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Cosmology, study of how the universe originated and how it has evolved. There are 2 main theories. The first, known as the evolutionary theory, pictures the universe as having been born, and as evolving and eventually dying. The popular name for this theory is the “big bang” theory, because it assumes that all the material in the universe was at one time packed tightly together and was then flung outward by an enormous explosion. This theory was first put forward in the 1920s by the Belgian astrophysicist and priest Georges LemaÎtre. It rested on the discovery by Edwin Hubble that the universe seemed to be expanding, like a balloon being blown up. Hubble showed that the galaxies are receding from each other with velocities increasing to nearly half the speed of light as they get farther away. After World War II, Hermann Bondi and Thomas Gold put forward the opposing steady state theory, which says that new material is continuously created to fill the space between the galaxies. Although the universe is expanding, the matter in it was never concentrated. New galaxies and old galaxies would exist side by side, and the universe would always look the same. The theory was later developed by the English astronomer Fred Hoyle. Observational tests for these theories rest on the fact that radiation takes time to cross very long distances. By looking deep into space, one looks backward in time, because the radiation now recorded may have left the object long ago. The farthest visible objects are so far away that their radiation has taken billions of years to reach earth.

As one looks back in time, the character of the universe seems to alter. Instead of galaxies one sees quasars, which may be galaxies in the process of formation. At distances past 8 billion light-years, the number of objects seems to decrease, indicating a time when the universe was just forming, which seems to support the evolutionary theory.

Another piece of evidence against the steady state theory is that radio astronomers record a faint radiation from all over the universe. This so-called “background” radiation, discovered in 1965 by Arno Penzias and Robert Wilson, may be the heat left over from a “big bang” explosion. Though it still has adherents, the steady state theory has now largely been abandoned.

See also: Astronomy.

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