Dark matter

Matter that is nonluminous, neither emitting nor reflecting any detectable electromagnetic or other radiation, is known as "dark" matter. Dark matter has potentially important implications for elementary particle physics and cosmology and may account for 85 percent or more of all matter universe.

Overview

The concept of dark matter plays a fundamental role in modern cosmology, both theoretical and observational. Dark matter is that part of the material universe that does not reflect or emit any electromagnetic or other known form of radiation at a detectable level. Since all information concerning remote portions of the universe is obtained by detection and analysis of such radiation, the presence, and indeed very existence, of dark matter must be indirectly inferred. Nevertheless, there are compelling observational and theoretical reasons not only to contemplate the existence of dark matter but also to consider its nature, properties, total amount, and distribution in the universe. Approximately 85 percent of all matter is believed to be dark matter; the combination of dark matter and dark energy may account for as much as 95 percent of the total mass-energy in the universe. The existence of dark matter is of primary importance for three areas of cosmology: the geometry of space-time, the evolutionary track of the universe, and the formation and clustering of galaxies.

Isaac Newton's theory of universal gravitation has been superseded by Albert Einstein's general theory of relativity. One of the important features of general relativity is that solutions of the field equations of the theory allow for the existence of curved space (indeed, of curved space-time). Prior to Einstein's development of the general theory, the universe was thought to be framed in a Euclidean, or flat, space—the type of space familiar from ordinary elementary geometry. While non-Euclidean geometries had been studied for some time by mathematicians, they had not been seriously considered as possible frameworks for the universe. Additionally, the solutions of the field equations indicate that it is mass-energy density that determines the type of space and the magnitude of the curvature, if any, within the geometry of that space. This mass-energy density must include all types of matter, both luminous and dark. It should be noted that since mass-energy is responsible for the curvature of the surrounding space-time, a concentration of mass may cause the local curvature and geometry of space-time to differ significantly from the global.

There are three main types of solutions to Einstein's field equations of general relativity, each characterized by an average mass-energy density of the universe. The first, which requires the global density of the universe to be equal to the critical, or closure, density, is known as the Einstein–de Sitter model. In this model, the space-time that frames the universe is flat, or Euclidean. The second and third solutions, the closed Friedmann and open Friedmann models, have global mass-energy densities that are larger and smaller, respectively, than the closure density. The closed Friedmann model corresponds to a non-Euclidean curved space described as spherical or hyperspherical, while the open model corresponds to a non-Euclidean curved space described as hyperbolic.

An equally important feature of these solutions is that space-time is dynamic, evolving over time. Each of the three models evolves differently: the open Friedmann universe expands indefinitely; the closed Friedmann universe expands at a decreasing rate until a maximum size is reached, at which point it collapses; and the Einstein–de Sitter universe expands at an ever-decreasing rate into the infinite future. The total amount of average mass-energy density in the universe, including dark matter, is the critical factor differentiating these three evolutionary tracks.

It is not surprising that mass would have such an effect on the evolution of the universe. An analogous example is the launch of an object upward from the surface of a planet. If the object has a velocity greater than or equal to the escape velocity of the planet, which is determined by the planet's mass and size, the object will continue to move away; in effect, the object-planet system continues to expand. If the object velocity is less than the escape velocity, the object slows in its trajectory until it reaches a maximum distance and then falls back to the surface of the planet; in other words, the object-planet system expands to its maximum size, then collapses.

One of the most difficult unresolved issues in cosmology concerns the mechanism behind the formation and clustering of galaxies. Dark matter is one of the distinguishing characteristics of the various proposed scenarios. In order for structures to have formed from the near-perfect homogeneity of the early stages of the big bang, gravitational instabilities caused by density fluctuations (i.e., variations in density) would have to have occurred. In the very early universe, the temperature was very high, and ordinary matter existed only in the form of a plasma. The energy density of radiation dominated the energy density resulting from matter; thus, fluctuations could not grow, since the intense radiation that is strongly coupled to ionized matter would have prevented any contraction relative to the overall expansion of the universe. In fact, only the largest fluctuations would have survived this period without decaying away. About three hundred thousand years after the big bang, when the universe had cooled to about four thousand kelvins, the electrons and positive ions of the plasma combined to form neutral matter, and the radiation and matter were effectively decoupled. At about the same time, the energy density of matter began to dominate that of radiation. After this, fluctuations could only have grown if they were greater than a certain size and had survived the previous epoch.

There are two main types of density fluctuations that may have led to the formation of galaxies: isothermal fluctuations, in which the temperature remains constant; and adiabatic fluctuations, in which the specific entropy, or entropy per unit of mass, remains constant. Estimates indicate that the smallest isothermal fluctuations would have masses comparable to globular clusters of stars, while the smallest adiabatic fluctuations would have masses comparable to giant galaxies or large clusters of typical galaxies, depending on various assumptions, one of which is the type of dark matter present. If the dark matter is not baryonic (i.e., composed of baryons) but rather composed of exotic elementary particles with presently unknown properties, then assumptions must be made as to when and if this exotic matter decouples from the radiation field. There are two extreme possible cases. Hot dark matter (HDM)—matter composed of low-mass particles, such as neutrinos, that travel near the speed of light—would probably decouple quite early from the radiation field, as such particles streaming freely through the early universe at high speed would effectively erase all density fluctuations smaller than some critical size. Cold dark matter (CDM)—particles that interact only weakly, if at all, with radiation and ordinary matter—would travel at speeds much less than the speed of light at the stage when structure formation would begin. Density fluctuations of CDM grow similarly to the isothermal fluctuations of ordinary matter after decoupling. In either case, the dark-matter component forms a lumpy background of density concentrations into which ordinary matter is collected to produce galaxies and clusters.

These various scenarios lead to two very different views of how large-scale structures came to exist. Theories that support the formation of structures via adiabatic fluctuations of ordinary matter are known as top-down theories; according to these, the regions of greatest size, corresponding to clusters and superclusters of galaxies, collapsed first into large gaseous disks, which later fragmented to form individual galaxies. Theories based on isothermal fluctuations of ordinary matter, known as bottom-up theories, argue that the smallest-scale structures, corresponding to galaxies or smaller structures, condensed first, and these freely moving units were later gravitationally attracted to form clusters and then superclusters. When dark matter is included in these scenarios, it is found that HDM tends to result in a top-down picture, and CDM yields a bottom-up picture.

In order to carry out the simulations needed to compare theory with observations, the initial spectrum of density fluctuations must be specified. Satellite measurements of the anisotropy of the cosmic microwave background radiation place limits on these spectra. Within these constraints, top-down theories tend to give a better, but not completely correct, model of the observed spatial distributions, which include sheets, voids, and streaming motion of galaxies. Although the spatial distribution is roughly correct, such models also have galaxies forming much too recently to agree with observational data. The limits on the magnitude of the initial fluctuations would cause the growth of structures to take too long. Additionally, the most likely constituent of HDM, neutrinos, with their large correlation lengths, cannot account by themselves for the large amount of dark matter inferred to be in galactic halos. Models of bottom-up theories that include CDM are able to produce objects with approximately the right masses (dark galactic halos), distributions, and angular momenta to account for observed galaxies. However, these models tend not to be able to account for the existence of the largest-scale structures, such as clusters.

Applications

The visible-mass density in the universe is estimated to be only about 4 percent of the critical density. Owing to its totally nonradiative nature, dark matter can be detected only indirectly, through its gravitational influence on ordinary luminous matter. There is an apparent correlation between the scale of the observation and the amount of dark matter required to explain the observation.

By looking statistically at the velocities of stars within a few thousand light-years of the sun in a direction perpendicular to the galactic plane, it is possible to determine the local vertical component of the gravitational field of the Milky Way. From the strength of this field, the amount of mass required to produce it can be determined; this figure, known as the Oort limit, has been found to be approximately twice the amount of visible mass in the galaxy.

By measuring the rotation rates of stars and interstellar gas about the galactic center at various positions in the galactic plane, the mass interior to any radius can be determined. These rotation curves for the Milky Way, and virtually all other spiral galaxies, indicate that the mass of a galaxy measured as a function of radial distance does not converge to a fixed value but rather continues to rise linearly out to the greatest distances measured. A similar plot of the total light intensity does asymptotically approach a finite fixed value. The mass-to-light ratio increases toward the outer part of these galaxies. Thus, it may be inferred that a large amount of dark matter is concentrated in the galactic halos, which extend out to as much as five times the diameter of the visible galactic disk. Estimates place the amount of halo dark matter at two to ten times the amount of luminous matter in the galaxy.

On a somewhat larger scale, the nearby Andromeda Galaxy, which along with the Milky Way and several smaller galaxies forms what is known as the Local Group, is one of the few galaxies in the universe that is actually approaching the Milky Way. If it is assumed that this approach results from the gravitational reversal of an earlier recession, then an estimate of the mass of the Local Group can be made. This estimate again indicates that dark matter exceeds luminous matter in the Local Group by a factor of five to ten. Similar results are found by considering the relative velocities of other pairs of gravitationally bound galaxies. As early as 1933, Fritz Zwicky had inferred the existence of dark matter by considering the motion of galaxies in rich clusters such as the Coma cluster. When the random (peculiar) velocities of the galaxies in such a cluster are measured, it is found that some of the velocities are much too large for the galaxies to remain gravitationally bound in the cluster if only the visible mass is present. This type of measurement, which has been made for many rich clusters, yields a dark-to-luminous-matter ratio of about ten to one. Additional supporting evidence for dark matter in rich clusters comes from observations of the x-rays emitted by the very hot gas found between galaxies in such clusters, as well as the gravitational microlensing of more distant objects by intervening galaxy clusters.

Precise measurements of the dipole asymmetry of the cosmic microwave background radiation imply that the velocity of the Local Group of galaxies is nearly 600 kilometers per second in a direction that is about 45 degrees from the Virgo cluster of galaxies. Part of this peculiar velocity is probably caused by the gravitational attraction of the Virgo cluster; the proposed source of the remainder is the Great Attractor, a very large mass at a distance of about 100 million light-years that is connected with the local supercluster of galaxies. Many other galaxies and clusters are also being drawn toward this mass concentration. In fact, the bulk motions of many groups of galaxies—a phenomenon known as galaxy streaming—seem to require densities near the critical value. Angular-size-versus-redshift measurements for very distant objects have enabled observers to determine the rate at which the universal expansion has been slowing over time. These observations also appear to require that the total average mass-energy density of the universe be very near its critical value. Thus it is not the existence of dark matter but its total amount, distribution, and makeup that is in question.

One of the striking successes of the big bang model is its prediction of the abundance of certain light isotopes. Based on this prediction, there should be more baryons in the universe than can be accounted for by visible baryonic matter; such a discrepancy suggests that a small portion of the dark matter in the universe may be composed of baryons. This baryonic dark matter might be in the form of hot gas between galaxies, large masses of gas that have not formed into stars or galaxies ("stillborn" galaxies), or Massive Astrophysical Compact Halo Objects (MACHOs) such as Jupiter-sized planets, brown dwarf stars, meteoroid swarms, and stellar-mass black holes. In fact, several MACHOs have been found in the halo of the Milky Way galaxy due to their microlensing effects.

Given the amount of dark matter implied by observations, there must exist another, even more plentiful, form of dark matter. This exotic dark matter is nonbaryonic and is categorized as either hot dark matter (HDM) or cold dark matter (CDM). The HDM consists of particles that travel near the speed of light. HDM is not strongly affected by gravity and so tends to clump only on the largest scales. The prime HDM candidate is neutrinos, which have a small rest mass. CDM consists of particles that travel slowly in relation to the speed of light and so are better able to clump together on smaller scales. Possible forms of CDM include Weakly Interacting Massive Particles (WIMPs), which are hypothetical particles that have yet to be observed but are predicted to exist by theories such as supersymmetry, and axions, which also hypothetical and are less massive than WIMPs.

Context

One of the underlying themes of physical science has been the search for the building blocks of the universe, for the nature of matter. This quest began with the concept of a fundamental unit, the atom, proposed by Democritus and others. The atomic idea was brought to its modern form in the nineteenth century with the discovery of many new elements and their systematic arrangement in the periodic table. During the late nineteenth and early twentieth centuries, with the discoveries of the proton, neutron, and electron, the atom itself was found to have structure. In the 1960s and 1970s, the field of particle physics came to the fore with the development of the quark theory of matter, the unification of the electromagnetic and weak nuclear forces that act on elementary particles, and the initial investigations of further unification schemes, such as supersymmetry, which would put all matter on an equal footing. Although the observational evidence has been accumulating for some time, the dark-matter hypothesis is the latest frontier in the search for the true nature of matter.

Although which particles are viewed as fundamental has changed over time, the one force to which all such particles are subject has always been gravity. In the late seventeenth century, Isaac Newton developed his theory of universal gravitation. Applying this theory to the universe at large, Newton came to believe that the universe had to be infinite, since a static finite universe would collapse under the attractive force of gravity. Later, Immanuel Kant realized that a finite universe was possible if the components of that universe were set in motion. At the beginning of the twentieth century, Sir James Jeans calculated the minimum density-fluctuation size necessary for the universe's contents to ultimately collapse under the influence of gravity and form structures. Einstein's development of the general theory of relativity, a theory of gravitation that superseded Newton's, implied a dynamic, evolutionary universe. So strong was his belief that the universe must be static that Einstein introduced an ad hoc cosmological constant to counterbalance the implied cosmic expansion. However, Edwin Hubble's observation of the recession of galaxies later established the reality of an expanding universe. On the theoretical side, the big bang model and its successor, the inflationary theory, have provided very successful explanations for most of the observed global properties of the universe. The inflationary theory requires the existence of dark matter to supplement visible matter, bringing the overall density of the universe up to its critical value.

Starting in the 1960s, several new windows on the universe were opened with the advent of satellite x-ray, gamma-ray, ultraviolet, and infrared observatories and the continuing improvement of ground-based optical and radio telescopes. The increased diversity and amount of information have brought the question of dark matter to the fore. Computer-simulation capabilities allow various structure-formation scenarios to be tested and refined. The identity of dark matter remains to be determined. The question of dark matter is an important one, for the fate of the universe literally depends on the answer.

Principal terms

BARYONIC MATTER: matter formed from protons, neutrons, and electrons; also known as ordinary matter

COLD DARK MATTER (CDM): heavy particles that travel at speeds that are small compared to the speed of light

COSMOLOGY: the study of the origin, evolution, and large-scale structure of the universe

CRITICAL DENSITY: the average mass-energy density of the universe required to halt the universal expansion; also called closure density

EXOTIC MATTER: elementary particles or larger matter structures, some hypothetical and yet to be observed, that are nonbaryonic; many of these are required by various unified theories of elementary particle physics

HOT DARK MATTER (HDM): low-mass particles that travel at very near the speed of light

NONBARYONIC MATTER: any form of matter that is not baryonic

SPACE-TIME: the combined spatial and temporal framework within which physical events and processes take place and are described

Bibliography

"Astronomers Map the Shape of Galactic Dark Matter." Astronomy Magazine. Kalmbach, 6 Jan. 2010. Web. 16 Jan. 2014.

Cohen, Nathan. Gravity's Lens: Views of the New Cosmology. New York: Wiley, 1988. Print. Provides a broad introduction to the so-called new cosmology, with significant sections on dark matter. Written at an introductory level by one of the pioneers of the gravitational microlensing effect.

Davies, Paul, ed. The New Physics. New York: Cambridge UP, 1989. Print. A collection of eighteen chapters that each deal with an area of then-recent advance in physics, written by experts in the various fields. Includes chapters on the inflationary universe, by Alan Guth and Paul Steinhardt, and the new astrophysics, by Malcolm Longair. Should be accessible to the motivated general reader.

Kinjo, Nori, and Akira Nakajima, eds. Recent Developments in Dark Matter Research. Hauppauge: Nova Sci., 2013. Print.

Krauss, Lawrence M. The Fifth Essence: The Search for Dark Matter in the Universe. New York: Basic, 1989. Print. An entire book devoted to dark matter and written at a popular level. Contains a chapter detailing the properties of the various candidate dark-matter particles and a fine account of the historical context of the dark-matter question.

Lightman, Alan. Time for the Stars: Astronomy in the 1990s. New York: Viking, 1992. Print. A brief survey of modern cosmology, with one chapter devoted to dark matter.

Longair, Malcolm S. The Origins of Our Universe. New York: Cambridge UP, 1991. Print. An elaboration of the author's Royal Institution Christmas Lecture for young people. Contains a number of useful photos and diagrams and an excellent introductory bibliography of cosmology in general.

Matarrese, Sabino, et al., eds. Dark Matter and Dark Energy: A Challenge for Modern Cosmology. Dordrecht: Springer, 2011. Print.

Pasachoff, Jay M., et al. The Farthest Things in the Universe. New York: Cambridge UP, 1994. Print. A short survey of the examinations of distant objects that provided much of the observational evidence for the large values of the mass density of the universe.

Prakash, Nirmala. Dark Matter, Neutrinos, and Our Solar System. Singapore: World Scientific, 2013. Print.

Riordan, Michael, and David N. Schramm. The Shadows of Creation: Dark Matter and the Structure of the Universe. New York: Freeman, 1991. Print. An introductory-level book that takes a broad but selective look at modern cosmology, focusing on those aspects connected with dark matter. Written by an active researcher of the dark-matter question who has also done important work in neutrino physics. A good general introduction to the subject.

Sanders, Robert H. The Dark Matter Problem: A Historical Perspective. New York: Cambridge UP, 2010. Print.

Schramm, David N. "Dark Matter and the Origin of Cosmic Structure." Sky & Telescope Oct. 1994: 28–35. Print. Provides a summary of observation-based estimates of dark-matter contributions to the total mass density and a description of how the various types of dark matter would affect structure-formation scenarios.

Silk, Joseph. The Big Bang. Rev. ed. New York: Freeman, 1989. Print. A standard introductory survey of big bang cosmology. Includes superb diagrams, a mathematical appendix, an extensive bibliography, and a glossary.