QNAS QNAS QnAs with Alan Guth Paul Gabrielsen Science Writer The announcement in March 2014 that a few minutes of the history of the universe. telescope near the South Pole had detected Inflation comes before all that. possible evidence of gravitational waves PNAS: Whatwasyourreactiontothe brought renewed attention to inflationary results of the BICEP2 experiment, suggesting theory, which describes the earliest moments evidence of gravitational waves? of the universe. According to inflationary Guth: I was incredibly impressed that they theory, exotic matter present at the birth of had discovered what, at that time, appeared to the universe exerted repulsive gravitational be a completely unambiguous signal of gravi- effects, driving extremely rapid expansion of tational radiation coming from the early uni- the universe and leaving behind traces of verse, presumably from inflation. Since then gravitational waves in the cosmic background things have changed. The observations were radiation. National Academy of Sciences certainly a tour de force, but when other people member Alan Guth, a physicist at the Mas- looked more carefully at the ways the experi- sachusetts Institute of Technology, first out- ment could go wrong, the possibility that the lined inflationary theory in 1981 and has since signal could have been entirely due to dust been working out the details and implications couldnotberuledout.SorightnowIthink Alan Guth. Image courtesy of Jenny Guth. of the theory with his colleagues. The South it is waiting to be confirmed. I still hope that it Pole experiment, called BICEP2 (Background willturnouttobereal,butatthispointIthink ImagingofCosmicExtragalacticPolarization we don’tknowifitwasarealsignalofgravita- Henry Tye got himself interested in a class of 2), is undergoing further scrutiny to assess tional radiation from the early universe or not. particle theories called Grand Unified Theo- the possible influence of cosmic dust on the PNAS: Apart from BICEP2, what evidence ries. And he asked me whether these theories gravitational wave signal. Guth, however, supports inflation theory? would predict that magnetic monopoles points to other evidence that illustrates how Guth: Since the proposal of inflation, there should exist, a magnetic monopole being a inflationary theory can accurately describe the have been a growing number of measure- particle with a net magnetic charge, north earliest moments of matter and energy in the ments that have supported exactly what in- or south. We came to the conclusion that if universe. In May 2014 Guth, along with flation predicts. One of those is the prediction we combined conventional cosmology with fellow inflationary theorists Andrei Linde about the overall mass density of the uni- Grand Unified Theories, the universe would and Alexei Starobinsky, was awarded the verse. When inflation was first proposed, be predicted to be swimming with magnetic Kavli Prize in astrophysics. To commemorate the measured value seemed to be lower than monopoles, but there couldn’t possibly be the honor, PNAS spoke with Guth. what inflation predicted, by a factor of about nearly that many. So Henry and I started PNAS: What does winning the Kavli Prize 3 or 4. Then, in 1998 astronomers discovered thinking about whether or not it’s possible mean to you? that the universe is not slowing down under to change anything to make the Grand Uni- Guth: I think the prize helps to solidify the the influence of gravity but rather is acceler- fied Theories compatible with cosmology. This acceptance of inflationary theory, which has ating. This acceleration gives a demonstration inquiry ultimately led to the idea of inflation. a great deal of evidence behind it now, and that gravity can act repulsively and not just FromthetimeIwasakidIwasquite I’m glad to see the institutions of the world attractively. But once we knew the universe fascinated with the idea that mathematics can agreeing with that conclusion. was accelerating, we had to invent a new kind describe the real world. In high school a friend PNAS: Which came first, inflation or the of matter to be responsible for that. And of mine was doing a physics experiment, big bang? that’swhatwecallthe“dark energy,” which taking a yardstick and punching holes in it at Guth: The word “big bang” is used incon- we now think makes up most of the energy in different locations, and pivoting the yardstick sistently. In the eyes of most of the public, the the universe. Once the dark energy is added around those holes, letting it swing back and “big bang” indicates the instant of creation of to the other matter that we already knew forth, and measuring the frequency of the the universe. But in fact we don’t even know about, the empirical estimates of the mass swinging as a function of where the hole was. if the universe had an instant of creation. Nev- density of the universe agree precisely with I had learned just enough Newtonian me- ertheless, if the phrase “big bang” is used this what inflation predicts. So what had been a chanics to calculate what the answer was way, then certainly it comes before anything. significant problem for inflation has turned supposed to be. We took my friend’sdata However, among scientists, the big bang the- into an important success. and a slide rule and plugged in the numbers oryisreallyonlythetheoryoftheaftermath PNAS: Why have you devoted your life to to compare his data with my calculation, and of a bang. It’s a theory of how the early uni- unraveling the secrets of the universe? we were both shocked to see that it actually verse expanded and cooled. It includes, for Guth: I certainly did not set out to invent worked: his measured frequencies were very example, a description of how the lightest a new theory of cosmology. When I was a close to what my formula predicted. It was chemical elements were produced in the first postdoc at Cornell, a fellow postdoc named great fun. 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