spread from overdense to underdense regions, while larger fluctuations are unaffected; therefore this length sets a minimum scale for structure formation. The categories are divided in comparison to the size of a protogalaxy (an object that later evolves into a dwarf galaxy). Cold dark matter's FSL is much smaller,[81] warm's is similar and hot's is much larger.[82]

A fourth category called mixed dark matter was discarded (in the 1990s) following the discovery of dark energy.

Davis et al. wrote in 1985:

Candidate particles can be grouped into three categories on the basis of their effect on the fluctuation spectrum (Bond et al. 1983). If the dark matter is composed of abundant light particles which remain relativistic until shortly before recombination, then it may be termed "hot". The best candidate for hot dark matter is a neutrino ... A second possibility is for the dark matter particles to interact more weakly than neutrinos, to be less abundant, and to have a mass of order 1 keV. Such particles are termed "warm dark matter", because they have lower thermal velocities than massive neutrinos ... there are at present few candidate particles which fit this description. Gravitinos and photinos have been suggested (Pagels and Primack 1982; Bond, Szalay and Turner 1982) ... Any particles which became nonrelativistic very early, and so were able to diffuse a negligible distance, are termed "cold" dark matter (CDM). There are many candidates for CDM including supersymmetric particles.[83]

Another approximate dividing line is that warm's became non-relativistic when the universe was approximately 1 year old and 1 millionth of its present size and in the radiation-dominated era (photons and neutrinos), with a photon temperature 2.7 million K. Standard physical cosmology gives the particle horizon size as 2ct in the radiation-dominated era, thus 2 light-years. A region of this size would ultimately expand to 2 million light years (absent structure formation). The actual FSL is roughly 5 times larger than the above length, since it continues to grow slowly as particle velocities decrease inversely with the scale factor after they become non-relativistic; therefore, in this example the FSL would correspond to 10 million light-years or 3 Mpc today, around the size containing an average large galaxy.

2.7 million K gives a typical photon energy of 250 electron-volts, thereby setting a typical mass scale for "warm" dark matter: particles much more massive than this, such as GeV – TeV mass WIMPs, would become non-relativistic much earlier than 1 year after the Big Bang and thus have FSL's much smaller than a proto-galaxy, making them cold. Conversely, much lighter particles, such as neutrinos with masses of only a few eV, have FSL's much larger than a proto-galaxy, thus qualifying them as hot.

Cold dark matter[edit]

Main article: Cold dark matter

Cold dark matter offers the simplest explanation for most cosmological observations. It is dark matter composed of constituents with an FSL much smaller than a protogalaxy. This is the focus for dark matter research, as hot dark matter does not seem to be capable of galaxy or galaxy cluster formation, and most particle candidates slowed early.

The constituents of cold dark matter are unknown. Possibilities range from large objects like MACHOs (such as black holes[84]) or RAMBOs (such as clusters of brown dwarfs), to new particles such as WIMPs and axions.

Studies of big bang nucleosynthesis and gravitational lensing have convinced most scientists[24][85][86][87][88][89] that MACHOs including black holes[85][87] cannot make up more than a small fraction of dark matter.[22][85] According to A. Peter: "... the only really plausible dark-matter candidates are new particles."[86]

The DAMA/NaI experiment and its successor DAMA/LIBRA claimed to directly detect dark matter particles passing through the Earth, but many scientists remain skeptical, as negative results from similar experiments seem incompatible with the DAMA results.

Many supersymmetric models offer dark matter candidates in the form of the Lightest Supersymmetric Particle (LSP). Separately, heavy sterile neutrinos exist in non-supersymmetric extensions to the standard model that explain the small neutrino mass through the seesaw mechanism.

Warm dark matter[edit]

Main article: Warm dark matter

Warm dark matter refers to particles with an FSL comparable to the size of a protogalaxy. This leads to predictions that are similar to those for cold dark matter on large scales, including the CMB, galaxy clustering and large galaxy rotation curves, but with less small-scale density perturbations. This reduces the predicted abundance of dwarf galaxies and may lead to lower density of dark matter in the central parts of large galaxies; some researchers consider this to be a better fit to observations. A challenge for this model is the lack of particle candidates with the required mass ~ 300 eV to 3000 eV.

No known particles can be categorized as warm dark matter. A postulated candidate is the sterile neutrino: a heavier, slower form of neutrino that does not interact through the weak force (unlike other neutrinos). Some modified gravity theories, such as scalar-tensor-vector gravity, require warm dark matter to make their equations work.

Hot dark matter[edit]

Main article: Hot dark matter

Hot dark matter consists of particles whose FSL is much larger than the size of a protogalaxy. The neutrino qualifies. They were discovered independently, long before the hunt for dark matter: they were postulated in 1930, and detected in 1956. Neutrinos' mass is less than .000001 that of an electron. Neutrinos interact with normal matter only via gravity and the weak force, making them difficult to detect (the weak force only works over a small distance, thus a neutrino triggers a weak force event only if it hits a nucleus head-on). This makes them 'weakly interacting light particles' (WILPs), as opposed to cold dark matter's theoretical candidates, the WIMPs.

The three different known flavors of neutrinos are the electron, muon, and tau. Their masses are slightly different. The resolution to the solar neutrino problem demonstrated that neutrinos oscillate among the flavors as they move. It is hard to determine an exact upper bound on the collective average mass of the three neutrinos (or for any of the three individually). For example, if the average neutrino mass were over 50 eV/c2 (less than .00001 of the mass of an electron), just by the sheer number of them, the universe would collapse due to their mass. Other observations have served to estimate an upper-bound for the neutrino mass. Using CMB data and other methods, their average mass probably does not exceed 0.3 eV/c2. Thus, the normal forms of neutrinos cannot explain dark matter.[90]

Hot dark matter was popular in the early 1980s, but it suffers from a severe problem: because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies that the first objects that can form are huge supercluster-size pancakes, which then fragment into galaxies. Deep-field observations show instead that galaxies formed first, followed by clusters and superclusters as galaxies clump together.

Mixed dark matter[edit]

Main article: Mixed dark matter

Mixed dark matter is an obsolete model, with a specifically chosen mass ratio of 80% cold and 20% hot content. The two do coexist in nature. However, this ratio was chosen because, during the early 1990s COBE and large-scale galaxy clustering observations excluded a universe with critical density of cold dark matter; the 80/20 mixed dark matter model could reconcile the two. The discovery of the accelerating universe (from supernovae) and more accurate measurements of CMB anisotropy and galaxy clustering then excluded this model (leaving LambdaCDM).

Detection[edit]

If the dark matter within the Milky Way is made up of WIMPs, then millions, possibly billions, of WIMPs must pass through every square centimeter of the Earth each second.[91][92] Many experiments (current and planned), aim to test this hypothesis. Although WIMPs are popular dark matter search candidates,[24] other experiments search for other particle candidates; the Axion Dark Matter eXperiment (ADMX) searches for the dark matter axion. Another candidate is heavy hidden sector particles which only interact with ordinary matter via gravity.

These experiments can be divided into two classes: direct detection experiments, which search for the scattering of dark matter particles off atomic nuclei within a detector; and indirect detection, which look for the products of WIMP annihilations.[41]

Synthesis[edit]

An alternative approach to the detection of WIMPs in nature is to produce them in the laboratory. Experiments with the Large Hadron Collider (LHC) may be able to detect WIMPs produced in collisions of the LHC proton beams. Because a WIMP has negligible interaction with matter, it may be detected indirectly as (large amounts of) missing energy and momentum that escape the detectors, provided other (non-negligible) collision products are detected.[93] These experiments could show that WIMPs can be created, but a direct detection experiment must still show that they exist in sufficient numbers to account for dark matter.

Direct detection experiments[edit]

Direct detection experiments operate in detectors deep underground to reduce the interference from cosmic rays. These include: the Stawell mine (Australia); the Soudan mine; the SNOLAB underground laboratory at Sudbury, Ontario (Canada); the Gran Sasso National Laboratory (Italy); the Canfranc Underground Laboratory (Spain); the Boulby Underground Laboratory (United Kingdom); the Deep Underground Science and Engineering Laboratory, South Dakota (United States); and the Particle and Astrophysical Xenon Detector (China).

These experiments mostly use one of two detector technologies: cryogenic detectors, operating at temperatures below 100mK, detect the heat produced when a particle hits an atom in a crystal absorber such as germanium. Noble liquid detectors detect the flash of scintillation light produced by a particle collision in liquid xenon or argon. Cryogenic detector experiments include: CDMS, CRESST, EDELWEISS, EURECA. Noble liquid experiments include ZEPLIN, XENON, DEAP, ArDM, WARP, DarkSide, PandaX, and LUX, the Large Underground Xenon experiment. Both of these detector techniques distinguish background particles (which scatter off electrons) from dark matter particles (which scatter off nuclei). Other experiments include SIMPLE and PICASSO.

The DAMA/NaI, DAMA/LIBRA experiments detected an annual modulation in the event rate,[94] which they claim is due to dark matter. (As the Earth orbits the Sun, the velocity of the detector relative to the dark matter halo will vary by a small amount depending on the time of year). This claim is so far unconfirmed and difficult to reconcile with the negative results of other experiments.[95]

Directional detection of dark matter is a search strategy based on the motion of the Solar System around the Galactic Center.[96][97][98][99]

A low pressure time projection chamber makes it possible to access information on recoiling tracks (3D reconstruction if possible) and constrain WIMP-nucleus kinematics. WIMPs coming from the direction in which the Sun is travelling (roughly towards Cygnus) may then be separated from background, which should be isotropic. Directional dark matter experiments include DMTPC, DRIFT, Newage and MIMAC.

On 17 December 2009, CDMS researchers reported two possible WIMP candidate events. They estimate that the probability that these events are due to background (neutrons or misidentified beta or gamma events) is 23%, and conclude "this analysis cannot be interpreted as significant evidence for WIMP interactions, but we cannot reject either event as signal."[100]

On 4 September 2011, researchers using the CRESST detectors presented evidence[101] of 67 collisions occurring in detector crystals from subatomic particles, calculating a less than 1 in 10,000 chance that all were caused by known sources of interference/contamination. It is possible that many were caused by WIMPs (and/or other unknown particles).

Indirect detection experiments[edit]

Collage of six cluster collisions with dark matter maps. The clusters were observed in a study of how dark matter in clusters of galaxies behaves when the clusters collide.[102]
Video about the potential gamma-ray detection of dark matter annihilation around supermassive black holes. (Duration 3:13, also see file description.)

Indirect detection experiments search for the products of WIMP annihilation/decay. If WIMPs are Majorana particles (their own antiparticle) then two WIMPs could annihilate to produce gamma rays or Standard Model particle-antiparticle pairs. If the WIMP is unstable, WIMPs could decay into standard model (or other) particles. These processes could be detected indirectly through an excess of gamma rays, antiprotons or positrons emanating from high density regions. The detection of such a signal is not conclusive evidence, as the sources of gamma ray production are not fully understood.[24][41]

The EGRET gamma ray telescope observed more gamma rays than expected from the Milky Way, but scientists concluded that this was most likely due to incorrect estimation of the telescope's sensitivity.[103]

The Fermi Gamma-ray Space Telescope, launched 11 June 2008, is searching for similar gamma rays.[104] In April 2012, an analysis[105] of previously available data from its Large Area Telescope instrument produced strong statistical evidence of a 130 GeV line in the gamma radiation coming from the center of the Milky Way. WIMP annihilation was the most probable explanation for that line.[106]

At higher energies, ground-based gamma-ray telescopes have set limits on the annihilation of dark matter in dwarf spheroidal galaxies[107] and in clusters of galaxies.[108]

The PAMELA experiment (launched 2006) detected excess positrons. They could be from dark matter annihilation or from pulsars. No excess anti-protons was observed.[109]

The Alpha Magnetic Spectrometer on the International Space Station is designed to directly measure the positron fraction of cosmic rays. The first results, published in 2013, indicate excess high-energy cosmic rays that could be due to dark matter annihilation.[110][111][112][113][114][115]

A few of the WIMPs passing through the Sun or Earth may scatter off atoms and lose energy. Thus WIMPs may accumulate at the center of these bodies, increasing the chance of collision/annihilation. This could produce a distinctive signal in the form of high-energy neutrinos.[116] Such a signal would be strong indirect proof of WIMP dark matter.[24] High-energy neutrino telescopes such as AMANDA, IceCube and ANTARES are searching for this signal.

WIMP annihilation from the Milky Way Galaxy as a whole may also be detected in the form of various annihilation products.[117] The Galactic Center is a particularly good place to look because the density of dark matter may be higher there.[118]

Alternative theories[edit]

Mass in extra dimensions[edit]

In some multidimensional theories, the force of gravity is the only force with effect across all dimensions.[36] This would explain the relative weakness of gravity compared to the other forces of nature that cannot cross into extra dimensions: electromagnetism, strong interaction and weak interaction.

In that case, dark matter would be a candidate for matter that would exist in other dimensions and that could only interact with the matter in our dimensions through gravity. That dark matter could potentially aggregate in the same way as ordinary matter, forming other-dimensional galaxies.[35]

Topological defects[edit]

Dark matter could consist of primordial defects ("birth defects") in the topology of quantum fields, which would contain energy and therefore gravitate. This possibility may be investigated by the use of an orbital network of atomic clocks, which would register the passage of topological defects by changes to clock synchronization. The Global Positioning System may be able to operate as such a network.[119]

Modified gravity[edit]

Alternative theories have been proposed to explain these observations without dark matter. Most of these theories modify the laws of gravity.

The earliest was Mordehai Milgrom's Modified Newtonian Dynamics (MOND) in 1983, which adjusts Newton's laws to create a stronger gravitational field when gravitational acceleration becomes tiny (such as near the rim of a galaxy). It had some success explaining rotational velocity curves of elliptical galaxies, and dwarf elliptical galaxies, but not galaxy cluster gravitational lensing. MOND was not relativistic: it was an adjustment of the Newtonian account. Attempts were made to bring MOND into conformity with general relativity; this spawned competing MOND-based hypotheses—including TeVeS, MOG or STV gravity and the phenomenological covariant approach.[120]

In 2007, John W. Moffat proposed a modified gravity hypothesis based on the nonsymmetric gravitational theory (NGT) that claims to account for the behavior of colliding galaxies.[121] This model requires the presence of non-relativistic neutrinos, or other candidates for (cold) dark matter, to work.

Another proposal uses a gravitational backreaction from a theory that explain gravitational force between objects as an action, a reaction, and then a back-reaction. Simply, an object A affects an object B, and the object B then re-affects object A, and so on: creating a feedback loop that strengthens gravity.[122]

In 2008, another group proposed "dark fluid", a modification of large-scale gravity. It hypothesized that attractive gravitational effects are instead a side-effect of dark energy. Dark fluid combines dark matter and dark energy in a single energy field that produces different effects at different scales. This treatment is a simplification of a previous fluid-like model called the generalized Chaplygin gas model in which the whole of spacetime is a compressible gas.[123] Dark fluid can be compared to an atmospheric system. Atmospheric pressure causes air to expand and air regions can collapse to form clouds. In the same way, the dark fluid might generally disperse, but it could collect around galaxies.[123]

Fractality of spacetime[edit]

Applying relativity to fractal non-differentiable spacetime, Laurent Nottale suggests that potential energy may arise due to the fractality of spacetime, which would account for the missing mass-energy observed at cosmological scales.[124][125]

Popular culture[edit]

Mention of dark matter is made in some video games and other works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties. Such descriptions are often inconsistent with the hypothesized properties of dark matter in physics and cosmology.

See also[edit]

Notes[edit]

  1. ^ Since dark energy, by convention, does not count as "matter", this is 26.8/(4.9 + 26.8)=0.845

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