Astrophysics Discovery of the Century!

An interview with Saul Perlmutter by Genevieve Shiffrar

June 30, 2003

Most astrophysicists believe we are entering a new "Golden Age of Exploration," every bit as exciting as when European explorers sailed to the New World or when Sputnik orbited Earth. Saul Perlmutter, head of the Supernova Cosmology Project at Lawrence Berkeley National Laboratory, is resolute.

He also believes that UC Berkeley's Physics and Astronomy Departments, UC Berkeley's Space Sciences Laboratory, and the Lawrence Berkeley National Laboratory are leading the world into this new age. He discussed some of the major discoveries made by his group and others at these institutions in an interview with Genevieve Shiffrar in May.

G S: Let's start with the "big picture." Astrophysics can seem like a rather intimidating field of study, but at its root lie perhaps universal questions. At the most basic level, what are the types of questions you are exploring?

S P: We are interested in very human questions—the kinds that people always have been asking. Cave men probably used to walk out of their caves, look up at the stars, and wonder, "Does the Universe go on forever? Will it last forever? Will it all come to an end someday?" For almost all of human history, the only way to answer such questions was to ask philosophers.

We live at an exciting moment. For the first time in human history, we actually can answer these questions in a scientific way. The advances have been both conceptual and technological.

The conceptual comes from the work of a number of scientists, in particular Albert Einstein and his theory of relativity. Relativity gave us a way of thinking. Now we can imagine taking measurements and writing down equations: "How should the Universe expand if you have mass in it? How should it expand if you have some energy in it?"

We also just happen to live in the one time in human history where we have advanced the technologies, including detectors, telescopes, and computers, to the point where we can use Einstein's concepts to measure what seem like philosophical questions. For me, living at this point in history is just amazing. We can simply make measurements to see if the Universe is going to last forever or not.

G S: You have measured if the Universe will come to an end someday? How did you begin to accomplish that?

S P: First, we wanted to determine how much the Universe's expansion was slowing down. This would indicate if it was going to last forever or someday slow to a halt and then begin to collapse to a final "big crunch" ending. We started working on this particular project over a dozen years ago. The project came out of work being done in the lab of Rich Muller in the Physics Department at UC Berkeley. (Our group included Berkeley physicists Carl Pennypacker and Gerson Goldhaber and now includes Eugene Commins.)

The supernova research community realized that a certain type of exploding star, or supernova, always explodes with essentially the same intensity. Therefore, we could use them as marker points across the huge distances of the Universe. It takes quite a long time for the light from these exploding stars to reach us. To bring this idea closer to home, the light from the sun we are seeing traveled for eight minutes to reach us. If the sun were to go out, we wouldn't know about it for eight minutes.

In the same fashion, the light from the supernova explosions takes millions, in fact, billions of years to reach us. And that means that anyplace you see starlight, you are seeing the past. The light from the next clusters of stars to us, the nearest satellite galaxy to our galaxy, left there around 150,000 years ago—around the time of the evidence of first humans on Earth. And, light has been traveling to us from the nearest cluster of galaxies since the time that the dinosaurs went extinct here on earth. So, it is like a time machine. The farther you look, the further back in time you get to see.

A lot of what we were doing over the years was developing the capabilities and techniques to find these distant exploding stars, these tiny pin pricks of light in this vast sky. We had to use the biggest telescopes to find the really faint ones. We needed to invent computer software and techniques and ways to screen, until eventually we were able to find just the supernovae we cared about.

Once we did that, we got to the point where we could find supernovae that were not just a few hundreds of millions of years back in time, but some that were billions of years back in time—some three billion, some five billion, some seven billion. We started using them to mark out what was going on at those different times in history. The beginning of the Universe was only 15 billion years ago and we are getting to see a good fraction of that time back in history.

G S: How far back can you see?

S P: There are other ways to go look even further back, but for these kinds of supernovae explosions, the furthest one discovered so far is about 10 billion years back.

G S: Wow, if you can see 10 billion years back, and the Universe is 15 billion years old, then you are almost there. You can almost see the beginning of the Universe! 

S P: Yes, it is incredible. They are explosions of light, like fireworks going off. A supernova brightens and then fades away over the course of a month or so. When the light reaches us, we see its brightening and fading away pulse from 10 billion years ago.

G S: I understand that finding and recording the supernova explosions create a way to see into the past. How does that play into figuring out the future?

S P: There is a really simple way of reading off how big the Universe was then compared to today. If the Universe is stretching, that is, while every distance is getting larger during all the time that the light has been traveling to us from that exploding star, the wavelength of the light gets stretched, just like the Universe is getting stretched.

So we look at the colors of the various features of the supernova light, and the more red they are (that is, the longer their wavelengths), the more we know the Universe must have stretched since whenever the supernova exploded. We can easily make a chart showing how much the Universe has stretched since 3 billion years ago, 5 billion years ago, and 7 billion years ago, and what the history has been.

If you know what the Universe was doing in the past, you can predict what it will be doing in the future. You can also learn about what the space is shaped like. If, for example, you found out that the Universe was slowing more and more and more, then you know that it actually might slow enough someday to come to a halt and then collapse. Everything could fall in together, the whole Universe, and come to an end in a "Big Crunch."

G S: This sounds like a pretty awful ending. Is this our fate?

S P: When we first started to plot these supernova we wondered if the expansion of the Universe was slowing down—the very first few data points suggested this. But we continued to develop the techniques and collect the data. And what we discovered was in fact, that the data indicated that we were not slowing down at all. It showed the Universe was actually speeding up over the last 7 billion years.

This was a completely different result than anybody thought. There are a number of significances to this result. The most important is that, as far as we can tell, the Universe is never going to come to an end. It is just going to expand and expand, faster and faster, forever. And it will get to be a really lonely place. If it expands really fast, everything is going to quickly separate from us far out of sight and you won't be able to do astronomy any more. That is sad, but it means we have mere billions years or so to get our work done!

G S: Wow, go on. You say there are other significances?

S P: Yes, there is another aspect of the expansion of the Universe which is very exciting. There has to be, we believe, some energy that is associated with all of empty space which is making the Universe speed up its expansion. And that is something which has never been identified before. In fact, we think that maybe most of the Universe is made up of this stuff. And we've never learned about it before. In the past two years, it seems we have identified, or noticed the effect of, something that is brand new that we have not yet included in the physics theories.

G S: What is it?

S P: This is something that we are calling "dark energy."

G S: What do you know about it?

S P: Right now, we know very little about dark energy. The only things we know about it are the effects on the expansion of the Universe. Dark energy is spread out over the Universe very, very uniformly. We think all of empty space intrinsically has this dark energy. The reason we are unaware of it is that it has an extremely tiny density to start with and its effect only builds up over huge amounts of space. The only place it has effect is across vast reaches of space where you have a lot of it. Its effect on things, as far as we can tell, is almost purely on the shape of space itself, and the fact that it makes space want to "reproduce."

G S: How do we know that dark energy affects the shape of space?

S P: In the next few years after our discovery, there were other kinds of measurements coming in. For example, there is the glow left over from the Big Bang, the cosmic microwave background. Measurements from the cosmic microwave background showed that the Universe is very flat. Flat is a technical term--not meaning two dimensional. It means the Universe doesn't have a weird curvature in it that would make it so that if you traveled as much as you wanted in one direction, you would find yourself back where you were before. They found that it doesn't have a bizarre curvature: you can travel as far as you want in any direction, and you'll just keep going. It is like we always thought; there is nothing weird about that space.

But the discovery says something about dark energy. We knew already that the quantities of energy in the Universe curve space. The Universe becomes more curved or less curved depending on it. One form of energy is all the mass that we see, that we're made out of, that everything is made out of. It turns out that if you want to flatten the Universe, you have to add something extra. And that would be dark energy.

G S: So the measurements of the remnants of the Big Bang indicate that the Universe is flat. The amount of mass in the Universe isn't enough to make it flat, but if you add dark energy there is enough total energy to "flatten" the Universe.

S P: Exactly.

G S: Excuse me for saying this, but it sounds so otherworldly, almost unbelievable. It must have been controversial.

S P: Yes, it was. But, there was another group that used the same technique in finding supernovae, the High-Z Supernova Search Team, headed by Australian Brian Schmidt and including UC Berkeley astronomer Alex Filippenko. We were racing each other, and within weeks of each other, we both made announcements at conferences back to back in January and February of 1998. And our collective data agreed perfectly—the two data were a perfect match to each other. Our interpretations of the data, that there is probably dark energy in the story, matched as well.

So now I'd say that people in the field accept the fact that we are stuck with the mystery of dark energy. It was important that two teams got the same results. Then, a second kind of measurement agreed. And by now, I think people believe they have to accept it and try to figure out what this dark energy is.

G S: Like everyone else, I want to know more about dark energy. What can you tell me?

S P: Not much. If you ask the great elementary particle physicists of the world today, "What do you think it is?" They all say, "I'm clueless. I absolutely have no idea what this is. It is one of the biggest mysteries in science."

The theorists need to see some properties of this stuff. We are trying to develop new experiments that will tell us more about dark energy: How did it start? When did it start? These historical details could change the story and determine which theories work.

G S: So you are trying to use the supernova to determine when dark energy came into existence.

S P: And when it became the most important thing. In other words, it could have been there all along, but it may be that the mass density of dark energy was getting diluted by the expansion of the Universe that happens anyway. It might have been just sitting there, waiting. What was the exact time that that happened? Or the exact way in which it started to speed up the Universe? Did it speed it up immediately, or did it slowly begin to speed it up and then really speed it up? Different theories predict different histories. There are such tiny differences in these theories that it is a big job to figure out which ones are more likely to be correct. And to tell that apart from mistakes in your measurements is also very difficult.

G S: You could go so wrong with the smallest mistake.

S P: Yes. We need ways of controlling those little errors at a level where we could begin to trust the final result. And that is such a big job that I think it is going to be another 10 years for us to get to this next bit of information. Now along the way we are going to learn things, too. But to get to the level of precision that we'd like, that is going to take a while.

UC Berkeley, Lawrence Berkeley Lab, and UC Berkeley's Space Sciences Lab are at the forefront with the most dramatic techniques, ideas to be played with, and data sets coming in. The SNAP satellite (Supernova/Acceleration Probe) has been developed between the Lawrence Berkeley Lab and the Space Sciences Lab. It will work like a huge fisheye camera lens in the sky, measuring thousands of different supernovas instead of just 10s or 100s. We'll be able to study them in a level of detail that that no one's ever been able to do before, even nearby ones. And, this will go out twice as far in depth. So in all respects, the data set is going to be dramatically different.

And there is exciting work going on with the cosmic microwave background. For it, you need very unusual detectors, and UC Berkeley Physics Department faculty members Adrian Lee and Bill Holzapfel are building the next generation of these technologies. We expect that when those projects are complete, Berkeley is going to be one of the main centers for that.

G S: What would Einstein think about these ideas? 

S P: It would be interesting to know how he would even start to think about the next problem. For him, it would be that whole extra realm of things to start thinking about. He spent most of his life, after he did his amazing first discoveries, working on the question of how to bring together everything into a single theory. Physicists talk about separate forces, like gravity, electromagnetism, and the strong and the weak forces. Einstein was busy trying to fit them into a single story in which they all might make sense with each other. He wanted to know, "Why is it that there are just these forces and not any other forces?"

G S: A whole story for the world? That was pretty ambitious. 

S P: It was, it was. There are still people working on this. They have made progress, bringing together a few of the forces into a story. But they haven't managed to get gravity into the mix. By the time somebody figures out what this dark energy is, maybe we'll have a chance to pull this whole thing together, since dark energy is so associated with gravity. But, it is pure speculation. We have no idea if dark energy will help us understand gravity or make it harder.

We are making just baby steps at this point, because this is the first time in human history that we could make almost any step at all. In some sense, we should expect surprises. What we have seen is that our picture of the cosmos was too simple. We already have had to add dark matter—it makes up 30% of the Universe and we don't know what it is made of. Now, dark energy is two-thirds of the Universe. It seems to me that we are adding lots of different elements to the story, but it isn't a very elegant story yet. I'm hoping that a really smart theorist will wake up one day and say, "Wow, I just had an idea." "If we look at it a completely different way, the whole thing makes sense." Alternatively, that might never happen. It could be that the Universe is a complicated place.

This is the kind of thing everyone can have fun participating in. For the first time in history we can hear scientists say, "Maybe in the next two years, we will know what the Universe is made of." I think people would like to feel like they were living in those times and share that sense of discovery, like when we first went to the moon. It can tie people together more, to share a sense of our capabilities.

G S: It has been great fun talking to you. Thank you.