The Amazing Expanding, Accelerating Universe
In 1998, teams led by astrophysicists Saul Perlmutter, Brian Schmidt, and Adam Riess announced a surprising finding: Rather than decelerating due to the gravitational pull of its matter, the universe’s expansion was actually accelerating. The trio would later win the Nobel Prize for this discovery, which set the stage for research into the dark energy thought to be driving the expansion.
In early 1999, when the discovery was still fresh, Ira sat down with astrophysicists Neta Bahcall, of Princeton University, and Wendy Freedman, then-director of the Hubble Key Project and a professor at Carnegie Mellon University, to digest the news and ponder other cosmic questions.
Neta Bahcall is a professor of astronomy at Princeton University Observatory in Princeton, New Jersey.
Wendy Freedman, a former team leader of the Hubble Key Project, is a professor of astronomy and astrophysics at the University of Chicago in Chicago, Illinois.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. As we mark our 25th year, it’s interesting to look back and discover just how many discoveries were announced on this program and we’re revisiting some of them today like this one nearly 20 years ago with astrophysicists Neta Bahcall and Wendy Freedman.
Teams led by Saul Perlmutter, Adam Riess, Brian Schmidt had just announced the discovery that the expansion of the universe was not slowing down, but accelerating. And that discovery, as you know, came to be known as “dark energy.” It was also a time when we believed the universe was much younger than recent calculations have suggested.
Welcome back to Science Friday. I’m Ira Flatow. We’re going to take you to the outer reaches of the universe now. And you won’t need to stand in line or get popcorn all over the floor, either, so don’t worry about that.
Long ago, say about 1929, Edwin Hubble discovered that the universe was expanding. And ever since then, astronomers and astrophysicists have tried to pin down exactly how fast that expansion is taking place. Because the answer to that has a lot to do with big, big questions like how the universe is, and what’s going to happen to it in the distant future? And we’re talking billions of years from now.
Well, this week, a team of astronomers working on a project on the Hubble Space Telescope– the key project of the Hubble, in fact– announced that after eight years, they have the best estimate for that number to date. And that number we’re talking about is called the Hubble constant. And it will help clear up a lot of mysteries about the universe.
And for the rest of this hour, we’re going to be talking about research into astrophysics, and the Hubble constant, the Hubble Space Telescope– same name, different kind of thing. And joining me now are Dr. Neta Bahcall, a Professor of Astronomy at Princeton University Observatory in Princeton, New Jersey. She’s also one of the authors of a paper in Science magazine this week, reviewing some of the most recent cosmological research. And she joins us from the Princeton campus. Welcome to the program.
NETA BAHCALL: Well, nice to be here, Ira.
IRA FLATOW: Nice to have you. Dr. Wendy Freedman, an Astronomer at Carnegie Observatories in Pasadena, California, and team leader of the Hubble key project. She joins us from Batavia, Illinois. Thank you for joining us, Dr. Freedman.
WENDY FREEDMAN: Very nice to be here.
IRA FLATOW: The project you’ve been working on, the key project of the Hubble, the main thing it was designed to do, right?
WENDY FREEDMAN: That’s right. When the Hubble Space Telescope was first being planned, in fact, this was the project that the Hubble was designed to do. In terms of priority, it was the number one ranked project.
IRA FLATOW: To find out what the Hubble constant is, to pin it down.
WENDY FREEDMAN: To pin it down to an accuracy of 10%. It’s something that astronomers and astrophysicists had been arguing about on a level of a factor of two.
IRA FLATOW: And what is the argument? Basically, give us the argument, and why that’s so important.
WENDY FREEDMAN: The unknown, what we’re trying to measure, is how fast the universe is expanding. The Hubble constant measures that rate at the current time. And if we can measure how fast the universe is expanding now, that gives us some information.
In a sense, it’s like playing a movie in reverse. If we know how fast it’s expanding, then we can tell how long it’s been expanding. And it gives us information about the age of the universe. If we also know what the density of the universe– of matter in the universe– is.
And it turned out to be much more difficult to measure than originally anticipated by Edwin Hubble when he discovered the expansion. And the real difficulty comes in measuring how far away galaxies are from us. And that’s what the Hubble Space Telescope allows us to do much more accurately than it was done in the past.
IRA FLATOW: So you pointed the telescope at some key stars to look for that constant?
WENDY FREEDMAN: The way we measure distances– one of the most accurate means that astronomers have to measure distances to galaxies– depends on a certain type of star known as a cepheids. And it has a unique property that it changes in its brightness. And that’s not true for most stars.
If we look at the sky, and if we just look up at the sky or stars at night, they don’t change, at least on a time scale of an ordinary human lifetime. But these cepheids change on timescales of maybe two days or to a few months. And how fast they change is directly related to how bright they are. And that gives us a unique predictor of the distance.
And it turned out that from the ground, it was very difficult to find these stars in very many galaxies. But by getting above the Earth’s atmosphere, which the Hubble does, we don’t have the blurring that takes place because the atmosphere is in motion. And so suddenly, we could survey a volume that was 1,000 times as great, or a distance 10 times as great, as we could do from the ground. And that was the real breakthrough that Hubble allowed.
IRA FLATOW: And what number did you come up with for the Hubble constant?
WENDY FREEDMAN: The number we came up with for the Hubble constant is 70. That’s in funny units that astronomers use of kilometers per second per megaparsec. And what that means in units that are more familiar is that a galaxy that at a distance of about three million light years from us would be moving at a speed of 160,000 miles per hour. And what the Hubble expansion or the Hubble law tells us is that the further a galaxy is away from us, the faster it’s moving. So an object 10 times further away is moving 10 times faster than that.
IRA FLATOW: So now that we know how fast the universe is expanding, does that narrow down the age of the universe?
WENDY FREEDMAN: It does narrow down the age. But I want to emphasize also that it’s not the only quantity that we need to know to determine the age. We need to know, one, how fast is the universe expanding? And two, how much matter is there in the universe?
And the reason you need to know how much matter there is, is that gravity will tend to slow the expansion down. And we call this quantity the Hubble constant, which only refers to it being constant at the time we’re measuring it now. That actually changes over time because of the presence of gravity, and then possibly other forces, which might be accelerating the universe.
And so we need to know all those quantities to accurately determine the age. But if we use the best estimates for what the matter density is– and that’s something that Dr. Bahcall is an expert in– then we come up with an age for the universe of about 12 billion years, given this Hubble constant that we’ve measured with the Space Telescope. And if there is an accelerating universe, that means that the universe is speeding up.
So in the past, the Hubble constant would have been smaller. And that leads to an older age for the universe. So it could be as high as 13 or perhaps even 15 billion years old, in that case.
IRA FLATOW: Dr. Bahcall, how confident are you in this new estimate of the age of the universe?
NETA BAHCALL: Well, it seems to be a quite robust estimate within the range that we just heard that Wendy was talking about. There are many different measurements now regarding the mass density of the universe, and some suggestions about whether the universe may be accelerating or not. Combining it with the measure of the Hubble Constant that we just heard from Wendy really gives quite a robust estimate for what the age of the universe is based on the model, which is ranging from about 12 billion years to about 14 or 15 billion years, if there was this extra energy– that sort of anti-gravity force– in the universe that pushes the expansion, and accelerates it. And that age seems to be very consistent with the direct observations of the age of the oldest stars in the universe.
IRA FLATOW: You haven’t been able to answer what that strange expansion force is.
NETA BAHCALL: Well, the interesting thing is when you put together all these recent observations, the data does suggest– and not only from one source, but from different types of observations– that such energy– dark energy, we call it, something that opposes gravity, an anti-gravity type energy– pushes back and accelerates the expansion rate of the universe, which is really quite puzzling. As Wendy just mentioned, if we had, as we have been thinking all the way since the days of Hubble, if there was only matter in the universe– and that was the expectation– the only thing that matter can do by its gravitational attraction is slow down the expansion of the universe.
IRA FLATOW: Right, because the gravity would pull itself back.
NETA BAHCALL: Pull itself back together. And what the observations suggest is that, instead of slowing down the expansion rate of the universe, the expansion rate may actually be speeding up. And that is very strange, because gravity can just not do it.
It’s like when you throw a ball in the air, you expect that at some point, depending on how fast you throw it, at some point gravity will pull it back down. And you will be terribly surprised if you throw the ball in the air, and it will keep just receding from you faster and faster and faster. Then you’ll ask, what’s doing it?
IRA FLATOW: I hate it when that happens. What is the universe expanding into?
NETA BAHCALL: Yes, I think that’s a fascinating question. And one way of explaining it in the sort of expansion of the universe or the history of the universe– it’s not a question of where is it going to– there is some other space outside there and we are moving into. But that the universe itself, the space-time itself generated by the matter and the energy in the universe, is the one that is stretching the universe, and creating the space and time.
IRA FLATOW: It’s hard to imagine. Our analogies just break down at some point, don’t they?
NETA BAHCALL: I think that’s exactly right, Ira. The analogy of what we are used to from our life here on Earth, or even getting a little bit outside of our Earth to the solar system and nearby stars, starts breaking down intuitively when we think about sort of the edges of the universe, or how the universe started, and where is it going, and so on.
IRA FLATOW: Falling Waters, West Virginia, go ahead.
FRED: If particles are traveling out into space, eventually, I figure they would, by theory, lose gravity of each other. And if there was any kind of a minor curve in the universe, it seems to me without gravity, they would loop around and come back to their– close to the original spot. I just wanted to ask your guests, have they measured any curve in the universe at all through their measurements?
NETA BAHCALL: That’s a very interesting question. This is one of the things that we address in our review article in today’s Science issue, where we combine the different observational results of the last couple of years or so, few years. And the combinations of the different observations suggests that the universe is flat– that we do not see any curvature in the universe, that there is some acceleration as we talked about, and that the mass of the universe is much too small to stop the expansion.
IRA FLATOW: So it is flat, you’re saying?
NETA BAHCALL: Yes, the suggestion of putting together all these current observations–
IRA FLATOW: So what happened to the whole idea of curved space and things? That’s done?
NETA BAHCALL: Yes, well, space is curved depending on how much matter and how much energy exists in it, as predicted by Einstein, of course, earlier in the century from general relativity. And these recent measurements combined together, both from supernova observations and from this old remnant from the Big Bang, the microwave background radiation, combining these together with the mass density that we observe suggests that the total mass plus energy in the universe is exactly balanced, to give it a flat space– in other words, no significant curvature.
IRA FLATOW: I’m Ira Flatow. And this is Science Friday from PRI. You’re listening to an interview with astrophysicist Neta Bahcall and Wendy Freedman from 1999 about the accelerating expansion of the universe. Dr. Freedman?
WENDY FREEDMAN: Yeah, another way of expressing that is that astronomers or cosmologists talk about something called a critical density, which corresponds to this flat case that Neta is referring to. And so if there is more matter in the universe than this critical density, then the expansion of the universe would be halted eventually, and the universe would collapse back on itself. If there is less matter than this critical density, then the universe could keep expanding forever. This critical boundary and so-called critical density is where it’s right at that value, equal to the critical density.
IRA FLATOW: You know, Fred was asking, so what would happen, then, if I send a light beam out into space, theoretically? It could go on forever and never come back?
NETA BAHCALL: Yes.
IRA FLATOW: And that is something different that we’ve been thinking about all these years.
NETA BAHCALL: Well, that depends exactly on what the curvature is, as we discussed– what the curvature of the universe is.
WENDY FREEDMAN: It’s a way of expressing the geometry. And if we live in a flat universe– and that’s what the best current theories suggest, is that somehow, very early in the universe, there was a very rapid, exponential, in fact, exponentially fast period of expansion that essentially flattened out the geometry.
IRA FLATOW: Fred, blow your mind, like Kramer would say? It’s blown my mind?
FRED: Stranger and stranger. Thanks a lot.
IRA FLATOW: I have to go back to this flat space idea, because it’s just something, I think, that’s shocking.
NETA BAHCALL: That’s the most mind-boggling thing, isn’t it?
IRA FLATOW: Well, does that all these Star Trek ideas, where you have wormholes and things, because space is curved, and you can go through these time– is now gone, because we don’t have curved space anymore?
WENDY FREEDMAN: Another thing we didn’t allude to is that in general, the basis of all this Big Bang Theory of the universe is general relativity, which essentially tells you that matter curves space. And there are regions of very high-density black holes, and other exotic, interesting objects, where there can be such effects that people are investigating, that touch on science fiction– wormholes.
IRA FLATOW: So that will happen at those local areas, then, where you have black holes and things like that.
NETA BAHCALL: Well, there are certainly curvatures every place. Matter is, as Einstein’s general relativity theory says, there’s curvature any time you have matter. But when we look– the question was about the global curvature of the universe. There, the suggestion is that there is probably no curvature– that it’s probably flat, possibly due to this very rapid inflationary state right after the Big Bang
IRA FLATOW: I have about a minute for one last question. What do you do now, Dr. Freedman? You’ve answered the key question. You’ve got the Hubble. Do you just close up the lid on the Hubble and say goodbye now? Is that it?
WENDY FREEDMAN: Well, I guess I would say we’ve answered one of the key questions. But I’d also like to mention that there are other telescopes that are currently on the drawing board that are going to come online in the next decade that are also very exciting. And Hubble still is continuing to do lots of interesting things, many of which were not anticipated when the telescope was built.
IRA FLATOW: And another key question the Hubble needs to answer, quickly? I mean, is there something else that–
NETA BAHCALL: There are many exciting things that the Hubble Space Telescope, of course, is doing. The Hubble constant is clearly one of the key projects from the beginning, but there are many exciting discoveries that are coming almost weekly or monthly from the Hubble Space Telescope. And we certainly expect it to keep doing that.
IRA FLATOW: And we’ll keep coming back and talking about it as these results come in, because it is truly exciting. And I’d like to thank both of you for joining me. Dr. Neta Bahcall, Professor of Astronomy at Princeton’s Observatory in Princeton, New Jersey, Dr. Wendy Freedman, Astronomer at Carnegie Observatories in Pasadena and team leader of the Hubble Project. Thank you both for joining me today.
NETA BAHCALL: Oh, thank you.
WENDY FREEDMAN: Nice to talk to you.
IRA FLATOW: You’re welcome. B. J. Leiderman composed our theme music. Our thanks to our production partners at the studios of the City University of New York. If you’d like to write us, please send your letters to Science Friday, 19 West 44th Street, Room 412, New York, New York, 10036.
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