The Evolution of Universe

     The universe is commonly defined as the totality of everything that exists, including all space, time, matter, energy, planets, stars, galaxies, intergalactic space, and beyond. Definitions and usage vary and similar terms include the cosmos, the world and nature. Scientific observation of earlier stages in the development of the universe, which can be seen at great distances, suggests that the universe has been governed by the same physical laws and constants throughout most of its extent and history. There are various theories, in which physicists have suggested the universe is one among many universes that likewise exist.

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: 

     The Big Bang Theory is the most accepted theory for the origin and evolution of our universe.

     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 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

     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. A steady state of the universe is not disturbed at all.

  3. Pulsating Theory

     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.