by Sam Lemonick Thursday, January 5, 2012
Astronomer Adam Riess and his team made a huge splash in 1998 when they announced they had discovered something called dark energy, the major component of our universe. They had been studying supernovae — exploding stars — to learn more about how fast our universe is expanding. By measuring the brightness and color of the light from a supernova, astronomers can tell how far the light has traveled and how its speed has changed over its journey. Riess' calculations led to the discovery that the expansion of the universe is accelerating due to the repulsive force of dark energy. His discovery was called 1998’s “Breakthrough of the Year” by Science magazine.
Riess, who works at Johns Hopkins University in Baltimore, Md., was awarded a MacArthur Fellowship, also known as the Genius Grant, in 2008. He talked with EARTH contributing writer Sam Lemonick about his incredible discovery and what dark energy is doing to the universe.
[SL:] You are famous for the discovery of dark energy. What exactly is dark energy?
[AR:] Dark energy is a really big mystery so we don’t know a lot about it. We know it accelerates the universe; it makes up about 70 percent of the universe; and it’s been around a long time. We also know that it is eerily similar to something Einstein first suggested in 1916: that the gravity of empty space could be repulsive. Dark energy may indeed be [what Einstein described], but we still haven’t quite understood the connection between the dark energy we see and the dark energy Einstein thought about.
[SL:] Could dark energy be called antigravity?
[AR:] I think it’s fair for people to think of it in terms of antigravity in the sense that its gravity is repulsive, not attractive. It’s a little different in that attractive gravity is very localized to some object — what we describe as the center of that gravity. That’s not really the way this repulsive gravity works. It’s more of a sort of springiness to space; it pushes space apart. It’s not like we have a blob of dark energy that causes antigravity or that you could have antigravity boots. It seems like a property of space itself that makes space act like it is repulsive to itself.
We don’t really understand this at a deep level yet, but it does match some very basic predictions of the theory of quantum mechanics, which says that something like this could exist. So in some ways we’re not completely shocked [to discover dark energy] because it doesn’t require a whole new concept of physics. It just requires us to understand some of the weirdest parts of physics that we never really got a good handle on.
[SL:] Has dark energy always existed in our universe?
[AR:] That’s one of the $64,000 questions about it. If we identify dark energy as being the vacuum energy — energy related to empty space — then it probably has always existed. It’s also possible it’s a transient phenomenon.
What we observe is that the universe was always expanding, but earlier [in its history] it was not accelerating. That’s only been a recent phenomenon that probably started about 5 billion years ago.
[SL:] How are you studying dark energy and its effects on the universe?
[AR:] I’m working on two projects, both using the Hubble Space Telescope. One is to measure the present rate at which the universe is expanding with higher precision and accuracy. The other is to look for exploding stars called supernovae that went off when the universe was very young, maybe 10 percent of its current age.
We’re using a new camera that the astronauts installed on Hubble in 2009 called the Wide Field Camera 3. It has infrared capability, which is a kind of light that we don’t see with our eyes at wavelengths even redder than red. It’s an important wavelength to look at very distant objects because the expansion of space stretches the wavelengths of light. Light may have left the supernovae at blue wavelengths but by the time it reaches Hubble, it has been, we say, red-shifted — stretched by the expansion of space all the way to the infrared. If we didn’t have a camera like this, we’d never see that light.
[SL:] Why do you look for supernovae?
[AR:] They are one of the best ways we have of measuring distances across the universe. We’ve been able to calibrate how luminous they are, so when we see one we can tell how far away it is based on how faint it looks. That’s a very powerful capability.
There are two major classes of supernovae. The first are core collapse supernovae. They occur when a massive star runs out of fuel and runs out of the ability to hold back gravity from crushing it. It implodes, then rebounds and explodes. Those come in a huge range of luminosities so they’re not very good at measuring distances. The ones that we have been measuring and that we focus on are thermonuclear supernovae. Those occur when the core of an old star, called a white dwarf, gathers material from a companion [star] in a binary system. When it reaches a critical mass, it will explode. That gives us a very uniform explosion. They’re like nature’s own 60-watt light bulbs.
[SL:] When did this work start?
[AR:] I went to graduate school at Harvard to get a Ph.D. in astrophysics, but I didn’t really know what I wanted to do. I met my advisor, Robert Kirshner, who suggested this project about finding out better ways to measure distances using supernovae. That turned out to be a really great project and the start of this hot field of using supernovae as distance indicators. Within a few years, that [research] led to our team discovering that the expansion of the universe is accelerating, and hence the existence of this very weird stuff called dark energy.
[SL:] Tell me more about the discovery.
[AR:] My colleagues and I, on a team called the High-z Supernova Search, were looking at very distant supernovae and using them to measure the rate at which the universe was expanding in the past and comparing that to the rate at which it’s expanding today. We thought the expansion would be slowing down because all the matter in the universe has attractive gravity and that should try to slow down the expansion.
Instead, it was speeding up. It was very exciting. There was a competing team that came up with the same result at the same time. I was lucky to actually be the person who first saw that result in our team’s data and got to write the paper, but it was really the work of a lot of my teammates.
[SL:] The Space Telescope Science Institute called your discovery — including that dark energy makes up close to three-fourths of the universe — the top achievement of Hubble. Is that deserved?
[AR:] I think it’s one of the top handful of things that Hubble has contributed to. It’s very profound and it really has a fundamental impact on our quest to understand the universe. There are only so many times you will find the majority of the universe. [Laughs.] So [the discovery of dark energy] is certainly a watershed moment and it certainly has to be one of the top hits of any telescope that contributes.
[SL:] You were a 2008 MacArthur Fellow. What are you doing with your grant?
[AR:] I’ve been able to work on this project to design a new class of astronomical filters that are specialized for finding supernovae. It’s probably something I could not have easily gotten a grant for so it’s nice that I can just say, “Well, I think that’s going to work. Let’s do it.” That’s the kind of thing [a MacArthur grant] helps with.
[SL:] You’ve already been part of a huge advance in our understanding of the universe. What do you hope your future research will yield?
[AR:] I’m hoping to contribute to our community’s efforts to try to understand the nature of dark energy. I feel like that is a really big question and if I could end up still playing a small part in that, it would be very enjoyable.
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