Here he explains what to expect from the James Webb Space Telescope, what makes him keep coming back to MIAPbP, and why his children like to jump into the creeks of the Englischer Garten.

Prof. Riess, you are a guest researcher at MIAPbP – once again.

Adam Riess: You can’t get rid of me.

Currently, you are a participating in the program “The Extragalactic Distance Scale and Cosmic Expansion in the Era of Large Surveys and the James Webb Telescope”. A pretty long program title. So maybe let’s start with the James Webb Telescope. In what way can it help to measure distances?

The things we know and understand the most are stars. We assume that stars in other galaxies are like our stars. So when we can see those stars in other galaxies far away and compare them to our stars, we can figure out how far away those galaxies are, because the stars will appear dimmer, proportionate to distance – actually, the inverse square of distance.

So that’s great, but you will need fantastic telescopes – like the James Webb Telescope – to be able to see individual stars. It’s like seeing individual people in a crowd. If you stand too far away from the crowd you can’t pick out the individual people. Telescopes give us the resolution that allows us to pick out individual stars in distant galaxies and compare them to our own to figure out how far away those galaxies are. And this is one of humankind’s greatest challenges: to figure out what is out there. To start figuring out what is out there you have to figure out: What do we mean by ‘there’? How far away is that spot, is that near, is it far? It’s getting a sense of proportion, of perspective. We are like babies, we have to learn our way around the universe by finding things that we can recognize, and stars are the best tools we have for that.

For a long time, the Hubble telescope was the only instrument in the world that can make IR measurements of Cepheids beyond the Local Group and that can find and follow supernovae at redshifts greater that one.

That is not true anymore. Now, the James Webb Telescope can do all the same things, and it can do them better than Hubble can.

Riess: "I always hope for surprises!"

The universe is accelerating at an increased rate due to a mysterious force called Dark Energy. If there would be something like a measure – say: type Ia supernovae at redshifts greater than one – would it be possible to pin down what Dark Energy actually is?

It would help, because it would allow us to answer the question: Is the thing we call Dark Energy changing with time? We would be looking back in time and seeing if it has the same effect all the time. If it is changing with time, that gives you some information that would be useful. It would allow us to distinguish between Einstein’s idea of what Dark Energy could be, versus a more generic phenomenon that we think gave rise to an inflationary period shortly after the Big Bang. We have these two classes of Dark Energy, if there was something that would allow us to distinguish between those, we would be able to know more than we do now.

Besides the James Webb Telescope, the current MIAPbP program deals with large astronomical surveys like the Dark Energy Survey, which collected data from 2013 until 2019. So far, only half of the data has been analyzed. What do you expect from the rest?

Surprises! I always hope for surprises! Science often moves along slowly and methodically, but then surprise come along, and they move things more rapidly.

What other surveys could help in the future to better understand Dark Energy?

The Vera T. Rubin Observatory, that used to be called LSST, a massive all-sky survey. Then there will be the Nancy Grace Roman Space Telescope, and also, in Europe, the Euclid Telescope. These are all big survey telescopes. Another European mission that is helping in all this is called Gaia, which is a survey of astrometry, of positions. It helps us to measure parallax and the like. We are really getting a huge amount of information from all this, and we are optimistic it will teach us new things.

Were you able to connect and talk about these surveys with other MIAPbP participants?

Yeah, so far we have been talking about certain results we have, techniques we have and how those techniques can be improved and why do we see different things sometimes.

How many times have you been to MIAPbP?

This is my third time. 2014, 2018, and now this year.

What makes you keep coming back?

There are very few places where you can get together with colleagues – many of whom I have never met before, I have only read their papers, they have read mine – and get to intensely focus on one subject. Particularly since the pandemic, I think all of us were reminded how important it is to interact with people directly rather than over Zoom, texting and whatnot. People are built to understand each other in many forms of communication. Some of them are verbal, some of them are, you know, over a beer. It’s hard to have a beer over Zoom.

                                                                                                                                                                                                                                                                                                                                                                                                                                                   You travelled to Munich with your family. Are you exploring the area after work?

Yeah, yeah! We went to – what’s that river that runs through the English Graden in Munich?

The Eisbach.

Yes, so we went in there and it was really cold. My kids wanted to float down it. I don’t float so well, but they did. I tried to swim. Also, we went to the Deutsches Museum and we have done some bike riding.

Final question: Your grandfather was from Würzburg. Have you ever been there?

No I haven’t. But I tell you something interesting: My grandfather was Curt Riess, and he was a famous author of many historical books. He wrote in the 1930s a lot about the rise of the Nazis, but not in a favorable way to them. So he had to leave in 1933, where he went to write for a Paris newspaper before leaving altogether. My father, Michael Riess, was born in Berlin, and he had to leave in 1937, he must have been six years old by that time. I learned some of this only recently. My great-uncle was a PhD student of chemistry at Rostock University. There was a purging of Jews in the 1930s where he had to leave the program. So, interestingly, I have a lot of original roots in the area. Maybe that’s why I like coming back. (Laughs)

Prof. Riess, thank you for taking the time.

Note: The original interview has been slightly abridged for better clarity.