DARK encyclopedia topics

dark matter, material that is believed to make up (along with dark energy) more than 90% of the mass of the universe but is not readily visible because it neither emits nor reflects electromagnetic radiation, such as light or radio signals. Its existence would explain gravitational anomalies seen in the motion and distribution of galaxies. Dark matter can be detected only indirectly, e.g., through the bending of light rays from distant stars by its gravity.

Dark matter may consist of dust, planets, intergalactic gas formed of ordinary matter, or of MACHOs [Massive Astrophysical Compact Halo Objects], nonluminous bodies such as burned-out stars, black holes, and brown dwarfs; these are the so-called hot dark matter and would be dispersed uniformly throughout the universe. The discovery in 2001 of a large concentration of white dwarf stars in the halo surrounding the Milky Way indicates that these burned-out stars could represent as much as a third of the dark matter in the universe.

Other theories hold that it is made of elementary particles that played a key role in the formation of the universe, possibly the low-mass neutrino or theoretical particles called axions and WIMPs [Weakly Interacting Massive Particles]; these are the so-called cold dark matter and would be found in clumps throughout the universe. In 1996 a Japanese team at the Univ. of Tokyo led by Yasushi Ikebe reported on dark-matter clumping in the galactic cluster Fornax. Clumps were found in two distinct regions: around a massive galaxy in the center of the cluster and, in larger amounts, around the entire cluster. This suggests that the slower, cold dark matter might form the smaller clumps associated with the galaxy while the faster, hot dark matter might form the larger clumps associated with the galactic cluster.

Computer simulations of the formation of the universe favored the cold dark matter but tended to predict the formation of too many dwarf galaxies when compared to the observed universe. This led to the postulation of warm dark matter, which resolved the simulation problems. Unlike cold dark matter, which has mass but virtually no velocity or temperature, or hot dark matter, which has mass and is highly energetic, warm dark matter has mass and a low temperature corresponding to an extremely low velocity.

See also interstellar matter.

See R. Morris, Cosmic Questions: Galactic Halos, Cold Dark Matter and the End of Time (1995); T. Van Flandern, Dark Matter, Missing Planets, and New Comets (2d ed. 1998); M. Hawkins, Hunting Down the Universe: The Missing Mass, Primordial Black Holes and Other Dark Matters (1999).

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dark horse

dark horse, in U.S. politics, a person unexpectedly chosen by a major party as a candidate for public office, especially for the presidency. A presidential dark horse is usually chosen at a party national convention and often has acquired only a local or limited reputation at the time of his nomination. He is invariably the offspring of compromise after rival factions have deadlocked the convention. Probably the best-known example of a dark horse is James K. Polk, who was selected at the Democratic convention of 1844 on the ninth ballot, although he had not been nominated until the eighth ballot.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

dark energy

dark energy, repulsive force that opposes the self-attraction of matter (see gravitation) and causes the expansion of the universe to accelerate. The search for dark energy was triggered by the discovery (1998) in images from the Hubble Space Telescope of a distant supernova that implied an accelerating, expanding universe, which in turn required a new cosmological model (see cosmology). Although dark energy is predicted in particle physics, it has never been directly observed. It is generally agreed, however, that dark energy dominates the universe, which is projected to have a composition of c.70% dark energy, c.30% dark matter, and c.0.5% bright stars. By 2006, astronomers using the space telescope to examine more distant supernovas had found evidence of the effects of dark energy dating to 9 billion years ago.

The concept of dark energy was first proposed, and then discarded, by Albert Einstein early in the 20th cent. His theory of general relativity implied that the pull of gravity would make the universe collapse, but, like many scientists of his time, he assumed the universe to be static and unchanging. To make his equations fit these assumptions, Einstein added a "cosmological constant" whose effect was repulsive. When American astronomer Edwin Hubble discovered that the universe was expanding, it was assumed that the universe must be slowing down because of gravity and might even come to a halt. This led Einstein to remove the cosmological constant from his equations and to say that it had been the biggest blunder of his career.

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Dark Ages

Dark Ages: see Middle Ages.

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Licensed from Columbia University Press