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90-652: RAR or Rar may refer to: Radial acceleration relation (RAR) , a relation derived from studying the radial acceleration traced by rotation curves in galaxies Radio acoustic ranging , a non-visual technique for determining a ship's position at sea "rar", the ISO 639-2 code for the Cook Islands Māori language RAR (file format) , a proprietary compressed archive file format in computer software Rarotonga International Airport , Cook Islands, IATA code RAR Rarus , or Rar,

180-511: A quasar and an observer. In this case, the galaxy cluster will lens the quasar. 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 . By measuring the distortion geometry, the mass of the intervening cluster can be obtained. In the weak regime, lensing does not distort background galaxies into arcs, causing minute distortions instead. By examining

270-538: A Lambda-CDM framework that include baryonic feedback effects reproduce the same relation, without the need to invoke new dynamics (such as MOND). Thus, a contribution due to dark matter itself can be fully predictable from that of the baryons, once the feedback effects due to the dissipative collapse of baryons are taken into account. MOND is not a relativistic theory, although relativistic theories which reduce to MOND have been proposed, such as tensor–vector–scalar gravity (TeVeS), scalar–tensor–vector gravity (STVG), and

360-886: A figure in Greek mythology Retinoic acid receptor , a type of protein Rhodesian African Rifles , a unit of the Rhodesian Army Rock Against Racism , a UK movement Rock am Ring and Rock im Park , a German rock music festival Royal Australian Regiment , a unit of the Australian Army Ruger American Rifle , a line of centerfire bolt-action rifles Grupo RAR , a Portuguese company WinRAR , RAR, and UNRAR computer software for compressed archive files Ridotto Alpino Repubblicano , Republican Alpine Redoubt (RAR), intended last stronghold of

450-646: A flat rotation curve, a density profile for a galaxy and its environs must be different than one that is centrally concentrated. Newton's version of Kepler's Third Law implies that the spherically symmetric, radial density profile ρ ( r ) is: ρ ( r ) = v ( r ) 2 4 π G r 2 ( 1 + 2   d log ⁡ v ( r ) d log ⁡ r ) {\displaystyle \rho (r)={\frac {v(r)^{2}}{4\pi Gr^{2}}}\left(1+2~{\frac {d\log v(r)}{d\log r}}\right)} where v ( r )

540-420: A gravitational instability caused by primordial density fluctuations. Many cosmologists strive to understand the nature and the history of these ubiquitous dark haloes by investigating the properties of the galaxies they contain (i.e. their luminosities, kinematics, sizes, and morphologies). The measurement of the kinematics (their positions, velocities and accelerations) of the observable stars and gas has become

630-544: A great majority of them – may be dark bodies. In 1906, Poincaré used the French term [ matière obscure ] ("dark matter") in discussing Kelvin's work. He found that the amount of dark matter would need to be less than that of visible matter, incorrectly, it turns out. The second to suggest the existence of dark matter using stellar velocities was Dutch astronomer Jacobus Kapteyn in 1922. A publication from 1930 by Swedish astronomer Knut Lundmark points to him being

720-690: A greater degree of accuracy than had ever before been achieved. Together with fellow staff-member Kent Ford , Rubin announced at a 1975 meeting of the American Astronomical Society the discovery that most stars in spiral galaxies orbit at roughly the same speed, and that this implied that galaxy masses grow approximately linearly with radius well beyond the location of most of the stars (the galactic bulge ). Rubin presented her results in an influential paper in 1980. These results suggested either that Newtonian gravity does not apply universally or that, conservatively, upwards of 50% of

810-454: A large range of distances. In contrast, the orbital velocities of planets in planetary systems and moons orbiting planets decline with distance according to Kepler’s third law . This reflects the mass distributions within those systems. The mass estimations for galaxies based on the light they emit are far too low to explain the velocity observations. The galaxy rotation problem is the discrepancy between observed galaxy rotation curves and

900-416: A mass structure according to: v ( r ) = ( r d Φ d r ) 1 / 2 {\displaystyle v(r)=\left(r\,{\frac {d\Phi }{dr}}\right)^{1/2}} with Φ the galaxy gravitational potential . Since observations of galaxy rotation do not match the distribution expected from application of Kepler's laws, they do not match

990-478: A mass-to-light ratio of 50; in 1940, Oort discovered and wrote about the large non-visible halo of NGC 3115 . 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. The hypothesis of dark matter largely took root in

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1080-418: A rotation curve that rises from the center to the edge, and most low-surface-brightness galaxies (LSB galaxies) have the same anomalous rotation curve. The rotation curves might be explained by hypothesizing the existence of a substantial amount of matter permeating the galaxy outside of the central bulge that is not emitting light in the mass-to-light ratio of the central bulge. The material responsible for

1170-635: A seminal 1996 paper. The authors then remarked that a "gently changing logarithmic slope" for a density profile function could also accommodate approximately flat rotation curves over large scales. They found the famous Navarro–Frenk–White profile , which is consistent both with N-body simulations and observations given by ρ ( r ) = ρ 0 r R s ( 1 + r R s ) 2 {\displaystyle \rho (r)={\frac {\rho _{0}}{{\frac {r}{R_{s}}}\left(1+{\frac {r}{R_{s}}}\right)^{2}}}} where

1260-682: 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. Nonetheless, research and theories proposing dense dark matter accounts for dark matter continue as of 2018, including approaches to dark matter cooling, and

1350-491: A similar inference. Zwicky applied the virial theorem to the Coma Cluster and obtained evidence of unseen mass 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 the cluster had about 400 times more mass than was visually observable. The gravity effect of

1440-424: A smaller fraction, using greater values for luminous mass. Nonetheless, Zwicky did correctly conclude from his calculation that most of the gravitational matter present was dark. However unlike modern theories, Zwicky considered "dark matter" to be non-luminous ordinary matter. Further indications of mass-to-light ratio anomalies came from measurements of galaxy rotation curves . In 1939, H.W. Babcock reported

1530-572: 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. However, that study assumed a monochromatic distribution to represent the LIGO/Virgo mass range, which is inapplicable to the broadly platykurtic mass distribution suggested by subsequent James Webb Space Telescope observations. The possibility that atom-sized primordial black holes account for

1620-455: A tool to investigate the nature of dark matter, as to its content and distribution relative to that of the various baryonic components of those galaxies. The rotational dynamics of galaxies are well characterized by their position on the Tully–Fisher relation , which shows that for spiral galaxies the rotational velocity is uniquely related to their total luminosity. A consistent way to predict

1710-457: Is modified Newtonian dynamics (MOND), originally proposed by Mordehai Milgrom in 1983, which modifies the Newtonian force law at low accelerations to enhance the effective gravitational attraction. MOND has had a considerable amount of success in predicting the rotation curves of low-surface-brightness galaxies, matching the baryonic Tully–Fisher relation , and the velocity dispersions of

1800-452: Is a hypothetical form of matter that does not interact with light or other electromagnetic radiation . Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be observed. Such effects occur in the context of formation and evolution of galaxies , gravitational lensing , the observable universe 's current structure, mass position in galactic collisions ,

1890-441: Is a rather complicated process, it is straightforward to model the observables of rotating galaxies through this relationship. So, while state-of-the-art cosmological and galaxy formation simulations of dark matter with normal baryonic matter included can be matched to galaxy observations, there is not yet any straightforward explanation as to why the observed scaling relationship exists. Additionally, detailed investigations of

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1980-482: Is classified as "cold", "warm", or "hot" according to 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. Although the astrophysics community generally accepts the existence of dark matter, a minority of astrophysicists, intrigued by specific observations that are not well explained by ordinary dark matter, argue for various modifications of

2070-424: Is different from Wikidata All article disambiguation pages All disambiguation pages Galaxy rotation curve#Alternatives to dark matter The rotational/orbital speeds of galaxies/stars do not follow the rules found in other orbital systems such as stars/planets and planets/moons that have most of their mass at the centre. Stars revolve around their galaxy's centre at equal or increasing speed over

2160-427: Is not known to interact with ordinary baryonic matter and radiation except through gravity, making it difficult to detect in the laboratory. The most prevalent explanation is that dark matter is some as-yet-undiscovered subatomic particle , such as either weakly interacting massive particles (WIMPs) or axions . The other main possibility is that dark matter is composed of primordial black holes . Dark matter

2250-536: Is of particular note because the location of the center of mass as measured by gravitational lensing is different from the location of the center of mass of visible matter. This is difficult for modified gravity theories, which generally predict lensing around visible matter, to explain. Standard dark matter theory however has no issue: the hot, visible gas in each cluster would be cooled and slowed down by electromagnetic interactions, while dark matter (which does not interact electromagnetically) would not. This leads to

2340-474: Is revealed only via its gravitational effects, or weak lensing . 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). In 2015, the idea that dense dark matter was composed of primordial black holes made a comeback following results of gravitational wave measurements which detected

2430-399: Is the gravitational lens . Gravitational lensing occurs when massive objects between a source of light and the observer act as a lens to bend light from this source. Lensing does not depend on the properties of the mass; it only requires there to be a mass. The more massive an object, the more lensing is observed. An example is a cluster of galaxies lying between a more distant source such as

2520-417: Is the radial orbital velocity profile and G is the gravitational constant . This profile closely matches the expectations of a singular isothermal sphere profile where if v ( r ) is approximately constant then the density ρ ∝ r to some inner "core radius" where the density is then assumed constant. Observations do not comport with such a simple profile, as reported by Navarro, Frenk, and White in

2610-434: Is unknown, but there are many hypotheses about what dark matter could consist of, as set out in the table below. 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

2700-559: Is well fitted by the lambda-CDM model , but difficult to reproduce with any competing model such as modified Newtonian dynamics (MOND). 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

2790-580: The 2dF Galaxy Redshift Survey . Results are in agreement with the lambda-CDM model . 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. These constraints agree with those obtained from WMAP data. The identity of dark matter

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2880-456: The f(R) theory of Capozziello and De Laurentis. A model of galaxy rotation based on a general relativity metric was also proposed, showing that the rotation curves for the Milky Way , NGC 3031 , NGC 3198 and NGC 7331 are consistent with the mass density distributions of the visible matter, avoiding the need for a massive halo of exotic dark matter. According to recent analysis of

2970-547: The 1970s. Several different observations were synthesized to argue that galaxies should be surrounded by halos of unseen matter. In two papers that appeared in 1974, this conclusion was drawn in tandem by independent groups: in Princeton, New Jersey, U.S.A., by Jeremiah Ostriker , Jim Peebles , and Amos Yahil, and in Tartu, Estonia, by Jaan Einasto , Enn Saar, and Ants Kaasik. One of the observations that served as evidence for

3060-422: The 21 cm line of atomic hydrogen in nearby galaxies. The radial distribution of interstellar atomic hydrogen ( H ) often extends to much greater galactic distances than can be observed as collective starlight, expanding the sampled distances for rotation curves – and thus of the total mass distribution – to a new dynamical regime. Early mapping of Andromeda with the 300 foot telescope at Green Bank and

3150-434: The 250 foot dish at Jodrell Bank already showed the H rotation curve did not trace the decline expected from Keplerian orbits. As more sensitive receivers became available, Roberts & Whitehurst (1975) were able to trace the rotational velocity of Andromeda to 30 kpc, much beyond the optical measurements. Illustrating the advantage of tracing the gas disk at large radii; that paper's Figure 16 combines

3240-632: 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. The results support the Lambda-CDM model. 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 ;

3330-466: The CMB. The CMB 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 locations of these peaks depend on cosmological parameters. Matching theory to data, therefore, constrains cosmological parameters. The CMB anisotropy

3420-572: The Italian fascists at the end of World War II Rarh region , region in eastern India Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title RAR . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=RAR&oldid=1246518717 " Category : Disambiguation pages Hidden categories: Short description

3510-474: The Solar System. This is not observed. Instead, the galaxy rotation curve remains flat or even increases as distance from the center increases. 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

3600-403: The apparent shear deformation of the adjacent background galaxies, the mean distribution of dark matter can be characterized. The measured mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements. 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

3690-458: The central density, ρ 0 , and the scale radius, R s , are parameters that vary from halo to halo. Because the slope of the density profile diverges at the center, other alternative profiles have been proposed, for example the Einasto profile , which has exhibited better agreement with certain dark matter halo simulations. Observations of orbit velocities in spiral galaxies suggest

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3780-546: The dark matter halo of this galaxy. The Carnegie telescope (Carnegie Double Astrograph) was intended to study this problem of Galactic rotation. In the late 1960s and early 1970s, Vera Rubin , an astronomer at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington , worked with a new sensitive spectrograph that could measure the velocity curve of edge-on spiral galaxies to

3870-429: The dark matter separating from the visible gas, producing the separate lensing peak as observed. 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. Data indicates the universe is expanding at an accelerating rate, the cause of which is usually ascribed to dark energy . Since observations indicate

3960-403: The dark matter. However, multiple lines of evidence suggest the majority of dark matter is not baryonic: There are two main candidates for non-baryonic dark matter: new hypothetical particles and primordial black holes . Unlike baryonic matter, nonbaryonic particles do not contribute to the formation of the elements in the early universe ( Big Bang nucleosynthesis ) and so its presence

4050-520: The data produced by the Gaia spacecraft , it would seem possible to explain at least the Milky Way 's rotation curve without requiring any dark matter if instead of a Newtonian approximation the entire set of equations of general relativity is adopted. In March 2021, Gerson Otto Ludwig published a model based on general relativity that explains galaxy rotation curves with gravitoelectromagnetism . Dark matter In astronomy , dark matter

4140-420: 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,

4230-436: The diameter of the observable Universe via cosmic expansion , the scale, a , has doubled. The energy of the cosmic microwave background radiation has been halved (because the wavelength of each photon has doubled); the energy of ultra-relativistic particles, such as early-era standard-model neutrinos, is similarly halved. The cosmological constant, as an intrinsic property of space, has a constant energy density regardless of

4320-410: The distribution of luminous matter. This implies that spiral galaxies contain large amounts of dark matter or, alternatively, the existence of exotic physics in action on galactic scales. The additional invisible component becomes progressively more conspicuous in each galaxy at outer radii and among galaxies in the less luminous ones. A popular interpretation of these observations is that about 26% of

4410-482: The dynamics of galaxies. The same relation provided a good fit for 2693 samples in 153 rotating galaxies, with diverse shapes, masses, sizes, and gas fractions. Brightness in the near infrared, where the more stable light from red giants dominates, was used to estimate the density contribution due to stars more consistently. The results are consistent with MOND, and place limits on alternative explanations involving dark matter alone. However, cosmological simulations within

4500-516: 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 . Combining

4590-416: The existence of dark matter and to assume its distribution from the galaxy's center out to its halo . Thus the discrepancy between the two curves can be accounted for by adding a dark matter halo surrounding the galaxy. Though dark matter is by far the most accepted explanation of the rotation problem, other proposals have been offered with varying degrees of success. Of the possible alternatives , one of

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4680-408: The existence of galactic halos of dark matter was the shape of galaxy rotation curves . These observations were done in optical and radio astronomy. In optical astronomy, Vera Rubin and Kent Ford worked with a new spectrograph to measure the velocity curve of edge-on spiral galaxies with greater accuracy. At the same time, radio astronomers were making use of new radio telescopes to map

4770-510: The extra mass was dubbed dark matter , the existence of which was first posited in the 1930s by Jan Oort in his measurements of the Oort constants and Fritz Zwicky in his studies of the masses of galaxy clusters . The existence of non-baryonic cold dark matter (CDM) is today a major feature of the Lambda-CDM model that describes the cosmology of the universe . In order to accommodate

4860-505: The first to realise that the universe must contain much more mass than can be observed. Dutch radio astronomy pioneer Jan Oort also hypothesized the existence of dark matter in 1932. Oort was studying stellar motions in the 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 incorrect. In 1933, Swiss astrophysicist Fritz Zwicky studied galaxy clusters while working at Cal Tech and made

4950-456: The galactic center (the dashed line in Fig. 1). Observations of the rotation curve of spirals, however, do not bear this out. Rather, the curves do not decrease in the expected inverse square root relationship but are "flat", i.e. outside of the central bulge the speed is nearly a constant (the solid line in Fig. 1). It is also observed that galaxies with a uniform distribution of luminous matter have

5040-473: The galactic center. The luminous mass density of a spiral galaxy decreases as one goes from the center 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 Third Law , it is expected that the rotation velocities will decrease with distance from the center, similar to

5130-429: The galaxies and clusters currently seen. 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. The Bullet Cluster is the result of a recent collision of two galaxy clusters. It

5220-499: The galaxy. 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 do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits. As with galaxy rotation curves,

5310-400: 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. If there were only ordinary matter in the universe, there would not have been enough time for density perturbations to grow into

5400-453: The key to solving the dwarf galaxy problem of structure formation . Very importantly, the analysis of the inner parts of low and high surface brightness galaxies showed that the shape of the rotation curves in the centre of dark-matter dominated systems indicates a profile different from the NFW spatial mass distribution profile. This so-called cuspy halo problem is a persistent problem for

5490-433: The late 1970s the existence of dark matter halos around galaxies was widely recognized as real, and became a major unsolved problem in astronomy. A stream of observations in the 1980–1990s supported the presence of dark matter. Persic, Salucci & Stel (1996) is notable for the investigation of 967 spirals. The evidence for dark matter also included gravitational lensing of background objects by galaxy clusters ,

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5580-453: The mass of galaxies was contained in the relatively dark galactic halo. Although initially met with skepticism, Rubin's results have been confirmed over the subsequent decades. If Newtonian mechanics is assumed to be correct, it would follow that most of the mass of the galaxy had to be in the galactic bulge near the center and that the stars and gas in the disk portion should orbit the center at decreasing velocities with radial distance from

5670-427: The mass of the Universe is composed of dark matter, a hypothetical type of matter which does not emit or interact with electromagnetic radiation . Dark matter is believed to dominate the gravitational potential of galaxies and clusters of galaxies. Under this theory, galaxies are baryonic condensations of stars and gas (namely hydrogen and helium) that lie at the centers of much larger haloes of dark matter, affected by

5760-469: 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

5850-475: The most notable is modified Newtonian dynamics (MOND), which involves modifying the laws of gravity. In 1932, Jan Hendrik Oort became the first to report that measurements of the stars in the solar neighborhood indicated that they moved faster than expected when a mass distribution based upon visible matter was assumed, but these measurements were later determined to be essentially erroneous. In 1939, Horace Babcock reported in his PhD thesis measurements of

5940-445: The motion of galaxies within galaxy clusters , and cosmic microwave background anisotropies . 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 . Thus, dark matter constitutes 85% of the total mass, while dark energy and dark matter constitute 95% of the total mass–energy content. Dark matter

6030-408: The obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter. Galaxy clusters are particularly important for dark matter studies since their masses can be estimated in three independent ways: Generally, these three methods are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1. One of the consequences of general relativity

6120-464: The optical data (the cluster of points at radii of less than 15 kpc with a single point further out) with the H data between 20 and 30 kpc, exhibiting the flatness of the outer galaxy rotation curve; the solid curve peaking at the center is the optical surface density, while the other curve shows the cumulative mass, still rising linearly at the outermost measurement. In parallel, the use of interferometric arrays for extragalactic H spectroscopy

6210-513: 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. A black hole would ingest both baryonic and non-baryonic matter that comes close enough to its event horizon; afterwards, the distinction between the two is lost. These massive objects that are hard to detect are collectively known as MACHOs . Some scientists initially hoped that baryonic MACHOs could account for and explain all

6300-585: The potential number of stars around the Sun in the appendices of a book based on a series of lectures given in 1884 in Baltimore. He inferred their density using the observed velocity dispersion of the stars near the Sun, assuming that the Sun was 20–100 million years old. He posed what would happen if there were a thousand million stars within 1  kiloparsec of the Sun (at which distance their parallax would be 1  milli-arcsecond ). Kelvin concluded Many of our supposed thousand million stars – perhaps

6390-461: 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

6480-448: The rotation curve for Andromeda which suggested that the mass-to-luminosity ratio increases radially. He attributed that to either the absorption of light within the galaxy or to modified dynamics in the outer portions of the spiral and not to any form of missing matter. Babcock's measurements turned out to disagree substantially with those found later, and the first measurement of an extended rotation curve in good agreement with modern data

6570-531: The rotation curve for the Andromeda nebula (now called the Andromeda Galaxy ), which suggested the mass-to-luminosity ratio increases radially. He attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral, rather than to unseen matter. Following Babcock's 1939 report of unexpectedly rapid rotation in the outskirts of the Andromeda Galaxy and

6660-457: The rotation curves of low-surface-brightness galaxies (LSB galaxies) in the 1990s and of their position on the Tully–Fisher relation showed that LSB galaxies had to have dark matter haloes that are more extended and less dense than those of galaxies with high surface brightness, and thus surface brightness is related to the halo properties. Such dark-matter-dominated dwarf galaxies may hold

6750-402: The rotational velocity of a spiral galaxy is to measure its bolometric luminosity and then read its rotation rate from its location on the Tully–Fisher diagram. Conversely, knowing the rotational velocity of a spiral galaxy gives its luminosity. Thus the magnitude of the galaxy rotation is related to the galaxy's visible mass. While precise fitting of the bulge, disk, and halo density profiles

6840-607: The small satellite galaxies of the Local Group . Using data from the Spitzer Photometry and Accurate Rotation Curves (SPARC) database, a group has found that the radial acceleration traced by rotation curves (an effect given the name "radial acceleration relation") could be predicted just from the observed baryon distribution (that is, including stars and gas but not dark matter). This so-called radial acceleration relation (RAR) might be fundamental for understanding

6930-411: The standard cold dark matter theory. Simulations involving the feedback of stellar energy into the interstellar medium in order to alter the predicted dark matter distribution in the innermost regions of galaxies are frequently invoked in this context. There have been a number of attempts to solve the problem of galaxy rotation by modifying gravity without invoking dark matter. One of the most discussed

7020-465: 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 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. The hypothesis of dark matter has an elaborate history. Wm. Thomson, Lord Kelvin, discussed

7110-454: 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

7200-475: The system appears to bear almost no relation to that of light... one finds the ratio of mass to light in the outer parts of NGC 3115 to be about 250". On page 302–303 of his journal article, he wrote that "The strongly condensed luminous system appears imbedded in a large and more or less homogeneous mass of great density" and although he went on to speculate that this mass may be either extremely faint dwarf stars or interstellar gas and dust, he had clearly detected

7290-470: The temperature distribution of hot gas in galaxies and clusters, and the pattern of anisotropies in the cosmic microwave background . According to the current consensus among cosmologists, dark matter is composed primarily of some type of not-yet-characterized subatomic particle . The search for this particle, by a variety of means, is one of the major efforts in particle physics . In standard cosmological calculations, "matter" means any constituent of

7380-399: The theoretical prediction, assuming a centrally dominated mass associated with the observed luminous material. When mass profiles of galaxies are calculated from the distribution of stars in spirals and mass-to-light ratios in the stellar disks, they do not match with the masses derived from the observed rotation curves and the law of gravity . A solution to this conundrum is to hypothesize

7470-523: The universe is almost flat, it is expected the total energy density of everything in the universe should sum to 1 ( Ω tot ≈ 1 ). The measured dark energy density is Ω Λ ≈ 0.690 ; the observed ordinary (baryonic) matter energy density is Ω b ≈ 0.0482 and the energy density of radiation is negligible. This leaves a missing Ω dm ≈ 0.258 which nonetheless behaves like matter (see technical definition section above) – dark matter. Baryon acoustic oscillations (BAO) are fluctuations in

7560-439: The universe whose energy density scales with the inverse cube of the scale factor , i.e., ρ ∝ a . This is in contrast to "radiation" , which scales as the inverse fourth power of the scale factor ρ ∝ a , and a cosmological constant , which does not change with respect to a ( ρ ∝ a ). The different scaling factors for matter and radiation are a consequence of radiation redshift . For example, after doubling

7650-404: The visible galaxies was far too small for such fast orbits, thus mass must be hidden from view. Based on these conclusions, Zwicky inferred some unseen matter provided the mass and associated gravitational attraction to hold the cluster together. Zwicky's estimates were off by more than an order of magnitude, mainly due to an obsolete value of the Hubble constant ; the same calculation today shows

7740-399: The volume under consideration. In principle, "dark matter" means all components of the universe which are not visible but still obey ρ ∝ a . 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. The arms of spiral galaxies rotate around

7830-585: Was being developed. Rogstad & Shostak (1972) published H rotation curves of five spirals mapped with the Owens Valley interferometer; the rotation curves of all five were very flat, suggesting very large values of mass-to-light ratio in the outer parts of their extended H  disks. In 1978, Albert Bosma showed further evidence of flat rotation curves using data from the Westerbork Synthesis Radio Telescope . By

7920-556: 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 BOOMERanG experiment in 2000, the power spectrum was precisely observed by WMAP in 2003–2012, and even more precisely by the Planck spacecraft in 2013–2015. The results support the Lambda-CDM model. The observed CMB angular power spectrum provides powerful evidence in support of dark matter, as its precise structure

8010-423: 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

8100-520: Was published in 1957 by Henk van de Hulst and collaborators, who studied M31 with the newly commissioned Dwingeloo 25 meter telescope. A companion paper by Maarten Schmidt showed that this rotation curve could be fit by a flattened mass distribution more extensive than the light. In 1959, Louise Volders used the same telescope to demonstrate that the spiral galaxy M33 also does not spin as expected according to Keplerian dynamics . Reporting on NGC 3115 , Jan Oort wrote that "the distribution of mass in

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