EVOLUTION THEORIES

                                 Evolution theories of universe

                                   The branch of science which deals with the study of position, composition, motion and other related facts of the heavenly bodies, is called astronomy. It is an observational science and not experimental science like other sciences, because we cannot control the conditions of the experiments or events which are occurring in the heavenly bodies i.e., we cannot repeat these experiments in our laboratory since the conditions like temperature, pressure etc. in these heavenly bodies are too extreme and they cannot be artificially created in the laboratory. In other branches of science like physics, we can control over the experimental conditions in the laboratory of wavelength 4x10^7m to 8x10^7m and radio waves of wavelength 8mm to 17 mm reaching the earth from the heavenly bodies. These are called the 'two windows of astronomy '. it means the visible lights and radio waves coming from the heavenly bodies are the two windows to study about the sun ,the stars etc., about the universe. 

                Science at the beginning of the twenty first century can make some bold ,yet simple observations. It means the visible lights and radio waves   It means the visible lights and radio waves coming from the heavenly bodies are the two windows to study about the sun, the stars etc. i.e. about the universe. Science at the beginning of the twenty-first century can make some bold, yet simple observations:1) the universe has evolved;2) we are a result of that evolution. If being educated means having an informed sense of time and place, then it is essential for a person to be familiar with the scientific aspects of the universe and know something of its origin and structure.” (Project 2061, American Association for the Advancement of Science.
          Cosmology is the Study of the Universe: its structure, origin, evolution, and destiny The three main theories put forward to explain the origin and evolution of the universe are:

     1.    The Big Bang Theory

           2.    The Steady State Theory

           3.    The Pulsating Theory
      The main source of information about the heavenly bodies is the study of the electromagnetic radiations emitted from them. There are two distinct regions of radiations (i.e., visible radiations)
                        The Big Bang Theory               

                 Le.Maitre and Gammow proposed this theory. According to this theory, at the beginning of the universe, the whole matter of the universe was once concentrated in an extremely dense and hot fireball. Then about 20 billion years ago a vast explosion (big bang) occurred. The matter was broken into pieces, which were thrown out with high speed in all directions forming stars and galaxies; which are still moving away from one another. According to Hubble's law, the  velocity of recession of a galaxy becomes equal to the velocity of light at a distance equal of 20 billion light years. It means, the light rays from stars and galaxies, which are situated at a distance of 20 billion light years or more, can never reach us. Thus this distance becomes the boundary of observable universe. On account of continuous recession, more and more galaxies will go beyond this boundary and the will be lost. As a result of this, the number of galaxies per unit volume will go on decreasing and ultimately a time may come when we may have empty universe.

                       Steady State Theory 

                 Bondi Gold and Fred Hoyle developed this theory. According to this theory, the   number of galaxies in the observable universe is constant and new galaxies are continuously being created out of empty space, which fill up the gaps caused by those galaxies, which have crossed the boundary of the observable universe. As a result of the overall size of mass of the observable universe remains constant. Thus a steady state of the universe is not disturbed at all.

                                        Pulsating Theory

According to this theory, the universe is supposed 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 they 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. Since, long, scientists have been trying to know the universe around them. They have taken great interest in the motion of heavenly bodies like sun, planets, moon etc. Ptolemy, a Greek astronomer proposed theory regarding the planetary motion. His theory was called Geocentric  

Theory of Planetary Motion.

     

                  According to this theory, all the heavenly bodies like sun, stars, planets, moon etc. are moving around the earth, which is stationary and is taken as the centre of the universe. Copernicus challenged this theory. In 498 A.D. a great Indian mathematician and astronomer, Aryabhatta from his observations concluded that the various facts (like solar eclipse, moon eclipse, formation of day and night etc.) could be very easily explained by considering that earth revolves round the sun and also rotates on its own axis. However, due to lack of communication, his ideas could not be communicated to the western philosophers. In 1543, Copernicus suggested that description of motion of heavenly bodies becomes very simple and more properly explained if we assume that sun is stationary and earth as well as other planets is revolving around it. This theory was called Heliocentric theory. Galileo later on supported this theory from his experimental study on moon and other planets.