For a better understanding, the whole of the universe can be said to be made of certain smaller units of different sizes and shapes. The largest types of these universal units are called star Clusters. In fact, there are such millions and millions of star Clusters in the universe. And each of these clusters contains a group of galaxies, the second largest units of the universe. Again, these galaxies comprise a number of star-systems, like that of the Sun. Our Sun is only one star of such a galaxy, called Milky Way. And the Milky Way consists of about 100,000 million stars, along with clouds of dust and gas. So imagine the vastness of our universe compared to our small planet. Or, just the other way round.
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The divine discipline:
To what else can we compare this well ordered vastness?  It is fascinating to note that despite such huge vastness, there is an inherent discipline and orderliness. It is that orderliness that has bound everything inside it in their respective place. It is like a strange endless elastic super-string. Thus each of the insiders can move on freely in their path. But wherever they go the super string keeps them bound by the rule of the universe. This super-string like binder, that embraces all bodies in the universe, is called gravitation.

In fact, all these stars and planets, which appear fixed up against the night sky, follow certain course of motions. Yet, they are not displaced or derailed from their course due to this gravitation. Like the planets in the solar system, the Sun itself is in motion, Along with it carries the whole system. But over a much longer period. And so does all the star-systems like that of the Sun.

And, the galaxies made up of these star systems, are also in motion. Many rotate about their own axis, while the stars inside them move back and forth. Thus, rotate or not, each galaxy is also moving along as a whole. Even so, the distances between galaxies take a 1000 million years to move past each other. 

Besides, these individual motions, the distant galaxies have a rapid motions, the distant galaxies have a rapid motion in common. They all seem to be moving away from us. The farther they are, the faster they appear to move away from us. So the distant ones move faster than those nearer our own galaxy. These movement farther away points to the concept or the universe being inflated like a balloon. And it is called the expansion of the universe.

When the universe was first conceived of as an orderly unit, it was called cosmos. Just as opposed to chaos. The movement of the star against galaxy affects its light as seen by an observer. top

The expansion and the Big Bang:

Well, is there an end to this kind of expansion? 
This is where the theory of Big Bang came in. The idea holds that the universe began in an immense explosion of space-time. The legacy of that bang is the expanding universe, in which everything races away from everything else like raisins in a rising tea-cake.

Abbe Georges Lemaitre, a Belgian astronomer-priest, explained this process of expansion in the 'big bang' theory. He argued that billions of years ago, cosmic matter, or, the universe, was in an extremely compressed state. With all its energy and mass compressed into, what is called, a super dense ball. 

Then a kind of huge explosion took place. This explosion, called the 'Big Bang' because of its hugeness, spark-plugged the process of expansion. This explosion broke up the super dense ball and cast its fragments far out into the space. There they are still traveling at thousands of kilometers per second. It is from these speeding fragments of matter that our galaxies  have been formed.  The formation of galaxies and stars has not halted the speed of expansion. And, as it happens in all explosions, the farthest pieces are flying the fastest.

The explosion is of a primordial type. Because it was the first of its kind known to the universe. And this primordial explosion is the hallmark of the big bang theory. And it is regarded as one of the most viable theories that explain the birth and life of the universe. Based on this line of thought, scientists now believe that the universe not only expands. But it contracts as well. This expansion and contraction take place alternately between periods running into tens of billions of years.  top

The next bang and after:
Some scientists believe that it was about 12 billion years ago when the great explosion occurred in the universe. And that the universe has been expanding ever since. It is likely to expand for 29 billion years more. Following this the gravitation will halt further expansion. From then on all matters will begin to contract or collapse upon itself in a process known as 'implosion'. This will go on for some 41 billion years compressing matters into an extremely super dense state and then it will explode once again.

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