Dark Matter

In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy. Its existence is inferred from gravitational effects on visible matter and gravitational lensing of background radiation, and was originally hypothesized to account for discrepancies between calculations of the mass of galaxies, clusters of galaxies and the entire universe made through dynamical and general relativistic means, and calculations based on the mass of the visible "luminous" matter these objects contain: stars and the gas and dust of the interstellar and intergalactic medium. Many experiments to detect dark matter through non-gravitational means are underway.

According to observations of structures larger than solar systems, as well as Big Bang cosmology interpreted under the Friedmann equations and the FLRW metric, dark matter accounts for 23% of the mass-energy density of the observable universe. In comparison, ordinary matter accounts for only 4.6% of the mass-energy density of the observable universe, with the remainder being attributable to dark energy. From these figures, dark matter constitutes 83%, (23/(23+4.6)), of the matter in the universe, whereas ordinary matter makes up only 17%.

Dark matter was postulated by Fritz Zwicky in 1934 to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters. Subsequently, other observations have indicated the presence of dark matter in the universe; these observations include the rotational speeds of galaxies, 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 plays a central role in state-of-the-art modeling of structure formation and galaxy evolution, and has measurable effects on the anisotropies observed in 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 that which interacts with electromagnetic radiation. The largest part of dark matter, which does not interact with electromagnetic radiation, is not only "dark" but also, by definition, utterly transparent.

As important as dark matter is believed to be in the cosmos, direct evidence of its existence and a concrete understanding of its nature have remained elusive. Though the theory of dark matter remains the most widely accepted theory to explain the anomalies in observed galactic rotation, some alternative theoretical approaches have been developed which broadly fall into the categories of modified gravitational laws, and quantum gravitational laws.

Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, "empty space" is actually full of temporary ("virtual") particles that continually form and then disappear. But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong - wrong by a lot. The number came out 10^120  times too big. That's a 1 with 120 zeros after it. It's hard to get an answer that bad. So the mystery continues. 

Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the Universe is the opposite of that of matter and normal energy. Some theorists have named this "quintessence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it is like, what it interacts with, or why it exists. So the mystery continues.

A last possibility is that Einstein's theory of gravity is not correct. That would not only affect the expansion of the Universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein's theory is known to do, and still give us the different prediction for the Universe that we need? There are candidate theories, but none are compelling. So the mystery continues.

The thing that is needed to decide between dark energy possibilities - a property of space, a new dynamic fluid, or a new theory of gravity - is more data, better data.

So far, though, scientists don’t know what dark energy is. It could spring from the vacuum of space itself, becoming a more dominant force as the universe expands and gets more spacious. Dark energy could be exotic new particles or other undiscovered physics. Dark energy could mean that our understanding of gravity needs an overhaul. Or it could be something completely different — perhaps something that no one has even thought about. It could require scientists to revise their ideas about the Big Bang, or even develop an entirely new scenario to explain how the universe was born.

Learning about dark energy is far more difficult than sticking your toe in the ocean or toting a bucket of water back to the laboratory, though. Trying to find something that you didn’t even know existed until a few years ago will require scientists to devise clever ways of probing the universe and the history of its birth and evolution, and engineers to design new tools to study them.

In the coming years and decades, astronomers will study exploding stars, map millions of galaxies, and plot the gravitational influence of dense galaxy clusters. Particle physicists will probe conditions near the time of the Big Bang. And all of them will tweak their models of how the universe began, how it has aged, and how it will end.

Their work will help us understand the vast cosmic "ocean" of dark energy — an ocean that we are just beginning to explore.

Baryonic and Nonbaryonic dark matter

A small proportion of dark matter may be baryonic dark matter: astronomical bodies, such as massive compact halo objects, that are composed of ordinary matter but which emit little or no electromagnetic radiation. The vast majority of dark matter in the universe is believed to be nonbaryonic, and thus not formed out of atoms. It is also believed that it does not interact with ordinary matter via electromagnetic forces; in particular, dark matter particles do not carry any electric charge. The nonbaryonic dark matter includes neutrinos, and possibly hypothetical entities such as axions, or super symmetric particles. Unlike baryonic dark matter, nonbaryonic dark matter does not contribute to the formation of the elements in the early universe ("Big Bang nucleosynthesis") and so its presence is revealed only via its gravitational attraction. In addition, if the particles of which it is composed are supersymmetric, they can undergo annihilation interactions with themselves resulting in observable by-products such as photons and neutrinos ("indirect detection").

Nonbaryonic dark matter is classified in terms of the mass of the particle(s) that is assumed to make it up, and/or the typical velocity dispersion of those particles (since more massive particles move more slowly). There are three prominent hypotheses on nonbaryonic dark matter, called Hot Dark Matter (HDM), Warm Dark Matter (WDM), and Cold Dark Matter (CDM); some combination of these is also possible. The most widely discussed models for nonbaryonic dark matter are based on the Cold Dark Matter hypothesis, and the corresponding particle is most commonly assumed to be a neutralino. Hot dark matter might consist of (massive) neutrinos. Cold dark matter would lead to a "bottom-up" formation of structure in the universe while hot dark matter would result in a "top-down" formation scenario.

One possibility is that cold dark matter could consist of primordial black holes in the range of 1014 kg to 1023 kg.[8] Being within the range of an asteroid's mass, they would be small enough to pass through objects like stars, with minimal impact on the star itself. These black holes may have formed shortly after the big bang when the energy density was great enough to form black holes directly from density variations, instead of from star collapse. In vast numbers they could account for the missing mass necessary to explain star motions in galaxies and gravitational lensing effects.

Observational evidence

The first person to provide evidence and infer the presence of dark matter was Swiss astrophysicist Fritz Zwicky, of the California Institute of Technology in 1933. He applied the virial theorem to the Coma cluster of galaxies and obtained evidence of unseen mass. Zwicky estimated the cluster's total mass based on the motions of galaxies near its edge and compared that estimate to one based on the number of galaxies and total brightness of the cluster. He found that there was about 400 times more estimated mass than was visually observable. The gravity of the visible galaxies in the cluster would be far too small for such fast orbits, so something extra was required. This is known as the "missing mass problem". Based on these conclusions, Zwicky inferred that there must be some non-visible form of matter which would provide enough of the mass and gravity to hold the cluster together.

Much of the evidence for dark matter comes from the study of the motions of galaxies. Many of these appear to be fairly uniform, so by the virial theorem the total kinetic energy should be half the total gravitational binding energy of the 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. Galactic rotation curves, which illustrate the velocity of rotation versus the distance from the galactic center, cannot be explained by only the visible matter. Assuming that the visible material makes up only a small part of the cluster is the most straightforward way of accounting for this. Galaxies show signs of being composed largely of a roughly spherically symmetric, centrally concentrated 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 would otherwise impair observations of the rotation curve of outlying stars.

Gravitational lensing observations of galaxy clusters allow direct estimates of the gravitational mass based on its effect on light from background galaxies, since large collections of matter (dark or otherwise) will gravitationally deflect light. In clusters such as Abell 1689, lensing observations confirm the presence of considerably more mass than is indicated by the clusters' light alone. In the Bullet Cluster, lensing observations show that much of the lensing mass is separated from the X-ray-emitting baryonic mass.

Galactic rotation curves

Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). Dark matter can explain the velocity curve having a 'flat' appearance out to a large radius

For 40 years after Zwicky's initial observations, no other corroborating observations indicated that the mass to light ratio was anything other than unity. 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 presented 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 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). An influential paper presented these results in 1980. These results suggest 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. Eventually other astronomers began to corroborate her work and it soon became well-established that most galaxies were in fact dominated by "dark matter":

• Low Surface Brightness (LSB) galaxies. LSBs are probably everywhere dark matter-dominated, with the observed stellar populations making only a small contribution to rotation curves. Such a property is extremely important because it allows one to avoid the difficulties associated with the deprojection and disentanglement of the dark and visible contributions to the rotation curves.[7]

• Spiral Galaxies. Rotation curves of both low and high surface luminosity galaxies appear to suggest a universal density profile, which can be expressed as the sum of an exponential thin stellar disk, and a spherical dark matter halo with a flat core of radius r0 and density ρ0 = 4.5 × 10−2(r0/kpc)−2/3 M⊙pc−3 (here, M⊙ denotes a solar mass, 2 × 1030 kg).

• Elliptical galaxies. Some elliptical galaxies show evidence for dark matter via strong gravitational lensing,[15] X-ray evidence reveals the presence of extended atmospheres of hot gas that fill the dark haloes of isolated ellipticals and whose hydrostatic support provides evidence for dark matter. Other ellipticals have low velocities in their outskirts (tracked for example by planetary nebulae) and were interpreted as not having dark matter haloes. However simulations of disk-galaxy mergers indicate that stars were torn by tidal forces from their original galaxies during the first close passage and put on outgoing trajectories, explaining the low velocities even with a DM halo.[16] More research is needed to clarify this situation.

Note that simulated DM haloes have significantly steeper density profiles (having central cusps) than are inferred from observations, which is a problem for cosmological models with dark matter at the smallest scale of galaxies as of 2008. This may only be a problem of resolution: star-forming regions which might alter the dark matter distribution via outflows of gas have been too small to resolve and model simultaneously with larger dark matter clumps. A recent simulation of a dwarf galaxy resolving these star-forming regions reported that strong outflows from supernovae remove low-angular-momentum gas, which inhibits the formation of a galactic bulge and decreases the dark matter density to less than half of what it would have been in the central kiloparsec. These simulation predictions—bulgeless and with shallow central dark matter profiles—correspond closely to observations of actual dwarf galaxies. There are no such discrepancies at the larger scales of clusters of galaxies and above, or in the outer regions of haloes of galaxies.

Exceptions to this general picture of DM haloes for galaxies appear to be galaxies with mass-to-light ratios close to that of stars.[citation needed] Subsequent to this, numerous observations have been made that do indicate the presence of dark matter in various parts of the cosmos.[citation needed] 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 unifying concept, dark matter is one of the dominant features considered in the analysis of structures on the order of galactic scale and larger.