QNAS
QnAs with Alan Guth Paul Gabrielsen Science Writer
The announcement in March 2014 that a telescope near the South Pole had detected possible evidence of gravitational waves brought renewed attention to inflationary theory, which describes the earliest moments of the universe. According to inflationary theory, exotic matter present at the birth of the universe exerted repulsive gravitational effects, driving extremely rapid expansion of the universe and leaving behind traces of gravitational waves in the cosmic background radiation. National Academy of Sciences member Alan Guth, a physicist at the Massachusetts Institute of Technology, first outlined inflationary theory in 1981 and has since been working out the details and implications of the theory with his colleagues. The South Pole experiment, called BICEP2 (Background Imaging of Cosmic Extragalactic Polarization 2), is undergoing further scrutiny to assess the possible influence of cosmic dust on the gravitational wave signal. Guth, however, points to other evidence that illustrates how inflationary theory can accurately describe the earliest moments of matter and energy in the universe. In May 2014 Guth, along with fellow inflationary theorists Andrei Linde and Alexei Starobinsky, was awarded the Kavli Prize in astrophysics. To commemorate the honor, PNAS spoke with Guth. PNAS: What does winning the Kavli Prize mean to you? Guth: I think the prize helps to solidify the acceptance of inflationary theory, which has a great deal of evidence behind it now, and I’m glad to see the institutions of the world agreeing with that conclusion. PNAS: Which came first, inflation or the big bang? Guth: The word “big bang” is used inconsistently. In the eyes of most of the public, the “big bang” indicates the instant of creation of the universe. But in fact we don’t even know if the universe had an instant of creation. Nevertheless, if the phrase “big bang” is used this way, then certainly it comes before anything. However, among scientists, the big bang theory is really only the theory of the aftermath of a bang. It’s a theory of how the early universe expanded and cooled. It includes, for example, a description of how the lightest chemical elements were produced in the first
few minutes of the history of the universe. Inflation comes before all that. PNAS: What was your reaction to the results of the BICEP2 experiment, suggesting evidence of gravitational waves? Guth: I was incredibly impressed that they had discovered what, at that time, appeared to be a completely unambiguous signal of gravitational radiation coming from the early universe, presumably from inflation. Since then things have changed. The observations were certainly a tour de force, but when other people looked more carefully at the ways the experiment could go wrong, the possibility that the signal could have been entirely due to dust could not be ruled out. So right now I think it is waiting to be confirmed. I still hope that it will turn out to be real, but at this point I think we don’t know if it was a real signal of gravitational radiation from the early universe or not. PNAS: Apart from BICEP2, what evidence supports inflation theory? Guth: Since the proposal of inflation, there have been a growing number of measurements that have supported exactly what inflation predicts. One of those is the prediction about the overall mass density of the universe. When inflation was first proposed, the measured value seemed to be lower than what inflation predicted, by a factor of about 3 or 4. Then, in 1998 astronomers discovered that the universe is not slowing down under the influence of gravity but rather is accelerating. This acceleration gives a demonstration that gravity can act repulsively and not just attractively. But once we knew the universe was accelerating, we had to invent a new kind of matter to be responsible for that. And that’s what we call the “dark energy,” which we now think makes up most of the energy in the universe. Once the dark energy is added to the other matter that we already knew about, the empirical estimates of the mass density of the universe agree precisely with what inflation predicts. So what had been a significant problem for inflation has turned into an important success. PNAS: Why have you devoted your life to unraveling the secrets of the universe? Guth: I certainly did not set out to invent a new theory of cosmology. When I was a postdoc at Cornell, a fellow postdoc named
14640 | PNAS | October 14, 2014 | vol. 111 | no. 41
Alan Guth. Image courtesy of Jenny Guth.
Henry Tye got himself interested in a class of particle theories called Grand Unified Theories. And he asked me whether these theories would predict that magnetic monopoles should exist, a magnetic monopole being a particle with a net magnetic charge, north or south. We came to the conclusion that if we combined conventional cosmology with Grand Unified Theories, the universe would be predicted to be swimming with magnetic monopoles, but there couldn’t possibly be nearly that many. So Henry and I started thinking about whether or not it’s possible to change anything to make the Grand Unified Theories compatible with cosmology. This inquiry ultimately led to the idea of inflation. From the time I was a kid I was quite fascinated with the idea that mathematics can describe the real world. In high school a friend of mine was doing a physics experiment, taking a yardstick and punching holes in it at different locations, and pivoting the yardstick around those holes, letting it swing back and forth, and measuring the frequency of the swinging as a function of where the hole was. I had learned just enough Newtonian mechanics to calculate what the answer was supposed to be. We took my friend’s data and a slide rule and plugged in the numbers to compare his data with my calculation, and we were both shocked to see that it actually worked: his measured frequencies were very close to what my formula predicted. It was great fun. Maybe that’s what hooked me. www.pnas.org/cgi/doi/10.1073/pnas.1417149111
QnAs with Alan Guth Paul Gabrielsen Science Writer
The announcement in March 2014 that a telescope near the South Pole had detected possible evidence of gravitational waves brought renewed attention to inflationary theory, which describes the earliest moments of the universe. According to inflationary theory, exotic matter present at the birth of the universe exerted repulsive gravitational effects, driving extremely rapid expansion of the universe and leaving behind traces of gravitational waves in the cosmic background radiation. National Academy of Sciences member Alan Guth, a physicist at the Massachusetts Institute of Technology, first outlined inflationary theory in 1981 and has since been working out the details and implications of the theory with his colleagues. The South Pole experiment, called BICEP2 (Background Imaging of Cosmic Extragalactic Polarization 2), is undergoing further scrutiny to assess the possible influence of cosmic dust on the gravitational wave signal. Guth, however, points to other evidence that illustrates how inflationary theory can accurately describe the earliest moments of matter and energy in the universe. In May 2014 Guth, along with fellow inflationary theorists Andrei Linde and Alexei Starobinsky, was awarded the Kavli Prize in astrophysics. To commemorate the honor, PNAS spoke with Guth. PNAS: What does winning the Kavli Prize mean to you? Guth: I think the prize helps to solidify the acceptance of inflationary theory, which has a great deal of evidence behind it now, and I’m glad to see the institutions of the world agreeing with that conclusion. PNAS: Which came first, inflation or the big bang? Guth: The word “big bang” is used inconsistently. In the eyes of most of the public, the “big bang” indicates the instant of creation of the universe. But in fact we don’t even know if the universe had an instant of creation. Nevertheless, if the phrase “big bang” is used this way, then certainly it comes before anything. However, among scientists, the big bang theory is really only the theory of the aftermath of a bang. It’s a theory of how the early universe expanded and cooled. It includes, for example, a description of how the lightest chemical elements were produced in the first
few minutes of the history of the universe. Inflation comes before all that. PNAS: What was your reaction to the results of the BICEP2 experiment, suggesting evidence of gravitational waves? Guth: I was incredibly impressed that they had discovered what, at that time, appeared to be a completely unambiguous signal of gravitational radiation coming from the early universe, presumably from inflation. Since then things have changed. The observations were certainly a tour de force, but when other people looked more carefully at the ways the experiment could go wrong, the possibility that the signal could have been entirely due to dust could not be ruled out. So right now I think it is waiting to be confirmed. I still hope that it will turn out to be real, but at this point I think we don’t know if it was a real signal of gravitational radiation from the early universe or not. PNAS: Apart from BICEP2, what evidence supports inflation theory? Guth: Since the proposal of inflation, there have been a growing number of measurements that have supported exactly what inflation predicts. One of those is the prediction about the overall mass density of the universe. When inflation was first proposed, the measured value seemed to be lower than what inflation predicted, by a factor of about 3 or 4. Then, in 1998 astronomers discovered that the universe is not slowing down under the influence of gravity but rather is accelerating. This acceleration gives a demonstration that gravity can act repulsively and not just attractively. But once we knew the universe was accelerating, we had to invent a new kind of matter to be responsible for that. And that’s what we call the “dark energy,” which we now think makes up most of the energy in the universe. Once the dark energy is added to the other matter that we already knew about, the empirical estimates of the mass density of the universe agree precisely with what inflation predicts. So what had been a significant problem for inflation has turned into an important success. PNAS: Why have you devoted your life to unraveling the secrets of the universe? Guth: I certainly did not set out to invent a new theory of cosmology. When I was a postdoc at Cornell, a fellow postdoc named
14640 | PNAS | October 14, 2014 | vol. 111 | no. 41
Alan Guth. Image courtesy of Jenny Guth.
Henry Tye got himself interested in a class of particle theories called Grand Unified Theories. And he asked me whether these theories would predict that magnetic monopoles should exist, a magnetic monopole being a particle with a net magnetic charge, north or south. We came to the conclusion that if we combined conventional cosmology with Grand Unified Theories, the universe would be predicted to be swimming with magnetic monopoles, but there couldn’t possibly be nearly that many. So Henry and I started thinking about whether or not it’s possible to change anything to make the Grand Unified Theories compatible with cosmology. This inquiry ultimately led to the idea of inflation. From the time I was a kid I was quite fascinated with the idea that mathematics can describe the real world. In high school a friend of mine was doing a physics experiment, taking a yardstick and punching holes in it at different locations, and pivoting the yardstick around those holes, letting it swing back and forth, and measuring the frequency of the swinging as a function of where the hole was. I had learned just enough Newtonian mechanics to calculate what the answer was supposed to be. We took my friend’s data and a slide rule and plugged in the numbers to compare his data with my calculation, and we were both shocked to see that it actually worked: his measured frequencies were very close to what my formula predicted. It was great fun. Maybe that’s what hooked me. www.pnas.org/cgi/doi/10.1073/pnas.1417149111
