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IRA FLATOW, host:

This is Talk of the Nation: Science Friday. I'm Ira Flatow. A little bit later in the hour, we'll talk with tech guru Tim O'Reilly about social-media services like Twitter and Facebook. But first, one of the mysteries of astrophysics is that the universe is expanding faster and faster. Now, we always thought that after the initial rush hours from the Big Bang, the expansion of the universe should gradually slow down, braked by the force of gravity pulling matter inwards. So, for the rate of expansion to be increasing, some other force must be acting, countering gravity and pushing the universe outwards, and scientists have called that mysterious force dark energy.

Several years ago, researchers were able to put the first numbers to that dark energy by observing the movement of distant supernova. Now using different technology, different tools and looking at different objects, astronomers have been able to confirm the presence of dark energy in the universe, and their findings meshed nicely with the previous supernova base data. They always like it when that happens. Joining me to talk about this is William Forman. He's an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Welcome to the program.

Dr. WILLIAM FORMAN (Astrophysics, Harvard-Smithsonian Center for Astrophysics): Thank you very much for having me this afternoon, and thanks for inviting me to represent our group.

FLATOW: You're welcome. Was I going overboard when I said physicists are you always happy when they get those numbers to match up?

Dr. FORMAN: I think we're always happy - well, sometimes people are happy when they make a new discovery of finding something doesn't work. We'll talk about that a little bit as we go.

FLATOW: OK. Let's talk about dark energy again. How was it originally discovered?

Dr. FORMAN: I mean, originally the discovery, as you mentioned, was through the study of distant galaxies, and people used supernovae in those galaxies to measure their distances. They had built on previous work that used a particular kind of supernova that people thought was a standard candle, a well-known, absolute brightness. And what they did was to measure the apparent brightness, how bright (unintelligible), and if they knew absolutely how bright it was, they could determine how far away it was. And if you combine that with a red shift of the host galaxy in which the supernova was found, they then determine the shift of the spectral lines from its stars, and then these two teams were able to determine the expansion rate of the universe as a function of time. And as you mentioned, they found out that, in fact, rather than slowing under the force of gravity, as everyone had really expected, universe was really accelerating.

FLATOW: So, what did your group do differently than using the supernovious(ph) candle?

Dr. FORMAN: So, it's a very, very different kind of measurement. The previous measurements of supernovae and several others had all studied that geometry using distance to measure the expansion and how the expansion was behaving. And this was a very, very different experiment that we did with - that we had performed. And what we did was we studied the growth of structure in the universe. And it sounds a little complicated, but basically, we were measuring the competition between gravity's pull and the accelerated expansion.

What we did was to study clusters of galaxies, the largest collapsed objects that we know about in the universe, several hundreds of galaxies together. And we measured this competition on a new scale, about 100 million light-years. And the observatory that we worked with is the Chandra X-ray (unintelligible), and we studied these masses of clusters as they appeared about five billion light years - as these clusters of galaxies about five billion light-years away, (unintelligible) about five billion light years before, the present. And the goal was to compare the masses, the weight, of these clusters as they appeared about five billion years ago and as they appear at the present. So, we could actually measure this competition between accelerated expansion and the effects of gravity.

FLATOW: And so, it bore out the effects that there was the dark energy there.

Dr. FORMAN: And so, that's exactly what happened. What we found is as you go from five billion years ago to the present, you expect - if there was no dark energy, you expect a certain growth, and in fact, there was five times less growth. So, these clusters of galaxies were much less massive than we expected by a factor of five. And what's particularly nice about the study that we were able to do is that the x-ray is really uniquely suited to doing this kind of work. Mostly normal matter in clusters is hot, and it radiates in the x-rays, and the clusters are bright so you can see them far away. So, it's relatively straightforward to study them at these relatively large distances.

But what was really nice about the clusters is that they are especially sensitive to this competition between the acceleration and gravity. And it's sort of a little bit like the canary in the gold mine. The cluster growth, when the most of the growth of clusters is actually happening, is just when the supernova had predicted the effects of the dark energy was becoming important, about seven billion years ago. So, the clusters were like these canaries just sitting, there growing just at a time when the...

FLATOW: Wow.

Dr. FORMAN: Expansion was just taking off.

FLATOW: Well, let me ask the two obvious questions then. One is, confirming that there is this dark energy, number one, do we know why it kicks in about seven billion years ago? And two, does it bring us any closer to knowing just what it is?

Dr. FORMAN: So, I think we don't really - we certainly don't really know what it is. We can talk about that a little bit more. As far as kicking in, you know, at a certain time, the - as this universe is expanding, its density is decreasing. And in one of the models, if you think about Einstein's general theory of relativity, there was a place in there for a cosmological constant. So, as the density of the universe falls, when that cosmological constant becomes important, that's when the growth would start. So, I don't think anybody would be able to predict that it would happen at, you know, seven billion years in the past.

FLATOW: Right.

Dr. FORMAN: But you know, it might happen somewhere and that is where it just happened to occur.

FLATOW: Mm-hmm. And as far as knowing anything more about what it is...

Dr. FORMAN: So, as far as what it is it's a little bit, you know, uncertain. Basically, I think people don't know. You know, there're two basic, you know, possibilities. The one is that, you know, Einstein is correct and that this stuff that we see, this dark energy that drives the acceleration is what Einstein allowed for in his equation, this cosmological constant. And you know, it's just called this dark energy; it's a cosmological constant because it's constant with time; it doesn't change with time. And if Einstein is right, then this acceleration would make up about 70 percent, 72 percent of all the matter and energy in the universe. It's much more common, much more important than all the stuff that we're made out of. So, that's one possibility.

If you think about particle physics, the particle physicists would tell you that even empty space has some energy associated with it. And so, this could be the cause of the acceleration, this energy and mass associated with empty space. And the other possibility is, in fact, that Einstein is wrong, and that general relativity, although it does work over many, many different environments, does in fact break down when you get onto scales on the size of the universe. So, it could be something associated with Einstein's theory in particle physics and the properties of empty space, or it could be something totally new that we know nothing about.

FLATOW: Does it tell us anything more about how the universe will end?

Dr. FORMAN: Well, it certainly gives us a hint. So, we now have some measure of the magnitude of this dark energy, and people - you could think about it as a spring, springs embedded in space. And the stiffness of the spring, how much it's pushing, we've now sort of gotten a handle on. And so, we have a prediction; we can make some estimates of how the universe will now evolve. And so, what - based on what we know now, it certainly looks like that things that are already collapsed, things that already bound - Earth, our solar system, our own galaxy - those are going to stay pretty much as we see them. There's another in nearby galaxy, some of your listeners perhaps have heard of, called M31, Andromeda Galaxy. You can see it on a nice clear night the right time of year, and it's very likely that we're bound to that galaxy and that we will eventually merge. But objects that are much further away, a little bit further away, the nearest big cluster to us is the Virgo cluster. That cluster is actually going to, eventually, in a few more - about twice the age of the present universe, that's going to actually recede and become invisible to us.

FLATOW: Not something we have to worry about in the near future.

Dr. FORMAN: Not something that we have to worry about in the immediate near future, no.

(Soundbite of laughter)

FLATOW: So, where do you go from here?

Dr. FORMAN: Well, I think where we go from here is there're on-going studies certainly from the ground; there are plants from space to do much more detailed studies; currently the models of - people have made mostly have been able to test - mostly assume that this cosmos is constant, doesn't change very rapidly or in a complex way over time. And so, what we would like to be able to do is to have enough observations that you can actually map in great detail with precision how this - how the universe expands over time. That would allow you to really test in a much more critical way both Einstein's cosmological constant and also whatever other models people propose.

FLATOW: If Einstein is wrong, does that mean we have to throw away physics as we know it?

Dr. FORMAN: So, just as, you know, Einstein modified Newton a little bit, so someone will modify, you know, Einstein's the theory a little bit, presumably. You know, Einstein's theory applies to a, you know, huge variety of different phenomena. You actually use it when every time you use your GPS. There're, you know, general relative is the corrections that are needed, so it does work, you know, on scales on the size of the Earth, on the size of the galaxy, you know, to really quite great precision, but it may be breaking down, and there may be small corrections needed to make it apply to the universe as a whole.

FLATOW: Well Einstein looked at the universe as sort of geometric, you know, gravity being geometric; did he look at the Constant that way too? And would that mean we have to modify the geometry sort of the (unintelligible) of it?

Dr. FORMAN: What you say is exactly correct, that, you know, Einstein's theory was a geometric theory of gravity, and he compared to geometry of space with the matter and energy that was in it. And when he initially proposed his general theory of relativity, he wanted to try to make the universe static, and he had to put this extra cosmological constant in without any other real motivation. And he then thought that was perhaps one of his blunders, because it was later found out that, in fact, the universe was really expanding. So, (unintelligible) on one side of the equation, but now particle physicists would like to put it on the other side, and that's where the energy density associated with the vacuum. So, it's a slight modification to what Einstein put in, but still completely consistent with his concept of, you know, space time.

FLATOW: Dr. Forman, I want to thank you for taking time to be with us, and good luck to you.

Dr. FORMAN: It was a great pleasure. Thank you very much.

FLATOW: You're welcome. William Forman, astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, confirming the presence of dark energy. Still don't know what it is, but it is truly an exciting kind of thing to think about, and the fact that maybe Einstein's theory may have to be modified a little bit; maybe not. We'll see what happens as these pans out over the years. Stay with us. We're going to take a short break and come back and talk about the future of Facebook, Twitter, all those social communities, with Tim O'Reilly, who's written all kinds of journals and articles and the guru of all this kinds of stuff. You get to ask him questions, and we'll be right back after this short break.

(Soundbite of music)

FLATOW: I'm Ira Flatow. This is Talk of the Nation: Science Friday from NPR News.

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