This article is a sample from DISCOVER's special Extreme Universe issue, available only on newsstands through March 22.
Also see the video from DISCOVER's Mysteries of the Cosmos event, in which Perlmutter was a panelist.
Few scientists can say their work forever changed how we see the universe. Saul Perlmutter is one of them, for his central role in the 1998 discovery of dark energy. That invisible energy, which accounts for a whopping 73 percent of everything in the cosmos, is stretching the fabric of space and could cause a runaway expansion of the universe. Through his groundbreaking research, the then 38-year-old physicist at Lawrence Berkeley National Laboratory in California basically turned our model of the universe on its head.
Scientists had long assumed that atoms—the constituent parts of stars, planets, and people—dominated the universe. Now it is generally accepted that matter makes up merely 5 percent, its share dwarfed by the mysterious antigravity energy that is propelling space apart. (The remaining 22 percent of the cosmos is so-called dark matter, unrelated to dark energy except in its ability to defy all current methods of detection.) Scientists had also long assumed that the universe would either slow infinitely or eventually stop expanding and collapse in on itself. Perlmutter’s findings have forced them to consider that it might instead expand away into nothingness or, worse, end in a “big rip” as the ingredients of stars and galaxies are literally pulled apart.
Since 1998 Perlmutter has worked to refine his measurements of the accelerating universe and the dark energy causing it. Theories abound about the nature of this elusive energy, and Perlmutter is hotly pursuing observational evidence to help find the answer. He spoke to DISCOVER about his strange discovery, the latest ideas about dark energy, and the projects that have the best shot at making sense of this cosmic mystery.
What was the underlying motivation behind the research that led to your discovering dark energy in 1998?In the 1920s, Edwin Hubble showed that the universe is expanding. But the very next thing out of people’s mouths was more questions: Will it keep expanding? Could it stop expanding? Maybe it could turn around and collapse. How do we know the universe will last forever? These are the obvious things you want to know when you say we live in a changing, expanding universe. And the way you answer questions about the future is by looking at the past.
How do you approach such a complex problem as the history of the universe’s expansion?The basic idea is that when you look at farther and farther distances, you’re looking further and further back in time. There were some very early papers in the 1930s that proposed using supernovas—really, really bright exploding stars—to measure the universe’s expansion because it appeared there was consistency in how bright they got. If every supernova had almost exactly the same brightness, then you could use how bright it appeared from Earth to measure its distance. But it turned out that the more you looked at supernovas, the wider the variety you saw, and that consistency disappeared. It wasn’t until the 1980s that scientists realized there are subgroups of supernovas, and that one of them, called Type Ia, is very consistent in its brightness. Fortunately, it’s also the brightest of the group, so it’s the one you could follow farthest away.
How did those supernovas reveal the way the universe is expanding?We used these Type Ia supernovas as our distance indicators. Then you want to know how much the universe has expanded since each explosion occurred. There’s a really convenient way of getting that. The supernova sends out almost all its light in a specific wavelength of blue. But as that blue light travels, it gets stretched exactly as the universe stretches, so it looks red [with a longer wavelength] by the time it reaches us. How red the light looks tells you exactly how much the universe has expanded since the explosion of that supernova. Looking at different supernovas, you should be able to figure out how much the universe has expanded since, for example, 5 billion, 3 billion, then 1 billion years ago, and you would see how that expansion has changed over time. The expectation was that over time the universe’s expansion would be slowing down due to the gravitational attraction of all the mass of all the stuff in the universe. As it turned out, we found that the universe’s expansion was actually speeding up.
Why is it so significant that the universe is expanding faster and faster?It suggests that the universe is not just a single-parameter story. It can’t just be mass that is causing the change in expansion; the only thing mass can do is slow everything down. So we immediately knew that there was something else in the story. It turns out that most of the stuff in the universe is in the form of some energy in the vacuum that has an odd repulsive property. It makes space reproduce faster, accelerating the expansion of the universe. We don’t know what it actually is, but for now people use the term dark energy as a placeholder to describe the attributes of this mystery.
Was there a moment when the enormous implications of your research really hit you?Well, it’s funny. This had to be the slowest aha in history, an aha spread out over several months. And the reason is that these are really complex data analysis jobs and there are many steps you have to calibrate and get all straightened out before you get those nice, final data points. On the other hand, there was the very first time I went out to give a talk and present the data. After the talk a famous cosmologist, Joel Primack, stood up and said he just wanted to point out to the physicists in the audience that this is an amazing, absolutely flabbergasting result. I think at that moment I felt the extra sense of ah, that’s right, this is really shocking.
How are scientists now attempting to explain dark energy?Einstein originally put a term called lambda into his equation for general relativity that was meant to counteract the effects of gravity and create a static universe. Edwin Hubble’s discovery of an expanding universe convinced Einstein that lambda was unnecessary. But later, people realized that in quantum mechanics lambda could easily be identified with the effects of particles that spontaneously appear and disappear in all empty space. They’re called virtual particles, and the energy associated with the background hum of their constant appearance and disappearance became the way in which we understand the source of repulsive vacuum energy. Unfortunately, if you do the basic calculations for how much of that energy you expect, you get numbers that are way off—10^120 (1 followed by 120 zeros) times too large. You have a big problem when your answer is that far off. So it has been assumed that there must be a perfect cancellation that makes that discrepancy go right to zero. But now we see that the answer isn’t exactly zero. I think most of the particle theorists believe that we need new explanations.
What are some of the explanations?One explanation is that dark energy is a scalar field whose properties at every point in space “roll” from one value to another. While it is rolling, the effect of its energy is high enough to make the universe accelerate. Or perhaps the general relativity equations that Einstein gave us are not completely perfect, and we’re going to need to modify them a little bit. Another fun explanation is the possibility that there are extra dimensions and that gravity can leak away into those other dimensions that aren’t visible. For the last 10 years there have been two or three papers a week on dark energy explanations. But if you ask these theorists if they believe their particular model is the answer, I think almost every one of them would say, “No, I’m just trying out different ideas and hoping we can get some clues.” Then they turn back to us, the experimentalists, for more data.
How are we going to find out more about what dark energy really is?A good example is the Nearby Supernova Factory, an experiment that has turned up more than 500 Type Ia supernovas to help explore the nature of dark energy. I’m also involved in the Joint Dark Energy Mission, which is being funded by NASA and the U.S. Department of Energy. The plan is to send up a satellite that will search for supernovas and also pursue other techniques for exploring dark energy’s influence. I’m optimistic that if we can just do a really good project, then the theorists can have the other aha moment that we’re looking for within a half-dozen years. All we need to do is start getting some hints that will point people in the right direction. I think theorists are very creative and will be able to get the job done, but right now there’s simply too wide a scope. Dark energy could be anything.
How has the universe’s expansion, and hence the influence of dark energy, changed since the Big Bang?For cosmologists there’s this interesting moment in the very, very early universe—10^–35 seconds or so after the Big Bang—called the inflationary period. Inflation was another period of acceleration, and we don’t know what caused that acceleration, either. It’s possible that there was another kind of dark energy back then. After inflation there was so much mass so close together that gravity dominated and the expansion slowed. That lasted until about halfway through the life of the universe. It was some 7 billion years before the universe expanded to the point where matter was too scattered to keep the expansion slowing. At that point, dark energy’s power started to be felt and the universe started to accelerate again.
What does this discovery mean for the fate of the universe? Will dark energy ever let up?Well, you can just take the naive approach of saying that the universe is accelerating now, so that means it will accelerate forever and lead to a very dark, empty, cold end, and that’s all we have to look forward to. However, we should remember that we don’t know what’s causing the current acceleration, and we don’t know what caused that acceleration during inflation at the very beginning of the universe. That inflation turned around—it stopped and the universe started to decelerate. Who knows whether we’re seeing something now that might also decay away, and then the universe could collapse. So I would say that the fate of the universe has to remain in the category of unknown until we have any clue as to why it is currently accelerating.
See the video from DISCOVER's Mysteries of the Cosmos event, in which Perlmutter was a panelist.