An MIT Alumni Association Publication

Podcast: Inflationary Cosmology—Is Our Universe Part of a Multiverse?

  • Jay London
  • Slice of MIT

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MIT professor Alan Guth ’69, PhD ’72 pioneered the theory of cosmic inflation: a period of rapid expansion that that he posits occurred a fraction of a second after the Big Bang. And he also supports the idea that our universe is just one of many in a much larger multiverse.

“What we call the Big Bang Theory is really just a theory of the aftermath of some kind of a bang,” Guth says. “And inflation is a possible answer to what propelled this expansion. It's based on the idea that gravity itself can, under some circumstances, act as a repulsive force instead of an attractive force.”

Guth, the Victor F. Weisskopf Professor of Physics, shared his research on cosmic inflation, other ideas about the universe, and reflections on his career with an audience of MIT alumni in December 2019. He delivered his lecture at MIT’s Endicott House during a holiday luncheon event organized by the Alumni Association’s Cardinal and Gray Society.

“Does inflation suggest that we're perhaps living not in just an isolated, lonely universe but maybe our universe is, in fact, one of a whole set of universes, perhaps even an infinite set? “We certainly don't know the answer to that question, so I won't pretend to know,” Guth says. “But inflation does very much point toward the possibility of our universe not being unique but rather our universe being part of a much larger complex, which has come to be called the multiverse.”

Listen to the podcast above or on the Alumni Association’s SoundCloud page, or read the full transcript of Guth’s talk below.

Then learn more about the Cardinal and Gray Society, which invites MIT alumni who have reached the 50th anniversary of their MIT graduation to gather for social and intellectual events between and during their five-year reunion festivities.


Thank you. Thank you for that lovely introduction, Rick. It's a real pleasure to be here. I always enjoy speaking to my fellow MIT alumni. I thought I'd make today's talk a mix of experiences and talk about physics. So I thought I'd begin by saying a few words about how I came across this idea of cosmic inflation, the idea that Rick just spoke about, the idea that, in fact, did make my career work. In fact, enabled me to get back on the faculty at MIT. I received my PhD from MIT in 1972 and then, as Rick said, had a series of postdoc positions at Princeton, Columbia University, Cornell University, and then the Stanford Linear Accelerator Center. It was actually while I was at Cornell that I got involved in the work that led to inflation. And it was very much through the influence of a fellow recent MIT PhD, someone named Henry Tye, who at that time was also a postdoc at Cornell. And I had been working mostly on ideas that, in fact, turned out to be fairly old fashioned in physics. But Henry was up to date. And he knew that the new thing in physics at that time were a class of theories called grand unified theories. And he came to me one day and asked me whether these grand unified theories would imply that magnetic monopoles should exist. A magnetic monopole is a hypothetical particle. We still don't know if they exist. But it's a hypothetical particle that would have a net north or south magnetic charge. As you probably know, all the magnets that we actually have both a north and a south pole. But the possibility of magnetic monopoles is very real. And in fact, when he asked me if these grand unified theories would predict that magnetic monopoles should exist, he had to teach me what a grand unified theory was. But he did that. And then I worked on similar things. And the answer is, yes, grand unified theories do predict that magnetic monopoles should exist but they're outrageously massive, outrageously heavy. They weigh about 10 to the 16th times as much as a proton. And by and large, we discover new particles by producing them in particle accelerators. And the accelerator has to have enough energy to make them. So no hope of making these magnetic monopoles. So I told Henry that he should forget about it. But he immediately came back and said, well, why don't we try to figure out how many of them would have been produced in the Big Bang? I thought that was a crazy idea at the time, frankly, but eventually he convinced me to work with him on it. And that is what ultimately led to this concept of cosmic inflation.

There were some odd things going on. So Henry in fact left town just before I came to the idea of inflation or otherwise he certainly would have been a co-author of the original paper. But in December of 1979, I came up with this idea and started telling people about it, and started going around the country giving talks about it. And the idea was an immediate point of interest in the particle theory community. It took longer for the astrophysicists to get interested in it. I was still a postdoc at this time. This was my last postdoc at the Stanford Linear Accelerator Center.

And I went around the country giving talks at basically every place that had a job to offer about this new idea. And it went over very well. But one catch was that MIT did not have a job in particle theory that year. So I visited MIT during my travels and really felt wonderfully comfortable back at home at MIT. But since they did not have any job, they didn't mention anything about jobs. I didn't mention anything about jobs.

I gave a talk at Harvard that same time. They offered me a job. But by the end of the trip, I had a number of different job offers including the one from Harvard. But what I really wanted was to come back to MIT if that was possible. Everything hinged, it turned out, on a Chinese fortune cookie. It was the very last night of my six week trip around the country. I was at University of Maryland. And they took me out for a Chinese dinner, which was a very common thing among physicists then and now. And my fortune read-- hold on. I'll put it exactly to make sure I get it worded exactly right-- an exciting opportunity lies just ahead if you are not too timid.

I thought it was telling me something so I followed the fortune cookie's advice. And when I got back to California where I was working, it was on the weekend but the following Monday I worked up the nerve to call someone on the faculty in the particle theory group at MIT. I called Jeffrey Goldstone. And I didn't really know what to say, because I wasn't very good at these things. But I kind of got across the idea that I had a number of job offers but I really would be interested in coming back to MIT if that were a possibility. And he said he would talk to the rest of the group and get back to me.

The very next day, I got a phone call from the director of the CTP offering me an associate professorship which was fabulous. So it took a little while for the paperwork to go through and for me to receive an offer. And when I received the offer, I immediately accepted it. Interestingly, a week later, I was again having this time a lunch in a Chinese restaurant. And just immediately after I accepted this offer, the Chinese fortune cookie said, you should not act on the impulse of the moment. But I decided what would a Chinese fortune cookie know anyhow.

So I remained happy with my decision and have been very happy with that decision for the past 40 years or so since I've been back on the faculty at MIT. It's a great place to be, a great place to work, and I'm sure all of you feel the same way. So I thought I'd say a little bit now about this idea of inflation. What inflation is is a possible, and I would say very plausible, answer to the question of what propelled the Big Bang. It probably surprises many of you to know that, even though this theory that we always hear about called the Big Bang Theory, really says nothing whatever about the bang itself.

What we call the Big Bang Theory is really just a theory of the aftermath of some kind of a bang. It starts in its scientific version where we can write down equations, it starts with the universe already rapidly expanding and all of the matter already in place. So inflation is a possible answer to what propelled this expansion. And it's based on the idea that gravity itself can, under some circumstances, act as a repulsive force instead of an attractive force. This is certainly not the case for Newton's theory of gravity. That's always attractive.

But from the beginning, it was realized that Einstein's general relativity, which really is now our modern theory of gravity, does allow for gravity, under some circumstances, to act repulsively. And inflation is just based on the assumption that the early universe contained at least specks of matter of this peculiar kind that produce repulsive gravitational fields. And all you need is a tiny speck of this stuff. And because of its self-gravitational repulsion, a tiny speck will start to grow exponentially. And it will very rapidly become vastly larger than everything else around. And that's essentially what inflation is.

Now it turns out that inflation actually does make predictions which are testable, which is why you have handouts at your seats. So today I'd like to just focus on two of those predictions. One of them is inflation predicts what we should expect for the mass density of the universe, the average number of grams per CC of stuff, any kind of stuff, averaged over the universe. And it's generally measured in terms of what's called the critical density. The critical energy density is that density that would make the universe geometrically flat. According to general relativity, the universe doesn't have to be geometrically flat. It could be closed or open and a lot of other shapes if it doesn't have to be uniform. But if we assume it's uniform as it looks, it has to be the closed, open, or flat, flat being the borderline case. And inflation predicts we should be at that borderline case.

When inflation was first proposed, it looked like we were only at about 0.2 or 0.3 of this critical density. So it looked like inflation was doing badly. And during those years, I have to admit it was fairly uncomfortable for me to have dinner with astronomers because they would look at me and say, yeah, inflation is a beautiful theory but it predicts the wrong mass density.

All of that changed in 1998. In 1998, there was a crucial discovery by astronomers. They discovered that the universe is not slowing down under the influence of gravity as everybody certainly, including me, had thought. But rather now we know starting in 1998 that, for the last 5 billion years or so, the universe has actually been accelerating rather than slowing down.

And that acceleration is clearly a process very similar to the inflation itself that I spoke about. It's apparently repulsive gravity. And the stuff that drives that repulsive gravity is what we now call dark energy. We're not sure what it is. It's probably just the energy of empty space. But in any case, once one adds in this dark energy, the value of the total mass density, which is what inflation predicts, becomes equal to this critical density at least to an accuracy of about a half of a percent. So now inflation is right on the money for predicting the mass density of the universe.

In addition, and this is what your handout at your table is about, inflation gives a very plausible answer to where the nonuniformities in the universe came from. Our picture of the universe is that shortly after the Big Bang, the matter density in the universe was almost completely uniform. But it had small nonuniformities at the level of about one part in 100,000, incredibly small nonuniformities. And before inflation, nobody had any idea where these came from. But they were crucial for how the universe evolved afterwards. These small nonuniformities are amplified by gravity. In any region with a slightly higher than average mass density, there is a slightly stronger than average gravitational field that pulls matter in, creating a still stronger overdensity of matter, creating a still stronger gravitational field. And this process cascades and eventually produces all of the structure that we see in the universe. And astronomers think they understand that quite well now.

But there is still the question of where these nonuniformities came from and how would we ever predict what properties they might have. Inflation offers a possible answer to that. And the answer is rather dramatic, I think. The inflationary answer is that these nonuniformities are simply quantum fluctuations. As you probably know, classical physics, physics of Newton, is completely predictive. If you know the initial state, you can predict exactly where it's going to go, how it will evolve. But quantum theory is not like that. Quantum theory is fundamentally probabilistic.

You cannot predict exactly what will happen in a given situation. You can only predict the probabilities. So concerning the mass density of the universe, the classical prediction from inflation would have been that it would just be completely uniform. But that would never allow galaxies to form. There was no place for the matter to clump. But quantum mechanically, the classical prediction gets modified. So if the classical prediction was complete uniformity, when you look at the analog of that in quantum theory, the prediction is that in some places just at random, the mass density will be slightly higher than this classical average value. In other places, it will be slightly lower.

And that produces ripples on top of this almost uniform mass density. And that's exactly what was wanted to describe what we actually see in the early universe. We see the early universe mainly through what's called the cosmic microwave background radiation. This is radiation that just fills the universe. And we detect it on Earth coming from all directions.

It's thermal radiation. It looks like it's just the radiation of a hot body. And that's what we think it is. It's the radiation from the hot matter in the early universe. It has cooled by the expansion of the universe so now it's at a temperature of 2.7 degrees Kelvin. And now it's been measured to extraordinary precision.

And one of the things that inflation can predict is the spectrum of these fluctuations. And by spectrum, I mean pretty much the same thing we mean when we talk about the spectrum of a sound wave. When we talk about the spectrum of a sound wave, we imagine that we can break the sound wave up into components that have definite wavelengths. And the spectrum is just the description of how much intensity is there at each wavelength.

And the same thing can be done for the fluctuations that we see on the sky in the cosmic microwave background. And what you have in front of you is the graph of the most precise measurements we have to date, which come from a satellite called the Planck satellite, which flew from 2010. It's still up there although it's no longer taking data.

And what's shown is the intensity of the fluctuations as a function of the effective wavelength. In this case, the wavelength is measured as an angle because we're just seeing this pattern on the sky. And on the graph, the longest wavelengths are on the left and the shortest wavelengths are on the right.

And what's shown is the intensity of the fluctuations at each of those wavelengths. And as you see, there's just a marvelous fit between a blue line and a lot of red dots. And that fit between the blue line and the red dots says that inflation is telling us exactly what we should be expecting. The blue line is a description of a particular class of inflationary models. Inflation is not really a detailed theory but rather a class of theories. So the blue line describes what we think is the right class of inflationary theories. And the dots are the actual data with error bars shown where they're visible.

And as you can see, it's just a gorgeous-- it's my favorite graph in the whole world as you might expect. And I think it really is very strong evidence for inflation. And I quoted the Planck team at the bottom there saying that they regard it as powerful evidence for the simplest inflationary models.

OK. Let me divert to another anecdote now, which I think is fun. One of the marvelous things that happened as a result of my involvement in inflation was my winning an award called the Fundamental Physics Prize in 2012. And this was actually a $3 million award so it was very significant. I liked it.

And I thought I'd tell you a little bit about the story of how it happened. It started with my getting an email from a fellow, I don't think many of you can see this picture very well. We're unable to do slides here. But a fellow physicist named Nima Arkani-Hamed is at the Institute for Advanced Study. And on July 17 of 2012, I got an email from him which I'll read to you. It said, "Dear Alan, I hope all is going great with you. I am writing to you regarding a major new prize in fundamental physics. The matter is time sensitive so if you have any time to talk today, that would be wonderful. Please let me know where I can reach you. Hope to talk to you soon. All the very best, Nima."

So to be honest, it never occurred to me that they're talking about my winning an award here. I was sure that what he was talking about is getting me on a committee that choose people to win the award. That's much more common. I have been on many such committees. And I assumed this was going to be another case of that.

But I called him back later that day. And he told me very much to my surprise that a Russian billionaire named Yuri Milner was setting up this new prize. Yuri Milner himself had been a graduate student in theoretical physics but then turned to finance instead, made a billion dollars or so on internet companies, and wanted to get back to be involved in theoretical physics.

So they were initially awarding this award to eight or nine people. I think it was eight when I was first told and then one more was added before it became final. And when he told me it was $3 million, I assumed, of course, that the eight of us would be splitting the $3 million. But when I asked about that, he told me to my shock that, no, we'd each be getting $3 million.

So I of course didn't really know whether to believe it or not. My usual assumption is that if something is too good to be true, it's probably not. That's always been my experience. He told me that Yuri would call me in the next day or two. And indeed I did get a call the next day from someone with a Russian accent congratulating me on winning this award. And I said thank you and all that.

Then I guess a few weeks later, I got an email asking me to send my bank account information so they could deposit the money. And at that point, as you obviously realized from your laughter, I began to think about this. In fact, if I really looked at all my email, I probably get about one email a week telling me that I've won several million dollars if I only send my bank account information to some place or other. So I started thinking about the question of is this real or is this a hoax. I was quite sure that the person I was speaking to on the phone, the first phone call really was Nima Arkani-Hamed. He has a rather distinctive voice. And I know him quite well. But there was always the chance that we were all being duped, which seemed real. So I looked around and did discover-- I didn't want to pass up the opportunity of course. There was a chance it was real.

But I did discover that I had a bank account that we had abandoned but the bank account still exists. It had like $19 in it or something like that. So I thought I could easily risk $19. And I sent back the bank account number for that account. And then a few weeks later, I kept monitoring it, of course, on the web. And a few weeks later, the balance suddenly jumped from $19 to $3,000,004. It was a $15 wire transfer fee. At first, I was a little upset that they didn't pay that for me, but then I decided that was kind of silly. There's really no need to worry about the bank transfer fee. So it turned out to be real. And we eventually had a ceremony in Geneva and all that. And it's been awarded ever since. It now has actually grown. It's become part of what are called the family of Breakthrough Prizes, which now also includes biology and mathematics.

I think that's it. And they're awarded every year. One consequence is that the award now is decided by the previous winners so we have a committee that decides all this. For the first couple, we just got together and voted and it was all decided in five minutes. I and some other people thought that we probably should work harder and have nominations, and discussions, and things like that. So now it's gone full tilt the opposite direction. So now we really work hard with people being assigned to write reports on possible candidates, things like that. And then we have a full day phone conference each year to decide on the winners for that year.

And one thing we decided, by the way, in the last year was to award a-- I'm sorry. I'm going back a couple of years. But one of the things we decided, I guess it was last year really, was to award a special breakthrough prize for LIGO, the Laser Interferometer Gravitational Observatory, the big success of Ray Weiss at MIT and other people as well, the discovery of gravitational radiation. And that was, I think, a marvelous prize that we were able to award.

One feature of our awards which make it different from the Nobel Prize is that we have adopted the habit of recognizing full experimental groups. So in the case of LIGO, there were something like 1,000 people that were honored by the award. And $3 million, they still got something of significance. That wasn't a great deal.

So let's see. I should probably now get onto what was the subject of the title of my talk, which was the multiverse. Does inflation suggest that we're perhaps living not in just an isolated, lonely universe but maybe our universe is, in fact, one of a whole set of universes, perhaps even an infinite set. And we certainly don't know the answer to that question so I won't pretend to know. But inflation does very much point toward the possibility of our universe not being unique but rather our universe being part of a much larger complex, which has come to be called the multiverse.

The real difference is how many big bangs there are. That's how we count. So a multiverse means many big bangs and our big bang was just one of many. So what is the reason for this? I'd like to, first of all, maybe just say philosophically a lot of people think the idea of dreaming about multiple universes must be pure science fiction. And the hard-nosed scientific approach would be to assume that there's just one universe. But I would say it's exactly the opposite.

If we believe that there can be a scientific description of how the universe came into existence, and that's what the ultimate goal of theoretical cosmology is-- we're not there yet by any means but that is the goal. If we could ever imagine achieving that goal, then I think it becomes a relevant fact that everything that we know of that can happen in science always happens more than once.

There is absolutely nothing we know of that we think we understand in science which happens only once. In a religious context, once is a common concept. But certainly scientifically that's not the norm.

The norm is that anything that can happen can happen again and again. And I think it's natural to expect that that's the likely situation with regard to production of universes. So with more detail though, there are three occurrences that have happened recently in cosmology that have driven us towards very seriously considering the idea at least that our universe is part of a multiverse. And the first of those is properties of inflationary models.

As I mentioned, inflation is not really a unique theory. It's really a class of theories. But almost all inflationary models in fact lead to what we call eternal inflation, inflation that goes on forever. It's not eternal into the past, we do not think. But we do believe that once inflation starts, it appears to go on forever, producing not just one universe but in fact an infinite sequence of universes which in this context we call pocket universes just to make it clear that we're talking about the possibility of having many of them.

So almost all inflationary models do that. That's point number one. Point number two comes back to what I mentioned earlier about the fabulous 1998 discovery-- the sound system isn't working very well.

I was talking about the 1998 discovery that the universe is not slowing down under the influence of gravity but is rather accelerating. The simplest explanation for that by far is that the energy density of empty space, the vacuum, is positive, non-zero. And it turns out that a positive vacuum energy is an example of the kind of material that produces repulsive gravity. So that seems very likely.

Now the idea that the vacuum has a non-zero energy density is not so strange to physicists, because physicists view the vacuum as, in fact, a very complicated state. The quantum theory of the vacuum involves a system where particle/antiparticle pairs are appearing out of nowhere and disappearing.

Fields like the electromagnetic field are not just zero but rather are constantly fluctuating, which is required by the uncertainty of quantum mechanics. And furthermore, we now think our vacuum even has one field, the Higgs field, which you maybe heard about, which has a non-zero value everywhere.

So it characterizes the vacuum as just that it seems to be the state of lowest energy density. But it's not at all simple. And there's no reason to expect the energy density to be zero. So the fact that we discovered that the vacuum seems to have a non-zero energy density sound so far so good.

But there was a crucial surprise. And the surprise is that the vacuum energy that we observe is incredibly small compared to what particle physicists would expect. Now we don't really know how to calculate what we expect the vacuum energy to be but we know how to estimate it because we know how to calculate some pieces of it. And the pieces of it that we know how to calculate are larger than what we observe by an absolutely amazing 120 orders of magnitude, a fantastic mismatch.

So the big mystery, which is still a mystery since 1998, is why is the energy density of the vacuum so incredibly small compared to what we expect it might be. So that question needs an answer of some sort. And we don't have a very good one. But I do want to explain a possible answer that involves inflation and a multiverse. It was also discovered at about similar times in the past 10, 20 years that string theory, which seems to be our best bet for a quantum theory of gravity-- we certainly don't know that string theory is right but it certainly is the best item on the table for a quantum explanation of gravity, which by the way seems to be necessary. There doesn't seem to be any consistent way we can imagine that gravity behaves classically while all the things creating gravitational fields are fundamentally quantum mechanical.

So we need a quantum theory of gravity. And string theory is a possibility. An important feature of string theory is that it doesn't lead to a unique vacuum. But it was discovered maybe 15 years ago that string theory, in fact, leads to a fantastic number of possible vacuum-like states.

And by a fantastic number, I mean like 10 to the 500th power or perhaps even much more than that. And each one of these different kinds of vacuum would have a different vacuum energy. And that means that if there are that many of them, and if their typical value is 120 orders of magnitude larger than what we observe, but it can be positive or negative so you'd expect the values to range from plus 120 orders of magnitude times what we observed to minus 120 orders of magnitude times what we observed.

And if there are 10 to the 500 of them more or less uniformly spread, that would place a lot of them in the narrow band around zero. That's as close to zero as what we observe. So if string theory is right, there should be plenty of types of vacuum that have energy densities as small as what we observe.

But then, of course, there's the question of why should we expect to find ourselves living in one of these incredibly unusual vacuua with such incredibly small energy densities. And there the people who advocate this advocate what the people who like it call a selection effect argument.

People who don't like it call it an anthropic argument. Those mean the same thing in principle. But the idea of the selection effect is that life, one can claim, primarily forms in those universes that have incredibly small vacuum energy densities. And there is an argument behind this. It's not just made up.

The energy density of the vacuum drives the acceleration of the universe. So if we think about a universe that has an energy density of a vacuum as large as what we expect from particle physics, those universes would fly apart in a time about 10 to the minus 43 seconds. So it's pretty easy to convince yourself that life probably would never form in a universe with a vacuum energy density that large.

Similarly, if the vacuum energy density were that large in magnitude but negative, which is a possibility, the universe would just implode on the same timescale of about 10 to the minus 43 seconds with pretty clearly no possibility of life. So it's easy to believe that life forms dominantly in those rare universes where the vacuum energy is incredibly small.

It's still controversial whether it's a strong enough effect to explain what we observe, but it's certainly the right order of magnitude. It does appear that life, at least life anything like what we know about, requires a very small vacuum energy. And that would then be the explanation for why the energy of the vacuum is so small.

I'd like to close by giving you just a little bit of a sociological survey of physicists and their reaction to this selection effect argument and multiverse argument about the vacuum energy. A few years ago, there was a very interesting survey done by a reporter at a physics meeting asking various people what confidence they would have in the existence of a multiverse.

And the first person who was asked was someone named Martin Rees who's a very well-known theoretical astrophysicist. He had been president of the Royal Society. He's been knighted. He's been master of Trinity College. He's had most every honor you can imagine other than the Nobel Prize.

And he said that he has enough confidence in the multiverse to bet his dog's life on it. It sounds pretty significant. I have to admit, I've never met Martin Rees' dog so I don't know how cute he is or anything like that but pretty significant.

The next person the reporter asked was Andrei Linde of Stanford University, formerly of Russia, and one of my co-creators really of cosmic inflation. He's a tremendous fan of the multiverse. And he said that he has enough confidence in the multiverse to bet his own life on it, which is certainly very extreme.

Then the last comment I want to relay to you was made by Steve Weinberg who was not at this meeting but learned about these comments and commented about it himself in an article he was writing. Steve Weinberg, I should maybe preface this, had been a professor at MIT when I was a graduate student there.

I, in fact, learned general relativity from Steve Weinberg. He's someone who I think is really the quintessence of commonsense, someone who I've always had a lot of faith in. And what he wrote is that he has enough confidence in the multiverse to bet both the lives of Andrei Linde and Martin Rees' dog. So I'll stop there. Thank you very much.

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