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Inflationary theory and the early universe

Inflationary theory and the early universe - from Roger Penrose's 'Road to Reality'

The concept of inflation in the early universe is an important theme in recent cosmology. The inflationary period is seen as a time during which a multiverse of different universes could have been spun off, with our universe being just one amongst many, or even an infinity of universes.

The observed 'fine tuning' of our universe creates impossible odds against our universe having arisen on its own and by chance from a single fluctuation in the vacuum. The idea of a multiverse overcomes this problem by allowing an infinity of unverses of which ours is only one. An inflationary period in the very early universe is suggested to have allowed the creation of a multiverse.

As so often, Penrose is out-of-line with fashionable thinking, and argues against inflation theory in his book, 'Road to Reality.' Much of the discussion here deals with the second law of thermodynamics and the increase in entropy. Penrose conceives of entropy as different sizes in phase space, which is conceived as containing six dimensions, three for position and three for momentum. The amount of entropy and therefore phase space gets smaller and smaller as we go further back towards the Big Bang. The source of the second law, by which entropy increases lies in a tiny volume of phase space at the Big Bang. The uniformity of the Big Bang corresponds to very low entropy. Penrose discusses the 'horizon problem', the fact that the observed temperature of the universe is nearly the same in all directions. This can be explained by the universe having been in thermal equilibrium. This in turn can be explained by an inflationary proceess very rapidly blowing the universe up from a small to a large size.

Penrose's criticism of this approach relates to the second law. If there was thermal equilibrium at the start of the inflationary phase, this would represent an increase in entropy prior to the inflationary phase, and therefore an even lower state of entropy at the beginning of the universe, which makes it even more unlikley that it could have arisen from a chance fluctuation in the vacuum.