The universe comprises matter, light, and other forms of radiation and energy. It includes the earth and everything on it. It also includes everything in the solar system. The size of the universe is unknown. Most astronomers think that bright, unusual galaxies called quasars may be the most distant objects in the universe. It is believed that they be as far away as 10 billion light-years from the earth. Most cosmological theories about the universe are based on the idea that any part of the universe is like any other part having the same age. The basis for theories about the behavior of the universe is formed in part by Albert Einstein's general theory of relativity. This theory is based on tow beliefs; (1) that no signal can
faster than the speed of light, and (2) that the laws of physics are the same everywhere in the universe. Theories resulting from these ideas involve a universe that expands and contracts. From observing the red shift in distant galaxies, scientists conclude that the universe is expanding. But the overall behavior of the universe depends on the average density of matter in the universe at the present time. The other theory is based on the belief that any part of the universe is like all other parts at all times. According to this theory, matter is continuously created and formed into new galaxies that replace those which recede to infinite distances. The development of the telescope, the spectroscope, and the photographic plate led to a great expansion of astronomical knowledge.