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63 Dark Matter Primarily from Wikipedia, the Free Encyclopedia Dark Matter Primarily from Wikipedia, the free encyclopedia (https://en.wikipedia.org/wiki/Dark_matter) Mark Herbert, PhD World Development Institute 39-06 Main Street, Flushing, Queens, New York 11354, USA, [email protected] Abstract: Dark matter is a form of matter composed for approximately 85% of the matter in the universe and about a quarter of its total mass - energy density. Primary evidence for dark matter comes from calculations showing that many galaxies would fly apart, or that they would not have formed or would not move as they do, if they did not contain a large amount of unseen matter. Other lines of evidence include observations in gravitational lensing and in the cosmic microwave background, along with astronomical observations of the observable universe's current structure, the formation and evolution of galaxies, mass location during galactic collisions, and the motion of galaxies within galaxy clusters. In the standard Lambda-CDM model of cosmology, the total mass–energy of the universe contains 5% ordinary matter and energy, 27% dark matter and 68% of a form of energy known as dark energy. [Mark Herbert. Dark Matter. N Y Sci J 2020;13(11):63-79]. ISSN 1554-0200 (print); ISSN 2375-723X (online). http://www.sciencepub.net/newyork. 9. doi:10.7537/marsnys131120.09. Keywords: dark matter; universe; energy; galaxy; gravity; cosmic microwave background, cosmology Dark matter is a form of matter composed for while dark energy plus dark matter constitute 95% of approximately 85% of the matter in the universe and total mass–energy content.[9][10][11][12] about a quarter of its total mass - energy density (about Because dark matter has not yet been observed 2.241×10−27 kg/m3). Its presence is implied in a variety directly, if it exists, it must barely interact with of astrophysical observations, including gravitational ordinary baryonic matter and radiation, except through effects that cannot be explained by accepted theories gravity. Most dark matter is thought to be non- of gravity unless more matter is present than can be baryonic in nature; it may be composed of some as-yet seen. For this reason, most experts think that dark undiscovered subatomic particles.[b] The primary matter is abundant in the universe and that it has had a candidate for dark matter is some new kind of strong influence on its structure and evolution. Dark elementary particle that has not yet been discovered, in matter is called dark because it does not appear to particular, weakly interacting massive particles interact with the electromagnetic field, which means it (WIMPs).[13] Many experiments to directly detect and doesn't absorb, reflect or emit electromagnetic study dark matter particles are being actively radiation, and is therefore difficult to detect.[1] undertaken, but none have yet succeeded.[14] Dark Primary evidence for dark matter comes from matter is classified as cold, warm, or hot according to calculations showing that many galaxies would fly its velocity (more precisely, its free streaming length). apart, or that they would not have formed or would not Current models favor a cold dark matter scenario, in move as they do, if they did not contain a large amount which structures emerge by gradual accumulation of of unseen matter.[2] Other lines of evidence include particles. observations in gravitational lensing[3] and in the Although the existence of dark matter is cosmic microwave background, along with generally accepted by the scientific community, some astronomical observations of the observable universe's astrophysicists, intrigued by certain observations current structure, the formation and evolution of which do not fit some dark matter theories, argue for galaxies, mass location during galactic collisions,[4] various modifications of the standard laws of general and the motion of galaxies within galaxy clusters. In relativity, such as modified Newtonian dynamics, the standard Lambda-CDM model of cosmology, the tensor–vector–scalar gravity, or entropic gravity. total mass–energy of the universe contains 5% These models attempt to account for all observations ordinary matter and energy, 27% dark matter and 68% without invoking supplemental non-baryonic matter.[15] of a form of energy known as dark energy.[5][6][7][8] Thus, dark matter constitutes 85%[a] of total mass, 63 New York Science Journal 2020;13(11) http://www.sciencepub.net/newyork NYJ History 1940 Jan Oort discovered and wrote about the large Early history non-visible halo of NGC3115.[31] The hypothesis of dark matter has an elaborate history.[16] In a talk given in 1884,[17] Lord Kelvin 1970s estimated the number of dark bodies in the Milky Way Vera Rubin, Kent Ford, and Ken Freeman's work from the observed velocity dispersion of the stars in the 1960s and 1970s[32] provided further strong orbiting around the center of the galaxy. By using evidence, also using galaxy rotation curves.[33][34][35] these measurements, he estimated the mass of the Rubin and Ford worked with a new spectrograph to galaxy, which he determined is different from the mass measure the velocity curve of edge-on spiral galaxies of visible stars. Lord Kelvin thus concluded many of with greater accuracy.[35] This result was confirmed in our stars, perhaps a great majority of them, may be 1978.[36] An influential paper presented Rubin and dark bodies.[18][19] In 1906 Henri Poincaré in The Ford's results in 1980.[37] They showed most galaxies Milky Way and Theory of Gases used dark must contain about six times as much dark as visible matter.[20][19] mass;[38] thus, by around 1980 the apparent need for The first to suggest the existence of dark matter dark matter was widely recognized as a major using stellar velocities was Dutch astronomer Jacobus unsolved problem in astronomy.[33] Kapteyn in 1922.[21][22] Fellow Dutchman and radio At the same time Rubin and Ford were exploring astronomy pioneer Jan Oort also hypothesized the optical rotation curves, radio astronomers were making existence of dark matter in 1932.[22][23][24] Oort was use of new radio telescopes to map the 21 cm line of studying stellar motions in the local galactic atomic hydrogen in nearby galaxies. The radial neighborhood and found the mass in the galactic plane distribution of interstellar atomic hydrogen (H-I) often must be greater than what was observed, but this extends to much larger galactic radii than those measurement was later determined to be erroneous.[25] accessible by optical studies, extending the sampling In 1933, Swiss astrophysicist Fritz Zwicky, who of rotation curves – and thus of the total mass studied galaxy clusters while working at the California distribution – to a new dynamical regime. Early Institute of Technology, made a similar inference.[26][27] mapping of Andromeda with the 300 foot telescope at Zwicky applied the virial theorem to the Coma Cluster Green Bank[39] and the 250 foot dish at Jodrell Bank[40] and obtained evidence of unseen mass he called dunkle already showed the H-I rotation curve did not trace the Materie (dark matter). Zwicky estimated its mass expected Keplerian decline. As more sensitive based on the motions of galaxies near its edge and receivers became available, Morton Roberts and compared that to an estimate based on its brightness Robert Whitehurst[41] were able to trace the rotational and number of galaxies. He estimated the cluster had velocity of Andromeda to 30 kpc, much beyond the about 400 times more mass than was visually optical measurements. Illustrating the advantage of observable. The gravity effect of the visible galaxies tracing the gas disk at large radii, Figure 16 of that was far too small for such fast orbits, thus mass must paper[41] combines the optical data[35] with the H-I data be hidden from view. Based on these conclusions, between 20–30 kpc, exhibiting the flatness of the outer Zwicky inferred some unseen matter provided the galaxy rotation curve; the solid curve peaking at the mass and associated gravitation attraction to hold the center is the optical surface density, while the other cluster together.[28] Zwicky's estimates were off by curve shows the cumulative mass, still rising linearly more than an order of magnitude, mainly due to an at the outermost measurement. In parallel, the use of obsolete value of the Hubble constant;[29] the same interferometric arrays for extragalactic H-I calculation today shows a smaller fraction, using spectroscopy was being developed. In 1972, David greater values for luminous mass. Nonetheless, Rogstad and Seth Shostak[42] published H-I rotation Zwicky did correctly conclude from his calculation curves of five spirals mapped with the Owens Valley that the bulk of the matter was dark.[19] interferometer; the rotation curves of all five were very Further indications the mass-to-light ratio was flat, suggesting very large values of mass-to-light ratio not unity came from measurements of galaxy rotation in the outer parts of their extended H-I disks. curves. In 1939, Horace W. Babcock reported the A stream of observations in the 1980s supported rotation curve for the Andromeda nebula, which the presence of dark matter, including gravitational suggested the mass-to-luminosity ratio increases lensing of background objects by galaxy clusters,[43] radially.[30] He attributed it to either light absorption the temperature distribution of hot gas in galaxies and within the galaxy or modified dynamics in the outer clusters, and the pattern of anisotropies in the cosmic portions of the spiral and not to the missing
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