In a recent article in Scientific American, "Does the arrow of time run backward in other universes?" (May 21, 2008), Sean M. Carroll proposed that the multiverse may explain why time
only runs forward, never backward, a puzzle that clearly irritates him and other cosmologists:
The universe does not look right. That may seem like a strange thing to say, given that cosmologists have very little standard for comparison. How do we know what the universe is supposed to look like? Nevertheless, over the years we have developed a strong intuition for what counts as "natural"-and the universe we see does not qualify.
So ...
If the observable universe were all that existed, it would be nearly impossible to account for the arrow of time in a natural way. But if the universe around us is a tiny piece of a much larger picture, new possibilities present themselves. We can conceive of our bit of universe as just one piece of the puzzle, part of the tendency of the larger system to increase its entropy without limit in the very far past and the very far future. To paraphrase physicist Edward Tryon, the big bang is easier to understand if it is not the beginning of everything but just one of those things that happens from time to time.
One does not need to be a cosmologist to sense the difficulty with this form of reasoning. If we have only one example of a popkin, without any basis for comparison to another popkin - that is known to actually exist - it is difficult to justify the intuition that the popkin doesn't "look right."
If we inflate our intuition that the popkin doesn't look right into an argument for the existence of Popkin World - most of whose popkins look different - it is a tribute to our imagination. But only evidence would make such a proposition real. And then we must be careful that we are not seeing only what we believe.
Carroll identifies some problems with the concept of an inflationary early universe:
The inflationary paradigm has been very successful in many ways. Its predictions of slight deviations from perfect uniformity agree with observations of density variations in the universe. As an explanation for time asymmetry, however, cosmologists increasingly consider it a bit of a cheat, for reasons that Roger Penrose of the University of Oxford and others have emphasized. For the process to work as desired, the ultradense dark energy had to begin in a very specific configuration. In fact, its entropy had to be fantastically smaller than the entropy of the hot, dense gas into which it decayed. That implies inflation has not really solved anything: it “explains” a state of unusually low entropy (a hot, dense, uniform gas) by invoking a prior state of even lower entropy (a smooth patch of space dominated by ultradense dark energy). It simply pushes the puzzle back a step: Why did inflation ever happen?
So the problem commonly called fine-tuning (here called "a very specific configuration") remains.
Note: A popkin? Well, if you know what it is, you are doing better than me.
I tried to explain the concept of time running backwards to some kids a while back, via a jingle,
here.
