Often it becomes necessary in science that we have to make lots of assumptions otherwise we can't develop theories. But the problem is that often scientists make assumptions even if not really needed. This is what happened with big bang also. I remember having argued with many people if big bang was the beginning of time. My position was that big bang, if it happened, was just an event in universe and it could not be the very beginning. Definitely it was not the beginning of time. Most of Physics books that talk of big bang claim that big bang was the beginning of time itself. Now the question is, "Why did scientists believe that big bang was the beginning of time? Are there any experimental results that indicate this?". The answer is that there is absolutely no experimental result that shows that big bang is the beginning of time. In fact, big bang was a point of singularity. According to Einstein's general theory of relativity, all the currently known laws of Physics break down at singularity. So, no laws of Physics could tell what happened before big bang. They could not even tell if there was any 'before'. Then Physicists decided just to consider big bang as the birth of the universe. So far it was OK. But some went on to the extent of saying that there was absolutely nothing before big bang. Big bang was the beginning of time itself and therefore it is meaningless to ask what was before big bang because there was no 'before'. Many other physicists adopted this idea. This is very bad tendency. If we do not have anyway to know the answer to something, it is far better to accept that we do not know rather than making some assumption and thinking that assumption to be perfect and building theories after theories on that assumption.

This new model is very recent. So, scientists are still not sure if big bang should be considered as the beginning of everything including time. My knowledge of science is very little. I do not know what more discoveries will tell, but my personal feelings are that universe existed even before big bang and that big bangs have been happening and will keep on happening.