"Dark matter" refers matter of an unknown type that astronomers and cosmologists believe must make up the majority of the mass in the universe. It is called "dark" because it does not emit any light. We know of its presence because of the gravitational effects it has on objects that we can see. For example, galaxies in clusters move at speeds that are too high to be attributed just to the visible galaxies. In addition, astronomers measure high temperature gas in these galaxy clusters. This gas is at too high a temperature to remain bound to the cluster without some additional, hidden, mass. For galaxies and groups, the X-ray data have often indicated very extended dark matter halos far beyond the radius at which one sees starlight or galaxies. The total inferred dark matter mass is often 10 times that in the "visible" galaxies alone.
Dark matter also plays a role in the early universe. Astronomers theorize that the presence of dark matter helps to explain the relative amounts of light elements and isotopes produced in the Big Bang. Results from the Wilkinson Microwave Anisotropy Probe (WMAP) show that 23% of the Universe is made up of dark matter.
One of the best ways of determining the mass of a system, such as a cluster of galaxies, group of galaxies or a massive elliptical galaxy, is to measure the X-ray temperature and gas profiles. Astronomers start by assuming the gas is in equilibrium, which is borne out by the thermal spectra of the gas. Matching models of the distribution and temperature of the gas to the X-ray observations gives the mass of the gas. Use of this technique has shown that clusters of galaxies are gas and baryon rich, that is, the mass in gas exceeds the mass in stars by factors of 3-5 and that the total baryonic mass is ~15% of the total mass of the cluster (i.e. visible mass plus dark matter). Since clusters are supposed to be "fair samples of the universe" they should have a baryon fraction that corresponds to 4%, as inferred from Big Bang nucleosynthesis and results from WMAP. However, WMAP and other evidence now point to a new component in the universe, which is called dark energy. Dark energy makes up 73% of the energy and matter of the universe. By including this dark energy with the visible mass and dark matter, we find that clusters really do share in the same baryonic fraction as the rest of the universe.
2007-01-09 00:25:57
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answer #2
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answered by Tim C 4
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The composition of dark matter is unknown, but may include new elementary particles such as WIMPs, axions, and ordinary and heavy neutrinos, as well as astronomical bodies such as dwarf stars, planets collectively called MACHOs, and clouds of nonluminous gas. Current evidence favors models in which the primary component of dark matter is new elementary particles, collectively called non-baryonic dark matter.
Black holes are NOT synonymous with dark matter.
2007-01-08 23:42:49
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answer #3
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answered by gebobs 6
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Dark matter is incredibly dense. Gravity pulls matter together to a point of singularity where it is so dense that it has zero length, width, and height. At the point of singularity, the matter is so dense that not even light can escape its gravitation. Hence, dark matter is perceived as such as it does not reflect light but absorbs it. You can find more about dark matter and black holes at http://en.wikipedia.org/wiki/Black_hole
2007-01-08 23:32:23
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answer #4
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answered by suntoucher79 1
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For nearly 40 years after Zwicky's initial observations, no other corroborating observations indicated that the mass to light ratio was anything other than unity (a high mass-to-light ratio indicates the presence of dark matter). Then in the late 1960s and early 1970s, Vera Rubin, a young astronomer at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington began to present findings based on a new sensitive spectrograph that could measure the velocity curve of edge-on spiral galaxies to 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 astonishing discovery that most stars in spiral galaxies orbit at roughly the same speed, which implied that their mass densities were uniform well beyond the locations with most of the stars (the galactic bulge). This result suggests that either Newtonian gravity does not apply universally or that, conservatively, upwards of 50% of the mass of galaxies was contained in the relatively dark galactic halo. Met with skepticism, Rubin insisted that the observations were correct. (She was not unaccustomed to controversy — both her master's and PhD work were openly ridiculed and rejected for publication years earlier.) Eventually other astronomers began to corroborate her work and it soon became well-established that most galaxies were in fact dominated by "dark matter"; the exception appeared to be galaxies with mass-to-light ratios close to that of stars. Subsequent to this, numerous observations have been made that do indicate the presence of dark matter in various parts of the cosmos. Together with Rubin's findings for spiral galaxies and Zwicky's work on galaxy clusters, the observational evidence for dark matter has been collecting over the decades to the point that today most astrophysicists accept its existence as a matter of course. As a unifying concept, it is one of the dominant features considered in the analysis of structures on the order of galactic scales and larger.
[edit] Velocity dispersions of galaxies
Rubin's pioneering work has stood the test of time. Measurements of velocity curves in spiral galaxies were soon followed up with velocity dispersions of elliptical galaxies. While sometimes appearing with lower mass-to-light ratios, measurements of ellipticals still indicate a relatively high dark matter content. Likewise, measurements of the diffuse interstellar gas found at the edge of galaxies indicate not only dark matter distributions that extend beyond the visible limit of the galaxies, but also that the galaxies are virialized up to ten times their visible radii. This has the effect of pushing up the dark matter as a fraction of the total amount of gravitating matter from 50% measured by Rubin to the now accepted value of nearly 95%.
There are places where dark matter seems to be a small or totally absent component. Globular clusters show no evidence that they contain dark matter, though their orbital interactions with galaxies do show evidence for galactic dark matter. For some time, measurements of the velocity profile of stars seemed to indicate concentration of dark matter in the disk of the Milky Way galaxy, however, now it seems that the high concentration of baryonic matter in the disk of the galaxy (especially in the interstellar medium) can account for this motion. Galaxy mass profiles are thought to look very different from the light profiles. The typical model for dark matter galaxies is a smooth, spherical distribution in virialized halos. Such would have to be the case to avoid small-scale (stellar) dynamical effects. Recent research reported in January 2006 from the University of Massachusetts, Amherst would explain the previously mysterious warp in the disk of the Milky Way by the interaction of the Large and Small Magellanic Clouds and the predicted 20 fold increase in mass of the Milky Way taking into account dark matter.
Recently, astronomers from Cardiff University claim to have discovered a galaxy made almost entirely of dark matter, 50 million light years away in the Virgo Cluster, which was named VIRGOHI21.[9] Unusually, VIRGOHI21 does not appear to contain any visible stars: it was seen with radio frequency observations of hydrogen. Based on rotation profiles, the scientists estimate that this object contains approximately 1000 times more dark matter than hydrogen and has a total mass of about 1/10th that of the Milky Way Galaxy we live in. For comparison, the Milky Way is believed to have roughly 10 times as much dark matter as ordinary matter. Models of the Big Bang and structure formation have suggested that such dark galaxies should be very common in the universe, but none have previously been detected. If the existence of this dark galaxy is confirmed, it provides strong evidence for the theory of galaxy formation and poses problems for alternative explanations of dark matter.
[edit] Missing matter in clusters of galaxies
Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689 indicates the presence of dark matter - Enlarge the image to see the lensing arcs.
Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689 indicates the presence of dark matter - Enlarge the image to see the lensing arcs.
Dark matter affects galaxy clusters as well. X-ray measurements of hot intracluster gas correspond closely to Zwicky's observations of mass-to-light ratios for large clusters of nearly 10 to 1. Many of the experiments of the Chandra X-ray Observatory use this technique to independently determine the mass of clusters.
The galaxy cluster Abell 2029 is composed of thousands of galaxies enveloped in a cloud of hot gas, and an amount of dark matter equivalent to more than 1014 Suns. At the center of this cluster is an enormous, elliptically shaped galaxy that is thought to have been formed from the mergers of many smaller galaxies.[10] The measured orbital velocities of galaxies within galactic clusters have been found to be consistent with dark matter observations.
Another important tool for future dark matter observations is gravitational lensing. Lensing relies on the effects of general relativity to predict masses without relying on dynamics, and so is a completely independent means of measuring the dark matter. Strong lensing, the observed distortion of background galaxies into arcs when the light passes through a gravitational lens, has been observed around a few distant clusters including Abell 1689 (pictured right). By measuring the distortion geometry, the mass of the cluster causing the phenomena 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.
Perhaps more convincing, a technique has been developed over the last 10 years called weak lensing which looks at microscale distortions of galaxies observed in vast galaxy surveys due to foreground objects through statistical analyses. By examining the shear deformation of the adjacent background galaxies, astrophysicists can characterize the mean distribution of dark matter by statistical means and have found mass-to-light ratios that correspond to dark matter densities predicted by other large-scale structure measurements. The correspondence of the two gravitational lens techniques to other dark matter measurements has convinced almost all astrophysicists that dark matter actually exists as a major component of the universe's composition.
[edit] Structure formation
Main article: structure formation
Dark matter is crucial to the Big Bang model of cosmology as a component which corresponds directly to measurements of the parameters associated with Friedmann cosmology solutions to general relativity. In particular, measurements of the cosmic microwave background anisotropies correspond to a cosmology where much of the matter interacts with photons more weakly than the known forces that couple light interactions to baryonic matter. Likewise, a significant amount of non-baryonic, cold matter is necessary to explain the large-scale structure of the universe.
Observations suggest that structure formation in the universe proceeds hierarchically, with the smallest structures collapsing first and followed by galaxies and then clusters of galaxies. As the structures collapse in the evolving universe, they begin to "light up" as the baryonic matter heats up through gravitational contraction and the object approaches hydrostatic pressure balance. Ordinary baryonic matter had too high a temperature, and too much pressure left over from the big bang to collapse and form smaller structures, such as stars, via the Jeans instability. Dark matter acts as a compactor of structure. Amazingly, this model not only corresponds with statistical surveying of the visible structure in the universe but also corresponds precisely to the dark matter predictions of the cosmic microwave background.
This bottom up model of structure formation requires something like cold dark matter to succeed. Large computer simulations of billions of dark matter particles have been used to confirm that the cold dark matter model of structure formation is consistent with the structures observed in the universe through galaxy surveys, such as the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey, as well as observations of the Lyman-alpha forest. These studies have been crucial in constructing the Lambda-CDM model which measures the cosmological parameters, including the fraction of the universe made up of baryons and dark matter.
[edit] Dark matter composition
Unsolved problems in physics: What is dark matter? How is it generated? Is it related to supersymmetry?
Although dark matter was detected by its gravitational lensing in August 2006,[11] many aspects of dark matter remain speculative. The DAMA/NaI experiment has claimed to directly detect dark matter passing through the Earth, though most scientists remain skeptical since negative results of other experiments are (almost) incompatible with the DAMA results if dark matter consists of neutralinos.
Data from a number of lines of evidence, including galaxy rotation curves, gravitational lensing, structure formation, and the fraction of baryons in clusters and the cluster abundance combined with independent evidence for the baryon density, indicate that 85-90% of the mass in the universe does not interact with the electromagnetic force. This "dark matter" is evident through its gravitational effect. Several categories of dark matter have been postulated.
2007-01-09 02:21:27
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answer #5
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answered by Dushyant 1
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