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69-470: Officially discovered in 1994, by Rodrigo Ibata, Mike Irwin , and Gerry Gilmore , Sgr dSph was immediately recognized as being the nearest known neighbor to the Milky Way at the time. (The disputed Canis Major Dwarf Galaxy , discovered in 2003, might be the actual nearest neighbor.) Although it is one of the closest companion galaxies to the Milky Way, the main parent cluster is on the opposite side of

138-498: 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 the standard laws of general relativity. These include modified Newtonian dynamics , tensor–vector–scalar gravity , or entropic gravity . So far none of

207-511: 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

276-478: 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

345-531: A roughly polar orbit around the Milky Way as close as 50,000 light-years from the galactic core. Although it may have begun as a spherical object before falling towards the Milky Way, Sgr dSph is now being torn apart by immense tidal forces over hundreds of millions of years. Numerical simulations suggest that stars ripped out from the dwarf would be spread out in a long stellar stream along its path, which were subsequently detected. However, some astronomers contend that Sgr dSph has been in orbit around

414-682: 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

483-491: 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

552-424: 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

621-572: 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

690-660: Is an unusually low number of globular clusters, and an analysis of VVV and Gaia EDR3 data has found at least twenty more. The newly discovered globular clusters tend to be more metal-rich than previously known globular clusters. Sgr dSph has multiple stellar populations , ranging in age from the oldest globular clusters (almost as old as the universe itself) to trace populations as young as several hundred million years (mya) . It also exhibits an age- metallicity relationship, in that its old populations are metal poor ( [Fe/H] = −1.6 ± 0.1 ) while its youngest populations have super-solar abundances. Based on its current trajectory,

759-433: Is found to have galactocentric distances that oscillate between ≈13 and ≈41 kpc with a period of 550 to 750 million years. The last perigalacticon was approximately fifty million years ago. Also in 1999, Jiang & Binney found that it may have started its infall into the Milky Way at a point more than 200 kpc away if its starting mass was as large as ≈10 M ☉ . The models of both its orbit and

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828-434: Is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be observed. Such effects occur in the context of formation and evolution of galaxies , gravitational lensing , the observable universe 's current structure, mass position in galactic collisions , the motion of galaxies within galaxy clusters , and cosmic microwave background anisotropies . In

897-491: Is known worldwide for the leading role he plays in processing of digital optical and infra-red survey data. Currently, his efforts in processing of digital optical and infrared survey data of Vista Data Flow are being used for processing United Kingdom Infrared Telescope data. In 2012, Royal Astronomical Society awarded Michael Irwin the 2012 Herschel Medal , which recognises investigations of outstanding merit in observational astrophysics . He has also made contributions in

966-536: 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

1035-474: 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

1104-399: 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

1173-434: 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

1242-559: 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

1311-580: 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

1380-468: 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

1449-465: The Galactic Center from Earth, and consequently is very faint, although covering a large area of the sky. Sgr dSph appears to be an older galaxy with little interstellar dust, composed largely of Population II stars, older and metal-poor, as compared to the Milky Way. No neutral hydrogen gas related to Sgr dSph has been found. Further discoveries by astrophysics teams from both

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1518-542: The University of Virginia and the University of Massachusetts Amherst , drawing upon the 2MASS Two-Micron All Sky Infrared Survey data, revealed the entire loop-shaped structure. In 2003 with the aid of infrared telescopes and super computers, Steven Majewski, Michael Skrutskie, and Martin Weinberg were able to help create a new star map, picking out the full Sagittarius Dwarf presence, position, and looping shape from

1587-547: 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

1656-422: 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

1725-434: 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

1794-632: 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 ;

1863-466: 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

1932-500: The European Space Agency, designed primarily to investigate the origin, evolution and structure of the Milky Way, delivered the largest and most precise census of positions, velocities and other stellar properties of more than a billion stars, which showed that Sgr dSph had caused perturbations in a set of stars near the Milky Way's core, causing unexpected rippling movements of the stars triggered when it sailed past

2001-472: The Milky Way between 300 and 900 million years ago. A 2019 study by TCU Graduate Student Matthew Melendez and co-authors concluded that Sgr dSph had a decreasing metallicity trend as a function of radius, with a larger spread in metallicity in the core relative to the outer regions. Also, they did find evidence for the first time for two distinct populations in alpha abundances as a function of metallicity. A 2020 study concluded that collisions between

2070-433: The Milky Way for some billions of years, and has already orbited it approximately ten times. Its ability to retain some coherence despite such strains would indicate an unusually high concentration of dark matter within that galaxy. In 1999, Johnston et al. concluded that Sgr dSph has orbited the Milky Way for at least one gigayear and that during that time its mass has decreased by a factor of two or three. Its orbit

2139-518: The Milky Way's potential field could be improved by proper motion observations of Sgr dSph's stellar debris. This issue is under intense investigation, with computational support by the MilkyWay@Home project. A simulation published in 2011 suggested that the Milky Way may have obtained its spiral structure as a result of repeated collisions with Sgr dSph. In 2018, the Gaia project of

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2208-788: The Sagittarius Dwarf Spheroidal Galaxy and the Milky Way triggered major episodes of star formation in the latter, based on data taken from the Gaia project. Mike Irwin Michael J. Irwin is a British astronomer . He is the director of the Cambridge Astronomical Survey Unit and one of the discoverers of the Cetus Dwarf galaxy and the Sagittarius Dwarf Elliptical Galaxy . Irwin

2277-487: The Sgr ;dSph main cluster and its merger with the Milky Way stream is expected to be complete within a billion years from now. At first, many astronomers thought that Sgr dSph had already reached an advanced state of destruction, so that a large part of its original matter was already mixed with that of the Milky Way. However, Sgr dSph still has coherence as a dispersed elongated ellipse, and appears to move in

2346-415: The Sgr dSph main cluster is about to pass through the galactic disc of the Milky Way within the next hundred million years, while the extended loop-shaped ellipse is already extended around and through our local space and on through the Milky Way galactic disc, and in the process of slowly being absorbed into the larger galaxy, calculated at 10,000 times the mass of Sgr dSph. The dissipation of

2415-474: 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

2484-403: 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

2553-429: 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

2622-403: 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

2691-420: 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,

2760-436: 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

2829-516: 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

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2898-408: 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

2967-505: 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

3036-473: 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

3105-429: 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

3174-499: 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,

3243-400: 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

3312-437: 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 is classified as "cold", "warm", or "hot" according to velocity (more precisely, its free streaming length). Recent models have favored

3381-433: 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 ,

3450-470: The mass of background stars and finding this smaller galaxy to be at a near right angle to the plane of the Milky Way. Sgr dSph has at least nine known globular clusters . One, M 54 , appears to reside at its core, while three others reside within the main body of the galaxy: Terzan 7 , Terzan 8 and Arp 2 . Additionally, Palomar 12 , Whiting 1 , NGC 2419 , NGC 4147 , and NGC 5634 are found within its extended stellar streams . However, this

3519-469: 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

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3588-439: 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 a great majority of them – may be dark bodies. In 1906, Poincaré used

3657-408: 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

3726-464: 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

3795-513: 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

3864-531: 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 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

3933-461: 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

4002-531: 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

4071-547: The scientific community by writing and helping write several books. According to the Minor Planet Center 's official discoverer list, Irwin co-discovered 8 minor planets during 1990–1996. This article about a British astronomer is a stub . You can help Misplaced Pages by expanding it . Dark matter In astronomy , dark matter is a hypothetical form of matter that does not interact with light or other electromagnetic radiation . Dark matter

4140-471: 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 is not known to interact with ordinary baryonic matter and radiation except through gravity, making it difficult to detect in

4209-454: 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

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4278-470: 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

4347-523: 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

4416-439: 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

4485-404: 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

4554-399: 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

4623-585: 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

4692-556: 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

4761-423: 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

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