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 an unidentified type of matter distinct from dark energy, baryonic matter (ordinary matter), and neutrinos whose existence would explain a number of otherwise puzzling astronomical observations.[1] The name refers to the fact that it does not emit or interact with electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum.[2] Although dark matter has not been directly observed, its existence and properties are inferred from its gravitational effects such as the motions of visible matter,[3] gravitational lensing, its influence on the universe's large-scale structure, on galaxies, and its effects in the cosmic microwave background.

The standard model of cosmology indicates that the total mass–energy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.[4][5][6][7] 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.[8][9][10][11] The great majority of ordinary matter in the universe is also unseen, since visible stars and gas inside galaxies and clusters account for less than 10% of the ordinary matter contribution to the mass-energy density of the universe.[12] The most widely accepted hypothesis on the form for dark matter is that it is composed of weakly interacting massive particles (WIMPs) that interact only through gravity and the weak force.[13]

The dark matter hypothesis plays a central role in current modeling of cosmic structure formation and galaxy formation and evolution and on explanations of the anisotropies observed in the cosmic microwave background (CMB). All these lines of evidence suggest that galaxies, galaxy clusters, and the universe as a whole contain far more matter than that which is observable via electromagnetic signals.[14] Many experiments to detect proposed dark matter particles through non-gravitational means are under way;[15] however, no dark matter particle has been conclusively identified.

Although the existence of dark matter is generally accepted by most of the astronomical community, a minority of astronomers,[16] motivated by the lack of conclusive identification of dark matter, argue for various modifications of the standard laws of general relativity, such as MOND, TeVeS, and conformal gravity[17] that attempt to account for the observations without invoking additional matter.[18]

History[edit]

Lord Kelvin estimated the number of dark bodies in the Milky Way galaxy from the observed velocity dispersion of the stars, the speed the stars were orbiting around the center of the galaxy, which he used to estimate the mass of the galaxy, which was different than the mass of stars which can be seen, and concluded that “many of our stars, perhaps a great majority of them, may be dark bodies.”[19]

In 1906 Henri Poincaré in the “The Milky Way and Theory of Gases” used "dark matter,” or “matière obscure” in French in discussing Kelvin's work.[19]

The first to suggest the existence of dark matter (using stellar velocities) was Dutch astronomer Jacobus Kapteyn in 1922.[20][21][better source needed] Fellow Dutchman and radio astronomy pioneer Jan Oort also hypothesized the existence of dark matter in 1932.[21][22][23] 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.[24]

In 1933, Swiss astrophysicist Fritz Zwicky, who studied galactic clusters while working at the California Institute of Technology, made a similar inference.[25][26][27] Zwicky applied the virial theorem to the Coma galaxy 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.[28] Zwicky's estimates were off by more than an order of magnitude, mainly due to an obsolete value of the Hubble constant;[29] 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.[clarification needed][28]

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.[30] 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 provided further strong evidence, also using galaxy rotation curves.[31][32][33] Rubin worked with a new spectrograph to measure the velocity curve of edge-on spiral galaxies with greater accuracy.[33] This result was confirmed in 1978.[34] An influential paper presented Rubin's results in 1980.[35] Rubin found that most galaxies must contain about six times as much dark as visible mass;[36] thus, by around 1980 the apparent need for dark matter was widely recognized as a major unsolved problem in astronomy.[31]

A stream of observations in the 1980s indicated its presence, including gravitational lensing of background objects by galaxy clusters,[37] 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.[13][38] The search for this particle, by a variety of means, is one of the major efforts in particle physics.[15]

Cosmic microwave background radiation (CMB)[edit]

In cosmology, the CMB is explained as relic radiation which has travelled freely since the era of recombination, around 375,000 years after the Big Bang. The CMB’s anisotropies are explained as the result of small primordial density fluctuations, and subsequent acoustic oscillations in the photon-baryon plasma whose restoring force is gravity.[39]

The Cosmic Background Explorer (COBE) satellite found the CMB spectrum to be a very precise 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.[40]

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. During the 1990s, the first peak was measured with increasing sensitivity, and in 2000 the BOOMERanG experiment[41] reported that the highest power fluctuations occur at scales of approximately one degree, showing that the Universe is close to flat. 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,[42][43] and CBI provided the first E-mode polarization spectrum with compelling evidence that it is out of phase with the T-mode spectrum.[44] COBE's successor, the Wilkinson Microwave Anisotropy Probe (WMAP) provided the most detailed measurements of (large-scale) anisotropies in the CMB in 2003–2010.[45] ESA's Planck spacecraft returned more detailed results in 2013–2015.

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 six adjustable parameters: dark matter density, baryon (atom) density, the universe’s age (or equivalently, the Hubble constant), the initial fluctuation amplitude and their scale dependence.

Technical definition[edit]

In standard cosmology, matter is anything whose energy density scales with the inverse cube of the scale factor, i.e. ρ ∝ a−3. This is in contrast to radiation, which scales to the inverse fourth power of the scale factor ρ ∝ a−4, and dark energy, which is unaffected ρ ∝ a0. This can be understood intuitively: for an ordinary particle in a square box, doubling the length of a side of the box decreases the density (and hence energy density) by a factor of eight (23). For radiation, the decrease in energy density is greater, because an increase in spatial distance also causes a redshift. Dark energy, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.[46]

Dark matter is that component of the universe which is not ordinary matter, but still obeys ρ ∝ a−3.

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.[47]
Dark matter map of KiDS survey region (region G12).[48]

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.
Comparison of rotating disc galaxies in the distant Universe and the present day. The imaginary galaxy on the left is in the nearby Universe and the stars in its outer parts are orbiting rapidly due to the presence of large amounts of dark matter around the central regions. On the other hand, the galaxy at the right, which is in the distant Universe, and seen as it was about ten billion years ago, is rotating more slowly in its outer parts as dark matter is more diffuse. The size of the difference is exaggerated in this schematic view to make the effect clearer. The distribution of dark matter is shown in red.

The arms of spiral galaxies rotate around the galactic centre. The luminous mass density of a spiral galaxy decreases as one goes from the centre to the outskirts. If luminous mass were all the matter, then we can model the galaxy as a point mass in the centre and test masses orbiting around it (similar to the solar system). From Kepler's Second Law, we expect that the rotation velocities will decrease with distance from the centre. This is not observed.[49] Instead, the galaxy rotation curve remains flat as distant from the centre as the data is available.

If we assume the validity of Kepler's laws, then the obvious way to resolve this discrepancy is to conclude that the mass distribution in spiral galaxies are not similar to that of the solar system. In particular, there is a lot of non-luminous matter in the outskirts of the galaxy ("dark matter").

Velocity dispersions[edit]

Main article: Velocity dispersion

Stars in bound systems must obey the virial theorem. The theorem, together with the measured velocity distribution, can be used to measure the mass distribution in a bound system, such as elliptical galaxies or globular clusters. With some exceptions, velocity dispersion estimates of elliptical galaxies[50] do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits.[51]

As with galaxy rotation curves, the obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter.

Galaxy clusters[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.

Galaxy clusters are particularly important for dark matter studies since their masses can be estimated in three independent ways:

  • From the scatter in radial velocities of the galaxies within clusters
  • From X-rays emitted by hot gas in the clusters. From the X-ray energy spectrum and flux, the gas temperature and density can be estimated, hence giving the pressure; assuming pressure and gravity balance determines the cluster's mass profile.
  • Gravitational lensing (usually of more distant galaxies) can measure cluster masses without relying on observations of dynamics (e.g., velocity).

Generally, these three methods are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1.

Gravitational lensing[edit]

One of the consequences of general relativity is that massive objects should act 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. The more massive an object, the more lensing is observed.

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 many distant clusters including Abell 1689.[52] 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.[53] Lensing can lead to multiple copies of an image. By analyzing the distribution of multiple image copies, scientists have been able to deduce and map the distribution of dark matter around the MACS J0416.1-2403 galaxy cluster.[54][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, the mean distribution of dark matter can be characterized. The mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements.[56]

Cosmic microwave background[edit]

The cosmic microwave background by WMAP

Although both dark matter and ordinary matter are "matter", they do not behave in the same way. In particular, ordinary matter interacts with radiation, while dark matter does not. They therefore leave different imprints on the cosmic microwave background (CMB).

The cosmic microwave background is very close to a perfect blackbody, but contains anisotropies. The anisotropies can be decomposed into a power spectrum, whose peaks constrain cosmological parameters.[57] The first peak mostly shows 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.[57] The spectrum was first observed by WMAP, and the results support the Lambda-CDM model.[45]

The CMB angular power spectrum provides powerful evidence in support of dark matter, as its precise structure is difficult to reproduce with any competing model such as MOND.[58][59]

Sky surveys and baryon acoustic oscillations[edit]

Baryon acoustic oscillations (BAO) are regular, periodic fluctuations in the density of the visible baryonic matter (normal matter) of the universe. These are predicted to arise in the Lambda-CDM model due to the early universe's acoustic oscillations in the photon-baryon fluid and can be observed in the cosmic microwave background angular power spectrum. BAOs set up a preferred length scale for baryons. As the dark matter and baryons clumped together after recombination, the effect is much weaker in the galaxy distribution in the nearby universe, but is detectable as a subtle (~ 1 percent) preference for pairs of galaxies to be separated by 147 Mpc, compared to those separated by 130 or 160 Mpc. This feature was predicted theoretically in the 1990s and then discovered in 2005, in two large galaxy redshift surveys, the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey.[60] Combining the CMB observations with BAO measurements from galaxy redshift surveys provides a precise estimate of the Hubble constant and the average matter density in the Universe.[39] The results support the Lambda-CDM model.

Redshift-space distortions[edit]

Large galaxy redshift surveys may be used to make a three-dimensional map of the galaxy distribution. These maps are slightly distorted because distances are estimated from observed redshifts; the redshift contains a contribution from the galaxy's so-called peculiar velocity in addition to the dominant Hubble expansion term. On average, superclusters are expanding but more slowly than the cosmic mean due to their gravity, while voids are expanding faster than average. In a redshift map, galaxies in front of a supercluster have excess radial velocities towards it and have redshifts slightly higher than their distance would imply, while galaxies behind the supercluster have redshifts slightly low for their distance. This effect causes superclusters to appear "squashed" in the radial direction, and likewise voids are "stretched"; angular positions are unaffected. The effect is not detectable for any one structure since the true shape is not known, but can be measured by averaging over many structures assuming we are not at a special location in the Universe.

The effect was predicted quantitatively by Nick Kaiser in 1987, and first decisively measured in 2001 by the 2dF Galaxy Redshift Survey.[61] Results are in agreement with the Lambda-CDM model.

Type Ia supernova distance measurements[edit]

Type Ia supernovae can be used as "standard candles" to measure extragalactic distances, which can in turn be used to measure how fast the universe has expanded in the past. The data indicates that the universe is expanding at an accelerating rate, the cause of which is usually ascribed to dark energy.[62] Since observations indicate the universe is almost flat,[63][64][65] we expect the total energy density of everything in the universe to sum to 1 (Ωtot ~ 1). The measured dark energy density is ΩΛ = ~0.690; the observed ordinary matter energy density is Ωm = ~0.0482 and the energy density of radiation is negligible. This leaves a missing Ωdm = ~0.258 that nonetheless behaves like matter (see technical definition section above) – dark matter.[66]

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.[67] 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.[68]

Structure formation refers to the period after the Big Bang when density perturbations collapsed to form stars, galaxies, and clusters. Prior to structure formation, the Friedmann solutions to general relativity describe a homogeneous universe. Later, small anisotropies gradually grew and condensed the homogeneous universe into stars, galaxies and larger structures. Ordinary matter is affected by radiation, which is the dominant element of the universe at very early times. As a result, its density perturbations are washed out and unable to condense into structure.[69] If there were only ordinary matter in the universe, there would not have been enough time for density perturbations to grow into the galaxies and clusters that we see today.

Dark matter provides a solution to this problem because it is unaffected by radiation. Therefore, its density perturbations can grow first. The resulting gravitational potential acts as an attractive potential well for ordinary matter collapsing later, speeding up the structure formation process.[69][70]

Composition of dark matter: Baryonic vs. nonbaryonic[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)

Dark matter can refer to any substance which interacts predominantly via gravity with visible matter (e.g. stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard baryonic matter, such as protons or electrons. However, for the reasons outlined below, most scientists consider the dark matter to be dominated by a non-baryonic component, which is likely composed of a new fundamental particle (or similar exotic state).

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

Baryonic matter[edit]

Baryonic matter is made of baryons (protons and neutrons) that make up stars and planets. It also encompasses less common black holes, neutron stars, faint old white dwarfs and brown dwarfs, collectively known as massive compact halo objects (MACHOs).[71]

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

  • Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars.
  • The theory of Big Bang nucleosynthesis predicts the observed abundance of the chemical elements. If there are more baryons, then there should also be more helium, lithium and heavier elements synthesized during the Big Bang.[72][73] Agreement with observed abundances requires that baryonic matter makes up between 4–5% of the universe's critical density. In contrast, large-scale structure and other observations indicate that the total matter density is about 30% of the critical density.[66]
  • Astronomical searches for gravitational microlensing in the Milky Way found that at most a small fraction of the dark matter may be in dark, compact, conventional objects (MACHOs, etc.); the excluded range of object masses is from half the Earth's mass up to 30 solar masses, which covers nearly all the plausible candidates.[74][75][76][77][78][79]
  • Detailed analysis of the small irregularities (anisotropies) in the cosmic microwave background.[80] Observations by WMAP and Planck indicate that around five-sixths of the total matter is in a form that interacts significantly with ordinary matter or photons only through gravitational effects.

Non-baryonic matter[edit]

Candidates for nonbaryonic dark matter are hypothetical particles such as axions, sterile neutrinos or WIMPs (e.g. supersymmetric particles). The three neutrino types already observed are indeed abundant, and “dark”, and matter, but because their individual masses – however uncertain they may be – are almost certainly tiny, they can only supply a small fraction of dark matter, due to limits derived from large-scale structure and high-redshift galaxies.[81]

Unlike baryonic matter, nonbaryonic matter did not contribute to the formation of the elements in the early universe ("Big Bang nucleosynthesis")[13] 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").[81]

Classification of dark matter: cold, warm or hot[edit]

Dark matter can be divided into cold, warm and hot categories.[82] These categories refer to velocity rather than an actual temperature, indicating how far corresponding objects moved due to random motions in the early universe, before they slowed due to cosmic 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 later structure formation. The categories are set with respect to the size of a protogalaxy (an object that later evolves into a dwarf galaxy): dark matter particles are classified as cold, warm, or hot according as their FSL; much smaller (cold), similar (warm), or much larger (hot) than a protogalaxy.[83][84]

Mixtures of the above are also possible: a theory of mixed dark matter was popular in the mid-1990s, but was rejected following the discovery of dark energy.[citation needed]

Cold dark matter leads to a "bottom-up" formation of structure while hot dark matter would result in a "top-down" formation scenario; the latter is excluded by high-redshift galaxy observations.[15]

Alternative definitions[edit]

These categories also correspond to fluctuation spectrum effects and the interval following the Big Bang at which each type became non-relativistic. 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.[85]

Another approximate dividing line is that "warm" dark matter 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 (speed of light multiplied by time) in the radiation-dominated era, thus 2 light-years. A region of this size would expand to 2 million light years today (absent structure formation). The actual FSL is roughly 5 times the above length, since it continues to grow slowly as particle velocities decrease inversely with the scale factor after they become non-relativistic. In this example the FSL would correspond to 10 million light-years or 3 Mpc today, around the size containing an average large galaxy.

The 2.7 million K photon temperature 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 FSLs much smaller than a protogalaxy, making them "cold". Conversely, much lighter particles, such as neutrinos with masses of only a few eV, have FSLs much larger than a protogalaxy, 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 supporting 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[86]) or RAMBOs (such as clusters of brown dwarfs), to new particles such as WIMPs and axions.

Studies of Big Bang nucleosynthesis and gravitational lensing convinced most cosmologists[15][87][88][89][90][91] that MACHOs[87][89] cannot make up more than a small fraction of dark matter.[13][87] According to A. Peter: "... the only really plausible dark-matter candidates are new particles."[88]

The 1997 DAMA/NaI experiment and its successor DAMA/LIBRA in 2013, claimed to directly detect dark matter particles passing through the Earth, but many researchers 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 WIMPy Lightest Supersymmetric Particle (LSP).[92] 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. Predictions based on warm dark matter are similar to those for cold dark matter on large scales, 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.[citation needed]

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 as such particle. 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 10−6 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 WIMPs.

The three known flavours of neutrinos are the electron, muon, and tau. Their masses are slightly different. Neutrinos oscillate among the flavours 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 10−5 of the mass of an electron), the universe would collapse. CMB data and other methods indicate that their average mass probably does not exceed 0.3 eV/c2. Thus, observed neutrinos cannot explain dark matter.[93]

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.

Detection of dark matter particles[edit]

If dark matter is made up of sub-atomic particles, then millions, possibly billions, of such particles must pass through every square centimeter of the Earth each second.[94][95] Many experiments aim to test this hypothesis. Although WIMPs are popular search candidates,[15] the Axion Dark Matter eXperiment (ADMX) searches for axions. Another candidate is heavy hidden sector particles that 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 dark matter particle annihilations or decays.[81]

Direct detection[edit]

Direct detection experiments aim to observe low-energy recoils (typically a few keVs) of nuclei induced by interactions with particles of dark matter, which (in theory) are passing through the Earth. After such a recoil the nucleus will emit energy as e.g. scintillation light or phonons, which is then detected by sensitive apparatus. In order to do this effectively it is crucial to maintain a low background, and so such experiments operate deep underground to reduce the interference from cosmic rays. Examples of underground laboratories which house direct detection experiments include the Stawell mine, the Soudan mine, the SNOLAB underground laboratory at Sudbury, the Gran Sasso National Laboratory, the Canfranc Underground Laboratory, the Boulby Underground Laboratory, the Deep Underground Science and Engineering Laboratory and the China Jinping Underground Laboratory.

These experiments mostly use either cryogenic or noble liquid 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 scintillation 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 techniques focus strongly on their ability to distinguish background particles (which predominantly scatter off electrons) from dark matter particles (that scatter off nuclei). Other experiments include SIMPLE and PICASSO.

Currently there has been no well-established claim of dark matter detection from a direct detection experiment, leading instead to strong upper limits on the mass and interaction cross section with nucleons of such dark matter particles.[96] The DAMA/NaI and more recent DAMA/LIBRA experimental collaborations claim to have detected an annual modulation in the rate of events in their detectors,[97][98] which they claim is due to dark matter. This results from the expectation that as the Earth orbits the Sun, the velocity of the detector relative to the dark matter halo will vary by a small amount. This claim is so far unconfirmed and in contradiction with negative results from other experiments such as LUX and SuperCDMS.[99]

A special case of direct detection experiments covers those with directional sensitivity. This is a search strategy based on the motion of the Solar System around the Galactic Center.[100][101][102][103] A low pressure time projection chamber makes it possible to access information on recoiling tracks 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.

Indirect detection[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.[104]
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 the self-annihilation or decay of dark matter particles in outer space. For example, in regions of high dark matter density (e.g. the centre of our galaxy) two dark matter particles could annihilate to produce gamma rays or Standard Model particle-antiparticle pairs.[105] Alternatively if the dark matter particle is unstable, it 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 in our galaxy or others.[106] A major difficulty inherent in such searches is that there are various astrophysical sources which can mimic the signal expected from dark matter, and so multiple signals will likely be required for a conclusive discovery.[15][81]

A few of the dark matter particles passing through the Sun or Earth may scatter off atoms and lose energy. Thus dark matter 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.[107] Such a signal would be strong indirect proof of WIMP dark matter.[15] High-energy neutrino telescopes such as AMANDA, IceCube and ANTARES are searching for this signal.[108] The detection by LIGO in September 2015 of gravitational waves, opens the possibility of observing dark matter in a new way, particularly if it is the form of primordial black holes.[109][110][111]

Many experimental searches have been undertaken to look for such emission from dark matter annihilation or decay, examples of which follow. The EGRET gamma ray telescope observed more gamma rays in 2008 than expected from the Milky Way, but scientists concluded that this was most likely due to incorrect estimation of the telescope's sensitivity.[112]

The Fermi Gamma-ray Space Telescope is searching for similar gamma rays.[113] In April 2012, an analysis of previously available data from its Large Area Telescope instrument produced statistical evidence of a 130 GeV signal in the gamma radiation coming from the center of the Milky Way.[114] WIMP annihilation was seen as the most probable explanation.[115]

At higher energies, ground-based gamma-ray telescopes have set limits on the annihilation of dark matter in dwarf spheroidal galaxies[116] and in clusters of galaxies.[117]

The PAMELA experiment (launched 2006) detected excess positrons. They could be from dark matter annihilation or from pulsars. No excess antiprotons were observed.[118]

In 2013 results from the Alpha Magnetic Spectrometer on the International Space Station indicated excess high-energy cosmic rays that could be due to dark matter annihilation.[119][120][121][122][123][124]

Collider searches for dark matter[edit]

An alternative approach to the detection of dark matter particles in nature is to produce them in a laboratory. Experiments with the Large Hadron Collider (LHC) may be able to detect dark matter particles produced in collisions of the LHC proton beams. Because a dark matter particle should have negligible interactions with normal visible 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.[125] Constraints on dark matter also exist from the LEP experiment using a similar principle, but probing the interaction of dark matter particles with electrons rather than quarks.[126] It is important to note that any discovery from collider searches will need to be corroborated by discoveries in the indirect or direct detection sectors, in order to prove that the particle discovered is in fact the dark matter of our Universe.

Alternative theories[edit]

For more details on this topic, see Alternatives to general relativity.

Because dark matter remains to be conclusively identified, many theories that aim to explain the observational evidence without invoking dark matter have emerged. The obvious way to do this is to modify general relativity. General relativity is well-tested on solar-system scales, but its validity on galactic or cosmological scales is less certain. A suitable modification to general relativity can conceivably eliminate the need for dark matter. The most well-known theories of this class are MOND and its relativistic generalization TeVeS,[127] and f(R) gravity.[128] Alternative theories abound.[129][130]

A problem with alternative theories is that the observational evidence for dark matter comes from so many independent angles (see the "observational evidence" section above). Any alternative theory not only has to explain all the evidence, it also has to explain individual cases such as the Bullet Cluster,[131] wherein two colliding clusters of galaxies gave rise to an 8σ statistical significance spatial offset of the center of the total mass from the center of the baryonic mass peaks.[132] Nonetheless, there has been some scattered successes for alternative theories, such as a 2016 test of gravitational lensing in entropic gravity.[133][134][135]

The prevailing opinion among most astrophysicists is that while modifications to general relativity can conceivably explain part of the observational evidence, there is probably enough data to conclude there must still be some dark matter.[136]

In philosophy of science[edit]

In philosophy of science, dark matter is an example of an "auxiliary hypothesis", an ad hoc postulate that is added to a theory in response to observations that falsify it. It has been argued that the dark matter hypothesis is a conventionalist hypothesis, that is, a hypothesis that adds no empirical content and hence is unfalsifiable in the sense defined by Karl Popper.[137]

In popular culture[edit]

Mention of dark matter is made in 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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