In the late 1990s, two separate research teams—the High-Z Supernova Search Team and the Supernova Cosmology Project—presented data, gathered by studying exploding stars, that brought with it a startling implication: our universe is expanding at an accelerating rate.
Since this game-changing discovery, for which three researchers were just awarded the 2011 Nobel Prize in Physics, scientists around the world have been using increasingly sophisticated technology to form a more precise picture of this cosmic expansion, and why exactly it’s speeding up.
Two of these new Nobel Laureates, Brian Schmidt and Adam Riess, started their work as graduate students in the lab of Robert Kirshner at Harvard University. Kirshner, Clowes Professor of Science at Harvard and an ongoing member of the High-Z Supernova Search Team, will discuss our rapidly expanding universe during a special lecture at Northwestern University on Tuesday, November 1. We spoke with him for a preview.
In your talk, you’ll discuss a force known as “dark energy.” What is this?
Dark energy is one explanation for a phenomenon we see, which is that the universe is expanding faster over time.
The universe has been expanding since the time of the Big Bang, about 14 billion years ago. What’s new, and what I’ll talk about at Northwestern, is evidence that the expansion of the universe has actually been speeding up, and that the more recent history of cosmic expansion shows galaxies zooming away from each other faster than the galaxies were receding from one another in the distant past.
Basically, what we see is that over cosmic time, the recession of galaxies from one another has been accelerating. We attribute this to the "dark energy,” but having a name for it doesn't mean we know very much about it.
Do you have theories as to where dark energy is coming from?
Dark energy could be something that Einstein thought of, back when he was inventing general relativity, which is the theory of gravity that we use to explain the universe. At the time, in 1917, people thought the universe was static—they hadn’t heard about this expanding universe stuff—and Einstein knew that there would be attractive gravity that would make the universe clump up. So he put in by hand a kind of repulsive term that acts like the dark energy we talk about today.
Einstein put the cosmological constant into his equations by hand to make a static universe, which he thought would agree with the evidence from astronomers. But, about 10 years later, astronomers were seeing something new—a universe that was expanding, with all the galaxies receding from each other. The cosmological constant seemed pretty silly, since it was invented to explain something that wasn't true, so Einstein said "Away with the cosmological constant." The legend is that he called it his "greatest blunder." After that, most people were very cautious about inserting this term back into the discussion of the expanding universe.
But the interesting thing is that, starting in 1997, we began to put together the evidence for the accelerating universe, and a cosmological constant is just what you need to make that happen. So, one idea for the dark energy is that it is a modern version of the cosmological constant.
The modern idea is that empty space is not completely without properties. That is, if you look on a small-enough scale, there are particles being created and destroyed. That kind of picture—a quantum-mechanical picture—agrees very well with what physicists see for the properties of electromagnetism. [But] there is no really good quantum theory for gravity, and so there’s no really good way to predict exactly what the analogous effects ought to be. But when people have done it on the back of an envelope, they find that there ought to be an energy associated with gravity, and associated with empty space, but the theoretical computation for this number is 10120 times bigger than the value that we see in astronomical measurements. That is a bad disagreement between our understanding of the physics and what we see out in the universe. We know this means we are missing something very important in our understanding of gravity. The dark energy could be a property of empty space itself, but it’s not as simple as simple estimates would predict.
How does dark energy relate to dark matter?
Well, they are different things. We call them both “dark” because we don’t see them. But dark matter is real; it is something that slowed down the expansion of the universe when it was very young, and that governs the motion of stars in the galaxies. It turns out that the kind of matter that makes up the stars we see with our telescopes, and that we are made of, only amounts to a small fraction, about 5%, of the mass and energy in the universe. Most of the mass in the universe is in some other form that’s invisible to us. We see its effects as gravity tugs on stars as they orbit in galaxies. But we don’t know what it is yet, and we can’t see it in our telescopes.
The idea is that there’s much more mass out there than is visible to us, and the motions in the universe are determined by a balance—a tug of war—between the dark matter, which is gravitating, and dark energy, which has this kind of springy quality that’s trying to make the universe fly apart.
We’re pretty sure that the dark matter is not made of ordinary stuff, like neutrons and protons and electrons—the things that make up the physical world that we see. It must be some other thing that doesn’t scatter light, that doesn’t interact with protons and neutrons. It turns out that particle physicists know about other things that are not ordinary matter, like neutrinos, for example, which have been in the news a lot lately. Neutrinos are real, but they have no charge, they have almost no mass, they don’t interact, they go through walls—they’re a ghostly kind of particle. And they would be a good candidate for dark matter if they had a little more mass, but they don’t. So it could be something like a neutrino, only more massive.
Particle physicists are in search of such particles at the big particle accelerators, [like] the [Large Hadron Collider]. People are looking for candidates for the dark matter particle. That would be great because, at the moment, it’s only the astronomers who know about this stuff from the effects [of dark matter]. The actual measurements from particle accelerators or from physics experiments on Earth have not yet shown the nature of the dark matter.
What is the focus of your research now?
The way we found out about the cosmic acceleration was by measuring exploding stars—supernovae—at great distances. The nice thing about a particular class of supernova is that the brightness of the explosions is very similar from one to another. If they were all identical, then you could estimate the distance to each one by how bright it appears. [For example], you can look out at the lake and things—light from a ship, let’s say—look bright when they’re nearby and look faint when they’re far away. Astronomers can do the same thing, measuring distances in the universe using these exploding stars.
What we found was that the relationship between distance and time for these things was not the one you’d get in a universe that was expanding at a constant rate or one that was slowing down. It was the relation you’d get if the universe was speeding up. This means that the distant ones are a little farther away than you would otherwise get.
We’d like to [measure] this better than we did ten years ago. What I’ve been working on is making measurements using light in the infrared [wavelengths] instead of using visible light, the ordinary kind that your eye…can detect. It’s technology that has changed. The detectors for infrared have gotten much better.
There are two really good things about this. One is that the supernovae are more similar to one another in the infrared than they are in visible light. This is very good for measuring distances. The other thing, since we’re measuring distance from the apparent brightness of something, you have to be really careful that there isn’t something else in between you and the object that is absorbing the light. For example, if you were looking at ships at sea, if there were a fog bank out there, the lights would look dimmer than they really ought to. So you would say that [the] ship must be farther away than it really is. Of course, this is why all navigation books counsel you not use the brightness of a lighthouse as a tool for measuring distance, because it’s very tricky.
In galaxies, there is dust in the space between the stars. This interstellar dust is very interesting—it becomes material for new planets—but for us it’s a nuisance. It’s in the way, and it makes objects appear dimmer than they really are. If you don’t account for that correctly, it could give the illusion of bigger distances, and the illusion of cosmic acceleration. That’s very dangerous, and we have worked very hard to measure that and take it into account. The good thing about measurements in the infrared is that they’re less vulnerable to dust.
So the supernovae measured in the infrared give better distances, and a more precise measurement of the history of cosmic expansion. Following that through to the conclusion about dark energy, we can find out if the dark energy behaves like the cosmological constant or not. So far, every measurement we've made is consistent with dark energy acting like the cosmological constant, but they aren't very precise. It is a deep mystery, and we want to do better.
Kirshner will speak at Northwestern University on Tuesday, November 1, at 4:00 PM in the Technological Institute's Ryan Auditorium (2145 Sheridan Road) on the Evanston campus. The event is free and open to the public, and is sponsored by by Center for Interdisciplinary Exploration and Research in Astrophysics and the Department of Physics and Astronomy. Click here for more information.