Dark matter
What is dark matter? How was it generated?
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Physical cosmology |
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In
In the standard lambda-CDM model of cosmology, the mass–energy content of the universe is 5% ordinary matter, 26.8% dark matter, and 68.2% a form of energy known as dark energy.[4][5][6][7] Thus, dark matter constitutes 85%[a] of the total mass, while dark energy and dark matter constitute 95% of the total mass–energy content.[8][9][10][11]
Dark matter is not known to interact with ordinary
Dark matter is classified as "cold", "warm", or "hot" according to its velocity (more precisely, its free streaming length). Recent models have favored a cold dark matter scenario, in which structures emerge by the gradual accumulation of particles, but after a half century of fruitless dark matter particle searches, more recent gravitational wave and James Webb Space Telescope observations have considerably strengthened the case for primordial and direct collapse black holes.[14][16][17]
Although the astrophysics community generally accepts dark matter's existence,[18] a minority of astrophysicists, intrigued by specific observations that are not well-explained by ordinary dark matter, argue for various modifications of the standard laws of general relativity. These include modified Newtonian dynamics, tensor–vector–scalar gravity, or entropic gravity. So far none of the proposed modified gravity theories can successfully describe every piece of observational evidence at the same time, suggesting that even if gravity has to be modified, some form of dark matter will still be required.[19]
History
Early history
The hypothesis of dark matter has an elaborate history.[20] In the appendices of the book Baltimore lectures on molecular dynamics and the wave theory of light where the main text was based on a series of lectures given in 1884,[21] Lord Kelvin discussed the potential number of stars around the Sun from the observed velocity dispersion of the stars near the Sun, assuming that the Sun was 20 to 100 million years old. He posed what would happen if there were a thousand million stars within 1 kilo-parsec of the Sun (at which distance their parallax would be 1 milli-arcsec). Lord Kelvin concluded:
Many of our supposed thousand million stars, perhaps a great majority of them, may be dark bodies.[22][23]
In 1906, Henri Poincaré in The Milky Way and Theory of Gases used the French term matière obscure ("dark matter") in discussing Kelvin's work.[24][23] He found that the amount of dark matter would need to be less than that of visible matter.[25]
The second to suggest the existence of dark matter using stellar velocities was Dutch astronomer Jacobus Kapteyn in 1922.[26][27] A publication from 1930 points to Swedish Knut Lundmark being the first to realise that the universe must contain much more mass than can be observed.[28] Dutchman and radio astronomy pioneer Jan Oort also hypothesized the existence of dark matter in 1932.[27][29][30] Oort was studying stellar motions in the local galactic neighborhood and found the mass in the galactic plane must be greater than what was observed, but this measurement was later determined to be erroneous.[31]
In 1933, Swiss astrophysicist
Further indications of mass-to-light ratio anomalies came from measurements of galaxy rotation curves. In 1939, Horace W. Babcock reported the rotation curve for the Andromeda nebula (known now as the Andromeda Galaxy), which suggested the mass-to-luminosity ratio increases radially.[36] He attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral and not to the missing matter he had uncovered. Following Babcock's 1939 report of unexpectedly rapid rotation in the outskirts of the Andromeda galaxy and a mass-to-light ratio of 50; in 1940 Jan Oort discovered and wrote about the large non-visible halo of NGC 3115.[37]
1960s
Early radio astronomy observations, performed by Seth Shostak, later SETI Institute Senior Astronomer, showed a half-dozen galaxies spun too fast in their outer regions, pointing to the existence of dark matter as a means of creating the gravitational pull needed to keep the stars in their orbits.[38]
1970s
At the same time Rubin and Ford were exploring optical rotation curves, radio astronomers were making use of new radio telescopes to map the 21 cm line of atomic hydrogen in nearby galaxies. The radial distribution of interstellar atomic hydrogen (
1980s
A stream of observations in the 1980s supported the presence of dark matter, including
Technical definition
In standard cosmological calculations, "matter" means any constituent of the universe whose energy density scales with the inverse cube of the
In principle, "dark matter" means all components of the universe which are not visible but still obey ρ ∝ a−3 . In practice, the term "dark matter" is often used to mean only the non-baryonic component of dark matter, i.e., excluding "missing baryons". Context will usually indicate which meaning is intended.
Observational evidence
Galaxy rotation curves
The arms of
If Kepler's laws are correct, then the obvious way to resolve this discrepancy is to conclude the mass distribution in spiral galaxies is not similar to that of the Solar System. In particular, there is a lot of non-luminous matter (dark matter) in the outskirts of the galaxy.
Velocity dispersions
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[56] do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits.[57]
As with galaxy rotation curves, the obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter.
Galaxy clusters
- 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.[58]
Gravitational lensing
One of the consequences of general relativity is massive objects (such as a cluster of galaxies) lying between a more distant source (such as a quasar) and an observer should act as a lens to bend light from this source. 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.[59] 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.[60] 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.[61][62]
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.[63] Dark matter does not bend light itself; mass (in this case the mass of the dark matter) bends spacetime. Light follows the curvature of spacetime, resulting in the lensing effect.[64][65]
In May 2021, a new detailed dark matter map was revealed by the Dark Energy Survey Collaboration.[66] In addition, the map revealed previously undiscovered filamentary structures connecting galaxies, by using a machine learning method.[67]
An April 2023 study in Nature Astronomy examined the inferred distribution of the dark matter responsible for the lensing of the
Cosmic microwave background
Although both dark matter and ordinary matter are matter, they do not behave in the same way. In particular, in the early universe, ordinary matter was ionized and interacted strongly with radiation via Thomson scattering. Dark matter does not interact directly with radiation, but it does affect the cosmic microwave background (CMB) by its gravitational potential (mainly on large scales) and by its effects on the density and velocity of ordinary matter. Ordinary and dark matter perturbations, therefore, evolve differently with time and leave different imprints on the CMB.
The cosmic microwave background is very close to a perfect blackbody but contains very small temperature anisotropies of a few parts in 100,000. A sky map of anisotropies can be decomposed into an angular power spectrum, which is observed to contain a series of acoustic peaks at near-equal spacing but different heights. The series of peaks can be predicted for any assumed set of cosmological parameters by modern computer codes such as CMBFAST and CAMB, and matching theory to data, therefore, constrains cosmological parameters.[69] 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.[69]
The CMB anisotropy was first discovered by COBE in 1992, though this had too coarse resolution to detect the acoustic peaks. After the discovery of the first acoustic peak by the balloon-borne
The observed CMB angular power spectrum provides powerful evidence in support of dark matter, as its precise structure is well fitted by the lambda-CDM model,[71] but difficult to reproduce with any competing model such as modified Newtonian dynamics (MOND).[71][72]
Structure formation
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.[74][75]
Bullet Cluster
If dark matter does not exist, then the next most likely explanation must be that general relativity – the prevailing theory of gravity – is incorrect and should be modified. The Bullet Cluster, the result of a recent collision of two galaxy clusters, provides a challenge for modified gravity theories because its apparent center of mass is far displaced from the baryonic center of mass.[76] Standard dark matter models can easily explain this observation, but modified gravity has a much harder time,[77][78] especially since the observational evidence is model-independent.[79]
Type Ia supernova distance measurements
Type Ia
Sky surveys and baryon acoustic oscillations
Baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter (normal matter) of the universe on large scales. These are predicted to arise in the Lambda-CDM model due to acoustic oscillations in the photon–baryon fluid of the early universe, 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 130–160 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.[86] 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.[87] The results support the Lambda-CDM model.
Redshift-space distortions
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 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. Their angular positions are unaffected. This effect is not detectable for any one structure since the true shape is not known, but can be measured by averaging over many structures. It was predicted quantitatively by Nick Kaiser in 1987, and first decisively measured in 2001 by the 2dF Galaxy Redshift Survey.[88] Results are in agreement with the lambda-CDM model.
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.[89] These constraints agree with those obtained from WMAP data.
Theoretical classifications
Composition
The exact identity of dark matter is unknown, but there are many hypotheses about what dark matter could consist of, as set out in the table below.
Light bosons | quantum chromodynamics axions |
axion-like particles | |
fuzzy cold dark matter | |
neutrinos | Standard Model |
sterile neutrinos
| |
weak scale | supersymmetry |
extra dimensions | |
little Higgs | |
effective field theory | |
simplified models | |
other particles | weakly interacting massive particle |
self-interacting dark matter | |
atomic dark matter[91][92][93][94] | |
strangelet[95] | |
superfluid vacuum theory | |
dynamical dark matter | |
macroscopic | primordial black holes[13][14][16][15][96][97][98][99][100][101] |
massive compact halo objects (MACHOs)
| |
macroscopic dark matter (Macros) | |
modified gravity (MOG)
|
modified Newtonian dynamics (MoND) |
tensor–vector–scalar gravity (TeVeS) | |
entropic gravity |
Baryonic matter
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 neutrons. Most of the ordinary matter familiar to astronomers, including planets, brown dwarfs, red dwarfs, visible stars, white dwarfs, neutron stars, and black holes, fall into this category.[102][103] Solitary black holes, neutron stars, burnt-out dwarfs, and other massive objects that that are hard to detect are collectively known as MACHOs; some scientists initially hoped that baryonic MACHOs could account for and explain all the dark matter.[45]: 286 [104]
However, multiple lines of evidence suggest the majority of dark matter is not baryonic:
- Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars.
- The theory of large-scale structure and other observations indicate that the total matter density is about 30% of the critical density.[85]
- Astronomical searches for gravitational microlensing in the Milky Way found at most only 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.[107][108][109][110][111][112]
- Detailed analysis of the small irregularities (anisotropies) in the only through gravitational effects.
Non-baryonic matter
There are two main candidates for non-baryonic dark matter: hypothetical particles such as
Unlike baryonic matter, nonbaryonic particles do not contribute to the formation of the
In 2015, the idea that dense dark matter was composed of primordial black holes made a comeback[118] following results of gravitational wave measurements which detected the merger of intermediate-mass black holes. Black holes with about 30 solar masses are not predicted to form by either stellar collapse (typically less than 15 solar masses) or by the merger of black holes in galactic centers (millions or billions of solar masses). It was proposed that the intermediate-mass black holes causing the detected merger formed in the hot dense early phase of the universe due to denser regions collapsing. A later survey of about a thousand supernovae detected no gravitational lensing events, when about eight would be expected if intermediate-mass primordial black holes above a certain mass range accounted for over 60% of dark matter.[119] However, that study assumed a monochromatic distribution to represent the LIGO/Virgo mass range, which is inapplicable to the broadly
The possibility that atom-sized primordial black holes account for a significant fraction of dark matter was ruled out by measurements of positron and electron fluxes outside the Sun's heliosphere by the Voyager 1 spacecraft. Tiny black holes are theorized to emit Hawking radiation. However the detected fluxes were too low and did not have the expected energy spectrum, suggesting that tiny primordial black holes are not widespread enough to account for dark matter.[121] Nonetheless, research and theories proposing dense dark matter accounts for dark matter continue as of 2018, including approaches to dark matter cooling,[122][123] and the question remains unsettled. In 2019, the lack of microlensing effects in the observation of Andromeda suggests that tiny black holes do not exist.[124]
However, there still exists a largely unconstrained mass range smaller than that which can be limited by optical microlensing observations, where primordial black holes may account for all dark matter.[125][126]
Free streaming length
Dark matter can be divided into cold, warm, and hot categories.[127] 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 to their FSL; much smaller (cold), similar to (warm), or much larger (hot) than a protogalaxy.[128][129][130] 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 with galaxies forming first and galaxy clusters at a latter stage, while hot dark matter would result in a top-down formation scenario with large matter aggregations forming early, later fragmenting into separate galaxies;[clarification needed] the latter is excluded by high-redshift galaxy observations.[52]
Fluctuation spectrum effects
These categories also correspond to
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.
— Davis, Efstathiou, Frenk, & White (1985)[131]
Alternative definitions
Another approximate dividing line is warm dark matter became non-relativistic when the universe was approximately 1 year old and 1 millionth of its present size and in the
The 2.7 million
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 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.
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
Warm dark matter
Warm dark matter comprises 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 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
The three known flavours of neutrinos are the electron, muon, and tau. 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. 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.[135] 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.[136]
Because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies 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.
Dark matter aggregation and dense dark matter objects
If dark matter is composed of weakly interacting particles, then an obvious question is whether it can form objects equivalent to planets, stars, or black holes. Historically, the answer has been it cannot,[h][137][138][139] because of two factors:
- It lacks an efficient means to lose energy[137]
- Ordinary matter forms dense objects because it has numerous ways to lose energy. Losing energy would be essential for object formation, because a particle that gains energy during compaction or falling "inward" under gravity, and cannot lose it any other way, will heat up and increase velocity and momentum. Dark matter appears to lack a means to lose energy, simply because it is not capable of interacting strongly in other ways except through gravity. The virial theorem suggests that such a particle would not stay bound to the gradually forming object – as the object began to form and compact, the dark matter particles within it would speed up and tend to escape.
- It lacks a diversity of interactions needed to form structures[139]
- Ordinary matter interacts in many different ways, which allows the matter to form more complex structures. For example, stars form through gravity, but the particles within them interact and can emit energy in the form of electromagnetic interaction. There is no evidence that dark matter is capable of such a wide variety of interactions, since it seems to only interact through gravity (and possibly through some means no stronger than the weak interaction, although until dark matter is better understood, this is only speculation).
However, there are theories of atomic dark matter similar to normal matter that overcome these problems.[94]
Detection of dark matter particles
If dark matter is made up of subatomic particles, then millions, possibly billions, of such particles must pass through every square centimeter of the Earth each second.[140][141] Many experiments aim to test this hypothesis. Although WIMPs have been the main search candidates,[52] axions have drawn renewed attention, with the Axion Dark Matter Experiment (ADMX) searches for axions and many more planned in the future.[142] 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 dark matter particle annihilations or decays.[114]
Direct detection
Direct detection experiments aim to observe low-energy recoils (typically a few
These experiments mostly use either cryogenic or noble liquid detector technologies. Cryogenic detectors operating at temperatures below 100 mK, detect the heat produced when a particle hits an atom in a crystal absorber such as
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.[143] The DAMA/NaI and more recent DAMA/LIBRA experimental collaborations have detected an annual modulation in the rate of events in their detectors,[144][145] 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, SuperCDMS[146] and XENON100.[147]
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
Indirect detection
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.[153] Alternatively, if a 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.[154] A major difficulty inherent in such searches is that various astrophysical sources can mimic the signal expected from dark matter, and so multiple signals are likely required for a conclusive discovery.[52][114]
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
: 298 The detection byMany experimental searches have been undertaken to look for such emission from dark matter annihilation or decay, examples of which follow.
The Energetic Gamma Ray Experiment Telescope observed more gamma rays in 2008 than expected from the Milky Way, but scientists concluded this was most likely due to incorrect estimation of the telescope's sensitivity.[159]
The
At higher energies, ground-based gamma-ray telescopes have set limits on the annihilation of dark matter in dwarf spheroidal galaxies[164] and in clusters of galaxies.[165]
The
In 2013, results from the Alpha Magnetic Spectrometer on the International Space Station indicated excess high-energy cosmic rays which could be due to dark matter annihilation.[167][168][169][170][171][172]
Collider searches for dark matter
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.[173] 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.[174] Any discovery from collider searches must be corroborated by discoveries in the indirect or direct detection sectors to prove that the particle discovered is, in fact, dark matter.
Alternative hypotheses
Because dark matter has not yet been identified, many other hypotheses have emerged aiming to explain the same observational phenomena without introducing a new unknown type of matter. The theory underpinning most observational evidence for dark matter, general relativity, is well-tested on solar system scales, but its validity on galactic or cosmological scales has not been well proven.[175] A suitable modification to general relativity can in principle conceivably eliminate the need for dark matter. The best-known theories of this class are MOND and its relativistic generalization tensor–vector–scalar gravity (TeVeS),[176] f(R) gravity,[177] negative mass, dark fluid,[178][179][180] and entropic gravity.[181] Alternative theories abound.[182][183]
Primordial black holes are considered candidates for components of dark matter.[100][98][184][185] Early constraints on primordial black holes as dark matter usually assumed most black holes would have similar or identical ("monochromatic") mass, which was disproven by LIGO/Virgo results.[96][97][99] In 2024, a review by Bernard Carr and colleagues concluded that primordial black holes forming in the quantum chromodynamics epoch prior to 10–5 seconds after the Big Bang can explain most observations attributed to dark matter. Such black hole formation would result in an extended mass distribution today, "with a number of distinct bumps, the most prominent one being at around one solar mass."[13]
A problem with alternative hypotheses is that observational evidence for dark matter comes from so many independent approaches (see the "observational evidence" section above). Explaining any individual observation is possible but explaining all of them in the absence of dark matter is very difficult. Nonetheless, there have been some scattered successes for alternative hypotheses, such as a 2016 test of gravitational lensing in entropic gravity[186][187][188] and a 2020 measurement of a unique MOND effect.[189][190]
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 be some form of dark matter present in the universe.[19]
In popular culture
Dark matter regularly appears as a topic in hybrid periodicals that cover both factual scientific topics and science fiction,[191] and dark matter itself has been referred to as "the stuff of science fiction".[192]
Mention of dark matter is made in works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties, thus becoming inconsistent with the hypothesized properties of dark matter in physics and cosmology. For example:
- Dark matter serves as a plot device in the X-Files episode "Soft Light."[193]
- A dark-matter-inspired substance known as "Dust" features prominently in Philip Pullman's His Dark Materials trilogy.[194]
- Beings made of dark matter are antagonists in Stephen Baxter's Xeelee Sequence.[195]
More broadly, the phrase "dark matter" is used metaphorically in fiction to evoke the unseen or invisible.[196]
Gallery
-
DM map by the CFHT Lensing Survey (CFHTLenS) using the Canada–France–Hawaii Telescope (2012).[199][200] (COSMOS map at the center)
See also
- Related theories
- Dark energy – Energy driving the accelerated expansion of the universe
- Conformal gravity – Gravity theories that are invariant under Weyl transformations
- Density wave theory – A theory in which waves of compressed gas, which move slower than the galaxy, maintain galaxy's structure
- Entropic gravity – Theory in modern physics that describes gravity as an entropic force
- Dark radiation – Postulated type of radiation that mediates interactions of dark matter
- Massive gravity – Theory of gravity in which the graviton has nonzero mass
- Unparticle physics – Speculative theory that conjectures a form of matter that cannot be explained in terms of particles
- Experiments
- DEAP – Dark matter search experiment, a search apparatus
- LZ experiment – experiment in South Dakota, United States , large underground dark matter detector
- Dark Matter Particle Explorer (DAMPE) – Chinese science satellite, a space mission
- General antiparticle spectrometer
- MultiDark, a research program
- Illustris project – Computer-simulated universes, astrophysical simulations
- Future Circular Collider – Proposed post-LHC particle accelerator at CERN, Geneva, Switzerland, a particle accelerator research infrastructure
- Dark matter candidates
- Feebly Interacting Particles
- Light dark matter – Dark matter weakly interacting massive particles candidates with masses less than 1 GeV
- Mirror matter – Hypothetical counterpart to ordinary matter
- Exotic matter – Any kind of unfamiliar matter with highly unusual properties
- Neutralino – Neutral mass eigenstate formed from superpartners of gauge and Higgs bosons
- Dark galaxy – A hypothesized galaxy with no, or very few, stars
- Scalar field dark matter – Classical, minimally coupled, scalar field postulated to account for the inferred dark matter
- Self-interacting dark matter – Hypothetical form of dark matter consisting of particles with strong self-interactions
- Weakly interacting massive particle (WIMP) – Hypothetical particles that may constitute dark matter
- Weakly interacting slim particle (WISP) – Low-mass counterpart to WIMP
- Strongly interacting massive particle (SIMP) – Hypothetical particle
- Chameleon particle – Hypothetical scalar particle that couples to matter more weakly than gravity
- Other
- Galactic Center GeV excess – Unexplained gamma rays from the galactic center
- Luminiferous aether – A once theorized invisible and infinite material with no interaction with physical objects, used to explain how light could travel through a vacuum (now disproven)
Notes
- ^ Since dark energy does not count as matter, this is 26.8/4.9 + 26.8 = 0.845.
- ^ Some dark matter candidates interact with ordinary matter via the weak interaction, but the weak interaction is weak, making any direct detection very difficult.
- ^ A small portion of dark matter could be baryonic and/or neutrinos. See Baryonic dark matter.
- ^ However, in the modern cosmic era, this neutrino field has cooled and started to behave more like matter and less like radiation.
- ^ Dark energy is a term often used nowadays as a substitute for cosmological constant. It is basically the same except that dark energy might depend on scale factor in some unknown way rather than necessarily being constant.
- ^ This is a consequence of the shell theorem and the observation that spiral galaxies are spherically symmetric to a large extent (in 2D).
- ^ The three neutrino types already observed are indeed abundant, and dark, and matter, but because their individual masses are almost certainly too tiny to account for more than a small fraction of dark matter, due to limits derived from large-scale structure and high-redshift galaxies.[114]
- ^ "One widely held belief about dark matter is it cannot cool off by radiating energy. If it could, then it might bunch together and create compact objects in the same way baryonic matter forms planets, stars, and galaxies. Observations so far suggest dark matter doesn't do that – it resides only in diffuse halos ... As a result, it is extremely unlikely there are very dense objects like stars made out of entirely (or even mostly) dark matter." — Buckley & Difranzo (2018)[137]
References
- ^ Siegfried, T. (5 July 1999). "Hidden space dimensions may permit parallel universes, explain cosmic mysteries". The Dallas Morning News.
- (PDF) from the original on 18 July 2018.
- ^ "A history of dark matter". 2017.
- ^ "Planck Mission Brings Universe into Sharp Focus". NASA Mission Pages. 21 March 2013. Archived from the original on 12 November 2020. Retrieved 1 May 2016.
- ^ "Dark Energy, Dark Matter". NASA Science: Astrophysics. 5 June 2015.
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Further reading
- McGaugh, Stacy S. (August 2018). "Is dark matter real?". Scientific American. Vol. 319, no. 2. pp. 36–43.
- Weiss, Rainer, "The Dark Universe Comes into Focus: The LIGO experiment opened a whole new window to the universe. We asked [2017 Nobel laureate] Rainer Weiss, one of LIGO's lead architects, what gravitational-wave astronomy could reveal next" (sponsor feature), Scientific American, vol. 329, no. 1 (July/August 2023), between p. 7 and p. 8. "I... think that dark matter is made of black holes – really small black holes, a tiny fraction of a solar mass, that don't interact much with light so you can't see them.... According to [cosmic inflation theory], the universe was created by a fluctuation in the vacuum. That kind of fluctuation will have instabilities and explode asymmetrically – which will generate gravitational waves."
External links
- Dark matter at Curlie
- Tremaine, Scott. Lecture on dark matter (Video). IAS.
- Gray, Meghan; Merrifield, Mike; Copeland, Ed (2010). Haran, Brady (ed.). "Dark Matter". Sixty Symbols. University of Nottingham.