„I'm interested in things that don't fit” – interview with Nobel laureate astrophysicist Adam Riess

Lucky physics of supernovae, rapid technological advances of the 90s, and a brilliant young astrophysicist. These were the ingredients of the 2011 Nobel Prize in Physics. We had the opportunity to ask one of the laureates, what happened after that? And what is this tension that seems to mount around dark energy lately? Click for the video recording of the lecture and the interview!

2023. június 9.

Watch the lecture by Adam Riess recorded at the Palace of the Hungarian Academy of Sciences.

Understanding dark energy, as you mentioned in the lecture, is a key to understand the origin and the fate of the universe and the workings of the gravity. But how can you translate it to terms of real physics?

Dark energy is about the gravity of empty space, the gravity of the vacuum. And the vacuum is a concept that we address in quantum theory and quantum mechanics. Quantum mechanics is physics on microscopic scales, while Einstein's theory of general relativity (which in turn determines gravity ed.) is physics on macroscopic scales. These two theories are both great, but they don't work together. We don't have what's called a quantum theory of gravity, the way the two are united. However, dark energy actually requires you to use both of these branches of physics. So our hope is that by observing how the universe actually does physics at that interface, we will learn how to unify those. I hope someday we get a theoretical breakthrough – but at the moment the best thing you could do is do experiments. And when we look at the universe, we're essentially doing experiments. And we're finding things…

Adam Riess Photo: mta.hu/Szigeti Tamás

In your lecture you showed how we can step further by using the James Webb Space Telescope data. But if you peer into the future, do you think of some kind of an experiment or a way of measurement beyond JWST?

There's one that, although it's not the future, it just came online a few years ago – the gravitational wave facilities like LIGO, which are able to see the universe now in gravity. It can improve the measurements we currently have, but it can also allow us to see things like the gravitational wave background that can tell us things about how the universe started with a theory called inflation. It's like you have your five senses, and it adds a sixth sense to our senses for astronomers.

These measurements are right here, or this new technology is just taking it’s first baby steps?

Well, it's like Galileo's telescope. Galileo built a telescope and you could see a few things immediately that nobody could see before. And then quickly you go: “But I want to see more!”, just to realize quickly that Galileo's telescope doesn't let you do that much. So they keep turning off LIGO and trying to improve its sensitivity and turning it back on. In the last five years, I think it's been off as much as it's been on. It's just about to turn on again, it would be the fourth observing run.

You got the Nobel Prize at a very young age. How did your research career change after that and how you're feeling after that you achieved what a scientist can achieve?

Well, I was 28 at the time of the discovery. (The Nobel Prize come 13 years later in 2011, but nonetheless Riess was still quite young at that time - ed.) But I will say that every scientist I know loves science and goes into science to study science. And if you told them at the beginning of their career, you'll never win the Nobel Prize, they wouldn't stop, they would still do science. This is like winning the lottery, in a way. And so when you say, “Well, what are you going to do next? ” The answer is, just keep doing science, the thing I really enjoy. I think the biggest challenge after you win the Nobel Prize is that people try to stop you from doing science. They keep trying to offer you other positions. But I was 28 and I wanted to keep doing science. Now at this conference I'm right there with the other scientists, we’re doing the science, we’re talking about it, we're asking questions, and I'm just going the way I was before... Maybe they'll make me stop sometime. (laughs)

Can you name the next big step you want to take in your research?

I want to understand this new thing we found in the last few years, called the Hubble tension. I want to know, how come this new model that we have of the universe still doesn't quite fit. And I want to know, is that another clue about dark energy or dark matter or some yet unknown particle? I would like to know why things don't fit… I'm interested in things that don't fit.

And your measurements are still with type Ia supernovae?

Supernovae and Cepheid variables… These are the best classes of stars to actually make measurements about the universe, its expansion. But right now they're in conflict with the other best method we have for learning about the universe, which is to study the radiation left over from the Big Bang. The radiation left over from the Big Bang gives us an exquisite picture of the state of the universe – but shortly after the Big Bang. We're comparing, essentially, the way the universe looks shortly after the Big Bang, combined with our understanding of how the universe should evolve if we understand it, and then we're actually measuring it now. That's what we do with stars, as we measure it now here, and they don't agree. It's not a huge disagreement, but it is significant. The universe shouldn't care whether we are looking at it from the beginning to the end or the end to the beginning. It should be the same story. And it's not the same story right now.

Ha valóban ilyen mélyreható változás történt volna az univerzumban, lehetséges, hogy idő közben maguk a fizika törvényei is megváltoztak?

If there is such a profound change in the structure of the universe, is it possible that even the laws of the physics change?

We hope for that not to be the case.

I admit my suggestion is an easy way out...

We follow something in cosmology, which has always been called the cosmological principle that says: There's nothing special about us. There's nothing special about where we live. There's nothing special going on here that isn't going on over there. And you almost have to make that assumption to do science. Otherwise, you look out in your telescope and every time you saw anything, you would say, “Well, that's maybe just the way it’s over there.” Maybe their laws of gravity are different… And you would have to start looking at the universe like a fairy tale, like not something you could actually try to understand and interpret with your understanding of science. So we do the opposite. We go, “OK, let's assume it's exactly the same and see if everything makes sense.” And for the most part, although I'm telling you about this discrepancy, everything makes sense. Of course, we focus on the discrepancies – we don't have conferences about the things that fit. We've tested this cosmological principle, but I have some colleagues who will sometimes say, “Oh, maybe the cosmological principle fails”, and that makes us all sick. It’s almost like introducing religion in some way – “That's the way God wanted it.” And it would defeat your ability as a rational scientist to try to make progress using the laws of science you understand and experiments, this whole process, because everything would be a special case. So we don't do that, and it works very well.

Watch the lecture here: