Material in space which does not emit light, and which therefore cannot be seen with conventional astronomical instruments; also known as the missing mass. Over 95% of the universe is thought to be composed of dark matter. According to one view, most of this matter consists of machos (massive compact halo objects) - large structures which do not emit light (such as black holes and brown dwarfs). An opposing view conceives of dark matter as consisting of new kinds of sub-atomic particle which have so far not been detected, known as wimps (weakly interacting massive particles). Research published in 1993 reported evidence indicating the existence of brown dwarfs, but the issue remains controversial.
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In astrophysics, dark matter is matter that does not emit or reflect enough electromagnetic radiation (such as light, X-rays and so on) to be detected directly, but whose presence may be inferred from its gravitational effects on visible matter. Among the observed phenomena consistent with the existence of dark matter are the rotational speeds of galaxies and orbital velocities of galaxies in clusters, gravitational lensing of background objects by galaxy clusters such as the Bullet cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter also plays a central role in structure formation and Big Bang nucleosynthesis, and has measurable effects on the anisotropy of the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than is directly observable, indicating that the remainder is dark.
The composition of dark matter is unknown, but may include new elementary particles such as WIMPs and axions, ordinary and heavy neutrinos, dwarf stars and 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 nonbaryonic dark matter.
The dark matter component has vastly more mass than the "visible" component of the universe. Some hard-to-detect baryonic matter (see baryonic dark matter) makes a contribution to dark matter, but constitutes only a small portion. It has been noted that dark matter and dark energy serve mainly as expressions of our ignorance, much as the marking of early maps with terra incognita.
Observational evidence
The first to provide evidence and infer the existence of a phenomenon that has come to be called "dark matter" was Swiss astrophysicist Fritz Zwicky, of the California Institute of Technology (Caltech) in 1933.
Much of the evidence for dark matter comes from the study of the motions of galaxies. Experimentally, however, the total kinetic energy is found to be much greater: in particular, assuming the gravitational mass is due to only the visible matter of the galaxy, stars far from the center of galaxies have much higher velocities than predicted by the virial theorem. Galaxies show signs of being composed largely of a roughly spherical halo of dark matter with the visible matter concentrated in a disc at the center. Low surface brightness dwarf galaxies are important sources of information for studying dark matter, as they have an uncommonly low ratio of visible matter to dark matter, and have few bright stars at the center which impair observations of the rotation curve of outlying stars.
According to results published in August 2006, dark matter has been observed separate from ordinary matter through measurements of the Bullet Cluster, actually two nearby clusters of galaxies that collided about 150 million years ago. The individual galaxies and the dark matter did not interact and are further from the center.
Galactic rotation curves
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).
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. 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. The typical model for dark matter galaxies is a smooth, spherical distribution in virialized halos. 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. 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. 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.
Missing matter in clusters of galaxies
Dark matter affects galaxy clusters as well.
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 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. 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. 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.
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. 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.
Dark matter composition
Unsolved problems in physics: What is dark matter?Although dark matter was detected via optical means in August 2006 , 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.
Baryonic dark matter Non-baryonic dark matter which is divided into three different types: Hot dark matter - nonbaryonic particles that move ultrarelativistically Warm dark matter - nonbaryonic particles that move relativistically Cold dark matter - nonbaryonic particles that move non-relativisticallyDavis et al wrote in 1985:
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If the dark matter is composed of abundant light particles which remain relativistic until shortly before recombination, then it may be termed "hot". The best candidate for hot dark
matter is a neutrino [..]
A second possibility is for the dark matter particles to interact more weakly than neutrinos, to be less abundant, and to have a mass of order 1eV. Such particles are termed "warm dark matter", because they have lower thermal velocities than massive neutrinos [..] there are at present few candidate particles which fit this description. Bond, Szalay and Turner 1982) [..] Any particles which became nonrelativistic very early, and so were able to diffuse a negligible distance, are termed "cold" dark matter (CDM). |
Hot dark matter consists of particles that travel with relativistic velocities. However, bounds on neutrinos indicate that ordinary neutrinos make only a small contribution to the density of dark matter.
Hot dark matter cannot explain how individual galaxies formed from the Big Bang. Hot dark matter, while it certainly exists in our universe in the form of neutrinos, is therefore only part of the story.
To explain structure in the universe it is necessary to invoke cold (non-relativistic) dark matter. However, studies of big bang nucleosynthesis have convinced most scientists that baryonic matter such as MACHOs cannot be more than a small fraction of the total dark matter.
At present, the most common view is that dark matter is primarily non-baryonic, made of one or more elementary particles other than the usual electrons, protons, neutrons, and known neutrinos.
Experimental searches for these dark matter candidates have been conducted and are ongoing. These efforts can be divided into two broad classes: direct detection, in which the dark matter particles are observed in a detector; There are also several experiments claiming positive evidence for dark matter detection, such as DAMA/NaI, PVLAS, and EGRET, but these are so far unconfirmed and difficult to reconcile with the negative results of other experiments. Other experiments searching for dark matter include the Cryogenic Dark Matter Search in the Soudan mine or the ArDM experiment.
In research due to be fully published in spring 2006, researchers from the University of Cambridge Institute of Astronomy claim to have calculated that dark matter only comes in clumps larger than about 1,000 light-years across, implying an average speed of dark matter particles of 9 km/s, a density of 20 amu/cm³, and temperature of 10,000 kelvins.
Alternative explanations
A proposed alternative to physical dark matter particles has been to suppose that the observed inconsistencies are due to an incomplete understanding of gravitation.
In August 2006, a study of colliding galaxy clusters claimed to show that even in a modified gravity hypothesis, the majority of the mass must be some form of dark matter by demonstrating that when regular matter is "swept away" from a cluster, the gravitational effects of dark matter (which is thought to be non-interacting aside from its gravitational effect) remain . However, a study claims that TeVeS may be able to produce the observed effect but needs the majority of the mass to be dark matter, possibly in the form of ordinary neutrinos . Also Nonsymmetric Gravitational Theory has been claimed to qualitatively fit the observations without needing exotic dark matter .
Dark matter in popular culture
Mentions of dark matter occur in some video games and other works of fiction.
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