There are three main theories :-
1. The Steady State Theory
2. The Big Bang Theory
3. The Pulsating Theory
The Steady Sate theory: It states that the counting of the galaxies in our Universe is constant and new galaxies which are forming continuously are filling the empty spaces which are created by those heavenly bodies which have crossed the boundary lines of observable Universe.
The Pulsating Theory: In this theory it is assumed that there is continuous expansion and contraction in universe. A word which can replace the above definition is ‘Pulsating’.
Pulsating theory states that it is the possibility that after some passage of time the expansion in the universe may stop. Then their may be the possibility of contraction. When this contraction will approaches to a particular size. Again the explosion will take place. As a result of this explosion the expansion of universe will start again.
Hence it results in a pulsating universe in which there is alternate expansion and contraction of universe.
The Big Bang Theory: It states that all the matter of universe was present at a single place in the form of hot and dense fire ball, having a high temperature of nearly 1012K. After the passage of nearly 20 billion years an enormous explosion took place. All the matter which was concentrated at one place scattered into space with rapid speed. This scattering was along all directions. Then this scattered matter took the shapes of galaxies and stars.
According to Hubble’s, the velocity of light and the recession velocities of galaxies become similar at a distance of 20 billion light years. So, it is sure that these heavenly bodies which are present at a distance of more than 20 billion light years can never arrive near us. From the above discussion an observable boundary of the universe is drawn.
If the recession will remain in continuity then large number of heavenly bodies will move away from us. Hence as a result they will be lost one day. So, number of stars and galaxies will reduce continuously and a day will come when we will be surprised to see our empty universe.