Podcast Transcription

Welcome to Science Talk, the weekly podcast of Scientific American for the week of February 18th, 2009. I'm Steve Mirsky. I just got back from Chicago, where I was at the annual meeting of the American Association for the Advancement of Science. On Monday February 16th, I was privileged to attend an historic press conference, which I'm going to share with you, on the current state of cosmology. The participants were M.I.T.'s Alan Guth, the developer of the inflationary model of the universe, Lawrence Krauss, a frequent contributor to Scientific American magazine and director of the Origins Initiative at Arizona State University, John Carlstrom from the University of Chicago, who studies the cosmic microwave background radiation left over from the big bang and Scott Dodelson of the Fermi National Accelerator Laboratory, who studies the origin and structure of the universe. In part one of the podcast, we'll hear from the four cosmologists, who discuss our understanding of the universe, and later, in part two, they take reporters' questions. The first voice you hear is Lawrence Krauss.

Krauss: The last decade has seen more revolutions in cosmology than perhaps the century before it, and our picture of the universe has changed completely, but at the same time we are at the threshold of addressing questions that I think it would be fair to say, a generation ago we wouldn't have even, not only no one [that we wouldn't have] even thought of asking, but never would have imagined would be testable. We may be able to probe literally physics of the universe at the time when the universe was less than a millionth of a millionth of a millionth of a millionth of a second old and, in fact, the current best ideas about what happened then are due to this guy…

Steve: This guy refers to M.I.T.'s Alan Guth sitting to Krauss' immediate left.

Krauss: … who developed something, an idea, which is called inflation, which is really currently the best-founded picture of what may have happened and the only picture that, sort of, provides a possibility of explaining what we see from first principles. That doesn't mean it's right, and that's the key question that we need [to] know. Currently, right now, with all of the new observations in the cosmic microwave background and galaxy structure and weighing the mass in the universe, it is all completely consistent with the ideas that are developed associated with the idea of inflation. But the key question is, is there any way to definitely test the idea? And one of the ways, one of the predictions of inflation, potentially, is if there is a background of something called gravitational waves—literally undulations in space and time that exist throughout the universe—and two other gentlemen that are here, John Carlstrom, he is one of the experimental leaders in looking at the cosmic microwave background radiation, which is currently our best probe of the universe. It is the most, some people would say, "What if it turned cosmology into a precision science?" That instead of being able to measure things to an order of magnitude, we can measure things to as many decimal places. And one of the probes that we might actually, that the cosmic microwave background might actually provide for us, is a probe of gravitational waves. And we have been living in the golden age of the cosmology, as people say, and the question is, "What's going to happen in the near future?" And of course we don't know. We are getting so close to threshold questions, fundamental questions about the universe that we may be at the limits of what we would call falsifiability. Our ability to definitively rule out ideas, maybe [will have begun to] be limited, because the grandeur of the ideas that we are testing may become so real. Inflation is really a remarkable idea that is simple and beautiful. Right now, it is an idea more than a model. And it could be that we may end up with observations that are completely consistent with inflation, but we may not be able to say, for certain, whether it happened or not. We may to have live with that. But it gets worse. The good news about the universe is that as bad it is; now it is going to get a lot worse, so you should enjoy it. And the future of the universe is based on what we now have been able to measure, completely miserable. The longer we wait, the less we will see of the universe, the more of the universe will disappear. The fewer measurements we will be able to make on nature, and so even our presentability to falsify the universe might get a lot worse. And these crazy ideas have suggested mainly to a change in the nature of science, [the] most puzzling observation that has been made in the last decade is that the universe seems to be full of this, something called dark energy, empty space, it's full of energy. If you get rid of all the radiation and matter from the universe, empty space still weighs something. But the crazy thing about empty space, weighing something—[well,] there are many crazy things—it produces a gravitational repulsion, rather than the attractions so the expansion of the universe is speeding up; but this stuff is so mysterious and inexplicable—completely inexplicable right now—that many physicists have been driven wild and mad and have changed what we might mean by fundamental physics by suggesting, for example, that the fundamental concepts in nature are not really fundamental at all, they are accidental; they are an environmental accident; that the are many universes and we just happen to live in the one that has the values it does because if you changed it a little bit then we wouldn't be living. Namely the universe is way it is because there are astronomers who can go out and measure it. And that may sound like either a [tautology] or a religious statement, but it is neither. In fact, in honor of Darwin its almost like a kind of cosmic evolution, [a] kind of cosmic natural selection. It's not too surprising that bees can see the colors of flowers, not because they were designed to do so but if they didn't they wouldn't be able to find the food that they need, and similarly it has been argued that perhaps crazy things like this dark energy has the value it has because if it was any different, galaxies wouldn't form and if galaxies didn't form, stars wouldn't form and if stars didn't form, planets wouldn't form and if planets wouldn't form, astronomers wouldn't form. Now that has changed completely the nature of, if that's really true, it means the future of science is very different, because if there are many universes and in each universe the laws of physics are different, then may be we have to throw out fundamental ideas and the ability to make fundamental predictions in nature [and] have to start talking about probability. [And] if that's true, well, all hell breaks loose, I think, anyway. And then finally, the future of cosmology would get even worse. The amazing thing is that all evidence for the universe that we now see will disappear. All evidence of the big bang, this remarkable edifice of theory and observation that we built up over the past century, that has produced this cockamamy universe but one that we think we understand very well, all the data is consistent with a single picture of the universe. In the far future, observers will come up with the best laws of physics they can come up with. They will observe the universe, but they will in fact infer the universe is static and eternal and that we live in an island galaxy surrounded by nothing, which is precisely the picture of the universe that existed in 1900. And that will be the future of cosmology, which will be to come up with exactly the wrong picture of the universe.

Steve: The University of Chicago's John Carlstrom.

Carlstrom: You've just heard from a theorist (laughs) and as he said that in the last 10 years, all this information has been uncovered and our picture of the university is completely changed and I agree with that. I am an experimentalist, and if you think, I mean, the last 10 years are fantastic; it really is true. I mean we had this picture of the universe, but this picture of the universe has dark energy, dark matter, 95 percent of the stuff that makes up the universe, we really don't know what it is. So my view is it has been an incredible time for discovery and to think that we are somehow anywhere near done discovering or understand anything well, now to predict the next 10 [billion] to 20 billion years, I think it's not quite right and I am not as pessimistic. I think there are lots of discoveries to be had, maybe learn from the LHC what dark matter is—that's the Large Hadron Collider—and of course future cosmic microwave background has lot more to tell us. So, I'd say, stay tuned a little bit before you assume things [are the] way [they are because we're] there to view them and that's the end of the story.

Krauss: And I certainly hope that's the case. It's certainly true that every time we've opened a new window on the universe, we've been surprised. And my biggest fear, and I'm willing to bet, John, here in front of reporters, is that we've made remarkable discoveries in the last decade.

Carlstrom: Yeah.

Krauss: And we don't understand these things that we've seen. We don't understand dark energy, we don't understand a lot of, we've discovered the nature of the universe, but we don't understand [why] it is the way it is. And I'm concerned that we may, that experiment may have expired, I mean, in terms of being able to fundamentally eliminate these questions, and we may [rely] on theory—and if you are a scientist that's [a] dangerous thing to rely on—and so we may be at the threshold, we may require a new idea, and that's a lot harder.

Steve: Alan Guth.

Guth: I probably agree with John more than with Lawrence. I doubt very much that we have reached the limits of our ability to understand or to probe cosmology. I think that 10 years from now, we [will] almost certainly know what dark matter is.

Carlstrom: Yes.

Guth: I think those discoveries are just on the verge of being made; with the Large Hadron Collider and with the many detector experiments that are being set up, some scientists may have already detected something. I think that puzzle is going to be solved. With dark energy, it is harder to know what it is. It may very well just be vacuum energy. If it is just vacuum energy all we can do is, sort of, further constrain it to be more and more like vacuum energy. There's not going to be a new theory there, there is no one experiment that's going to show [that it] is vacuum energy, but there will be many experiments that will pin down its properties, so that we'll eventually perhaps decide [that it] is so much like vacuum energy that it just must be vacuum energy. I think there is also possibility of many other, you know, very exciting discoveries coming about. As far as Lawrence's criticism of this crazy idea that the universe is the way it is because astronomers are here, I think we may in fact have to live with that. There, I guess, I agree with what Lawrence said, that I think it's distasteful to think that science, or at least this particular aspect of science, might be reduced to the statement that the universe is the way it is because astronomers are here. The key thing that I think probably pushes us in that direction is this dark energy. I should maybe add, I don't think Lawrence really said it, the most peculiar thing about this dark energy, is really the fact that there is so little of it. Based on particle physics alone, if we were to naively guess what the energy density of the vacuum ought to be, you might think it was zero; and zero was sort of acceptable to us for a long time, but the quantum vacuum that physicists really know is not empty, so, zero was not really a very sensible answer anyway. But the natural answer that we would pick would be about a 120 orders of magnitude larger than the observed vacuum energy. So, the big question is "What in the world explains why the vacuum energy is so unbelievably small?" And one possible answer, I don't know if its right, and I think it would be a long time before we really settle on whether it's the best explanation or not—I think it is best explanation we have now, but that's not quite good enough to make it believable that this is the best explanation we are able to come up with. But I would like to explain that it really isn't very natural scientific explanation within the context of the way our theories of cosmology are moving. This theory of inflation, which gives us very good predictions for properties like the cosmic background radiation for the mass density of the universe, for things that we do really measure; that same theory in almost all of its forms predicts that there should not be just one universe, but the same mechanism that produced our universe [would] produce more and more universes without limit, producing an infinite number of universes. Furthermore, that idea gets combined with ideas coming out of string theory, which, I think, to most theoretical physicists is our best guess for the fundamental laws of nature. String theory tells us that there is not just one vacuum, but there is a huge number of vacua, possible vacua, 10500 or so, crazy numbers that people talk about, and that means that each one of them has a different vacuum energy. And if this idea of eternal inflation producing an infinite number of universes is right, those infinite number of universes could each have a different one of these 10500 vacua, so all of them would exist. And then you have to ask yourself, where in this multiverse would you expect to find astronomers to make the measurements of the energy density of the vacuum? And there are good arguments that you might only find them when the vacuum energy is incredibly small, because a larger vacuum energy blows the universe apart, [it] produces a repulsive force before galaxies could form, and if you believe that observers only form in their galaxies, no observers in those universes. Conversely, if the vacuum energy were negative, it would cause the universe to rapidly re-collapse and there would be no time for astronomers, at least the type we know, to come into existence.

Krauss: That's the key point. At least of the type we know.

Guth: Actually you're right, now there is a big uncertainty, I completely agree. So I would be much happier if all this will go away, but I will still argue that this is a very reasonable explanation, and it could even be the right explanation for why [the] vacuum energy that we observed is so unbelievably small compared to what we would theoretically expect.

Steve: Fermilab's Scott Dodelson.

Dodelson: I guess I may be a little more optimistic than all of you. (laughs) Over the last decade we've figured out how the universe got from A to B; A in this case is the picture that John and his collaborators have taken with the microwave background and the universe was only 400,000 years old and B is what the universe looks like today when it is 13.7 billion years old. So the amazing thing is we have so much confidence in our ability to understand how it got from A to B, I mean, [that we're now thinking of] extrapolating, past B and before A. So Alan has done that before A, maybe its inflation, and Lawrence has done that after B, saying maybe the universe will die this pessimistic death. I really don't know, and I think the most exciting thing is that the measurements that we are going to be taking over the next decade can inform us about both possible extrapolations. So, for example, by measuring properties of the dark energy, we can learn whether Lawrence is right or wrong, and those measurements are going to be taken over the next decade. And similarly by making more detailed measurements of this cosmic microwave background, we can learn whether this theory of inflation is correct. And just like on the ground it seems to me that observations are not dying out, there are more and more of them; every year there is another idea for how to test inflation. For example, gravitational way is being a real solid test of inflation, but over the last couple of years, there have been a number of other ideas studying these primordial seeds that John and his collaborators measured. The detailed properties of these seeds have been getting more and fine scale measurements for them. We can hope to test inflation in more and more precise ways. So I'm very optimistic about these extrapolations. We're asking grand questions, but we really have real hopes of answer[ing] them.

Steve: That's it for part 1. We will be back with the cosmologists answering reporters' questions in part 2 of this podcast. For Scientific American's Science Talk, I'm Steve Mirsky.

Science Talk is a weekly podcast, subscribe here: RSS | iTunes

In part 1 of this podcast, cosmologists Alan Guth from M.I.T., Arizona State University's Lawrence Krauss, John Carlstrom from the University of Chicago, and Fermilab's Scott Dodelson discuss the state of cosmology—and the universe's possible dismal future—at a press conference at the annual meeting of the American Association for the Advancement of Science in Chicago on February 16th.