Jim Peebles
All About Space chats to the 2019 Nobel laureate in physics about his involvement in many groundbreaking discoveries and how our theories have evolved since he first began studying the universe
The Nobel Prize winner on his groundbreaking discoveries and how he reinvented space and time
It takes a brilliant mind to imagine what the universe was like from its inception and try and model it over roughly 14 billion years. How have you been able to attempt this and be so confident in your results?
You should bear in mind that people were talking about this expansion of the universe from what used to be called a ‘primaeval atom’ since the
1930s. There was nothing new in this concept. It was not imagination so much; it was just inertia. People had no other ideas. What’s happened since then is an accumulation of an enormous amount of experimental evidence.
The Big Bang theory is widely accepted now, even by the general public. How was the theory received when you first came into the field of cosmology? Was it as widely accepted then as it is today?
Oh, no. It was widely known in the physics community. I think you could say it was not taken very seriously. It was doubted and not believed. Instead it was an idea that had very little basis in hard empirical evidence.
What can you tell us about your part in the discovery of the cosmic microwave background?
Let’s just consider the situation after World War II; there was an immense release of energy in science and technology. It gave us automobiles and it gave us particle accelerators. I guess it was natural that scientists turned to cosmology. The idea of an early, dense universe expanding into the present state was around, as I said, since the late 1920s and early 1930s.
People started taking up the idea, in particular Bob [Robert] Dicke, [former Albert Einstein professor in science at Princeton University]. In the Soviet Union, Yakov Zeldovich became interested independently. In the UK Fred Hoyle, Thomas Gold and Hermann Bondi became interested. The former Ukrainian, George Gamow, who had immigrated to the United States, also became very interested in the subject.
These people, each brilliant physicists, became interested in this independently. Gamow had proposed earlier, in 1948, the notion of a hot Big Bang, but with that came a distinct lack of interest in details. He put together many of the important ideas about a hot Big Bang. In particular he recognised that a hot Big Bang would have radiation in it. That radiation would be smoothly distributed. It would have a thermal spectrum. As the universe expanded, radiation would cool, but it would keep its thermal spectrum.
Meanwhile, Fred Hoyle in the UK became aware of the fact that astronomers were finding evidence of a remarkably high helium abundance, and the problem became where that helium came from. Although Hoyle loathed the hot Big Bang theory – remember he was a supporter of the steady state theory – he, as a good scientist, published a paper with Roger Tayler which had the title, The Mystery of the Cosmic Helium Abundance. He pointed out that this helium could have come from Gamow’s hot Big Bang.
Meanwhile, here at Princeton, Bob Dicke had decided, for his own reasons, that a hot Big Bang might be interesting. He advised two of the young postdocs in his research group to build a radiometer – of the kind he invented during World War II – to see if this radiation might be present.
He then turned to me and said, “Why don’t you think of the theoretical implication?” Peter Roll, one of his postdoc students, went on into education, whereas David Wilkinson and I spent the rest of our careers following up Dicke’s idea.
Meanwhile, Bell Laboratories in northern New Jersey was developing the technology for microwave communications. The engineers at Bell Laboratories received an anomaly. Their receivers were detecting more radiation than what they could account for.
By 1964 two of the young people there – Arno Penzias and Robert Wilson – were struggling to discover what might be the source of this radiation. They’re greatly accredited for refusing to give up and for complaining about it until finally the word got back to Princeton and everything came together.
So Penzias and Wilson just happened to stumble across it, and with your help managed to identify it?
That’s right. They had it. They didn’t know what it was. We were looking for it, and when we got together, well, there was radiation noise in the microwave receivers and there was the helium that Fred Hoyle knew about. You see, no one person can be credited with this, besides maybe Gamow. It was the work of many people coming together… almost stumbling together.
Would it be correct to say that this was the start of physical cosmology and applying known physics to the large-scale structure of the universe?
Yes, you could say that. People were applying physics earlier, but it was pretty schematic. The important thing about this radiation is that it was there. You could measure it, you could see how it was distributed, you could also measure its spectrum and you could, of course, knowing that there’s a temperature, do a lot of physics that was meaningful. We can’t say that the new physical applications started then, but they suddenly became a lot richer.
It has been stated that there is just five per cent visible baryonic matter in the entire universe. How do you get people to wrap their heads around that?
I could explain why I introduced that non-baryonic matter – the matter that you can’t touch. It’s all because the radiation is very smoothly distributed. The matter we see, galaxies and so forth, is very clumpy. As the universe expanded and cooled, how could matter gather into these great clumps of galaxies and leave the radiation so smooth? The thing is that as matter gathers together to make galaxies, you would drag the radiation with it and mess it up.
The postulate I introduced in the early 1980s is that most matter is not the sort of matter you and I are made of, but rather it doesn’t interact with matter and radiation. And so the radiation could slip through this non-baryonic dark matter. It was, at the time, an ad-hoc guess as to what allowed the radiation to be so remarkably smooth. At the time I didn’t argue very forcefully for it, because it was rather a demonstration of what could have happened. It was that easy in the 1980s and early 1990s because I could think of other ways to account for this phenomenon. This was just the simplest. People were very attracted to the idea because it’s simple.
There are some fantastic missions with cosmological objectives coming to fruition in the near future. Do you think these missions will play an important role in providing valuable observations for theories?
I’m really hoping so. There are a whole series of missions I’m looking forward to, and we shouldn’t forget the missions already in progress. Laboratory detection of dark matter has been going on since the 1980s. Year after year [there has been] improved sensitivity, bigger detectors, better insulation against cosmic rays.
At any time we might hear the breathless announcement: dark matter has been detected. If you hear that, you should be a little careful because it’s very likely, I think, that the dark matter is more complicated. Perhaps it will be a laboratory detection of a sub-dominant part of the dark matter. We’ll have to wait and see. Anyway, that will be exciting when it happens. Of course, there’s no guarantee it will.
The precise measurements of the radiation distribution will show us, among other things, whether we’ve assumed the right initial conditions. You have to bear in mind that this theory is not at all complete. It has a set of assumptions, and among others is the initial conditions, which I assumed were the simplest of all possibilities. They work pretty well. There have been adjustments already and maybe more adjustments are in store.
Regarding your recent Nobel Prize win, how did you receive the news that you’d just won?
A telephone call at a convenient time in Stockholm, but five o’clock in the morning here in Princeton.
When we spoke to fellow laureates Michel Mayor and Didier Queloz about your shared Nobel Prize win earlier in the year, they spoke about how their field of research will benefit from this acknowledgement, and they hope it will inspire future researchers. Is this something that resonates with you?
That certainly does. It’s a little hard to credit how influential the Nobel Prize is. There are other major prizes – major in the sense that there is big money for a lot of good work – but they have nowhere near the visibility, or the weight, of the Nobel
Prize. I have suddenly become this iconic figure, which makes me a little nervous. I’m still the same person I was before. But yes, the entire world is so impressed by this.
Do you think that with this acknowledgement, astronomy, and cosmology specifically, will gain fresh researchers?
I think there’s a mixed situation. On the one hand, the Nobel Prize carries great weight, not only in the general public, but upon my colleagues. Suddenly they’re more impressed. That’s great. But they’re doing work they would have done anyway, and I don’t think any practising scientist has been influenced by the Nobel Prize to do something different from what they’re doing now. And I don’t know; maybe some will be inspired to follow in my footsteps. The problem I caution them is that my footsteps have already been taken. You can’t go back and do it again in the same way. You’re going to have to follow your own path. It has to be different from mine.
Also, I should make an important point. Winning such a prize depends on a lot of things, a lot of hard work, but also a lot of eventualities that line up in just such a way as to make the case for the Nobel Prize clear to the committee. That means that a young person starting out in physical science should not judge his or her career by prizes and awards. They have to be capricious, because so many things have to line up to make it work. Judge yourself by your accomplishments, by your record in the field, but not by prizes and awards.
How does it feel for your name to be in the same sort of history book, as a Nobel Prize laureate, as such names as Albert Einstein, Niels Bohr and Richard Feynman?
[Laughs]. I think I was fortunate in the fact that I reached a good age. I was 85 when I received the prize. I knew what I’d done and I felt comfortable with it and I feel – without boasting – the Nobel committee made a good choice. But I’m no longer much affected by it, if you know what I mean. Life is the same for me, apart from the uncomfortable obligation to talk more to people. Normally we live a very quiet life and I enjoy it. I continue to work on research at a modest level, but now I’ve become a spokesperson, which I’d never planned to be.
“I have suddenly become this iconic figure, which makes me a little nervous. I’m still the same person I was before”