Evolution of Universe

The three main theories put forward to explain the origin and evolution of the universe are:
The Big Bang Theory
The Steady State Theory
The Pulsating Theory

1. 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 (~10 12K) 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 way 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 billing 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.

2. 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 it, the overall size of mass of the observable universe remains constant. Thus a steady state of the universe is not disturbed at all.

3. 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 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.