University of Chicago
As recently as the 1920s, astronomers thought the Milky Way galaxy was the entire universe. Now we know it's one of hundreds of billions of similar galaxies out there. Last fall Sean Carroll and graduate student Jennifer Chen published an article in the online journal High Energy Physics Theory (arxiv.org/archive/hep-th) arguing that our universe is just one of many, each one seeded by a random fluctuation of energy in the reaches of space.
Harold Henderson: A theory as big as yours should explain just about everything, right?
Sean Carroll: We started out trying to explain why our universe is so improbably well organized, why it has such low entropy.
HH: What's low entropy?
SC: Low entropy is an egg, high entropy is an omelet. You can turn an egg into an omelet, but not an omelet into an egg.
HH: OK so far. Entropy is a synonym for disorder.
SC: Left to their own devices, things tend to evolve toward increasing disorder. If you shuffle a deck of cards it's very unlikely that they will end up precisely in numerical order. That's just because there are many more ways to be disordered than to be ordered. Similarly, our universe is evolving from a very smooth state at early times to a messier, lumpy state at late times, as clouds of gas contract and form stars and galaxies. Our entropy is low but growing.
HH: So if our universe has low entropy now, in all probability it must have been even lower in the past.
SC: And that's a puzzle. If the starting conditions of the universe are randomly chosen, high-entropy states are by far the most likely to occur. How did we ever get such a low-entropy starting point?
HH: What would a high-entropy universe look like? Are we moving in that direction?
SC: Yes, the entropy is increasing as matter continues to clump together. Sir Roger Penrose has suggested that the universe will ultimately end up as a collection of black holes. But eventually those black holes will evaporate, as Stephen Hawking has pointed out. So we believe that the ultimate end state is nearly empty space, with everything very spread out. The question, then, is why we observe any matter in the universe at all, much less an extremely dense big bang.
HH: No wonder you need such an elaborate theory. This sounds like the idea that the universe would end up in "heat death," approaching absolute zero with nothing moving and nothing ever happening.
SC: But cosmologists have discovered that the universe is not only expanding, it's accelerating its expansion. The best explanation of that so far is that even empty space contains a tiny amount of vacuum energy, or some sort of dark energy. It's still fluctuating after everything else has quit. And it will keep the average temperature of the universe from ever quite reaching absolute zero.
HH: So your ultra-long-range weather forecast is very cold.
SC: Bring your mittens.
HH: But not quite cold enough for the universe to seize up in a static heat death. I still don't see how you're going to get a big bang or another universe out of this situation.
SC: Quantum mechanics tells us that if you jiggle a little bit, at some point you'll jiggle a lot. If you wait long enough, in some region all these jiggling fields will happen to come together and bounce up far enough to start a process called inflation.
HH: That sounds promising.
SC: Inflation is not at all a new idea. Alan Guth came up with it in 1981 as a theory to explain how our observed universe is as smooth as it is. It might even be a substitute for the idea of the big bang. It's a situation in which a relatively small part of space-time and quantum energy expands by a factor of 10 to the 30th power in about 10 to the minus 30th of a second, creating a fantastic number of particles from a microscopically small initial patch of space-time.
HH: Isn't this all a bit like us waiting around to see whether random quantum fluctuations might construct Rockefeller Chapel?
SC: Not quite. You don't have to produce a whole universe all at once. We think it's easier for random quantum fluctuations to produce the start of inflation than it is for them to produce anything else, including Rockefeller Chapel. A crucial and still debatable step is that we think it's more likely to fluctuate into an inflating universe than anything else.
HH: But not all that likely.
SC: We calculated the probability and got the smallest number in the history of physics: 10 to the minus 10-to-the-10-to-the-56th power.
HH: Is there any experiment that might show your theory to be more or less plausible?
SC: I'd love it if there were. Inflation has observable consequences, and we think this theory makes inflation make more sense. In particular, now it makes sense both forward and backward. From any imaginable starting point a universe naturally drifts toward high entropy--empty space with a little quantum energy fluctuating around. Eventually those fluctuations in one place produce an inflation and another universe.
HH: What do you call the collection of all these universes?
SC: We use the word universe both for our own 14-billion-year-old universe and for the complete collection of all of them.
HH: Aren't you missing a chance here? You could call the whole thing a multiverse--or a carroll.
SC: Actually I'm waiting for somebody else to do that.
Art accompanying story in printed newspaper (not available in this archive): Photo/Lloyd DeGrane.