To understand it, it helps to go back to the 1992 COBE paper. Basically what that said and what we learned was that the amplitude of the fluctuations in the cosmic microwave background [CMB] was approximately what we call a scale-invariant spectrum. That means that if you consider any length scale, whether 100 megaparsecs or 1000 megaparsecs, the gravitational potential of the fluctuations are the same. The fluctuations have the same magnitude: on the order of 10e-4 to 10e-5 in the natural units of c².

It told us how much gravitational force there was moving material around. We know that structure has grown in the universe with time; dense regions get denser. Vacuous regions get more void-like. But the question then would be whether there’s enough force in the gravitational potential of these fluctuations to move the material in the universe to the point that you get the superclusters of galaxies that we see around us today. Is there enough gravitational force to do that?

“ ...what people have been able to accomplish using unmanned missions in space has been pretty spectacular.”

The answer we got from COBE was yes, there is enough. But it came with a caveat, and that was that we needed to have dark matter to do that. With dark matter, the gravitational potential fluctuations measured by COBE are just right to produce by gravity the large-scale structure in the universe.

There has always been, depending on how you look at it, kind of an excess of superclustering or a deficit of clustering. There is a discrepancy between the simple models of the universe and the ratio of clustering to superclustering. We could explain that in several different ways. One was standard cold dark matter, which got the superclustering right but was wrong on the clustering of galaxies.

That left three alternatives that got both the superclustering and the clustering right. The first was a universe with mixed hot and cold dark matter. The second was a universe with a vacuum dominated by dark energy. This is called the lambda (Cold Dark Matter [CDM]) model. Lambda is what Einstein denoted the cosmological constant, which is equivalent to the energy density of the vacuum. The final alternative was an open universe, without enough matter to reach critical density. As of 1992, any one of these three alternatives could have worked.

Over the next 11 years, many people built and deployed and got data from CMB experiments measuring the CMB anisotropy at smaller angular scales. This was driven by a very strong prediction made in the context of the standard CDM model: that you should see acoustic oscillations in the angular power spectrum of the CMB. There would be a big acoustic peak of angular scale at which there was an excess of temperature anisotropy. But that scale was too small for COBE to resolve.

So you had many experiments trying to measure the CMB at half-degree or one-degree scales. The predictions for each of the three models are different at that scale. The open model, where there’s not enough matter to close the universe, gives the excess at an angular scale of about half a degree. The hot and cold dark matter, mixed dark matter model, gives an angular scale of about .8 of a degree and the lambda-CDM model gives you about .9 of a degree. By about 1994, people had seen that there was this acoustic peak and had started to get some information about where it occurred. By 1998, we had a pretty good idea.

Around .8 of a degree. The measurements were good enough to exclude the open model, but not to decide between the mixed dark matter model and the lambda-CDM model. We still required better data to distinguish between those two models. In 1998, the supernova results came out, indicating that the expansion of the universe was accelerating. Of the two alternatives for the CMB, the only one with an accelerating expansion was the lambda-CDM model. So the supernova results basically said it’s got to be lambda-CDM.

WMAP was launched in 2001. In 2003, we released the analysis from the first year of data and we were able to pin down L to 220.1 plus or minus 0.8. This was really getting into precision cosmology. Basically with WMAP, we could completely rule out the idea that the universe is filled with matter. The mixed dark matter model just died, except for the few diehards who are always willing to propose extreme measures to save a theory. I thought it was too bad, because having this dark energy—lambda—is really odd. That’s why there’s so much excitement about measuring it. I wasn’t ready to believe in it, even though the supernova data favored it. Not until the WMAP data came out and came down firmly against a critical density of one did I start to believe it.

It was surprisingly consistent with what people expected. So there was nothing dramatically new. At that time, we found we could fit all the data: the Hubble constant data from the Hubble space telescope, the supernova data, the data on clustering and superclustering of galaxies, and the CMB data. And we could do this with a model that had only five parameters. The model was the lambda-CDM model, and it had the scale-invariant spectrum exactly, a baryon density, a dark matter density, a Hubble constant, and an optical depth since the time of recombination.

WMAP is still observing and will continue to observe for a few more years. It already has five years in. And we’ll continue to get a better signal-to-noise ratio. That will help us untangle a question of the polarization of this CMB, which in turn will tell us when the universe became transparent. At large angular scales, we’re trying to measure a one percent polarization on a signal that is basically a 30 microKelvin anisotropy. We have five different frequencies, so we can see if the polarization is the same at all frequencies. If it is, then it’s most likely a real CMB signal. So we’re working on that. And then after WMAP has taken about seven years of data, we expect to see a new satellite launched.

It’s being built by Europeans and it’s called Planck. It has the very desirable characteristic that it’s far more sensitive than WMAP. It also has undesirable characteristics that may affect its capacity to measure this large angular scale polarization, but it will be able to measure a lot of very important stuff. It won’t be able to keep going for years and years like WMAP, because it requires helium coolant and that will run out.

Planck will be launched probably in 2008; it will observe in 2009, and we’ll probably see the data coming out around 2011. Assuming Planck comes in with good data, we’ll have very well-measured CMB anisotropy, very well-measured polarization at smaller angular scales, and maybe better data at larger angular scales, as well.

I would have to vote for knowing precisely what the dark energy is, the vacuum energy, the cosmological constant. I’d like to know precisely what that energy density is, and whether it’s constant. That seems to be the highest priority question in cosmology at the moment. That’s why people are still very excited about measuring supernovae. But there are plenty of other things that would be interesting to know as well.

There are a few missions that people are now proposing to address these questions. One is to measure supernovae. That’s the Joint Dark Energy Mission. The idea is to figure out the acceleration of the universe and whether it’s constant or not. One is an x-ray telescope to measure the property of matter accreting on black holes. We can learn a lot about black holes that way. One is to measure gravitational waves. It would be able to measure hundreds of fairly ordinary binaries in the Milky Way, plus binary black holes coalescing in the distant universe.

And then there’s CMBpol, which is designed to measure a kind of polarization that cannot be produced by electron scattering. In other words, electron scattering in the early universe produces all the polarization in the CMB that’s been measured to date. CMBpol could measure a kind of twisty or screwy polarization, in the sense of not being the same in its mirror image. If we could measure that, as CMBpol aims to do, we can determine the energy density of the universe during inflation, back 10e-35 seconds after the Big Bang. That’s another exciting thing people want to measure.

I don’t know that I’m surprised. Certainly we have learned a lot and we’ve learned some very surprising things by employing new ways of looking at the universe. Certainly, what people have been able to accomplish using unmanned missions in space has been pretty spectacular. I would say that it’s been a very interesting time to have a career in astrophysics. We learned an awful lot of new stuff. It’s not at all what we were learning back when I was an undergraduate.