The Spergel et al. paper ("First-year Wilkinson Microwave Anisotropy Probe [WMAP] observations: Determination of cosmological parameters," Astrophys. J. Suppl. Ser. 148[1]: 175-194, September 2003), with the most citations, is the end product of using the data from the WMAP mission to determine the fundamental parameters describing our universe—its history, content, and shape. There’s a lot of material there. In fact, this was a turning point after decades of cosmic microwave background work, following its prediction in the late 1940s, its detection by Penzias and Wilson in 1965, and the discovery of anisotropy by COBE (Cosmic Background Explorer) in 1992.

“WMAP left no doubt that dark energy exists.”

After COBE, there was a realization by many scientists who studied the cosmic microwave background (CMB) that smaller-scale measurements would produce a wealth of information about our universe. Cosmologists understood well the physics at play on these smaller scales. If we experimenters could measure temperature patterns on smaller scales, then we could determine parameters that describe our universe. That’s what WMAP did with unprecedented accuracy and precision, and the Spergel paper reports the parameter results. It derives, based on the WMAP data—often combined with other datasets—quantities such as the expansion rate of the universe, the age of the universe, the flatness of the universe, and the percent of the mass-energy of the universe that is in baryons (or atoms) verses the percent in cold dark matter versus dark energy.

WMAP left no doubt that dark energy exists. That was a major result. Science magazine declared the 2003 WMAP results the "breakthrough of the year." Charles Seife wrote, "Lingering doubts about the existence of dark energy and the composition of the universe dissolved when the WMAP satellite took the most detailed picture ever of the cosmic microwave background (CMB)." (Science 302[5653]: 2038-39, 19 December 2003). WMAP data were also only consistent with a substantial amount of nonbaryonic cold dark matter. WMAP also put inflation theory to a rigorous test. So WMAP has been important in many different ways.

Honestly, I don’t expect any to be overturned. I would caution, however, that to obtain cosmic parameters from the WMAP data means we’re fitting a model to the data. The model we describe fits perfectly well, but that does not mean the model is necessarily correct—there might be another model that proves to be better. We just don’t know what that might be, but it is possible we have an incomplete model.

I mean the ingredients that make up our understanding of the universe: its content and its history. If our cosmological model is fundamentally missing something, then in the future some other model will come along and augment or replace it. One of the nice things about the CMB is that it’s a pretty pristine way of measuring the early universe. We’re lucky to have it. I think it’s very impressive that one can start with fundamental laws of physics, measure the CMB, and find that it matches a simple model so closely.

But there is much we don’t know. We don’t know what the dark matter is, or what is driving the accelerating expansion. But WMAP is more than cosmic parameters. It’s the maps themselves—the data—that is primary. They’re the measurement of what our sky looks like. The quality of the maps is extremely high.

The Bennett et al. paper is the presentation of the maps—the basic data. WMAP, as I alluded to, has lot of aspects to it. When we went to write up the data descriptions, there was just an enormous amount of material. We divided the job up—hence the large number of papers. Bennett et al. was meant as an overview of all the papers and also as an introduction to the maps themselves.

There are two Bennett et al. papers. Another one, not quite as highly cited, describes how we distinguish the cosmic microwave background from foreground emissions (Bennett CL, et al., "First-year Wilkinson Microwave Anisotropy Probe [WMAP] observations: foreground emission," Astrophys. J. Suppl. Ser. 148[1]: 97-117, September 2003). It’s an essential step. There are also papers on statistical methods. They’re all important aspects of laying out the data, and how we go about understanding it. In total, the 2003 WMAP papers cover 241 journal pages, an enormous amount of material. Bennett et al. was the overview.

The thing that always comes to my mind is how amazing it is that scientists generated detailed physical models, and we then went out and measured the sky with precision and accuracy with WMAP, and that the basic model fit the data. It wasn’t obvious that this would happen. A lot of people were betting it wouldn’t, that the universe would turn out to be more complicated.

The fact that the physics was simple enough to calculate, and that the calculations fit the measurements, told us we really do understand a lot about the universe. Our basic picture of the hot big bang, the expanding universe, the accelerating expansion, components of baryons, cold dark matter, dark energy—all of that—seems to be needed. We would get different microwave patterns across the sky if the universe had different ingredients or different physics. I’m struck that the maps are in such close agreement with these prior theoretical predictions—with a relatively simple universe.

Observationally, we got it right by definition, because we we’re just reporting how the sky actually appears. That the theory matches the data is the surprising thing. The fact that a lot of the members of WMAP were part of the COBE team meant many of us have been in the thick of things together for a long time. We learned from the COBE experience. The fact that NASA, right at that time, decided to start a better, faster, cheaper program and was willing to do a follow-up to COBE was very fortunate for us. That we could turn around and so quickly go from COBE to proposing a new mission, to getting it approved and funded, was very lucky. There was a program ideally matched to what we wanted to do and we showed up at just the right time. I feel fortunate, very fortunate, to have had the opportunity to work on two important space missions.

There are several things. The basic cosmological model is pretty sound, but there are parts we don’t understand. One is the question of what happened at the very beginning of the universe. The earliest part about inflation is not as sound. It looks like a good idea. Its predictions look good. But it’s not trusted to the degree that other things are. Whether it’s the right idea or not, we don’t really know. And even if it is right, inflation is really the name for a very broad range of theories. We don’t have any idea which of those is the right one. So trying to get clues about whether inflation happened and, if so, which version is the right one is a fascinating problem. I have been involved with studying another, future CMB mission to try to get these measurements.

I also think the dark energy is fascinating. It was not at all predicted. When the supernova results came out, my impression was that most people didn’t believe it. Part of not believing it is that it seemed so weird, so unnatural. Now that, with WMAP data, we must accept its existence, it forces us to deal with the fact that we have no really good explanation for what’s going on. I’m also involved in studies of another future space mission to investigate the properties of dark energy—to try to figure out what it is. This is a fundamental question, and it potentially goes right to the heart of the laws of physics. We know that quantum mechanics and general relativity, our theory of gravity, are in some ways incompatible. Here we have something—dark energy—that may fall right at the nexus of those two theories. So it could be important for understanding not just cosmology, but understanding the laws of physics too.

Different people have different ideas. The original supernova technique is one way to do it, but it has a number of disadvantages. The one I like best, the one we’re focusing on for our current efforts, parallels what WMAP does, by measuring the tiny fluctuations initiated in the early universe that WMAP measured. They’re called, "baryon acoustic oscillations," and they’re basically sound waves in the early universe. They have a particular known scale size, which provides us a "standard ruler." A supernova is often referred to as a "standard candle" (although, in truth, they vary in brightness). The baryon acoustic oscillations are the standard ruler. Nature has provided us with a standard ruler against which we can measure how the universe evolved, right through the distribution of galaxies.

One measurable property of dark energy is whether it changes with time. Is it evolving or is it constant? One possibility is that it is the cosmological constant, which is the same all the time, but is so weak that for most of the development of the universe, it doesn’t dominate. It doesn’t matter, in effect. But as the universe expands, it cools, and the energy level drops and it reaches a point where now the cosmological constant does matter. Another possibility is that the dark energy is something dynamic; that it has actually grown in importance. Or the dark energy could be indicating a flaw in general relativity on very large size scales.

So what we want to do is measure what happened in the past. By having a standard ruler—these baryon acoustic oscillations—we can see how the universe developed step by step across time, and whether that development is consistent with a constant dark energy or a changing dark energy. Basically we’d be measuring the expansion rate of the universe as function of time.

Inflation makes several predictions and several of what we can call post-dictions. When it was first introduced, it mostly provided post-dictions. It provided natural explanations for some of the problems with the big bang theory: why there were no monopoles, why the cosmic microwave background has such equal temperature all over the sky, and why the shape of space has so little curvature.

But it also made some predictions. One thing we know about inflation is that at some point it ended. It stops in the early universe. To stop it had to slow down, and this slowing down of the expansion manifests itself in the amplitude of the CMB fluctuations at different scale sizes. Before we had the theory of inflation, scientists thought that the CMB fluctuations should naturally be independent of angular scale size. This is called a Harrison-Zel’dovich spectrum. Since inflation has to slow down, though, it creates a slight imbalance, so different scale sizes have slightly different amplitudes. The latest WMAP 2006 data prefer such an inflation model.

There is one big remaining prediction of inflation has not been tested, and not because of lack of effort. Inflation produces gravitational waves in the early universe. These gravitational waves cause a certain pattern of polarization in the CMB. If we could detect that pattern of polarization, that would be considered a smoking gun for inflation. It’s hard to imagine what else could generate such a thing, if not inflation itself.

A lot of scientists are looking for this specific polarization pattern that inflation predicts. WMAP measured the microwave polarization all across the sky for the first time, and it put some limits on that pattern. Continued WMAP observations and the upcoming European Planck mission hope to press that boundary. Several groups are now working on ground-based experiments and a future space mission specifically designed to detect this polarization, if it exists. Just getting upper limits on the measurement of this pattern will help us sort out which inflation models could or could not have happened. So even the limits of this polarization measurement are useful for sorting between models. Detection would be smoking gun evidence for inflation.

Two things come to mind. One is that I get many questions from the general public—and one thing they ask is how we can possibly know of all of these things about the universe. People say, "How can you possibly know that the universe is 4.4 percent baryons or that its age is 13.7 billion years? You must be making this up." I think that cosmology is naturally thought of as the result of armchair speculation. It used to be, but now it is not. It’s a science—we have ideas and we test them—we can and do make measurements of the universe. And the point I want to make is that we can make precision measurements of our universe, and we can do it even though we’re stuck here on (or near) this speck of dust that’s Earth. I would like the public to understand that we’re following the scientific method; we’re making measurements and using them to test our ideas. It’s truly amazing that we can do this, given the enormity of the universe.

The other thing to convey is that this is fun! I’m naturally very curious. And I think a lot of people want to know more about the universe. What’s out there? How much of it is there? What is it? It’s a great adventure to make these measurements and learn about the universe. We’re lucky to be able to do it. And it’s a team sport. The teams that do this have many people, and these people complement one another with expertise in many different areas. Teams pull together to accomplish what no individual could do. In the end, knowledge of our universe is a marvelous human achievement. But the knowledge is not just for scientists; it’s for everybody.

Charles L. Bennett, Ph.D.
Department of Physics and Astronomy
The Johns Hopkins University
Baltimore, MD, USA