Cosmologist David Spergel—Decoder of the Cosmos

Thursday, May 01, 2003
On June 30, 2001, NASA launched a probe to confront some of the biggest questions we can ask: How old is the universe? How will it end? Is it finite or infinite? (See "The Magnificent Mission" by Tim Folger in Discover,

May 2000.) The spacecraft — recently renamed the Wilkinson Microwave Anisotropy Probe in honor of astrophysicist David Wilkinson — is sifting for clues in the cosmic microwave background, a remnant glow of microwaves from the early universe. The first results spectacularly corroborate the prevailing ideas about cosmic origins, including the theory that the universe began with an episode of runaway growth. Contributing editor Tim Folger discussed the implications of these findings with David Spergel of Princeton University, a leading cosmologist and a member of the WMAP team.

What is the most significant aspect of the new cosmic measurements? 
The thing I'm most excited about is the precision. We now know the age of the universe — 13.7 billion years — to an accuracy of 1 percent. We know that ordinary matter accounts for only 4 percent of the mass of the universe. The rest consists of dark matter. It confirms many of the predictions we've been making.

At the beginning of your career, did you ever imagine it would be possible to answer questions like this?

Where do we go from here — have we answered all the big questions?
Well, there are a bunch of big questions. What is dark matter? What is dark energy, the unseen thing that seems to be driving the universe to speed up? Those are two really fundamental questions. Another very big one is understanding what caused inflation, the extremely rapid expansion that occurred in the universe's first moment of existence. With WMAP we've actually ruled out the simplest, most popular model of inflation. We've begun the process of starting to probe the complicated physics of the early universe. That's something that's just beginning. We've just gotten to the sensitivities where we can start constraining different cosmological models. Looking further ahead, Planck [a European Space Agency probe scheduled for launch in 2007] will be another big step forward in constraining inflationary parameters. Planck may detect gravity waves from the early universe in the microwave background.

Does WMAP's results also rule out alternatives to inflation, more far-out physics?
Yes, they do. We ruled out a whole broad class of theories. MAP really shows that the universe underwent this early period of acceleration like you get in the inflationary model.

So WMAP confirms most of inflation's predictions?
It's a triumph for inflation. I also think it's a real triumph for the work of people like Jim Peebles at Princeton, Rashid Sunyaev in Moscow, and Joe Silk at Berkeley. Soon after the cosmic microwave background was discovered, they basically predicted 30, 40 years in advance what we would see. That's really quite amazing.

Is there anything in the future that will match what we've learned about the universe in the past 30 years?
Inflation may turn out to be a very simple version of a deeper theory that, let's say, comes out of string theory. Something like that would be, I think, more profound than anything we've had up to now. It's true there are areas where physics has been too successful, where the big questions have been answered. But I think when we end up having a model in which 4 percent of the universe is atoms and 96 percent is something else, I think it's hard to claim that we know it all!

Will WMAP help us learn about the fate of the universe?
The preliminary results support the idea that hidden energy is spurring on cosmic expansion. Until we understand the nature of the dark energy, we don't really know what will happen to it in the future. But if we take the simplest model that fits our data, then the universe will accelerate forever.

Will the probe reveal if the universe is finite or infinite?
Well, there is a way we could tell if the universe is finite. We can't really tell if it's infinite. There's always the possibility that the universe is very, very large, but still finite. If the universe is finite and is smaller than or comparable to the size of the visible universe —13 billion light-years in radius — then we would actually see a younger version of our galaxy, because its light would have traveled all around a finite universe and back. One of the things we're actually looking for now in the WMAP data is the signature of the universe being finite.

What's your gut feeling? Do you suspect that the universe will be too big to detect whether it's finite or not?
Yes. That looks most likely. What we will be able to do with the data is put a limit on the size of the universe — the universe will have to be bigger than a certain minimum size.

Have WMAP's results been surprising in any other ways?
I think the surprise that is definitely there is the evidence for early star formation, just 100 million years after the Big Bang. Another result that we don't really understand is that we don't see any temperature fluctuations in the microwave background on scales larger than 60 degrees [the angular size in the sky of the fluctuations]. There seems to be a maximum size for hot and cold spots. We're left with this intriguing anomaly that we don't quite know what to do with. [Jeff Weeks, a mathematician who is working with Spergel to analyze WMAP's data, says, "The large-scale temperature correlations across the sky are much less than one would expect in an infinite universe but exactly what one would expect in a finite one."]

How does it feel to work on such a historic mission? 
I realize now that I'll never be on a project like this again. I'll do a lot of interesting things, I hope, but this one was really special. It was a very small team, which meant we all had a lot of responsibility. I've never worked harder than I did on this project. I was here until 2 in the morning, waking up at 7, every day for months. It was really intense, a small team working like that. And the results are really exciting. It was a very special experience. This will have a very big impact on the field — it will occupy cosmologists for the next 10 years. And this is only our first year of data. We have at least three more years of operation. I think a lot of the numbers will get more exact.

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