Galaxies are the building blocks of the universe. An astronomer called Edwin Hubble observed in 1920 from the Mount Wilson observatory in California (USA), that galaxies are not stationary in the universe. All the galaxies are travelling away from one another at a great speed. The speed with which the galaxies run away from one another is directly proportional to the distance between them. Larger the distance between two galaxies, the faster they run away (or recede) from one another. The farthest galaxies observed are travelling at more than half the speed of light. This means that our universe is expanding !
Image Source: spiff.rit.edu/classes/phys230/lectures/local_group/m31_gendler_big.jpg
A simple way to understand the expanding universe is to imagine the millions of galaxies as dots painted on a balloon. When such a balloon is blown up, its surface expands, and the dots become farther apart, as if they were running away from one another. The galaxies are moving away from one another like dots painted on the surface of an expanding balloon. Since the galaxies are racing away from one another at present, so many astronomers believe that they must have originally started from a single point in space. The evidence for the hypothesis that the universe began in a big-bang (big explosion) is given below.
It is now known that the various galaxies are racing away from one another. Now, to find out how these galaxies were formed in the first place, let us reverse the motion of these galaxies so that they start coming near one another. It has been found that if the present positions of galaxies are brought back to about 15 billion years in the past, then all the galaxies would shrink to a single point, that is, the whole universe would shrink to a single point, called primeval atom.
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So, most of the astronomers believe that at some time in the past, some event like the big-bang (big explosion) of the primeval atom must have taken place. And this explosion of the primeval atom scattered the matter all around forming the universe consisting of galaxies of stars which started expanding and moving away from one another since that time. The astronomers place the time of formation of the universe (or time of birth of this universe) at around 15 billion years back (15 x 109 years back). We shall now discuss the big-bang theory of the origin of this universe in somewhat detail.
The Big-Bang Theory. One of the most important theories about the beginning of this universe is the big-bang theory. The big-bang theory explains the formation of heavenly bodies like galaxies, stars, and planets, etc., as follows : According to the big-bang theory, all the matter in the universe was originally concentrated in one vast lump called primeval atom which was 100 million light years wide. About 15 billion years ago, this superdense and extremely hot primeval atom (which was a concentrate of neutrons and protons) exploded with a big-bang, and the matter started flying in all directions through space leading to the formation of galaxies of stars and other heavenly bodies.
The big-bang theory suggests that the universe has a definite age and a definite size. The astronomers have estimated that the universe was formed (or born) about 15 billion years ago. Now, compared to the age of this universe, the formation of earth and the origin of life on earth have occurred comparatively much more recently. An idea of the gap between the formation of universe, the formation of earth and the evolution of life on earth can be had from the following example.
Let us choose a scale and consider the age of universe which is 15 billion years, equal to one earth year.Now, if we assume that the universe was formed on 1st of January in a big-bang, then the solar system (including earth) would have been formed in the beginning of tenth month, October, and the human life would have evolved on earth as late as 11.30PM on the last day of the year, 31st of December !