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Robert P. Kirshner is Harvard College Professor of Astronomy and Clowes Professor of Science at Harvard University. He graduated from Harvard College in 1970 and received a Ph.D. in Astronomy[…]

Robert Kirshner’s research into supernovae overturned decades of scientific assumptions about the universe, and how it is mysteriously expanding at a rapid rate.

Question: What are supernovae and why does your research focus on them? 

Robert Kirshner: Yeah. So, stars have a lifetime. It’s very long compared to ours, so stars seem permanent. For poets, stars are kind of symbols of permanence. The speed of light is a symbol of something going fast. But for astronomers, the speed of light is slow, that’s what lets us see the past and the stars are not permanent; they are changing over time. It’s just the time scales are much longer than human lifetimes. So, a star, like the Sun, gets its energy from nuclear fusion. Down in the center of the Sun, hydrogen, which is what the sun is made of, is being converted into helium, the next element up. When you do that – when the sun does that, it generates energy. Energy is released because the helium in the nucleus has a little less mass in it then the ingredients, the hydrogen nuclei that went into it.  So, hydrogen goes in and helium comes out, and the difference shows up as energy. 

You know that equation that Einstein has, E=MC2, that’s not just a symbol of scientific inscrutability. That is actually an equation where the thing on the left is equal to the thing on the right, the amount of energy that you get out is equal to the change in mass times the speed of light, squared. Well, the speed of light is a pretty big number and when you square it, it’s big, big. And that means you get a lot of energy out from a small change in the mass. This is really important. So this is nuclear fusion. This is how the sun works and since the sun is made of that fuel, it can last for a very long time.

We think the lifetime for a star like the sun is about 10 billion years. So, the sun formed about 5 billion years ago, along with the planets like we’re on, and it has about 5 billion years to go. So check you’re homeowner’s policy, but I think we’re going to be okay for awhile. The interesting thing is that the more massive stars have shorter lifetimes. So, a star that has ten times the mass that the sun has a lifetime that is not measured in billions of years but only millions of years. And the very massive stars have very short lifetimes compared to the age of the earth, or compared to the age of the sun. so that means that there’s a complicated story. When stars form, the massive stars live fast, dies young; blow up, it turns out at the end. They explode as supernova explosions. 

There’s another path that also leads to explosion where a star uses its fuel and crunches down in the center to become a white dwarf, which is a very dense kind of star that is left over after an ordinary star has used its nuclear fuel. The sun will probably be a white dwarf five billion years from now. In between, it will become a red giant. It will swell up and it will vaporize all of the planets, so if you’re worried about this sort of thing. You should worry. It will heat up the earth; it will boil off the oceans, melt the mountains, all that apocalyptic stuff will happen. But anyway, the lifetimes of stars are finite. The massive stars live short times, and the thing that is left over from a star like the sun is this white dwarf, which it turns out, is actually a thing that is held up by quantum mechanical forces. It’s stable if it’s left to its own devices and the white dwarf that is left after the sun has burned out, this kind of ash, this clinker that’s left over, will last indefinitely into the future.  Except, some of these white dwarfs – a lot of stars are in binaries. So, the sun is a single star, it’s all by itself and has a few planets around it, but there are other stars that when they are born are born in multiple systems. In a litter of other stars. And it’s interesting because the other star can affect what happens. For example. If there’s a white dwarf in a binary system with another star, some mass from that other star, when it swells up to become a red giant can fall onto the white dwarf. And it turns out, that can make it explode as a thermonuclear bomb. So, there are stars which, left to their own devices will be fine, but in these binary systems they will explode. 

So there’s a particular kind of explosion, these thermonuclear supernovae and these exploding white dwarfs, which have turned out to be incredibly important for measuring the size of the universe and for learning what the history of cosmic expansion has been. 

It comes to something like this, that we judge the distances to objects from their brightness.  You can go outside at the seashore and you look out and you see that there are ships out there. And you judge – you can’t really see much detail at night, but you can judge which are near and which are far with the apparent brightness of their lights. And in something like that, you can use the brightness of objects to judge their distance. The trouble with that is, it only works well if the objects really are the same brightness. Otherwise, if you have something that is intrinsically very bright, you’ll think, oh it must be nearby. And if something is intrinsically faint, you’ll say, oh that must be very far away. 

So if you have objects of different brightness, it’s much harder to sort them out. And it turns out that these exploding stars, these thermonuclear supernovae, the ones where a white dwarf blows up suddenly and for a little while shines as brightly as about four billion stars like the sun, these things turn out the have a fairly narrow range in brightness. They’re not all exactly the same, but it turns out it’s even better than that. By studying them we can tell which ones are the 100 watt bulbs, and which are the 50 watts, and which are the 25 watt light bulbs. We can actually measure other properties of the explosion. And that is extremely valuable. So, they’re very bright, they have a narrow range of brightness, and when we see a supernova like that we can measure how bright it appears and figure out how far away it is. 

This is very good because we want to figure out what the universe has been doing over time. What we know is that the universe is expanding. People have measured that the nearby galaxies are moving away from us. More distant galaxies are moving away from us more rapidly. It’s as if the whole universe is expanding in all directions. And we think that this is the result of, or anyway, the aftermath of the Big Bang, this sort of beginning of cosmic expansion about 14 billion years ago. And what we’re trying to do is to figure out how it’s changed over time. Has the universe expanded at a constant rate? Has it been slowing down due to gravity, as everyone expected it to? Or, as once – I don’t want to spoil the suspense, but what we’ve found is that the universe is actually speeding up, which is very mysterious. 

So. The supernovae are really important because we use them to measure distances. If you can figure out the relation between velocity and distance, that is the expansion and time, you can actually trace the history of cosmic expansion. 

Question: How did you determine from studying supernovae that the universe’s expansion is accelerating? 

Robert Kirshner: Right. So, in the 1990’s, everybody thought that we would be able to use the supernovae to measure the slowing down of the universe because there had to be enough matter in there for the universe to be just bound and slowing down. Everybody kept telling us that’s what we would find. 

So, we started to make the measurements and it was kind of interesting because the other group, the LBL group, published their results and said, “The universe is slowing down. We measured it with the supernovae and it seems to be slowing down.” So, that seemed good, except when we did our analysis of our data, we found that it didn’t seem to be slowing down. So, this was very awkward. We didn’t know what to do. We thought maybe we had made a mistake, or something. I don’t know. 

But as time went on, going from 1997 into 1998, both teams started to talk about their results and both of us saw that in our data, things were not slowing down. It’s as if the universe had been speeding up over time. So it was slowing down at the beginning, I suppose, but then there’s been a switch from slowing down to speeding up. A kind of acceleration in the expansion of the universe. 

Well, this was a big surprise to us. But it’s something that people had been talking about and had been thinking about for actually quite a long time. It turns out, Einstein, when he first applied General Relativity to the universes thought that the universe was equal to our Milky Way Galaxy because people didn’t know that these other galaxies were out there. So, when it came time to construct a model, a theory for how the universe would expand over time, he thought, static would be a good kind of – no expansion, that would be good. And he knew that if he had gravity that things would clump up, so he put into his equations another term to make things kind of a static universe. That was this so-called cosmological term, or cosmological constant. And there was no real reason that he did it, except it made it kind of easy to think about the universe as having a kind of finite volume and it avoided some technical problems and it made a static universe. 

But what astronomers found in the next decade was this business of the galaxies rushing away from us. The expansion of the universe. So, at that point, say in the 1930’s, Einstein said, well forget the cosmological constant. If it doesn’t serve its purpose, then let’s not talk about it. But it does have this quality of making things speed up. The modern way to talk about it is as if it’s an energy associated with empty space. So you know, in quantum mechanics, there are a lot of weird things. And one of the weird things is that, if you look on smaller, and smaller and smaller scales, the uncertainty in the energy gets bigger and bigger. And that means, you don’t know when you’re looking at a little tiny piece of space whether it might have in it a particle and it’s anti-particle that get created out of nothing and then annihilate. On average, the value is zero, but the fluctuations are big. That’s what the quantum mechanics tells you. And the idea is, that there could be, the vacuum, is not just emptiness with no properties, but it could have, be sort of interesting properties. And that’s the modern theory of electro-magnetism. It’s like that with particles and there are anti-particles being created and destroyed all the time. And that picture fits the data, which have been exquisitely measured in laboratories on the earth, much better than a picture of a vacuum with no properties. 

So this would be something similar for gravity. Now, nobody knows how to do this. Nobody knows how to make a proper quantum theory for gravity, so making the right prediction about what the properties of empty space ought to be is a little tricky. If you do it more or less the way people have for other forces in nature, it turns out you get a really bad answer because the energy associated with the vacuum gets big as the length that you’re looking at gets small. And it turns out that for gravity, the appropriate length is so small, and the energy that is associated with that is so big, that people have known since the first time they wrote this down that the universe isn’t like that. 

So, what this says is, the cosmological constant is very embarrassing to theoretical physicists. The number they got was, first of all Einstein had said it was a bad thing, then when you did this calculation, the number they got was way too big, like 10 to the 60th, instead of .7, which is really bad. So, for a while, people just didn’t want to talk about it. They didn’t want to think about it and they thought, well, if it’s not infinite, it must be zero.  That was one way to deal with it. And for an observational astronomer, like me, the idea was not to worry about cosmological constant because we thought we were going to measure slowing down. 

Anyway, we measured cosmic speeding up, the natural way to get that is to have energy associated with the vacuum that is like the cosmological constant that can make the universe speed up over time. And that’s the idea that people have used as kind of a beginning to think about how to understand these observations that we really see a universe that’s expanding faster now than in the past. So, the idea is, there is something. We can call it the dark energy that resembles Einstein’s cosmological constant, more or less, that is really propelling the universe. That is making it expand. The trouble is, the way we talk about it, but we don’t understand it very well, is we don’t know – so if you do a calculation of what is the value of that number, you get a very wrong answer. So that’s telling you something’s not right. And there are plenty of other possibilities; it doesn’t have to be constant. There could be something that changes with time. You know, the dark energy could be any of a very large number of things that people have thought of. So there’s this kind of vast, weedy garden of ideas that people have thought of. Well, what about this, what about that, what about the other thing. And as an observer, what you want to do is do the best you can to put constraints on this and kind of weed out the garden. Show that some of these ideas are wrong and people shouldn’t waste their time pursuing them. 

So the idea is with the observation – from the observational side was to kind of weed out this garden and find out which ideas are right, or anyway, could be right and show which ones are wrong so that people don’t waste their time working on those. So, that’s where we’re getting with this. We’ve seen the effect; the universe has been speeding up in the last five billion years or so. We’ve also looked far enough back into the past to see that the universe was slowing down before that, so that’s good. That’s a prediction of this kind of gravity and dark energy picture is that if you look far enough into the past, gravity should have been winning, that density should have been higher. So things sort of fit together. 

But there was this very bad feeling that everybody has that we don’t understand in any basic way what this dark energy is. What’s more, there’s no physics experiments you can do. There’s no laboratory experiment that you can do, or that people have thought of anyway, that really tells you whether this idea is right – what the properties of the dark energy really are. So, it’s a very interesting thing where in astronomy, we see the effects, we talk about it as if it’s real, and we can try to pin down what the properties of the dark energy are. Is it like the cosmological constant? Does it change with time? That’s a good question you can try to work on that. And we are working on that. But there’s no laboratory experiment. It’s kind of a funny sort of thing where only the astronomy is telling us about this very big phenomenon that turns out to be very important. It’s two-thirds, or maybe three-quarters of the universe is in this form of the dark energy. Now that’s really weird and we discovered three-quarters of the universe in the last 10 years. We don’t know what it is. And there’s no laboratory experiment that helps us kind of sort this out.

Recorded on February 17, 2010
Interviewed by Austin Allen