Steady State Theory and Pulsating Theory

In cosmology, the Steady State theory (also known as the Infinite Universe theory or continuous creation) is a model developed in 1948 by Fred Hoyle, Thomas Gold, Hermann Bondi and others as an alternative to the Big Bang theory (known, usually, as the standard cosmological model). In steady state views, new matter is continuously created as the universe expands, so that the perfect cosmological principle is adhered to.Theoretical calculations showed that a static universe was impossible under general relativity, and observations by Edwin Hubble had shown that the universe was expanding. The steady state theory asserts that although the universe is expanding, it nevertheless does not change its appearance over time (the perfect cosmological principle); it has no beginning and no end. The theory requires that new matter must be continuously created (mostly as hydrogen) to keep the average density of matter equal over time.The amount required is low and not directly detectable: roughly one solar mass of baryons per cubic megaparsec per year or roughly one hydrogen atom per cubic meter per billion years, with roughly five times as much dark matter. Such a creation rate, however, would cause observable effects on cosmological scales. Dust-Cloud Theory. Between 1940 and 1955 the German astronomer Carl f. von Weizsaccker, the Dutch-American astronomer Gerald P. Kuiper and the U. S. chemist Harold C.Urey worked out a theory that attempted to account for all the characteristics of the solar system that need to be explained. According to their dust-cloud theory, the solar system was formed from a slowly rotating cloud of dust and gas that contracted and started to rotate faster in its outer parts, where eddies formed. These eddies were small near the center of the cloud and larger at greater distances from the center. The distances corresponded more or less to the Titius-Bode relation.As the clouds cooled, materials coagulated near the edges of the eddies and eventually formed planets and asteroids, all moving in the same direction. The slowly rotating central part of the cloud condensed and formed the sun, and the sun's central temperature rose as gravity further compressed the material. When nuclear reactions eventually began in the suns interior, about 5 billion years ago, much of the nearby gas was blown away by the pressure of the sun's emitted light.Nevertheless the earthy retained an atmosphere consisting of methane, ammonia, carbon monoxide, water vapor, and nitrogen, with perhaps some hydrogen. In this primitive atmosphere and in the seas below it, organic compounds were formed that eventually resulted in living organisms. The organisms evolved in the next 2 billion years into higher plants and animals, and photosynthesis by plants and the weathering of rock produced the oxygen in the earth's atmosphere.Although free gases near the sun were blown outward 4 to 5 billion years ago, according to the dust-cloud theory, the giant planets were too distant to be much affected. They are large, therefore, and contain a great amount of hydrogen. The comets, in turn, are thought to be the outer part of the primordial nebula, left behind as the inner part condensed to form the sun and the planets. The Dutch astronomer J. H. Oort speculated that this material condensed into chunks that continue to move along with the sun through space.Now and then a chunk is perturbed and falls slowly toward the sun. As it is heated by sunlight, it grows a coma and tail. The dust-cloud theory thus explains the solar system characteristics listed above. It is most weak in detailing the process whereby the planets and asteroids formed from solids that made up only a small percent of the primordial nebula. However, this is essentially a chemical problem, strongly dependent on the sequence or timing of events such as eddy formation, temperature changes, and the start of solar luminosity. Pulsating Theory: According to this theory, the universe is su pposed to be expanding and contracting alternately i. e. pulsating. At present, the universe is Expanding. According to pulsating theory, it is possible that at a certain time, the expansion of the universe may be stopped by the gravitational pull and the may contract again. After it has been contracted to a certain size, explosion again occurs and the universe will start expanding. The alternate expansion and contraction of the universe give rise to pulsating universe.