3 Did the universe have a beginning?

At this point, it seems that the answer to this question is probably yes.2 Here we
have addressed three scenarios which seemed to offer a way to avoid a beginning,
and have found that none of them can actually be eternal in the past. Both
eternal inflation and cyclic universe scenarios have H_av > 0, which means that
they must be past-geodesically incomplete. We have also examined a simple
emergent universe model, and concluded that it cannot escape quantum collapse.
Even considering more general emergent universe models, there do not seem to
be any matter sources that admit solutions that are immune to collapse.

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He goes on to discuss various thermodynamic arguments that suggest the universe cannot have existed for ever. The bottom line is that the inevitable increase of entropy over time ensures that a past eternal universe ought to have long since lost any semblance of order. Since we can see order all around us, the universe cannot be eternal in the past.

He finishes with this: "We may conclude that there is a beginning, but in any kind of inflating cosmology the odds strongly (infinitely) favor the beginning to be so far in the past that it is effectively at minus infinity."

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His argument is equivalent to saying that the cosmos must have had a beginning even if it looks eternal in the past, which is rather similar to Mithani and Vilenkin's view. The distinction that Susskind does make is that his focus is purely on the practical implications of this--although what he means by 'practical' isn't clear.

That the universe did or did not have a beginning is profoundly important from a philosophical point of view, so much so that a definitive answer may well have practical implications for humanity.

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