Pre-Big Bang models, cannot be extended into the infinite past if they are taken to be realistic descriptions of the universe 1
The Big Bang theory seems to suggest it did, but in recent decades, cosmologists have concocted elaborate theories – for example, an eternally inflating universe or a cyclic universe – which claim to avoid the need for a beginning of the cosmos. Now it appears that the universe really had a beginning after all, even if it wasn’t necessarily the Big Bang.
Disorder increases with time. So following each cycle, the universe must get more and more disordered. But if there has already been an infinite number of cycles, the universe we inhabit now should be in a state of maximum disorder. Such a universe would be uniformly lukewarm and featureless, and definitely lacking such complicated beings as stars, planets and physicists – nothing like the one we see around us.
One way around that is to propose that the universe just gets bigger with every cycle. Then the amount of disorder per volume doesn’t increase, so needn’t reach the maximum. But Vilenkin found that this scenario falls prey to the same mathematical argument as eternal inflation: if your universe keeps getting bigger, it must have started somewhere.