EVOLUTION OF THE BIG BANG THEORY

It seems the idea of god was not liked by the people who have their chips working back in their heads. So, they started the quest for an answer to the question, how and when did this all start?

The reply to the question was started when Edwin Hubble made his landmark observation, that is wherever you look in the open sky, distant galaxies are moving away from us. It just means the universe is expanding. That is revolutionary because until then, people believed that the universe is static, infinite and existed forever, as there was no observable proof to state their beliefs wrong.

Hubble’s observations made all realize that at earlier time they would have been closer together. In fact, it seemed that there was a time about ten or twenty million years ago they were all at exactly the same place.

This discovery finally brought the question of the beginning of the universe into the realm of science. Hubble’s observations suggested that there was a time called the Big Bang. But we can’t just directly rely on that idea without any evidence.

On the other side, Friedmann made two simple assumptions about the universe that the universe looks identical in whichever direction we look, and that this would also be true if we were observing the universe from anywhere else. On the basis of general theory of relativity and his two assumptions, he predicted the Edwin Hubble’s discovery years ago.

Basing on the Friedmann’s assumptions, three different kinds of models of Universe are found. In the first kind, the universe is expanding so sufficiently slowly that the gravitational attraction between the different galaxies causes the expansion to slow down and eventually to stop. Second, the universe is expanding so rapidly that the gravitational attraction can never stop it, though it does slow down a bit. The third is, the speed at which universe is expanding is only fast enough to avoid recollapse.

In the meantime, Friedmann’s assumptions are proved to be nearly true with some corrections.

But which Friedmann model describes our universe? Will the universe eventually stop expanding and start contracting, or will it expand forever?

After all the necessary calculations, present evidence suggests that the universe will probably expand forever. But we can’t bank on it. All we can really be sure is if it’s going to recollapse, it won’t do so for at least another ten thousand million years since it has already been expanding for at least that long.

However, the point to be concentrated is, all models have the feature that at some time in the past, between ten and twenty million years ago, the distance between neighbouring galaxies must have been zero. At that time, which we call the big bang, the density of the universe and the curvature of space-time would have been infinite.

But the question that disturbed to come to a conclusion is, does the general theory of relativity predict that our universe should have had a big bang, a beginning of time? Since, our conclusions were drawn, treating the general theory of relativity to be true.

The answer to this came out of a completely different approach started by a British physicist, Roger Penrose, in 1965. He showed that a star that collapses under its own gravity is trapped in a region whose boundary eventually shrinks to zero size. Which means that all the matter in star compresses into a region of zero volume (singularity), so the density of matter and curvature of space-time would be infinite. This singularity is also called as Blackhole. From this theorem, Stephen Hawking realised, if one reversed the direction of time in Penrose’s theorem so that the collapse became an expansion, the conditions of his theorem would still hold, provided the universe was roughly like Friedmann model on large scale at present time.

Penrose theorem had shown that any collapsing star must end in singularity; the time-reversed argument by Stephen Hawking shows that any Friedmann-like expanding universe must have begun with a singularity.

Finally, a joint paper by Penrose and Hawking in 1970, proved there must have been a big bang singularity provided only that general theory of relativity is correct and universe contains as much matter as we observe.

So, it is now generally accepted that the universe must have a beginning.

References: Stephen Hawking's The Theory of Everything and some TED speeches…