For most of us, "inflation" is a term that comes up only in conversations about the economy or flat tires. But for many cosmologists, inflation is the ultimate word in understanding how the universe was created.
In the beginning, according to inflation theory, the universe was tinier than an atom. Then, in an unimaginably brief period of time -- .00000000000000000000000000000001 second, to be precise -- it expanded ("inflated") to a size trillions of times bigger than what we can observe today.
A handful of scientists in the United States and the former Soviet Union independently came up with the idea of inflationary cosmology in the late '70s and early '80s -- among them, Russian cosmologist Andrei Linde, now a professor of physics at Stanford; former SLAC researcher Alan Guth, now at the Massachusetts Institute of Technology; Paul Steinhardt, now at Princeton University; and Andreas Albrecht, now at the University of California-Davis.
When first proposed more than two decades ago, the inflation hypothesis was considered somewhat exotic, but subsequent experiments have provided a wealth of data verifying many of its underlying principles.
In February, scientists announced the results of the latest cosmological experiment -- a NASA satellite called the Wilkinson Microwave Anisotropy Probe (WMAP). Launched in 2001, WMAP captures light waves that were produced during the Big Bang some 13.7 billion years ago. NASA scientists used the light wave data to create a detailed map of the universe that confirms many aspects of inflationary theory while ruling out others.
The WMAP experiment was a featured topic of discussion during a conference on cosmic inflation at UC-Davis in March -- a three-day event attended by many leading cosmologists including Albrecht, Guth, Linde, Steinhardt and Stephen Hawking of Cambridge University. Researchers presented new findings on a wide range of hotly debated cosmological issues, including string theory -- the idea that the universe was built from infinitesimally small string-like particles; multiverses -- Linde's hypothesis that our universe is but one of many interconnected universes that inflate and contract like so many soap bubbles; and the cyclic model -- Steinhardt's competing proposal, which states that there is one universe that undergoes an endless sequence of cosmic epochs that always begin with a "bang" and end with a "crunch."
While conference participants clashed over specific theories, they were unanimous in their praise for the WMAP experiment and what physicists call the new era of precision cosmology, in which speculation about the nature of the universe is confirmed or rejected by solid experimental data.
During a break at the conference, Linde discussed some of the latest trends in cosmology with Stanford Report science writer Mark Shwartz.
Q: How did the results of the WMAP experiment affect cosmic inflation theory?
A: WMAP makes a big leap in confirming many of the predictions of inflationary cosmology, and this places the theory on much firmer ground that it was before.
When inflationary theory was first proposed about 20 years ago, nobody really expected that in our lifetime we would have any serious tools developed to verify whether we were right or wrong. Of course, we knew all the way that we were right, because the theory was beautiful! But it's one thing to have moral assurance that you're doing a decent thing, and another thing to see that your predictions are confirmed experimentally.
At today's conference there was a competing scenario -- I would not really call it a theory -- the cyclic model of the universe. This is a very exotic model, although it has some chance to work.
Q: Is the idea that the universe recycles over and over?
A: Yes, but the idea that the universe may go in cycles is actually a very old idea. There are many people who have studied an oscillating model of the universe. The problem is that all of them have failed to describe what happens at the moment that the universe collapses. How exactly does it start expanding again?
Many statements made by the authors of the cyclic scenario during the last two years were quite controversial, and some of them were simply incorrect. Also, they do postulate the stage of inflation before the collapse of the universe. Therefore, from my perspective, the cyclic model is just an extremely complicated and, I would say, baroque version of inflationary cosmology.
Q: It seems like string theory also took a few hits at this conference from Stephen Hawking and others.
A: Well, since 1985, string theorists were telling us, "We really know how to do things. We're going to explain our world soon." This may be right -- string theory is very powerful; but it is also immensely complicated -- and it changes every two years.
One of the main challenges to string theory has been cosmology. If string theory is unable to explain the present acceleration of the universe, it will be too bad. If string theory is unable to explain inflation, it will be too bad -- unless we find some alternative mechanism explaining where the galaxies came from and why our universe is so large and homogenous. So string theory has come under scrutiny and attack by some people.
However, I do not think that there is any reason to be pessimistic with respect to string theory and its relation to cosmology. Just a month ago, physicists Shamit Kachru, Renata Kallosh [both of Stanford], Sandip Trivedi [of India] and I found a way to describe the present acceleration of the universe in string theory. The next challenge is to find a good mechanism describing inflation in string theory. This is a very complicated problem, but I think that it is solvable. In fact, we are working on it now.
Q: Did WMAP throw out some of your own theories or cause you to change them in any way?
A: If an experiment is good, then it not only confirms something but it also rules out something. WMAP tends to rule out one of my lovely theories -- I have many! However, a much simpler version of the same theory is quite fine and doing quite well.
So one thing WMAP is capable of is ruling out some theories, leaving alive some others -- and that's wonderful, because in this way, we can really sort out many different versions of the same theory and pick out one that agrees with the observational data better.
But so far, if you ask me, I do not really know of a class of theory that I would consider as a decent competitor of inflation at the moment.
Stanford Report, April 9, 2003