Further Reading

Dana, James Dwight. "On the Areas of Subsidence in the Pacific as Indicated by the Distribution of Coral Islands." American Journal of Science 45 (1943): 131-135.

-. "Certain Parallel Relations between the Classes of

Vertebrates and Some Characteristics of the Reptilian Birds." American Journal of Science 36 (1863): 315-321.

-. "A New Mineralogical Nomenclature." Annals of the Lyceum of Natural History of New York 8 (1837): 9-34.

-. "Manual of Geology: Treating of the Principles of the

Science with Special Reference to American Geological History, for the use of Colleges, Academies, and Schools of Science." Philadelphia: T. Bliss and Co., 1863.

-. A System of Mineralogy. New York: John Wiley

dark matter The mass in galaxies and galactic clusters inferred to exist by the rotational properties of galaxies, the bending of light, and other techniques but has not been confirmed to exist by observations at any electromagnetic wavelength is known as dark matter. Dark matter is mysterious; with an unknown composition it interacts only very weakly with normal matter and has been decoupled from the rest of the universe since before the time primordial nucleosynthesis began. Scientists believe that dark matter has experienced large fluxuations in its density distribution since early in the history of the universe, without affecting the background radiation of the universe, but effectively forming large-scale clumping and other mass distributions in the universe. In this way dark matter is able to control the overall mass distribution in the universe without affecting the microwave background radiation or any other observational constraints on the universe.

The large gravitational attraction of dark matter is theorized to have drawn gas and other matter into the vicinity of peaks in its distribution over the history of the universe, accounting for the distribution of galaxies and clusters observed today. One of the shocking features of dark matter is that, although it cannot be seen or directly observed, it is thought to make up the bulk of the universe. About 4 percent of the universe is thought to be made of visible matter, about 22 percent is estimated to be dark matter, and a remarkable 74 percent of the universe is thought to be dark energy. Dark energy permeates the entire universe and is thought to be the cause of the recently detected increase in the rate of expansion of the universe. Dark energy has been proposed to consist of two forms, including the cosmological constant, a constant form of energy that fills space homogeneously, and other more exotic forms of energy (known as scalar fields with names such as moduli and quintessence) that vary in time and in space.

Astrophysicists have classified dark matter into two basic theoretical types, hot and cold, based on its temperature at the time the galaxies began to form. Whether the dark matter was hot or cold at the time the galaxies formed results in vastly different structures for the universe in later times. Astrophysicists and cosmologists have used this variation to model the evolution and structure of the universe using different combinations of hot and cold dark matter as gravitational building blocks. This work is purely theoretical, carried out by simulations in supercomputers, since dark matter has never been directly observed.

Hot dark matter is thought to be made of very lightweight particles, even lighter than electrons. some astrophysicists think that hot dark matter may be composed of neutrinos. Models for the evolution of the universe using hot dark matter account for the development of very large-scale structures, such as superclusters, and vast empty regions called voids, but they do not explain the smaller-scale structures very well. This is because small amounts of hot material tend to disperse, not to group together to help form smaller-scale structures. Therefore most astrophysicists suggest that models for the evolution of the universe that rely only on hot dark matter are not feasible.

Cold dark matter is thought to consist of heavy particles that formed in the earliest microseconds (10-43 second after the big bang) at a time when the strong, weak, and electromagnetic forces were still unified (a time known as the grand unified theory time). Unlike the (theoretical) hot dark matter, computer models for the origin of the universe that use cold dark matter can explain the formation of both large-scale and small-scale structures in the universe. Most astrophysicists and cosmologists therefore prefer models of dark matter that suggest it consists of heavy cold particles that formed very soon after the big bang.

some new models for the universe suggest that dark matter may consist of both hot and cold particles. supercomputer simulations can match the theoretical evolution of the universe and its current structure with what might have happened by specific mixtures of hot and cold dark matter.

See also astronomy; astrophysics; cosmic microwave background radiation; cosmology; origin and evolution of the universe.

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