Astrophysicist Adam Riess shared the Nobel Prize in Physics in 2011 for his work on distant supernovae, which demonstrated that the universe was not only expanding—but that its expansion was accelerating. Now he's hunting for clues that might explain why, and one of the prime suspects is a mysterious force known as dark energy.
IRA FLATOW, HOST: This is SCIENCE FRIDAY. I'm Ira Flatow. We're broadcasting from the Grosvenor Auditorium at the National Geographic Society in Washington. And later in the hour, we're going to take a look at some of the scientific mysteries of Mount Everest - talk about National Geographic.</s>IRA FLATOW, HOST: But first, my next guest shared the Nobel Prize in Physics last year for the unexpected discovery that the universe is not only expanding but that it's expanding at an accelerating rate, which means everything is zooming away from everything else faster and faster every minute.</s>IRA FLATOW, HOST: And that spooky, repulsive force has become known as dark energy. Here to shed light on the dark energy, fresh from his visit to Stockholm, is Adam Riess, winner of the 2011 Nobel Prize in Physics. He's also a scientist and professor of astronomy at Johns Hopkins University and the Space Telescope Science Institute in Baltimore. Welcome to SCIENCE FRIDAY.</s>ADAM RIESS: Thank you.</s>IRA FLATOW, HOST: Get over that ceremony yet, or are you still playing it out...</s>ADAM RIESS: The time change is tough, but we've had a few months to adjust now.</s>IRA FLATOW, HOST: How surprised were you to discover that things were not - the universe wasn't working the way we thought it was working?</s>ADAM RIESS: Oh, the discovery was shocking, really. I liken it to if you took a pair of keys and threw them up in the air with the purpose of measuring how fast they fall back down, to measure how much the Earth tugs on your keys, and then they went up instead, you would be very confused, and that's the position my colleagues and I were in, in 1998, when we saw the universe not slowing down as we expected but actually speeding up, implying the existence of this very mysterious dark energy.</s>IRA FLATOW, HOST: And where does the dark energy live?</s>ADAM RIESS: It's here with us in the room. I brought some with me.</s>ADAM RIESS: It's everywhere, really. It's between the galaxies. It is in this room. We believe that everywhere that you have space, empty space, that you cannot avoid having some of this dark energy.</s>IRA FLATOW, HOST: So our concept that we're taught about, that space is empty, is not really empty.</s>ADAM RIESS: That's right. Your chemistry high school teacher lied to you when they told you...</s>IRA FLATOW, HOST: Not the first time.</s>ADAM RIESS: ...that there was such a thing as a vacuum, that you could take space and move every particle out of it. Now, the strange rules of quantum mechanics tell us that that's impossible, that would violate the Heisenberg Uncertainty Principle. And so we believe, and we've verified, that there are virtual particles, particles that just flip in out of existence and for very short periods of time borrow energy and create this vacuum energy.</s>IRA FLATOW, HOST: So how do you measure it? How do you know it's there?</s>ADAM RIESS: Right, well, what we do is we look at distant exploding stars called supernovae, and we've developed techniques to measure how far away they are and how fast they're moving away from us. So like in the example when I toss the keys, in order to know that the keys were going up instead of down, you would want to measure how far away they are at different points in time, in that trajectory.</s>ADAM RIESS: And so supernovae are the tools, are some of our best tools for making those measurements.</s>IRA FLATOW, HOST: If you'd like to ask Dr. Riess a question, you can come up to the microphones here. He is the superstar in supernovae work and dark energy. If you ever wanted to know something about it, now's the time to do it. What's the difference between dark energy and dark matter?</s>ADAM RIESS: Yes, that's a good question. So when astronomers talk about something being dark, they mean it doesn't emit light, but also we detect it by its gravity. So in that way, they're similar. But the way they're different is the direction of that gravity. So dark matter attracts things, and dark energy repels things.</s>ADAM RIESS: They're also different in their location. So dark matter, which we believe is made up of small particles, are particles that are drawn to where we see luminous matter. So we see them in galaxies, in clusters. Dark energy is everywhere, but since it fills empty space, and most of empty space is between galaxies, most of the dark energy is actually between galaxies.</s>IRA FLATOW, HOST: So most of the universe then is made of dark energy.</s>ADAM RIESS: That's right. About 70 percent of the energy matter budget of the universe is dark energy.</s>IRA FLATOW, HOST: Because there's so much empty space in the universe, but it's - as you say, it's not empty.</s>ADAM RIESS: It's not empty.</s>IRA FLATOW, HOST: This is part of the fabric of the emptiness.</s>ADAM RIESS: That's right.</s>IRA FLATOW, HOST: This expansive pushing-out force. But what is fascinating, most fascinating about this, and the more you talk about it, the more fascinating it gets, is that we didn't always - you just discovered it a few years ago, relatively speaking, and the dark energy didn't kick in as a force until relatively recently.</s>ADAM RIESS: That's right. Yeah, in fact it's quite interesting. If we had lived many billions of years ago, we could not have inferred the presence of dark energy, as it only started accelerating the universe a few billion years ago. And as Lawrence Krauss(ph) points out in his new book, there will come a point in the future when, if the universe keeps accelerating, galaxies will be so far away from us, most of them, we will not be able to see them anymore, and so we will lose the means to actually infer the universe is accelerating.</s>IRA FLATOW, HOST: So before dark energy sort of kicked in, then we had gravity as a greater force pulling the universe back?</s>ADAM RIESS: That's right, attractive gravity, right.</s>IRA FLATOW, HOST: And then what happened?</s>ADAM RIESS: And then as the universe got larger, the space between galaxies increased, and ordinary attractive gravity of the dark matter declined, just like you learn in physics class, as one over the distance squared. So the matter density is always dropping, but the density in this dark energy is about constant. So if something's falling, and something's staying the same, at some point the thing that's staying the same will win, and we are in that winning period.</s>IRA FLATOW, HOST: And as you say, because the universe is expanding, that space has the dark energy in it, sooner or later it got greater than the attractive force of the gravity because space is getting bigger.</s>ADAM RIESS: That's right.</s>IRA FLATOW, HOST: And why didn't we know about this until a few years ago?</s>ADAM RIESS: Well, you might say Einstein gave us a clue. Einstein wrestled with a problem back before we even knew the universe was expanding, and he was looking for a way to keep the universe from collapsing. And so he discovered, in his theory of gravity, something like this dark energy - he called it a cosmological constant - could play this role, pushing things away.</s>ADAM RIESS: Now, he gave up on it once he learned the universe was expanding, but it was always sort of waiting in the wings as a possibility. So once we saw this, that was the first thing we thought of.</s>IRA FLATOW, HOST: Yeah, and he later said that was the biggest blunder in his life.</s>ADAM RIESS: That's right, that's right.</s>IRA FLATOW, HOST: Yeah, let's go to the audience here, a question here. Yes, sir.</s>UNDENTIFIED MAN: Can I ask two questions since I got squeezed out in the last segment?</s>IRA FLATOW, HOST: OK, we'll see. Start out with question one.</s>UNDENTIFIED MAN: The first question is: Do you envision this dark matter, this dark energy, to be able to be beneficial or used by man like gravity? We can use the force of gravity for particular things. Do you ever see that force, the dark force getting - the dark force...</s>UNDENTIFIED MAN: ...getting to be significantly large enough, in particular in our area, that we can harness it for something, space travel?</s>ADAM RIESS: Right. I think the greatest likelihood is that by following dark energy, we're likely to develop a deeper understanding of physics. So dark energy is so interesting to us because it lies at the nexus of two incompatible theories in physics. One is quantum theory, which is physics of small objects, and the other is general relativity, which describes gravity on large scales in the universe.</s>ADAM RIESS: And dark energy requires us to use both those theories together, though they're incompatible. So by understanding dark energy, we think we may get to a deeper theory, something people refer to as quantum gravity. And I would say any time we understand physics more deeply, you know, all bets are off, we learn all kinds of interesting things that are very practical in many cases.</s>ADAM RIESS: But I don't see us specifically harnessing, you know, a bag of dark energy or a barrel of dark matter.</s>UNDENTIFIED MAN: Well, not harnessing but using it in the way that we use gravity to...</s>IRA FLATOW, HOST: If you throw up a key, it comes down, you know it's going to come down. Can we make it so if I throw it up, we know it's going away?</s>UNDENTIFIED MAN: You can send a space ship around the moon, for instance, and use the gravity of the moon. Can you use the dark energy in that way?</s>ADAM RIESS: Right, so the problem then in that scenario is dark energy is weak enough that we never even saw it until we added it up across most of the universe. So we don't see any real measurable effects of dark energy within, let's say, the solar system.</s>ADAM RIESS: And so it's very difficult to imagine the scenario where we would collect on a large scale of the universe to do something.</s>IRA FLATOW, HOST: So it's so weak, that's why we're still sitting on our seat and not being pushed away because it's not strong enough around us to push us apart. OK, you only get one on now, because that was a double question.</s>IRA FLATOW, HOST: Yes, go ahead. It was a good question though.</s>UNDENTIFIED MAN: Are there different theories, you know, about the nature of dark energy? And what sort of research is being done to try to distinguish between them or determine what it is?</s>ADAM RIESS: Right, there are. The three leading ideas are similar to what Einstein had described, that vacuum energy is a static property of space itself and that it has to do with space's ability to host virtual particles that appear and disappear.</s>ADAM RIESS: Another possibility is that dark energy is a kind of a field. You could think of the electric field or the magnetic field. This would be a kind of temporary forcefield that has the property of making the universe expand faster and faster.</s>ADAM RIESS: Another possibility is we have finally broken Einstein's theory of general relativity. He gave it to us in good condition, but we worked with it for so long, and in particular we've now finally tested it in a realm, at a distance that it had never been tested before, and we may have broken it.</s>ADAM RIESS: So we have a sort of whole armada of surveys that are going to be undertaken over the next 10 years to make very precise measurements, probably 100 times more precise than what we've done so far, to measure a couple of properties of dark energy - its strength and its longevity. And we will use those measurements to test them against these different possibilities.</s>IRA FLATOW, HOST: Could it be there are whole new areas of physics we don't know about?</s>ADAM RIESS: Yes.</s>IRA FLATOW, HOST: And new ways of describing the universe we haven't thought of, or we haven't figure out yet?</s>ADAM RIESS: It is. You know, we think, we think the most likely possibility is that we actually discover the nature of dark energy, but it could be embedded in the laws of physics. It could be something deeper that we find and learn about by following dark energy.</s>IRA FLATOW, HOST: And Einstein's theory of gravity, which is a geometrical design, isn't it? Does dark energy follow the geometry of that, or is it something else?</s>ADAM RIESS: Well, you can explain it in terms of Einstein's geometry as just another term affecting the curvature or bending of space, but it is also possible that it sits on what we'd say the other side of the equation, not the side that - where you collect all the matter and energy but the side where you describe the physics.</s>ADAM RIESS: And so we're very excited to try to understand on which side of the equation really this dark energy phenomenon lives.</s>IRA FLATOW, HOST: And there are real experiments you can do to figure it out?</s>ADAM RIESS: Oh yeah, absolutely. I mean, the kinds of experiments we first did in 1998 to see that it was there, we're doing those to a much greater degree with much better statistics, with five or six different techniques. People are even starting to test gravity at very small scale lengths, you know, at the sub-sub-sub-millimeter to see if gravity behaves as it should on the smallest scales.</s>IRA FLATOW, HOST: All right, Adam Riess, we're going to take a short break. He'll stay with us after the break, and you can still ask some questions. He is the 2011 Nobel Prize-winner in Physics. You can ask us questions right here in our audience. You can tweet us @scifri, @-S-C-F-R-I - S-C-I-F-R-I, don't know how to spell it myself. We'll be right back after this break, so stay with us.</s>IRA FLATOW, HOST: I'm Ira Flatow. This is SCIENCE FRIDAY. We're talking about dark energy with Adam Riess, the 2011 Nobel Prize-winner in Physics and the discoverer of this mysterious, expansive force.</s>IRA FLATOW, HOST: One of the things you always hear about, Adam, when scientists talk about the frontiers of science, and they try to come up with explanations, and one of them is something like: Well, maybe it's the multiverse. What is the multiverse, and why are they so eager to throw that in?</s>ADAM RIESS: Right, so the...</s>IRA FLATOW, HOST: You don't sound too happy about that.</s>ADAM RIESS: Well, I'm not.</s>ADAM RIESS: As an observer, I'll tell you about the challenges of the multiverse. So some of our theories of how the universe began, in a period known as inflation, which would have occurred right after the Big Bang, it is possible that there were many universes created, that they sort of bubble out, a number far too large to count, maybe 10 to the 500 universes.</s>IRA FLATOW, HOST: Billions of them.</s>ADAM RIESS: Billions, many billions. And some speculate that each of these universes could have different physical conditions, different physical properties. And so while the greatest struggle we have with dark energy, the problem is to try to understand actually why is it so small because...</s>IRA FLATOW, HOST: It should be bigger.</s>ADAM RIESS: Our quantum theory says it should be 120 orders of magnitude bigger.</s>IRA FLATOW, HOST: Wow, so you have problems: What is it, and why don't we have more of it?</s>ADAM RIESS: Right, and so the folks who subscribe to the multiverse view say, well, that's not really a problem because maybe this dark energy takes many different possible values in all these different universes. Most of those values are too high to allow galaxies to form, planets to form and us to be here. So those are unlucky universes.</s>ADAM RIESS: But us lucky few, in the good universes with the lucky values, are here to contemplate our existence.</s>IRA FLATOW, HOST: We're in a Goldilocks universe.</s>ADAM RIESS: That's right. What's - what I don't really love about this is while it is possible that it's true, it doesn't follow the usual scientific method of the tight coupling between ideas and experiment in that it's very difficult to do an experiment in another universe.</s>ADAM RIESS: We may not actually have access to an experiment like that. And so I think it's been such a fruitful way to continue to come up with theories that are testable and test them, I'm concerned that we become a little disconnected from that process.</s>IRA FLATOW, HOST: So in other words, as scientists say, you can come up with all kinds of theories, but if you can't test out the theory, it's just a nice idea.</s>ADAM RIESS: Right.</s>IRA FLATOW, HOST: It's not - it may be nice philosophically to think about it, but if you can't come up with a way to test it, it's not a valid scientific...</s>ADAM RIESS: Right, and I have another objection, I would say, too to this idea. If I asked a theoretical physicist to derive the distance between the Earth and the sun, they would say, ah, well, you can't do that from first principles. There are lots of planets around lots of stars at lots of separations. And so we are here because there is a good separation between the sun and the Earth, and so we have water here.</s>ADAM RIESS: That is an anthropic argument in the same sense of the multiverse except in that case you can actually say yes, you're right, we have seen lots of planets, we have seen lots of stars. They come at many separations. We have all the prerequisites for a good understanding of why this is anthropically set. I don't feel like we're at the same point yet with this multiverse story, that we know that each universe gets a different set of physical constants and how those are distributed.</s>ADAM RIESS: So I think we have at least some more prerequisites to accomplish.</s>IRA FLATOW, HOST: The fact that we're discovering all these exoplanets now, and more, hundreds of them, that we never thought existed, doesn't that lend more credence that we don't need a multiverse to explain why we're here? There are a lot more of out there.</s>ADAM RIESS: Right, but the problem is, well, we think that within our universe, the whole universe is stuck with this same dark energy problem. On all these exoplanets are advanced civilizations, and when they reach our level, they say: Ugh, we can't understand this dark energy either.</s>ADAM RIESS: In fact, maybe the best solution is we make contact with the more advanced ones at some point, and they tell us the answer.</s>IRA FLATOW, HOST: There you go. That's like an experimental physicist thinking. Yes, sir.</s>IRA FLATOW, HOST: UNDENTIFIED MAN #2: You mentioned - you brought up Einstein earlier, and of course E equals MC squared, so energy and matter, sort of two sides of the same coin. Is the same true with dark energy and dark matter?</s>ADAM RIESS: It's true only insomuch as they both live along that equality of energy and matter. But we don't - when we talk about E equals MC squared, we are frequently using it as a conversion in stars, when you combine matter in fusion to produce energy.</s>ADAM RIESS: We're not aware, and we don't believe, that the universe makes use of E equals MC squared in the same way because we don't see any process going on like that.</s>IRA FLATOW, HOST: What was it, I can't think of the physicist who said, you know, the universe doesn't care if we like to do math or...</s>ADAM RIESS: That's right.</s>IRA FLATOW, HOST: Yes, ma'am, step up to the mic.</s>UNDENTIFIED WOMAN: Hi, I had a question about what the relationship is between dark energy and dark matter and string theory and if there is a relationship. And the other question I had is: If all those what we thought were empty spaces are filled with dark energy and dark matter, what does that mean in terms of traveling at the speed of light and time travel and wormholes, that whole issue?</s>IRA FLATOW, HOST: Only simple things.</s>UNDENTIFIED WOMAN: Just things I was just sitting and wondering about.</s>IRA FLATOW, HOST: Yeah, on the way home tonight in a car ride.</s>ADAM RIESS: Let's see, can I use all of those in a sentence?</s>ADAM RIESS: Well, let me just say that dark energy and dark matter, while they seem like two extra pieces, couldn't we just figure out how to combine them and make them one thing, it's really been a struggle to do that. They look so different in the universe, as I said before.</s>ADAM RIESS: Dark matter is clumpy and seems to be made of small particles, and dark energy seems to be smooth and located everywhere, and so therefore they just don't seem like they're part of the same phenomenon. Rather, I would say, to our great surprise, although we are made of atoms, it turns out most of the universe is not.</s>And although we see each other and communicate by light and sound, it turns out most of the universe does not: It communicates by gravity. And so this is a great surprise to us.</s>IRA FLATOW, HOST: Well, would it communicate by dark energy now?</s>ADAM RIESS: Well, right. I mean, it's the gravity of the dark energy that we actually see. And so...</s>IRA FLATOW, HOST: Heavy.</s>ADAM RIESS: Yes, and so in this case, it's just, you know, we're the - I guess you could say we're the lucky four percent that is just different.</s>IRA FLATOW, HOST: Well, let me see if I understand. You said it's the gravity of the dark energy. Do you mean that literally, the gravity, the repulsiveness, the repulsive...</s>ADAM RIESS: That's right. So just to give you an...</s>IRA FLATOW, HOST: So gravity, we think of pulling together.</s>ADAM RIESS: Right, I mean, if you had a wind tunnel, you would not be able to see the wind, but you could release a little smoke in the wind tunnel, and then you could track it. And so these supernovae ride around in the dark energy, and we see the gravity as it affects the supernovae in the universe, although we cannot literally see the dark energy, we cannot literally see that stuff.</s>IRA FLATOW, HOST: And she asked about string theory. Can that explain any of this?</s>ADAM RIESS: So string theory has not predicted or explained very much to date, although...</s>IRA FLATOW, HOST: Another skeptic.</s>ADAM RIESS: Yeah, well, I'm an observer. So I sort of look at what is it predicting, what can I look at. But string theory has predicted these 10-to-the-500 universes, and so...</s>IRA FLATOW, HOST: The multiverse.</s>ADAM RIESS: That's right. And so perhaps one day that will be a very profound conclusion, but I think it's premature.</s>IRA FLATOW, HOST: Let's see if we can get the last question here. Well, then we'll go to here.</s>UNDENTIFIED MAN: My question is about dark matter, if we thought we had any testable theories or envisioned any experiments where we may be able to sort of figure out what the nature of this dark matter is.</s>ADAM RIESS: Right. We're getting very close, actually. There are direct detection experiments, where they actually try to build detectors. It's very difficult...</s>ADAM RIESS: ...usual way. It's flitting through all of us. But you could produce detectors, made out of very dense materials, and locate them in mines below the surface of the Earth, where the surface of the Earth acts to filter out the ordinary background particles.</s>ADAM RIESS: And a number of experiments have started to find just a few of these dark matter particles, so few that we're still not sure; it's going to be another year or two whether we know they were just unlucky, that they had some errors, or whether they're actually there. But if it's not this year, it'll be in a couple years.</s>IRA FLATOW, HOST: How do you know it's not a neutrino flying through?</s>ADAM RIESS: Well, that's what they're very good at, is trying to figure out - they know the mass scale of neutrinos. That's very different. And so it creates different products. Also the Large Hadron Collider, which is working in CERN, is expected to produce dark matter particles as part of the collisions. And so I think we're probably within hopefully just a few years of actually directly measuring and seeing dark matter.</s>IRA FLATOW, HOST: Physics, it's always just a few years.</s>ADAM RIESS: That's what we say to the funding agency.</s>IRA FLATOW, HOST: Yes, go ahead.</s>IRA FLATOW, HOST: UNIDENTIFIED MALE #3: So in the terms of the multiverse, if our universe is expanding, wouldn't it eventually collide with others and expand into other universes?</s>ADAM RIESS: That's a really good question. It is the one way, maybe the only way, we could actually discover that this multiverse story is true, is if there are collisions between what they call the branes - and this is B-R-A-N-E, like a membrane, but the surfaces of the different universes.</s>ADAM RIESS: And if they collide, it would look like a giant explosion. So we could be sitting here one day, and there would be a giant explosion coming from a dimension that we don't see. People look for a relic of this in the radiation leftover from the Big Bang, and nobody has really seen that yet. And so it's a very long shot, but that's the only chance we think we would have.</s>IRA FLATOW, HOST: It's a longshot. Last question here, yes, he's back.</s>IRA FLATOW, HOST: UNIDENTIFIED MALE #4: In string theory, dark energy and dark matter, they both - string theory says that everything is both a particle and a wave, right? So how would dark energy and dark matter both act as particles and waves? I mean, if...</s>IRA FLATOW, HOST: Yeah. Yeah.</s>IRA FLATOW, HOST: UNIDENTIFIED MALE #4: Or is this one of the ways that string theory and this dark energy stuff just don't go together?</s>ADAM RIESS: Right.</s>IRA FLATOW, HOST: I think he's on your side on this.</s>ADAM RIESS: I think so. That's a good question. So actually, it's not just string theory that says that objects act like particles and waves. In actually quantum theory - that's a very mainline theory of physics - all particles act as particles and waves. And so it is true that dark matter and dark energy would have this aspect. But we are mostly interested, at this point, in the gravity of dark energy and dark matter. So we don't often get to see interactions...</s>IRA FLATOW, HOST: Yeah. Yeah.</s>ADAM RIESS: ...collisions between dark matter particles that would actually illustrate the particle and wave nature.</s>IRA FLATOW, HOST: Adam Riess, everybody. Thank you very much for taking time to be with us...</s>IRA FLATOW, HOST: ...winner of the 2011 Nobel Prize in physics, also a scientist and professor of astronomy at Johns Hopkins University and the Space Telescope Science Institute in Baltimore.