Dark Matter

Resources

Dark matter is the term astronomers use to describe material in the universe that is non-luminous—that is, material that does not emit or reflect light and that is therefore invisible. Everything seen when looking through a telescope is visible because it is either emitting or reflecting light; stars, nebulae, and galaxies are examples of luminous objects. However, luminous matter appears to make up only a small fraction of all the matter in the universe, perhaps only a small percent. The rest of the matter is cold, dark, and hidden from direct view.

Because dark matter is invisible, it can only be detected through indirect means, primarily by analyzing its effect on visible material. Although dark matter does not shine, it still exerts a gravitational force on the matter around it. For example, it is possible to measure the velocities of many stars in the Milky

Way galaxy and in other galaxies. The measured velocities do not agree, in general, with those calculated on the assumption that the visible material of the galaxies (i.e., their stars and clouds of glowing gas) constitute all or even most of their mass. Additional, unseen mass, therefore, must exist in the vicinity of the galaxies, tugging on their stars. Such data seem to indicate the presence of massive halos of dark matter surrounding the galaxies that would account for most of their mass. Recent observations of the shapes formed by galaxies clumping together throughout the universe have confirmed that dark matter does not pervade space uniformly or form structures independently of the galaxies, but is concentrated around the galaxies.

The identity of the universe’s dark matter remains a subject of research and dispute among physicists. A number of possibilities have been proposed. One proposal that astronomers hold is that supermassive black holes exist at the centers of most galaxies, contributing several hundred million or even on the order of a billion solar masses to each galaxy. (One solar mass is a quantity of matter equal to the mass of the sun.) In 2001, observations of x-ray bursts from the center of the Milky Way galaxy confirmed the presence of a large black hole. In 2004 that report was confirmed by scientists using the Very Long Baseline Array (VLBA), ten radio telescopes located across the United States. They reported that the size of the black hole is likely the size of the Earth’s orbit about the sun. Such black holes supply invisible mass and count as dark matter. Another proposal maintains that multitudes of non-luminous brown dwarfs or machos (massive compact halo objects)—dim blobs of gas not massive enough to initiate fusion reactions at their centers and thereby become stars—may orbit each galaxy. Such objects have been detected using gravitational lensing, but not in sufficient numbers to account for the amount of dark matter that is believed to exist.

A third proposal is that subatomic particles, known as neutrinos, which pervade the universe in very great numbers, were shown in 1998 to have a small mass, ending a decades-long dispute among physicists about whether they are without mass (massless). It had been thought that neutrinos, if they have mass, might account for the universe’s dark matter; however, calculations now show, as of 2002, that each neutrino’s mass is so small that neutrinos can account for at most a fifth of the dark matter in the universe. Other research shows that only sterile neutrinos (those formed immediately after the Big Bang) could possibly make up dark matter. Fourth, particles of some unknown kind, generically termed wimps (weakly-interacting massive particles), may permeate the space around the galaxies, held together in clouds by gravity. Fifth, some scientists believe that MACHOs, or massive combo halo objects) may make up dark matter. MACHOs can be anything such as black holes, neutron stars, brown dwarfs, and various undetected planets. Lastly, some cosmologists favor models where the primary ingredient of dark matter is nonbaryonic dark matter. As opposed to baryonic dark matter, which does not emit light and is composed of baryons (such as protons, neutrons, and a number of unstable, heavy particles called hyperons), nonbaryonic dark matter, which can emit light, is not composed of baryons. Even though some cosmologists think nonbaryonic dark matter makes up most of dark matter, they still just do not know for sure.

Dark matter, which may turn out to be a combination of the above factors, has long been thought to play a crucial role in determining the fate of the universe. The most widely accepted theory regarding the origin and evolution of the universe is the Big Bang theory, which provides an elegant explanation for the well-documented expansion of the universe. One question is whether the universe will expand forever, propelled by the force of the Big Bang, or eventually stop expanding and begin to contract under its own gravity, much as a ball thrown up into the air eventually turns around and descends. The deciding factor is the amount of mass in the universe: the more mass, then the more overall gravity. The universe will eventually turn around and begin to contract (a closed universe) if above a critical mass threshold. Below this threshold, the expansion will continue forever (an open universe). It turns out that the luminous material currently observed throughout the universe does not amount to nearly enough mass to halt the expansion. However, what if there is a huge quantity of unseen mass out there, invisible but with a profound gravitational effect on the universe? Dark matter, it was long thought, might supply the missing gravity necessary to halt the universe’s expansion.

Debate over this question persisted for decades, but has probably been resolved by observations made during the last ten years that indicate that the expansion of the universe, far from slowing down, is accelerating. If this result is confirmed, then the fate of the universe is at last definitely known: it will expand forever, becoming darker, colder, and more diffuse.

To account for the observed acceleration, physicists have postulated a dark energy, still mysterious in origin, that pervades the universe and actually helps to push things apart rather than keep them together. Since energy (even dark energy) and matter are interchangeable, some of the universe’s dark matter may thus turn out to be not matter at all, but energy.

Resources

BOOKS

Freeman, Kenneth C. In Search of Dark Matter. Berlin, Germany, and New York: Springer, 2006.

PERIODICALS

Glanz, James. “Evidence Points to Black Hole At Center of the Milky Way.” New York Times, September 6, 2001.

Glanz, James. “Photo Gives Weight to Einstein’s Thesis of Negative Gravity.” New York Times, April 3, 2001.

Overbye, Dennis. “Dark Matter, Still Elusive, Gains Visibility.” New York Times January 8, 2002.

Wilford, John Noble. “Constructing a More Plausible Universe With ‘Warm Dark Matter.”’ New York Times, January 2, 2001.

OTHER

Elgaroy, Oystein, Jaqueline Mitton, and Peter Bond. “Neutrinos Only Account for One Fifth of Dark Matter.” Daily University Science News April 10, 2002 <http://unisci.com/stories/20022/0410026.htm> (accessed October 6, 2006).

PhysOrg.com. “Did ‘Dark Matter’ Create the First Stars?.” <http://www.physorg.com/news11798.html> (accessed October 6, 2006).

DARK MATTER

Despite the enormous amount of progress made during the last century in physics and astronomy, scientists still cannot identify over 90 percent of the overall composition of the universe. Indeed, what has been learned is that the visible matter forming planets, stars, and galaxies only makes up a relatively small fraction of the total mass-energy of the universe. In the past, this unseen component was called the missing matter or missing mass of the universe. However, many different astronomical observations have confirmed its presence. Most of the mass of the universe is not missing, it is just dark, and what it is remains unknown.

Observational Evidence

The presence of dark matter was suggested by Fritz Zwicky in 1933. Typical velocities v of galaxies in a cluster of galaxies can be related to the total mass M and size R of the system through the simple Newtonian relation

where G_N is Newton's constant. Measurements of velocities of galaxies in the Coma and Virgo clusters of galaxies indicated that there was significantly more matter than could be accounted for by the individual light-producing galaxies. This discrepancy is often accounted for by computing a mass-to-light ratio. A mass-to-light ratio is the mass of the system divided by the luminosity of the system and is usually expressed in solar units (i.e., it is compared to the mass and luminosity of the Sun). In the solar neighborhood, the mass-to-light ratio is about 2 M_⊙/L_⊙ The solar neighborhood, however, is not typical of either the galaxy or the universe as a whole. In the bright central parts of galaxies, the ratio is (10–20) h₀. For the Coma cluster, this ratio is about 400 h₀, indicating that there is significantly more matter on the large scales associated with clusters of galaxies than in galaxies themselves. Both of the mass-to-light ratios above carry an uncertainty as a result of the very large distance to other galaxies or clusters. The uncertainty is qualified by the Hubble parameter H (see discussion below). The quantity h₀ is a scaled value of the present Hubble parameter and is given by h₀ = H₀/100 km Mpc^-1 s^-1. The present value of the scaled parameter is h₀ = 0.71 ± 0.07.

There are several modern astronomical techniques for establishing the existence of dark matter. The same Newtonian relation expressed above can be applied to measurements of the rotation velocities of the disks of spiral galaxies. Rotation velocities can be determined from the Doppler shifts of 21-cm hydrogen lines in the far outer reaches of spiral galaxies. At radii far beyond the point where most of the light is concentrated, one would expect that the velocities diminish as ν∝ 1/√R Instead, one finds the following surprising result: the velocities remain constant at large radii, producing what are called flat rotation curves and indicating that the mass of galaxies continues to increase as M ∝ R . This provides direct evidence for the presence of dark matter.

Additional evidence is available from X-ray observations of elliptical galaxies and clusters of galaxies. The X rays are emitted by the hot hydrogen gas surrounding these systems. The atoms in a gas, with a temperature of order 10⁶ K, have velocities that would enable them to escape the system unless there was a sufficient amount of dark matter to gravitationally bind the gas to the system.

It is also possible to infer the presence of dark matter in clusters of galaxies through gravitational lensing. The large mass associated with clusters causes the trajectories of light from background galaxies to be bent. The degree of bending is related to the amount of dark matter in the cluster.

There is also a strong theoretical argument for dark matter arising from the theory of structure formation. Galaxies and clusters of galaxies are thought to have grown from primordial density fluctuations produced in the very early universe. These fluctuations begin to grow as a result of gravitational collapse that occurs when the universe becomes dominated by nonrelativistic matter. Without dark matter and, more importantly, without nonbaryonic dark matter (see below), there is not enough time for these perturbations to grow into the structures observed today.

The Density of Matter in the Universe

It is often convenient to relate the overall density of matter in the universe to a critical density ρ_c that is given in terms of Newton's constant and the Hubble parameter H : The ratio Ω = ρ/ρ_c is called the cosmological density parameter and is related to the overall spatial geometry of the universe. If Ω > 1, the universe is closed and finite in spatial extent; if Ω < 1, the universe is open and infinite; and if Ω = 1, the universe is spatially flat and infinite. The universe is observed to be expanding so that the distances to far-away objects are increasing. Furthermore, the expansion is uniform so that the increase in distance can be related to a common scale factor for the universe a (t ). The Hubble parameter expresses the rate of change of the scale factor and determines the velocities of distant objects: where ȧ is the time rate of change of a, v is the velocity of a distant object, and d is the distance to that object. This is known as Hubble's law.

Tghe density parameter may have several different types of contributions. Ordinary matter made up of neutrons and protons is referred to as baryonic matter. (Baryons are a class of particles composed of three quarks or three antiquarks. Neutrons and protons are the lightest-and longest-lived baryons.) Other forms of matter that may contribute to Ω can be collectively called nonbaryonic dark matter. The energy densities of both baryonic and nonbaryonic matter scale with the expansion of the universe in such a way that the density decreases inversely with the volume expansion, ρ_m ∼ a_-3.

Baryonic Dark Matter

The total amount of baryonic matter can be determined from the observations of the light element abundances D, ³He, ⁴He, and ⁷Li. These elements were produced within the first 3 minutes after the Big Bang. In particular, the abundance of deuterium is particularly sensitive to the baryon density. The value of the baryon density ρ_b is commonly expressed relative to the critical density. Therefore, one can define Ω_b = ρ_b/ρ_c. A firm upper limit to the quantity is 0.03. By the same token, Big Bang nucleosynthesis also requires that The detailed spectrum of microwave background fluctuations can also determine the baryon density. Current estimates yield However, the observed baryon density is much smaller. By adding up the densities of baryons in stars (both living and dead) as well as the observed gas, it has only been possible to find a fraction There is still the possibility that baryons contribute to some of the dark matter, particularly on the scale of galaxies. In general, baryons make poor dark matter candidates since they are typically associated with luminous objects such as stars or X-ray-producing gas. However, it is possible that dark baryons reside in the dead remnants of stars, such as black holes, neutron stars, or white dwarfs. Gravitational microlensing has been useful in limiting the amount of dark matter in these forms.

Particle Dark Matter

The data so far indicate that most of the dark matter in the universe is nonbaryonic. Various techniques, which include the study of the dynamics of galaxies and clusters, show that the total density of matter is roughly Ωmatter ∼ 0.3 - 0.5. The difference between this density and the baryon density yields the nonbaryonic component to the density of the universe. Fortunately, there are many potential candidates for dark matter in the well-studied models of particle physics.

From the standpoint of the Standard Model of particle physics, the simplest candidate for dark matter would be a neutrino. Originally, the theory of electroweak interactions was constructed so that the neutrino was massless. Recent data on solar neutrinos, and neutrinos produced by cosmic-ray collisions in the atmosphere, strongly indicate that neutrinos of different types oscillate and thereby have mass. As such, they could, in principle, contribute to the dark matter.

Even a small neutrino mass can make a large contribution to the overall density of matter. If m_ν is the mass of neutrino, then so long as m_ν < 1 MeV, its contribution to the density parameter is = m_ν/(94 eV). However, a neutrino-dominated universe is strongly disfavored by the observed large-scale structure of the universe. Light neutrinos remain relativistic until relatively late times. By the time they come to dominate the mass density of the universe, they have traveled across immense distances, erasing the possibility of the growth of structures on smaller scales. Thus, a neutrino-dominated universe inevitably produces too much large-scale structure. Furthermore, it appears that the oscillation data require neutrino masses that are too small to make a dominating contribution to the overall mass density.

In contrast to light neutrinos, which are typically labeled hot dark matter because of their relativistic velocities, an ideal dark matter candidate should be almost at rest with respect to the cosmic expansion. These cold dark matter candidates lead to the formation of smaller structures (galaxy size and smaller) first and fit the observations reasonably well.

Beyond the Standard Model of particle physics there are many particle candidates for cold dark matter. One of the best-studied candidates is found in an extension of the Standard Model based on supersymmetry. Supersymmetry is a symmetry that relates particles of different spin (an internal quantum number assigned to all particles). The supersymmetric Standard Model contains many new particles and predicts that one of these will be stable. This new stable particle is normally a neutralino (it is the supersymmetric partner of the photon and Z gauge boson) and may have a mass of order 100 times the mass of the proton. Another potential candidate is called the axion, and it arises in a theoretical solution to what is known in particle physics as the strong CP problem. Fortunately (or not), many other particle candidates are found in theories extending the Standard Model.

Dark Energy

It is also possible that there is a component of Ω for which the energy density is constant (with respect to the expansion of the universe). This is called the cosmological constant Λ. It was originally introduced by Albert Einstein in order to cancel the expansion he found in his cosmological models. When the expansion of the universe was observed, he described the introduction of the cosmological constant as his biggest blunder. Nevertheless, its potential contribution must be considered.

The total density parameter has been established to be very close to unity by microwave background experiments, which indicate that Ωtotal = 1.03 ± 0.06. As mentioned above, it appears that the contribution of Ω due to matter (both baryonic and nonbaryonic) is approximately 0.3–0.5. The difference between Ωtotal and Ωmatter has been called dark energy. The existence of dark energy has also been suggested by recent supernovae observations, which detect that the universe may be accelerating (i.e., its expansion rate is increasing). The cosmological constant could play the role of dark energy as could any smoothly distributed energy associated with the vacuum.

See also:Astrophysics; Cosmological Constant and Dark Energy; Cosmology; Outlook

Bibliography

Borner, G. The Early Universe: Facts and Fiction (Springer-Verlag, Heidelberg, Germany, 1993).

Krauss, L. M. The Fifth Essence: The Search for Dark Matter in the Universe (Basic Books, New York, 1989).

Riordan, M., and Schramm, D. N. The Shadows of Creation: Dark Matter and the Structure of the Universe (W. H. Freeman, New York, 1992).

Srednicki, M. Particle Physics and Cosmology: Dark Matter (North-Holland, Amsterdam, 1990).

Keith Olive

Dark matter

Dark matter is the term astronomers use to describe material in the Universe that is non-luminous—that is, material that does not emit or reflect light and that is therefore invisible. Everything seen when looking through a telescope is visible because it is either emitting or reflecting light; stars, nebulae, and galaxies are examples of luminous objects. However, luminous matter appears to make up only a small fraction of all the matter in the Universe, perhaps only a few percent . The rest of the matter is cold, dark, and hidden from direct view.

Because dark matter is invisible, it can only be detected through indirect means, primarily by analyzing its effect on visible material. Although dark matter does not shine, it still exerts a gravitational force on the matter around it. For example, it is possible to measure the velocities of many stars in our galaxy and in other galaxies. The measured velocities do not agree, in general, with those calculated on the assumption that the visible material of the galaxies (i.e., their stars and clouds of glowing gas) constitute all or even most of their mass . Additional, unseen mass, therefore, must exist in the vicinity of the galaxies, tugging on their stars. Such data seem to indicate the presence of massive "halos" of dark matter surrounding the galaxies that would account for most of their mass. Recent observations of the shapes formed by galaxies clumping together throughout the Universe have confirmed that dark matter does not pervade space uniformly or form structures independently of the galaxies, but is concentrated around the galaxies.

The identity of the Universe's dark matter remains a subject of research and dispute among physicists. A number of possibilities have been proposed: (1) Astronomers hold that supermassive black holes exist at the centers of most galaxies, contributing several hundred million or even on the order of a billion solar masses to each galaxy. (One solar mass is a quantity of matter equal to the mass of the Sun.) In 2001, observations of xray bursts from the center of our galaxy confirmed the presence of a large black hole there. Such black holes supply invisible mass and count as "dark matter." (2) Multitudes of non-luminous brown dwarfs or machos (massive compact halo objects)—dim blobs of gas not massive enough to initiate fusion reactions at their centers and thereby become stars—may orbit each galaxy. Such objects have been detected using gravitational lensing, but not in sufficient numbers to account for the amount of dark matter that is believed to exist. (3) The subatomic particles known as neutrinos, which pervade the universe in very great numbers, were shown in 1998 to have a small mass, ending a decades-long dispute among physicists about whether they are massless. It had been thought that neutrinos, if they have mass, might account for the Universe's dark matter; however, calculations now show that each neutrino's mass is so small that neutrinos can account for at most a fifth of the dark matter in the Universe. (4) Particles of some unknown kind, generically termed wimps (weakly-interacting massive particles), may permeate the space around the galaxies, held together in clouds by gravity.

Dark matter, which may turn out to be a combination of such factors, has long been thought to play a crucial role in determining the fate of the Universe. The most widely accepted theory regarding the origin and evolution of the universe is the big bang theory , which provides an elegant explanation for the well-documented expansion of the universe. One question is whether the universe will expand forever, propelled by the force of the big bang, or eventually stop expanding and begin to contract under its own gravity, much as a ball thrown into the air eventually turns around and descends. The deciding factor is the amount of mass in the universe: the more mass, the more overall gravity. There is a critical mass threshold above which the universe will eventually turn around and begin to contract (a "closed" universe). Below this threshold the expansion will continue forever (an "open" universe). It turns out that the luminous material currently observed throughout the Universe does not amount to nearly enough mass to halt the expansion. But what if there is a huge quantity of unseen mass out there, invisible but with a profound gravitational effect on the Universe? Dark matter, it was long thought, might supply the "missing gravity" necessary to halt the Universe's expansion.

Debate over this question persisted for decades, but has probably been resolved by observations made during the last five years that indicate that the expansion of the Universe, far from slowing down, is accelerating. If this result is confirmed, then the fate of the Universe is at last definitely known: it will expand forever, becoming darker, colder, and more diffuse.

To account for the observed acceleration , physicists have postulated a "dark energy," still mysterious in origin, that pervades the Universe and actually helps to push things apart rather than keep them together. Since energy (even "dark energy") and matter are interchangeable, some of the Universe's dark matter may thus turn out to be not matter at all, but energy.

Resources

periodicals

Glanz, James. "Evidence Points to Black Hole At Center of the Milky Way." New York Times, September 6, 2001.

Glanz, James. "Photo Gives Weight to Einstein's Thesis of Negative Gravity." New York Times, April 3, 2001.

Overbye, Dennis. "Dark Matter, Still Elusive, Gains Visibility." New York Times January 8, 2002.

Wilford, John Noble. "Constructing a More Plausible Universe With 'Warm Dark Matter.'" New York Times, January 2, 2001.

other

Elgaroy, Oystein, Jaqueline Mitton, and Peter Bond. "Neutrinos Only Account for One Fifth of Dark Matter." Daily University Science News. April 10, 2002 [cited January 9, 2003]. <http://unisci.com/stories/20022/0410026.htm>.

Dark matter

Dark matter is the term astronomers use to describe material in the universe that does not emit or reflect light and is, therefore, invisible. Stars, nebulae, and galaxies are examples of luminous objects in the sky. However, luminous matter appears to make up only a small fraction of all the matter in the universe, perhaps only up to 10 percent. The rest of the matter is cold and dark, hidden from people's direct view.

The principal way dark matter can be detected is by observing its gravitational effect on nearby objects. Although dark matter does not shine, it still exerts a gravitational force on the matter around it. Astronomers believe that dark matter is a "cosmic glue" holding together rapidly spinning galaxies and controlling the rate at which the universe expands.

How can we know what we cannot see?

Understanding something that cannot be seen is difficult, but it is not impossible. Present-day astronomers study dark matter by its effects on the bright matter that can be observed. It was in the 1930s that Swiss astronomer Fritz Zwicky first pointed out that dark matter must exist. Zwicky claimed that the mass of known matter in galaxies is not great enough to generate the gravitational force to hold a cluster of galaxies together. Each independent galaxy moves at too great a speed for galaxies to remain in a cluster. Yet the galaxies were not spinning away from each other. They had to be held together by a gravitational field created by undetected mass.

More than 40 years later, American astronomer Vera Rubin found that the same principle is true within a single galaxy. The mass of stars alone do not exert enough gravitational pull to hold the galaxy together. She discovered that stars in the far reaches of the galaxy rotate about the galactic center at the same speed as stars close to the center. Rubin concluded that some invisible, massive substance surrounds a galaxy, exerting gravitational force on all its stars.

Words to Know

Black dwarf: Remains of a white dwarf star that has stopped glowing.

Black hole: Single point of infinite mass and gravity formed when a massive star burns out its nuclear fuel and collapses under its own gravitational force.

Brown dwarf: Blob of celestial gas not massive and hot enough to trigger the nuclear reaction at its core that would classify it as a star.

Subatomic particle: Basic unit of matter and energy smaller than an atom.

White dwarf: Dense, faintly glowing core left when a medium-sized star stops burning.

Sources of dark matter?

Astronomers have only been able to speculate on the composition of dark matter. One source of dark matter might be the diffuse, dark "halo" of gas in and around galaxies that increases their mass. Another might be dark objects called MACHOs (MAssive Compact Halo Objects) that lurk invisibly in the halos surrounding galaxies and galactic clusters. MACHOs are planets or stars made up of ordinary matter that are too faint to be observed directly. However, they can act as a gravitational lens and magnify the brightness of brighter stars behind them.

Astronomers have also considered that dark matter may be supermassive black holes (single points of infinite mass and gravity formed from the collapse of burned-out massive stars) at the centers of galaxies. These black holes may be contributing several hundred million or even a billion solar masses to the galaxies in which they reside. One more possible source of dark matter could be multitudes of WIMPs (Weakly Interacting Massive Particles), exotic, unfamiliar particles that may exist all around us. Astronomers have theorized about the existence of these particles, although experiments have not yet confirmed their presence.

The search for the truth about dark matter is inextricably tied in with other outstanding questions in cosmology: How much mass does the universe contain? How did galaxies form? Will the universe expand forever? Dark matter is so important to the understanding of the size, shape, and ultimate fate of the universe that the search for it will very likely dominate astronomy until an answer is found.