Dark Matter

Dark Matter

Dark Matter

Dark matter is a still unidentified substance that makes up about 23 percent of the universe. It is thought to surround most galaxies, affecting their shapes by its gravity. Ordinarily it cannot be seen. This composite picture of galaxy cluster 1E 0657-556 was created from images taken by different space telescopes in X-ray and visible light. The cluster is actually two giant groups of galaxies that collided head-on. The dark matter around the groups of galaxies is indicated by the blue regions, which bend the light from more distant galaxies in the background. The pink regions are hot gas stripped away in the collision.

Dark Matter, in astronomy, designation for matter that does not give off or reflect detectable electromagnetic radiation, the radiant energy that includes visible light, radio waves, infrared radiation, X rays, and gamma rays. Although dark matter is practically invisible, astrophysicists have determined its existence by detecting its gravitational interaction with matter that does give off detectable electromagnetic radiation, such as stars, galaxies, and clusters of galaxies. Dark matter has become a vital component of modern theories of cosmology and elementary particle physics. Along with the phenomenon of dark energy, the puzzle of what dark matter is represents one of the most important questions in physics today.

II

THE DISCOVERY OF DARK MATTER

The existence of dark matter was first suggested in the early 20th century by the Swiss American astronomer Fritz Zwicky, but convincing and overwhelming evidence of its existence was gathered by the American astronomer Vera Rubin in the 1970s. In the early 1930s, Zwicky studied the rotational motions of thousands of galaxies clustered together in a large group of galaxies known as the Coma Cluster. He found that the orbital motion of the galaxies around their common center of mass could only be explained by the presence of unseen matter, which astronomers now call dark matter. Zwicky’s suggestion was not taken very seriously at first because there was not a great amount of evidence to support such a radical suggestion.

In the early 1970s, however, Rubin studied the orbital motions of stars in a large number of galaxies. As these stars orbited their galactic centers, Rubin noticed that the outlying stars in the galaxies were moving so fast that they should have been flung out of the galaxies. But since they were still part of the galaxies, Rubin proposed that unseen matter was keeping them gravitationally bound to the galaxies. A similar observation was reported in the early 1930s for the stars in our own Milky Way Galaxy by Dutch astronomer Jan Oort, but the dark matter interpretation was not considered at that time.

Following up on the observational data gathered by Oort, Zwicky, Rubin, and other astronomers, two American theoretical astrophysicists, Jeremiah P. Ostriker and P. J. E. Peebles, contributed important theoretical analyses. The data and the analyses helped scientists determine that dark matter probably constitutes as much as 90 percent of all the matter in the universe. Scientists verified that the orbital motions of stars in galaxies cannot be explained by the mutual gravitational influence of all the other visible stars. To explain this orbital motion, dark matter must be present. Rubin and other astronomers and astrophysicists showed that this dark matter seems to be distributed in a large envelope or “halo” around the visible matter of the galaxy. As a result the galaxies are much larger than what can actually be observed through a telescope.

Some scientists have theorized that there may be other explanations for the orbital motions of stars besides dark matter, such as a new type of force in nature that exerts itself over vast distances or a modification of the law of gravity. But so far, neither a new force nor a new understanding of gravity has been found. The hypothetical existence of dark matter, however, does explain the observed interactions with ordinary matter extremely well without resorting to a new long-range force.

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WHAT IS DARK MATTER?

To gain a fuller understanding of our universe, it is vital to determine exactly what dark matter is made of. Scientists think that dark matter occurs in several different forms. Moreover, observations and experiments place limits on the quantity and distribution of each type. There are two broad categories of dark matter: “hot” dark matter, which moves at speeds comparable to the speed of light (about 299,000 km per second or 186,000 mi per second), and “cold” dark matter, which moves at speeds well below that of light.

A

Hot Dark Matter

The elementary particle called the neutrino, discovered in 1956, is an example of a hot dark matter candidate. Various experiments and observations, such as those reported in 1998 by the Super-Kamiokande experiment in Japan, have shown that the neutrino has mass. Mass is the quality that causes gravitational attraction. The mass of the neutrino is extremely small, which is why the particle travels at speeds comparable to that of light. Neutrinos are extremely abundant in the universe because they are produced in enormous numbers in nuclear interactions that take place at the core of every star. For example, several trillion neutrinos pass through each person on Earth each second as a result of the nuclear reactions that cause the Sun to shine. Because neutrinos are electrically neutral they can pass easily through ordinary matter, such as through people, and so are able to spread throughout a region near ordinary matter. Their large numbers could enable them to be a significant component of dark matter despite the tiny amount of mass in an individual neutrino.

However, there is evidence that dark matter cannot be made up mostly of neutrinos. This is due to two reasons. First, their likely mass is still too small to provide enough matter to account for the gravitational effects seen in the orbital motions of stars. In addition, some form of dark matter was necessary in the early universe to create the early structures that eventually led to the formation of stars and galaxies. Neutrinos could not have played this role, in part because they could not have been created in the required quantities until stars actually formed.

Secondly, neutrinos are too energetic to have helped seed the process of star and galaxy formation. Some other form of dark matter must have contributed to star and galaxy formation, which developed from localized structures, or lumps, known as anisotropies. These lumps have been detected in the cosmic microwave background radiation, the radiation left over from the formation of the universe in the big bang about 13.7 billion years ago.

Detailed observations of the cosmic background radiation show that these localized lumps in the early universe were too small to be seeded by fast-moving particles such as neutrinos. For the anisotropies to have formed in the early universe, a large component of slower, cold dark matter must have been present.

B

Cold Dark Matter

Another candidate for the dark matter is known as baryonic cold dark matter. Baryonic cold dark matter is made of protons and neutrons, the subatomic particles known as baryons that make up ordinary matter and combine with electrons to form atoms. Baryonic cold dark matter could be found in celestial objects that were not massive enough to initiate the fusion processes that make stars shine. It could also be made of matter that collapsed to form dense objects such as neutron stars or even black holes (objects so dense that not even light can escape their gravitational field). Such objects are collectively referred to as “MACHOs,” which means “massive compact halo objects.”

Astronomical observations limit the amount of MACHOs that could exist. For example, if they were numerous, such objects would inevitably pass close to the line of sight between an observer on Earth and a distant visible object. Albert Einstein’s theory of gravity, known as the theory of general relativity, describes how light is bent by the presence of mass and energy. Mass and energy set up a gravitational field through which light passes. The MACHO object’s gravitational field would therefore bend the light of a more distant visible object and produce a distortion of its image. Such a process is called a gravitational lens. Several such gravitational lenses have been observed, and the measured frequency of such events has placed limits on how much dark matter can take the form of MACHOs. From the observations that have been made by astronomers, it is now known that MACHOs cannot be the dominant constituent of dark matter. There are simply not enough such gravitational lenses.

Physicists suspect that a more exotic form of cold dark matter must exist. This form is not baryonic. Like neutrinos, this form barely interacts with ordinary matter, but is some type of massive particle. Such candidates are often called WIMPs (for “weakly interacting massive particles”).

Various theories of the fundamental elementary particles and their interactions in nature predict the existence of new particles that have not yet been discovered. These hypothetical particles are excellent candidates for WIMPs. For example, supersymmetric theories propose that a new fundamental symmetry of nature was present when the universe was very young and energetic. If this symmetry existed in the past, it requires the hypothetical particles to exist. The symmetry would have disappeared as a result of natural dynamical effects as the universe aged and became less energetic. See also Theory of Everything; Unified Field Theory.

As a result we do not observe this symmetry today. The loss of symmetry over time is very similar to what happens as water freezes and turns to ice as it is cooled. Water is more symmetrical than ice since its molecules point in all directions in space, whereas ice forms a crystal lattice with the molecules pointing in the preferred directions of the lattice. As the universe expands, it also cools, and this can give rise to the disappearance of important symmetries in later epochs.

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FUTURE EXPERIMENTS

The supersymmetric theories predict that relics of the earlier, more symmetric phase of the universe exist and that they exist in the form of a very specific family of massive particles. New experiments, such as those planned for the Large Hadron Collider (LHC), a particle accelerator at the European Organization for Nuclear Research (CERN) in Switzerland, are expected to be able to test some of these theories. The experiments will try to create the predicted particles directly, using the energetic collisions produced in the LHC. If the particles are discovered, the LHC and other proposed experiments will systematically study their properties to determine if they are indeed the sought-after principal components of dark matter. This type of dark matter would be the key agent of structure formation that gave rise to the galaxies and clusters of galaxies that constitute our home in the universe.

In 2006 astronomers announced what may be the first direct evidence for dark matter in the cosmos, based on observations from the Chandra X-Ray Observatory telescope, the Hubble Space Telescope (HST), and advanced telescopes on Earth. A collision between two clusters of galaxies over 4 billion light-years away apparently caused visible matter and dark matter to separate on a gigantic scale. Huge gas clouds between and around the galaxies in each group smashed together as the two clusters raced past each other, heating the gas to a plasma state and dragging it away from the dark matter halos thought to surround the galaxies. The massive presence of dark matter was detected by gravitational lensing that bent the light from more-distant background galaxies in the telescopes’ line of sight. The findings appear to support theories that dark matter exists and that gravity behaves the same in giant clusters of galaxies as it does on Earth.

In early 2007 an international team of astronomers published the results of the first search for dark matter in a wide region of the sky using high-resolution images from the HST. Findings from the Cosmic Evolution Survey (COSMOS) conducted with the HST were used to create a three-dimensional map that shows the distribution of dark matter through periods of time as far back as 6.5 billion years ago, almost halfway to the big bang. The scientists studied the shapes of half a million galaxies to find distortions in their apparent shapes caused by gravitational lensing around large concentrations of mass lying between very distant galaxies and Earth. Such concentrations of mass indicate the presence of dark matter. The new map reveals how areas of dark matter provided the scaffolding for clusters of galaxies made of visible matter. The galaxies accumulated in the densest regions of dark matter as gravity gradually drew the dark matter into more compact filaments that form a massive networklike structure.