The Origins Podcast with Lawrence Krauss - Solving one of cosmology's biggest conundrums with Wendy Freedman

Episode Date: August 21, 2024

Wendy Freedman, the former director of the Carnegie Observatories and now distinguished professor at University of Chicago, has been a leading figure in observational cosmology and astronomy for over 30 years. I have known her as a friend and colleague, and have learned much from her over the years, and was very excited to be able to snag her amidst her busy schedule to record a podcast a week or two before the release of a new blockbuster result her team had produced. I am very happy that Critical Mass listeners will be among the first to get the detailed lowdown on the likely resolution of a problem that has been plaguing cosmology for the past decade. In the 1990’s Wendy led a major international team of astronomers in carrying the Hubble Space Telescope Key Project. The Hubble Space Telescope (HST) was named in part because of this project, to establish the distance scale of the universe and measure its current expansion rate, a quantity not coincidentally called the Hubble Constant, first measured by Edwin Hubble in 1929. Since that time, different groups have measured this most important single observable in our universe and gotten widely different values. In the 1980’s and early 90’s two different groups got values that differed by a factor of 2, even though each claimed errors of less than 10%. In 2001, Freedman’s team published their result, truly accurate to 10%, and the value, perhaps not surprisingly, fell right in the middle between the previous two discrepant values. All was good, until inferences based on the Cosmic Microwave Background, the most precise observable in modern cosmology suggested that measurements at a time when the universe was 300,000 years old, when extrapolated forward using the best current theory of cosmology today, would give a value that different from the HST value. The difference was statistically significant, and as time proceeded, and error bars got smaller, the discrepancy between the HST (and then the James Web Space Telescope (JWST)) measurement, and the CMB measurement got more significant. Was our current model of cosmology simply wrong?Such was the claim in various places over the past few years. Most recently, Wendy led a team to measure cosmic distances in 3 different ways using JWST, and as she describes in our discussion, it looks like the problem may now be solved, although not without leaving other mysteries.We talked about a lot more than this though. Wendy’s background, what got her into astronomy, her experiences throughout her career, and her leadership in a new project building the Giant Magellan Telescope, what will be the largest telescope in the world in Chile. The discussion was as fun as it was exciting. Wendy is a wonderful popular expositor, and as always, I really enjoyed talking to her. Tune in to hear, for the first time, about the newest and most important recent result in cosmology from one of my favorite colleagues and a world class scientist.As always, an ad-free video version of this podcast is also available to paid Critical Mass subscribers. Your subscriptions support the non-profit Origins Project Foundation, which produces the podcast. The audio version is available free on the Critical Mass site and on all podcast sites, and the video version will also be available on the Origins Project YouTube. Get full access to Critical Mass at lawrencekrauss.substack.com/subscribe

Hi, and welcome to the Origins Podcast. I'm your host Lawrence Krause. In this episode, I had the remarkable chance to talk to one of my favorite scientists, an old friend, and one of the most well-known and accomplished observational astronomers alive today, Wendy Friedman, who's now at the University of Chicago. She was the director of the Carnegie Observatories for many years, almost 30 years working there. And she has been involved in some observations of the most important features of cosmology, in particular, the measure of something called the Hubble constant, something measured by Edwin Hubble, the first observation in 1929 and 1930 that the universe was expanding. And you might think, well, we've observed the universe is expanding. Why do we have to keep looking at it?

Because that number is central to our understanding of how the universe works. And in particular, for anyone who's read the newspapers, there seems to have been a tension. We're measuring this expansion rate of the universe, two different. ways by looking at the very early universe and using theory to propagate forward to get what you should measure today and then measuring it today gave different numbers. Different numbers that didn't seem to differ by very much, by one or two percent. And you might think, well, that's good. But the observers in each case claimed that they were measuring things to much better accuracy and the disagreement was significant. And if that disagreement is real, then some people have

argued you might require vastly new physics. And because you're extrapolating from the early universe to the present universe by using the physics we now understand. And if that leads to disagreement with you actually observe today, then maybe the physics we think we understand is not what it was. And so this has become an even more urgent problem. And Wendy has been at the center of this by measuring the Hubble constant for the very first time with high accuracy, at that time at 10% accuracy in early 2000s. And has been working on this since then. And I'm happy to say in our program,

in fact, talks about a new result, which I'm not going to reveal now, that may, that will address this very important question. At the same time, she's been the founding director of the biggest new telescope being built, the giant Magellan telescope that's going to be ready in the 2030s to look at everything from new planets to maybe even, Earth-like planets. And so we discussed all of that, including her own career, as a young woman

in science, initially interested in biophysics, moved into astronomy. It was a fascinating discussion. She's a charming and lovely individual, I know that, as her friend, but also a great scientist. And it was a wonderful chance to really dig into the details in a way that perhaps you won't get to hear anywhere else. So I had a wonderful time. I hope you enjoy it as much as I did. And you can watch it, add-free, on our Critical Mass Substack site, or you can watch it later on on our YouTube channel, our Origins Project, our Origins Podcast YouTube channel, or you can listen to it on any podcast listening site.

And however you listen to it or watch it, I hope you'll consider either way supporting the Origins Project Foundation, which produces this podcast and the many other things that we do to try and discuss the most exciting things that are happening to humanity in the 21st century. Thanks again, with no further ado, Wendy Friedman. Well, thank you, Wendy Friedman, for coming on the program. I've wanted to have you on for so long. You were one of my favorite people and favorite scientists,

and it's a great thrill to have you here. Well, thanks very much for your patience, Lawrence, and I'm pleased to be here. Well, you know, this is going to be a lot of fun because we're going to talk about some interesting issues that are really at the forefront of cosmology. And in fact, you've been at the forefront of observation and cosmology your whole career. But this is the origins podcast, and I want to begin with your origins. I like to find out how people got to the starting point of where they were for what we know them for.

And I want to do that. And, of course, it's been fun for me to learn a little more about you. I know we spent time together and talked with each other about our history, but I now know a little bit more. I did not know that your parents spanned the two cultures, for example. So your father was a doctor and your mother was a concert pianist? Is that right? That's right. That's right.

And I felt fortunate in that because my father really inspired and nurtured my interest in science. And my mother really introduced me to the arts and literature. And it's just made for a very, you know, a life that has been a well-balanced individual. Maybe that's one reason I like you. much. You're so well-balanced. But I would have loved it if you'd said your mother nurtured your interested in science and your father in the arts. That would have even more fun. Well, maybe the next generations that will happen. But I know a lot of women scientists whose fathers were the ones who inspired them. I think it's a very common story.

So you're, so let's go. So you grew up in Toronto where I grew up and just a little bit younger than me, but we may be crossed paths somewhere in that time. And I won't ask where in Toronto because I'll ask you later. I was wondering. But your, so did your father want you to be a doctor ever? Or was it just an interest in science? I think I was very lucky. Not everyone who has parents who say, do what you love to do. And so he didn't push me to do anything,

but he certainly was a supporter. And it was unusual for a girl to be interested in science and for them to pursue it as a career. And my whole family was supportive. My grandparents thought I was stark reading mad. Why would a girl do that? Where are your grandparents from? Originally from Eastern Europe.

Yeah, Eastern Europe. Yeah, settled in Canada. And yeah, so I'm second generation Canadian. On both sides? Were grandparents on both sides from Eastern Europe? Both sides. Oh, okay. And they probably wanted you to have a real profession.

Yes, well, or not. Yeah, or not, exactly, or be a, yeah, that's why. What kind of doctor was your father? He was a psychiatrist, and the reason that... Psychiatrist. Yeah, became a psychiatrist was that he was interested in human nature and interested in science himself, and that was a way for him to keep learning. Okay, when you say your dad got you interested in science, I mean, in what sense, did he talk to you about it?

I mean, I'm assuming you read books or saw TV or what other things as well. He encouraged it. but in what way did he encourage it? Do you like reading when you're younger? I love reading. I read every book on the shelf on astronomy and in the library. But he would do things like for my birthday, he would buy me a microscope. In the next year, he'd buy me a chemistry set.

So I spent hours in our basement looking at cell. We all had chemistry sets. I had terrible smelling things. I loved it. And for whatever reason, I didn't really care for dogs. and so he was still nurturing my interest in in the things that was your mom at all interested in did she have an interest in science at all or or no I don't think so I mean and she's of course very pleased and interested in my career but but it certainly wasn't anything that was an interest

of hers and that's what I think we really did have a nice balance my father was really interested in science and he you know he was the one who introduced me to the night sky we went to a one summer was the first dark sky I had ever seen and we were looking up. And he was fascinated by astronomy. And he told me about the light travel time. You know, looking at these distant stars. And the experience for me looking up at this really dark sky away from the lights in Toronto. I was going to say, if you're in Toronto, you don't see the night sky.

It's right. It's a person who really seen the dark sky north of the city. And this idea that, you know, as he explained, if there might not be those stars that we were looking at, they might not be there anymore because they might have, you know, gone long before the light reached us. And I just, it clearly stuck with me because that's amazing. Well, I mean, that's wonderful. I mean, I think we know, you know, me. Either my parents graduated high school, so I didn't get that for them. They wanted me, be a doctor. And very disappointed in become one. But for a while. But did, so.

Yeah, but he also, you know, I really like math. And I think it was just, one of those things that the girls around me, we didn't have to continue math in high school in Toronto at that time. And most of the girls that I knew dropped it. Really? Yeah. But he again, just sort of, he liked the fact that I was good at math, that I liked math. And I think you get a lot of messages in our culture and particularly girls that math is hard and it's not for girls. And so it just really helps if you have people in your corner who are saying, gee, that's terrific that you like it. It's terrific. And it's not so hard if you're good at it and you work at it.

Yeah. Yeah. No, that's great. Yeah, I feel very, very fortunate in that. Was he, was he, was he, was he, was he, was he, was he, was he born in Toronto. Oh, he was born in Toronto. Did your mom, did you, do you play the piano? Did your mom?

I did play the piano until grade 13, which we had in Canada. Me too. At that time. And, and then, you know, astronomy and astrophysics kind of won. I stopped practicing by the time I got to university. But, yeah, I actually did. well. I liked playing the piano. So you're musical as well. You got something from both of them. I did. I did. And you have siblings. I know at least one. How many siblings do you have?

I have two. I have one brother and one sister. I know for your sister. And they, were you the only one to go into science? Yes. We all did very different things. So my sister went into literature. Yeah, I know she went. We're the ones who joked that between us, we have a balanced education. I went the science route. She went the literature and humanity's route. And my brother ended up in architecture and law. So very different.

Oh, why it's spanned. Okay, excellent. Well, so your father encouraged it. What about, did, were there, did school have an impact? Did you have great teachers? Or was it, or is it one of those things if you were so motivated, it didn't matter?

All of the above. I had great teachers. I had terrible teachers. I had the opportunity to be bored to death in some of my classes. And then I had really great and very supportive teachers too. Were the science teachers good or? Again, mixed. Yeah, me too.

I went to the public school system and in 10th grade, the physics teacher that I had, and people don't believe this when I say this now, but he would be describing something to the class and he would get to a point where it would get a bit technical and he turned to us and say, the girls don't have to listen to this. Wow, really? Yeah. Yeah. Huh, interesting.

Maybe I wasn't aware of it. I mean, I think I'm three years older than you. And I, we both went to public school. Because in Canada, there really weren't many, at least when I grew up, I never even heard of a private school. I know there were in down to Toronto and stuff. But, you know, kids just went to public school. And I think I did get on balance, a very good education.

Yeah, me too. but it was, you know, there were some teachers who were just... He said, wow, yeah. I mean, I had a crappy physics teacher in a way, but it wasn't that that, it wasn't, I don't remember him being sexist. He was just, uh... I think he was actually inspiring because it irritated me. I thought it was.

And I was interested, so I did pay attention. Do you think he did that on purpose? Maybe not. No, no, he was definitely. Absolutely not. But then in 12th grade, I was lucky to have a physics teacher who had actually gotten a master's in physics from Oxford, and he loved the subject. He was a fantastic teacher, and he really encouraged a group of us to think and challenged us.

And it was, for me, really the first experience I'd had of really being challenged. And I just really, really, that's wonderful. And I'm sure I wouldn't be where I am today if I hadn't taken his class. It was just, you know, it's funny. That's great to hear. For me, it was a grade 13, some grade 13 class, but not in science, actually. was in history where I learned how to write. A very demanding teacher,

we had to write an essay every week or two. Demanding teacher, the right kind of demanding, they're priceless. And then those of us who were lucky enough to have grade 13 meant we went to university really prepared. At least I think that way. Yeah, I wish it.

And okay, well, you, you, where in public, you went, you say, was it downtown Toronto, did you live? It was a Von Road collegiate institute. Oh, okay, I know where that is. A few years ago, yeah. Okay. But then you decided to stay in Toronto to go to university.

Do you live at home when you went to? I lived at home as an undergraduate, and then I lived in graduate residence. I was going to ask you, you did something that's somewhat unusual, not just staying at home in Toronto, but that's, I mean, University of Toronto had certainly one of the best physics departments

and probably a strong department. I don't know. But although I learned that you started out in biophysics, we'll get there in a minute. I want to find out what caused you to go from one another. Because it's interesting, when you've talked so far, it's all about stars and your father turning you up towards the sky. But I suspect biophysics may have been partly because you're also father was a doctor,

but we'll get there. But you also stayed at the same place for graduate school, which isn't that usual. So I want to talk about both those. First, why biophysics? Why did you want to do biophysics? well at that time so i had figured out in high school that i wanted to do science i knew i was headed to science and i really did enjoy biology and i was very interested in the question of memory that was something that that intrigued me and i thought it would be a nice way of combining an interest in

biology with my interest in in physics and so yeah when i entered university of toronto i that's where I thought I would head. But I took the introductory astronomy and astrophysic course because I loved astronomy. And then it turned out that the biology course, you may be familiar with Convocation Hall at the University of Toronto, that's where it was taught for undergraduates. Giant, I mean a thousand-seat auditorium. And it was mind-numbing. It was just dreadful.

And in fact, the lab for the course, they gave us a headset, and we had to go along with this headset and do the lab. And the TAs for the course just struck me as bored as tears with it. It was so uninspiring. I'm sure if I had ended up in biology, I would have been fascinated. You know, look at biology today. It's just blossoming. But it was just really deadly. And the astronomy and astrophysics course was terrific.

and fortunately because I really liked physics, I took the upper level physics classes because I wanted to. I didn't need them for biophysics, but I had them, so it was easy. You always tell people, I don't know if you found this, but I've always found,

and I was, even when I was, when I was chairman of department, I tried to make sure this wasn't the case, but most departments, most physics departments, when they have a physics course for engineers or a physics course for biologists, it's never,

it's never taught as well as a physics course for physicists. So I used to tell kids, even if you don't want to be a physicist, take the honors physics class because it's going to be better taught and more interesting instead of just sort of people thinking, oh, the kids only need to know X, Y, or Z, and therefore we don't have to, you know, really understand things. So, yeah, I guess that was a good choice for you to do that. Yeah. Also, again, although you're a little younger me, you're not a lot younger.

I wanted to be a doctor until about grade 12 when I dropped biology. And I dropped biology at the time for the same reason. Biology course was just memorizing parts of frog and things, and it was just awful. But part of that was just maybe the teacher and the course. But it was also that at the time, biology is blossomed. But at the time, we didn't even know about DNA, I think, at that time. I mean, at least it hadn't filtered down to the high schools. And so it wasn't scientifically.

I mean, now it is really, really blossom. But I'm still interested in it. I'm surprised you've even had a biophysics program because it wasn't that big back then. You know, honestly, I don't even know if they had a biophysics program. I don't know that, but I never got that far. And in my family, nobody had gone to graduate school. I wasn't thinking when I got to University of Toronto, okay, I'm going to go into graduate school.

This is where I'm going to end up today sitting, you know, University of Chicago. And it just wasn't part of my thinking. And I didn't know how to do it. And it wasn't until many years into the program where I realized, and people were encouraging me to apply to graduate school. But I had no idea how to do that or what was involved. I just didn't. No examples.

Yeah. Yeah. Well, I knew enough at the time that, yeah, I didn't know about, I never, I knew some people that went to the U.S. later on to graduate, but the whole thought of for undergraduate, but the whole thought of even that at the time, I never even thought it was a possibility to go to the U.S. And they didn't encourage us either.

And getting to how I ended up staying in Canada, I, for a graduate school, I applied to Yale because I was interested in star formation at the time in galaxies and Beatrice Tinsley was there. Yeah, where are they in thought? So I know that be. Yeah. And the people that Toronto were not encouraging me to apply to the U.S. And when I got into Yale and all I did was apply to Toronto and Yale. And what I think of it now and the numbers of places that people apply to. And then there was, you know, they kind of thought, you know, come here, it's better here. Well, that's nice to be wanted. I guess, but it was a little bit disconcerting at the time.

And, you know, you asked why I stayed. And that was the candidate France-Hawai telescope, hands down, was just being commissioned. And in Hawaii, you know, 14 feet and the opportunity to observe there. and Yale didn't have observing facilities. Ah, so that was the reason. And that was another good choice, not obvious, but a really good choice for me to. And at least, okay, while it is nice to experience it, I always tell, I tend to advise students to go to a different place to graduate school, just because it's good to see different institutions, different people, get it to experience the whole thing.

But also, but Toronto's a heck a lot nicer place to be than New Haven having a dismal. Yeah, you know, you don't know at the time, you make a decision, and, you know, sometimes it'll work, sometimes it won't. But I didn't really have access, you know, a lot of access to a big telescope. It's interesting because my, when I would, when I graduate, I don't think any, see, I was interested in particle physics and fundamental theoretical physics and none of my professors

even suggested I consider Canada at the time. Yeah, and I think if it hadn't been for a CFHT, they just would, you know, definitely. Yeah, yeah, absolutely. Definitely would have gone to Yale. And then just one little aside. I know it's about you and not me, but I will, I did, consider biophysics when I was in graduate school when I became very depressed. And I thought of doing

a joint MD PhD. I was at MIT and you could do a PhD at Harvard and a PhD at MIT. And I thought, first of all, my mother would be so happy. But it was interesting. But then, but you know, and this was again, that was probably around 1980 or so, maybe 78, 79, something like that. But I went to someone who was head of cell biology at Harvard, who was an uncle of a friend of mine. He said, don't do bio physics because it's not of interest to biologists and it's not of interest to physicists. Interesting. It's changed dramatically, but at the time. But it was true.

Yeah, it was true at the time. So you made the right choice. Well, it was it. I've never regretted it. I've never looked back and, you know, as I said, I'm sure I could have enjoyed a life, a career in biology, but I've really enjoyed my career. Well, that's great. And did you always, I guess in astronomy, I guess, yeah, there,

really aren't, there really weren't, I mean, there aren't many, there are theoretical astronomers, but I mean, you always wanted to look in the telescopes. You always wanted to, is that. My heart is, you know, actually, part of me, I don't know, I think both are important. You know, you often have people who are arguing. It's an area where you can do a little bit of both, not like. You need both, right. And they're both really important aspects coming at things from a different way, but, yeah. Actually, let me ask you about that, because, again, I don't think the public realize that, In the field that I was in, which is theoretical particle physics,

there's no way you could do experimental particle physics and theoretical particle physics. I mean, I mean, Fermi was the last person who probably did, you know, experimental and theoretical and theoretical baggage of building machines and intellectual baggage of mathematics. You can't do it. But I think in astronomy, you can do what you. It's easier to, I mean, it's, of course, like all fields that have evolved, specialties.

But you could do some, you know, theoretical, work while you were, you know, doing observing, which is be a... Yeah, well, I was in graduate school. I mean, one of the things that I got the opportunity to do was to work with Ray Carlberg and he was, no, I was in graduate school. And so we did some in-body simulations to add gas into an inbody code. And that was a really fun project and I enjoyed it. But it's sort of like piano.

Once you get going in something and I sort of took the observational route, you know, I just, I really enjoyed it. And I think, again, you know, you can, there are people who do both. So when you were graduate school, you went to CFHT or? I took a CFHT a lot. Yeah, Ken in France, Hawaii Telescope. So that must have been fun as a graduate student. Oh, yeah.

Yeah, it was terrific. Yeah. That was in the days. That was the days of prime focus cages. Yeah. And people would actually be in the same building they were observing in, right, back then. Yes. Oh, yeah.

We go to the summit at 14. thousand feet, right? Yeah. And people don't... No, I'm glad they've had the opportunity to do that. People don't do that anymore, right? I'm such an ignoramus, but people just, yeah, they're down at sea level. Down at sea level. And, of course, now we can be anywhere in the world, right?

I have driven from, you know, here, same place. Yeah. So, well, and since you're using, and we'll talk about using the Hubble Space Telescope, those are two telescopes. You're definitely not nearby when you're using, and so that's the way to us. Yeah. So you... you moved to to uh carnegie at uh in pasadena uh right away to do your postdoc right after after i don't know

and then stayed there for much of your career until like you know 30 years um and uh why there uh for observational astronomy i don't think there was a better place in the world to be we had access to the 200 inch telescope which you know the time was the largest telescope in the world and access to the 100 inch telescope at Las Campanis in Chile and people who were, you know, phenomenal. So, yeah, it was my first choice, hands down. Hands down. And obviously, their first choice, too. And, but then, but then you couldn't use a telescope you'd already worked on because now you're at a new place, right? But it didn't matter. I didn't need that me use it for a while. Okay. I didn't know how it worked for a while. I didn't know how it worked in the, okay. But I mean, look, I've known, I mean,

you're most well known for leading the the key project of the Hubble Space Telescope to measure the Hubble constant had had measuring obviously that's actually I've said this I don't know if I've ever said in print but Feynman used to you know once asked what's the most important if you had to know one number you know and he might

I don't know if it was the fine structure constant but I've always said it's the Hubble constant in a way because that one thing determines basically all the properties of our universe, I mean on largest scales. So it's obviously the most important number to know. And to put things in perspective, when you were starting, and when I was a young professor at Yale, there were measurements to Hubble Conson,

and it was either 100 plus or minus 5 or 42 plus or minus 5, which automatically as an outsider, you know, I came into astronomy as an outsider, made me suspicious of error estimates in the field. because, you know, these two fields that, you know, and all of us said, well, if it's 100 or, you know, 50, it's got to be 75 and tended out to be close to that. So the field has changed a lot. And in fact, I never really quoted him by name, but I will because he went to Carnegie and might have been director before you, Gus Omler, was my colleague at Yale. And didn't he move to become director there?

Yeah, he did. Yeah, he. Yeah, but he, when I was a young assistant, and I'd come and talk to him because, you know, and I was, as I say, a theoretical physicist and he, and I'd be talking about measuring the Hubble constant or omega or any, and he said to me, nature will always conspire so you'll never, ever be able to measure any of the fundamental consciousness in cosmology. That's interesting. But they'd always, up to that point, what it always happened is people make claims like 42 or 100,

and it was systematic uncertainties that always screwed you up. Same measuring a flat universe. measuring an open universe, always, you know, with definitively these wonderful new measurements, but then you discover there'd be systematics or things you didn't. I'm not sure it's that different now, but I think, you know, what we're arguing about are much smaller differences and much smaller level errors, but we still have to worry about the systematic. Absolutely.

I mean, the point is an observation you can't twiddle the dials like you can an experiment. And the reason I'm stressing this is we're going to get to it because people are arguing about 1% or 2% uncertainties. and my you know when people ask me about it I say well I'm an old guy and I grew up when there's a factor too I'd certainly you know but each one would each person would um each group would be convinced that they were right to a few percent and so I'm always a little skeptical but but getting things to find details is really where the hard work has been involved in and that's been much of your career yeah had what got you interested in so early on you were interested in star formation is that what

you did when you went to to Carnegie? No. By that time, I was interested in, in, in, in, in, in the distance scale and implications for cosmology. And that happened. What got you interested in that is what I wanted to ask. What got you made me in that direction? So when I started observing with Canada France-White telescope and with my interest in star formation, it was to study the, the high mass end of the initial mass function in

nearby galaxies. So galaxies that were close enough to actually resolve individual stars. And it was a field that people had measured in some cases, you know, literally only 20 stars and they were measuring a mass function. And so another field that was, you know, big error bars, people arguing about whether it was a universal initial mass function and so on, you know, the distribution of stars as a form with mass. And so, and I live through the transition between In photographic plates, which is what the Canada-France Hawaii Telescope started with, it had a really big field of view, so a degree on the sky, which is an unusually big field. So you could get things like the Andromeda Galaxy almost entirely on a plate.

That one was even a little bigger, but most of the nearby galaxies you could measure. And then when CCDs started to become available in the 1980s, early 1980s, in order to calibrate the photographic plates, I took CCD images in these nearby galaxies, and rather than just point in some random place, it was, okay, let's look at the sepheids in these galaxies. And what happened, so this was the last year of my thesis, measuring the period luminosity relation for sephiids, and using sephiads that had been discovered by Hubble or Vada and earlier astronomers, The beauty of Cepheid is they keep changing, you know, their variability.

You can measure them now, even though they were discovered a long time ago, unlike the supernovae, which go off once and you're done. So, you know, you come at them with different instrumentation. We're observing some of the same ones now with James Webb. But it was the first time that it was possible to make measurements at more than one wavelength, or at least accurately, there's been some in the visual. So in the blue filter, the visual filter and the near infrared eye and R band filter, And then what turned out to be the case was that if you measured the apparent distance modulus as a function of wavelength, they weren't the same.

The galaxy was clearly at one distance. But when you plotted them as a function of the inverse wavelength, it turned out to have the same functional form as the interstellar extinction law. You were seeing the effects of reddening by dust. And it was the most time that it became possible to make a correction then for the interstellar dust. And that was one of the reasons why there had been this factor of two. And you could do that better with CCDs than with, then with photographic place. Yeah, the photographic plays were sensitive to the B-band. That's what people were measuring with.

They weren't getting multi-wavlink data. And so that was the first time that it became possible to make a correction. Okay, I want to step. And that's what we use with the key project and use it today. And that's, I mean, this is a great importance. And so I want to step back a little bit for the, for the listener who may not know what a sefay it is, what extinction is, what distant module is, is.

Because these are going to be these are important concepts. And it was around, when, and CCDs first came in a route about the time you say when you were getting your junior. 82, 83 is when they came to CFHT and I think started to become available at Tarotillo and Kit Pete. It's interesting because I guess photographic plates were still used.

I remember I became a professor of Yale in 85 and someone I knew Margaret Geller used to come down. Yale must have had a big repository of photographic plates because Margaret Gell would come down from Harvard to use them. So, you know, went to where there were repositories. And so they were still being, and that would have been in the, in least 85, 86, 87, 88. So the transition must have been relatively still.

In the 80s, yeah, yeah. And by the end of the 80s, the transition had completely taken place. And it was, you know, and people, it was amazing. There were these machines that read these plates, Some of which had been around for years and years and years and years, right? I spent two summers at Cambridge when I was a graduate student and used an automatic plate measuring machine there that had been developed by Ed Kibblewhite. Yeah, it was great.

It was fun. Okay. And, well, oh, yeah, yeah, I mean, it must, yeah, it must have been amazing to be able. It's still weird for me to think that. And I guess it's been useful. Some people have gone back and looked at old plates to be able to compare to new things to see for transients and other things. Of course, you have the problem that. as you said, the calibration between those things and.

You know, and that's what I had the chance to do was actually do the transformation between the photographic photometry and the CCD photometry, which was linear, right? The photons in were directly proportional to the electrons you measured and the electronic signal. And so the light levels, low light levels for photographic plates, they're just, you know, it's just unreliable. And where they were bright, they were saturated. it was it was you know the photographic photometry had a lot of problems okay so now made it better okay so now i want to go back for people who don't know why sefayeds are important why were sefayids important let's go

through the history of this so the short answer is that they for the first time allowed you to measure accurate distances to galaxy so if you want to go back in time if you go to before edwin hubble we didn't know what the distances to what people called nebulae were at the time. So there were regions on photographic plates that had spiral-like features, and they had been known. So, you know, many of them had names Messier with a number behind them. So M-31 is Messier-31, the Andromeda Galaxy. And the question was, were these objects that might be within our Milky Way,

regions of star formation, gas and dust that might be condensing to form new stars, or were they like the Milky Way itself at greater distances? And so there was a great debate in 1920, the National Academy of Sciences, and we didn't know. I mean, there was just no way of gauging. It was a star faint because it was far away or, you know, was a star bright because it was nearby. We didn't know. So Henrietta Levitt, who was an astronomer working at the Harvard College Observatory in the 1920s, she was measuring stars in what turned out to be one of our nearest neighbor galaxies, the large Magellanic Cloud. What she noticed was that some of the stars were varying in their brightness. And when she looked closely, it turned out some of these stars were increasing in brightness pretty quickly

and then leveling off more slowly. And that was a characteristic shape of something that had been known, a type of star, a variable star called the sepheid variable. We've known about those since the 1700s. So it's 70. Yeah, yeah. What she discovered was that there was a relationship between how bright these sephiads were and how fast they were changing in their luminosity, their period of variation. So what that ultimately meant was that if you could measure the period, how, rapidly the star was changing and its apparent brightness. Then if you could measure the brightness of a

sephiates, say, in the Milky Way, by some other more accurate technique, then you know how intrinsically bright the sepheid was at a given period. So then you go to another object like the large Magellanic cloud. You measure how apparently bright they are and at a given period. And then you just compare it so that use the inverse square law of light. Light falls off, the intensity. of light falls off as the square of the distance from us. And so there was suddenly a way of measuring the distances. And that's what Edwin Hubble used when he discovered that there were galaxies outside of the Milky Way. That's how he discovered it, was making use of this Levit law. And then Hubble went on to discover that when he used velocities that had been measured by Vesto Slifer,

Arizona, and plotted those velocities versus distance, there was then a correlation between velocity and distance. And the implication of that, add in Einstein's general theory of relativity, was that, okay, the universe is actually expanding galaxies in the past. As the universe was expanding, they would have been closer and closer together in the past if the universe is expanding now. and with Einstein's theory led to this picture of a Big Bang universe, the universe that began in a hot phase and has been expanding. And the expansion was eventually confirmed with the discovery of the background radiation in microwaves in the 1960s. But that wasn't known at that time.

And as you know, Hubble, of course, knew he'd measured that, but didn't, I don't know if he, well, for a long time, he didn't believe it had any to do with. expansion. He was very careful. He wasn't so sure that it actually had cosmological implications. And just, and again, just to put this in perspective of how much has changed, I mean, I've looked at Hubble's data. It's nice to say there's a correlation. I often tell people if, if you had a first-year physics lab and you drew a straight line through that data set, and you might not, you might not be past it. It was certainly noisy. And, you know, with a straight line guides the eye, but it wasn't clear that it was the best thing to do. It was kind of lucky.

But he also, but just to make it clear, he got the number wrong by a factor of 10 almost, in fact, eight or so compared to now. So, and so, you know, things have changed. Yeah. And it comes back to the photographic photometry and not being able to correct for dust. And not having enough galaxies to measure which Hubble, the Hubble the space telescope allowed us to do. And that's one, that's another one of the terms he used about reddening.

And the point is that, you know, you're absolutely right. brightness is a great measure of distance unless there's a cloud between you and the object. And then it's not such a good, you know, then brightness is not such a good. So you've got to know what's between you and the object. And that's, and how can you know you're not out there? Well, you know, you're looking at light and the stuff that it's going through will absorb in certain frequencies differently. And therefore, if you see more absorption, I guess, in the red end, then you know

it's going through more dust, right? More in the blue end, right? More in the blue end. Sorry, look redder. Yeah, yeah, look redder. The dust grains, the size of the dust grains actually comparable to the wavelength of blue light. And so that it gets scattered and absorb. But the long wavelength red light sort of goes through.

Well, actually, now that, I mean, that's the same reason we tell people that the sky is blue. And when you look at sunset, the sun looks red because you're looking through more of the atmosphere. It's exactly the same argument, more or less. You're looking through more of the atmosphere and the blue's been filtered out. And you look up, it's been scattered down. and so it's the same in the in the in the in the in the galaxy but one when it comes to you mentioned so the period luminosity relationship with sepheids was fundamental but of course in order to calibrate that you have to be able to have another way of measuring the

distance to a sepheid right and you can't because you have to know you have to know you have to know you know you know I have to know the zero point right and how and how's that determined so now we have the advantage that we have parallax measurements for a sepheids. So that wasn't something that Hubble or early astronomers had access to. But now with satellites like Gaia, the European satellite, so you know, you can measure the position of a sepheid. So as the Earth is revolving about the sun, you're looking at a distant star from different angles as you're going through the annual orbit of the earth. And so you can literally use high school geometry to, you know, measure angles and get a distance.

And parallax for people, again, you know, if you just, if I'm looking at you on the screen, if I close one eye or the other, you jump back and forth, but the back of the room doesn't change as much. And because the baseline is the width, distance between my eyes. And the earth you have baseline, you know, June versus December or whatever. and but you say that wasn't available then. When was, when you know what it started to be able?

Well, so the first more reliable measurements for Cepheid at least. So you had some stars. The first, you know, stars that had parallax measurements is I think 1851 by Bessel, right? It took a long time to, but for Cepheid, you know, they're rare, they're super giants. It took a long time to be able to get accurate

enough measurements to measure for the Cepheid distance scale. And those are still being refined. And I think the ultimate goal of Gaia is to get 1% parallaxes. You still have to worry about reddening by dust, by the way. So they measure parallaxes, not distances. And there have been some calibration issues that have been hard to overcome. But that will come, 1920s. Is that one of this chief uncertainties when you get distances to Sefay?

Yes, yes, it is. And there aren't many opportunities to measure parallel. axes or get a geometric distance. You can use stars in the Milky Way. You can use stars in the large Magellanic Cloud, which was, you know, the galaxy that Henrietta Levitt studied. And there's another galaxy, NGC 4258. And those three are the main anchors now for the zero point for the calibration of the distance scale and the Hubble constant. But just nature hasn't given us very many nearby objects that you have really accurate measurements for. And that's one of the remaining uncertainty and where

when Henry Adelaide did this, it was purely phenomenalogical. We use the word phenomenological. Namely, it was discovered to be there, but people didn't know why. They didn't even know that the sepheas were pulsating. Yeah. Now, I was going to ask though,

it's always nice to have a theoretical understanding of something you're using at the fundamental basis to measure the universe. Are sephiads now, is that period luminosity relationship extremely well understood now theoretically? the extremely well I think probably doesn't fit we can't start yet from first principles and use you know just ordinary physics and and say you know we have a Hubble constant to 1% and predict that

the stars are pulsating they have atmospheres or again we have to worry about scattering how do we know that they're pulsating and have atmospheres now is that is that theoretical or is it can you I mean, how do you know they're pulsating? It's both. So theoretically, the, I'm actually trying to remember now. So it was, and I'm trying to remember who did this. I think it was Shat. Yeah, almost certainly was Shappley who showed that it was pulsation.

And I just not remembering in the moment what it was exactly that led him to rule out all the other, because there was a whole long list of possibilities. Yeah. Yeah. But they are definitely pulsating. and we can see how the radial velocities track. I mean, that's the other way now we have. We can measure radio velocities.

We see where the star is actually moving in and out. So I don't remember his argument, but we can see this empirically. So you can now see that you can actually see the chain, the oscillations and the radio velocity using the Doppler effect. So you see the front of the star moving back and forth as it, as it right. Okay, so that's important. So, I mean, I'm asking all these questions. It may sound like it's a little detailed and technical. but but these are important questions because of the consequences and the fact that so your interest in Cepheid's was you know obviously to measure these accurately and um and yeah as you say you know the Hubble constant impacts everything we do in cosmology and so there was this factor of two uncertainty when I started and and and so yeah it was a question that really intrigued me and continues to this day because now there are other reasons to

make more accurate measurements. And you know, this is really, so, but you became, you headed the Hubble, the Hubble Space Telescope Key Project to measure the Hubble constant. When, so Hubble was launched when, what year I forget now, isn't it awful? 1990, but it was supposed to be launched in 1986. So you started, you became a postdoc around 84 or something like that? Postdoc in 84, and then I joined the car. Carnegie professional staff in 87.

So you, so every, so the Hubble was on everyone's mind. So you were already looking forward and planning to use Hubble, even when you were a postdoc, I assume. Yeah. So somebody asked me this the other day, you know, how did the key projects come about? And it was, you know, that was interesting too, because it was Ricardo Giaconi, who was the director at that time of the Space Telescope Science Institute. And he wanted to make sure that, that what was unique to Hubble, could get done by the Hubble Space Telescope. And he knew that if he gave a time allocation committee,

which is the group that will decide the allocation of time on the telescope, people would have been waiting for decades to get access to the telescope. He knew it was going to be oversubscribed. And so he formed a committee to decide what were the most important projects that Hubble could do. And there were three projects that were selected an extra galactic distance scale was one of them.

And there was a medium-deep survey, and there was a quasar project. And he invited the community to propose for the key projects. And we did. And at the time, in 84, I think this was, maybe it was 83, but Mark Errenson was the leader of the group. And tragically, Mark was killed in an accident at Peak Observatory. 1987. And so by the time, oh, so then the Challenger accident happened in 1986. And Hubble was the due to be on the next shuttle. So everything got set back after that. And so the telescope wasn't

launched until 1990. And then the spherical collaboration, the problem with the focus of the telescope was discovered. So it didn't start getting data that was, you know, high resolution until December 93. Wow, so that's almost a decade after you had begun. Now, let me see, yeah, so I forgot that Mark Erickson died. So he's the first head of that.

Yeah, I was his deputy. You were his deputy and, yeah, so tragic. But let's go back then, there are two things I want to then ask about the science and then about your career. One thing you also mentioned, I just want to go back to it because you mentioned it and

people are going to be puzzled by this thing called apparent distance modulus. Do you want to explain that too? Okay, so distance modules is an arcane term that is a logarithmic measurement of the distance. And it's based on, yeah, sorry, there are different ways of measuring distance modulus, right? Yes.

And so with the sepheids, what we do is we measure how apparently bright a sepheid is. And then having determined what it's entreat Zieg, or it's absolute magnitude is another logarithmic measurement of the brightness. The difference between the apparent and the absolute magnitude is what we call the distance modulus. But put simply, it's a logarithmic measurement of distance. It's basically tells you a difference between how bright it appears and how bright it actually is. And a star that has a given brightness is a lot less bright when it's farther away. So this is a lot, this is a mathematical way

of just trying to calibrate the relationship between how bright it actually is and how bright you see it. Right. And then, and as you say, you have to be worried because things can get a little dimmer, not because they're far their way, but because there's stuff between you and them. All the things that, all the bugaboo's of observational astronomy. Dust in the interstellar medium and metals, astronomers call metals, anything heavier than hydrogen and helium, which is, but, you know, those are in the atmosphere.

They also scatter radiation. So these are the kinds of things we have to worry about that it can be systematic and cause problems. We're trying to make it. Okay, two things. One question of it's a personal one. And then I want to go to Hubble, why it was good to measure, why you need to go to Hubble and not the earth. You're sort of already mentioned one. But young scientists are worried about getting permanent positions for obvious reasons.

And you were a postdoc working on. a project starting to work in a postdoc in a project in 1984 and it didn't even and and the Hubble Space Telescope didn't even you know begin to get science so what 1993 you're saying basically yeah we started how do you survive as a scientist in the interim well there was plenty to do in fact it turned out to be really important in terms of learning how to best do the observations with the Hubble Space Telescope and and actually I'm grateful for the that there was a times in between. So, you know, we're talking about CFHT, the Canada of France-Wight telescope, one of the projects that I took on as a postdoc was to try and understand whether

metallicity, the abundance of these sephiates affected the luminosity. Because again, if you're trying to measure an accurate distance and you have a different metallicity for your calibrator than you do for the galaxy you're measuring, you're going to get the distance wrong. And just, I'm just going to interrupt one more time. Just so people know, stars have different abundances of different, you know, depending upon where they come from, what gas cloud they collapse from, they can have different abundances at some level of all, of all, you know, of all the elements. And that's right. And within a galaxy, there's more star formation in the center. There's just more activity. And so more metals get thrown

back out. So these stars expel their metals after they die. And the new stars that form are, you know, have these metals in them. And so the idea was to look as a, of the distance from the center of the Andromeda galaxy. These were observations made at CFHT and correct for reddening now for the dust using this multi-w wavelength observations and see whether there was something left over, a difference that could be attributed to the metal abundance. Oh, I see. We knew that all the sephiads were at the same distance.

They're all of the same galaxy. And so that was the first test that was done for a metallicity. And we repeated the test with Messier 101 and 101. one with the key project. And we're still, you know, to this day in the literature, people are arguing about how important is this effect of metallicity? How important is the effect? We don't actually know.

We don't know it to one or two percent. Yeah, that people have different. I mean, you know, that's the point of, you know, we're talking about one or two percent. And there's so many things you have to try and understand. It's really amazing. It's amazing. People like you can do it. I was going to say, amazing that we can do it, but it's unfair because it makes me seem like I'm, all right, but I certainly couldn't.

Well, it's interesting. You know, our technology has completely changed. We're now using infrared instrumentation. We've got this beautiful facility, James Webb, which I'm sure we'll talk about. And, you know, the ability to actually get to the bottom of these things now is so different than when I started. Yeah. I mean, the whole, who would have thought when we were young? I'm going to say that together for a few.

But, I mean, I would never, well, I would never have thought we knew the Hubble We know the density of the universe, all these things to, you know, accuracy that it's unfathomal. You know, I never thought in the early part of my career that these things would ever be known. It's really amazing how the revolutions in the field have taken place. And it's how I feel now. I think we meet this enormous progress. And at some level, we haven't stopped to actually appreciate the progress, right? Maybe we don't have it to 1%.

But if we have it to a few percent, look at the distances that we're covering. It's amazing. You can measure the universe at this level. And the other thing I try and emphasize as a theorist, especially, is that science is an empirical field. And what drives the progress every now and then, you know, new computers help and all of that. What drives the process is new instruments. And when you turn on new instrument, not only do you have a new window on the universe, but you're often surprised. And so that leads me to the next question.

What is it about the Hubble Space Telescope then that makes it better to do this? off than Earth-based telescopes? Well, you get above the Earth's atmosphere. And the Earth's atmosphere is an annoyance. It's turbulent. And so the light that's coming to us from these distant stars, as it's coming through the atmosphere, it gets smeared. And so the resolution that you have from ground-based telescopes,

and certainly when we started, it was about a factor of 10 better when we went to Hubble. Now we have techniques called adaptive optics that weren't available then. or we can do somewhat better, but not over a wide field. And so once you have the better resolution, you can then see sepheids more clearly against the background of the stars that they reside. Separate the brightness from the sepheed from the background around it. Right. And get a more accurate measurement again of the brightness, which is what you need to measure the distance.

And that also means that your resolution is 10 times better. The volume that you can cover is a thousand times bigger. And so the supernovae, which now has become the favored, the best method for stepping out into the distant, what we call Hubble flow, there were no calibrators when Hubble was launched. There just weren't any galaxies near enough to measure sephiids where they had also had type 1A supernova. And so it just opened up the volume of galaxies that were accessible. And then we had CCDs on Hubble and we could take advantage of the multi-wavelling. capability and yeah so and more galaxies yeah and it um so the key project itself officially began when i guess officially i would say 1990 well i what do you call official i mean we we came together

as a group in the collaboration was formed in 1990 basically no it's formed after geoconi got us together there was a meeting in aspen in either 83 or 84 and that was the nucleus of the group that got together. Then Challenger happened and then we put in a proposal for 1990 and we observed two nearby galaxies, M81 and M-M-1-1-1 with the sphere collaboration. But then in 1993, the telescope had corrective optics and that's when we really got going. And the definitive, I'm trying to remember the paper that, you know, there was the quote-unquote final key project paper. Was that like in early 2000, wasn't it? 2001. So it was based on almost a decade of not quite of observing.

Yeah, it was a decade. And that gave the famous number 72 for this Hubble. With a 10% uncertainty. And that had been our goal, which at the time seemed almost unattainable. Because we were looking at this factor of two. But we did design the program very deliberately to use five different methods. and the numbers of galaxies that we use for each method was set to give us a statistical uncertainty for each of those measurements at the 5% level. And our feeling was that in order to get the systematics and get a robust estimate of the overall systematics, that that's what we needed to do. And we did make the measurement.

And you did that. And it has, it's, it's survived over time, right, since that time. the Cepheid calibration of the Hubble constant has really remained quite constant. Yeah, in fact, we're going to get there. I see, I was just looking at a paper that's coming out of yours. But, and it's still about the same. The, and by the way, 72 was, that was what the number that was so amusing for us who are observing from the outside, because there was the 50 crowd and the 100 crowd,

and we figured 75, and we thought, here you did all this, all this work for 10 years, and I could have told you if two groups were measuring 50 and 100, it was going to come about it's 75 plus or minus a few. So it seemed to me like a lot of work to come up with what it seemed to be the obvious answer. It's always good to rely on data. Yeah, I know. It's always good to rely on data. It was nice to see that the data came out the way it should anyway.

And so that was great. And everyone was happy and that gave us a, you know, well, that more or less happy. That gave us an age of the universe. And as you know, I spent a lot of my time worrying about the age of the universe and what it might do for cosmology. and ultimately arguing that it argued for cosmological constant, among other things. But that was a fundamental important, because the expansionary of the universe gives you the age and many other things, and it was essential. But then along, so the Hubble, the big, I mean, there have been many instruments, I don't want to put anyone down, but Hubble was one of the biggies.

The other big thing that changed cosmology, again in the early 1990s, was the discovery of, of, of, of, measurements in the cosmic microwave background that you could actually see fluctuations that were inherent in that, that were around the universe when it was 300,000 years old. And that, between Hubble and that, that changed cosmology, that changed it and turned it from an art to a science in a way. I mean, obviously people thought they were doing science before, but now you could measure things incredibly accurately and do statistical analysis, get lots of data and not just one data point or other things. And so, you know, that changed. changed a lot of thing. That, again,

it allowed us to go back and try and understand the early universe and after the initial Kobe satellite, there was many other satellites that measure the cosmic microwave background and you measure things to three or four decimal place accuracy. It's

kind of amazing. Temperature and other things. And we won't go there, but of course it allows you to measure other, well, it allowed you to measure the other cosmological parameters incredibly accurately. like the density of the universe and whether the flatness of the universe and also the abundance of how much dark matter there might be

and how much normal matter there might be. But it also allowed you to measure, not directly measure the Hubble constant, but it allowed, since you, since the way we get from the early universe to now is gravity. It may sound simple, you know, gravity. But you start with these very small lumps and they end up being galaxies and stars.

and in principle, if you know enough, you can try and extrapolate from back there to hear. And the good thing is that when you try and do that with dark matter and this cosmological constant or this dark energy, and you use the theory, you come up with the universe that looks today like the universe we see, more or less. And there were a lot of bumps in that road, and a lot of times when people said, we've ruled it out,

you know, compared to numerical simulations, it's wrong, and then you find out the miracle simulations are wrong. And I've been through, you know, we've all lived through that. I've, I've often said that, you know, dark matter has, has, has, non-baric dark matter has, has been reborn many times. It's been a savior and saviors are always, you know, are reborn. But, but it did allow you not just to do that, but to also infer what you might measure to be the Hubble Consta today. And when that was done, and I don't know when the tension, I forget what was year now,

when that was done the original so WMAP of course was the first satellite all-sky measurements of the microwave background and they got a value of 71 I think in 2003 so there was this brief period as you say

where things looked like they agreed everything came together yeah that independently they came up with 71 everything it seemed like grand synthesis was in the air and then came I guess the success That's a bunch of a plank satellite. Yeah, and that was 2013. They came out with their first results,

and they came down to 67 with a smaller error bar. Yes, and so it's hard for remember when you're as old as me, you know, it seems like yesterday. But 2013, a decade ago, suddenly there seemed to be 67 and 72. And 72 at that, well, at 72 had already come down from 10% by 2013. What was the uncertainty in? So it was, you know, three to five percent. There was some disagreement about how low it had come down.

So 72 plus or minus two or three. And then 67, I don't know what they're quoted air bars and plank were. Well, by the time of plank, 2018, it became, you know, 67.4 plus or minus 0.5. So it was better than 1% precision. And that was new in cosmology when you use the term Hubble constant and 1% precision in the same sentence. And it really, I think it was good. It focused the community that's working on the local distance scale to think hard about how to improve it. But it also set a challenge, which I think is important.

It became no double tension. And some people push you more than others. But so since 2018, you know, five or six years now, people said, hey, one measure way of measuring this gives you 68 or 69. I think now, I don't know what the number is. It's somewhere in that plus or minus less than one. and then you've got 72 plus or minus a few. And that's a huge crisis. Again, I would smile because I'd say I grew up when it was, you know,

and my assumption was that they're systematics always, it's the problem. But some people have argued, including, you know, some of our mutual colleagues, people who measure supernovas in particular one of them, had argued, look, this demonstrates that we need, there's something fundamentally wrong with cosmology. And you've had to deal with it.

I get asked all the time. You know, does this mean the Big Bang isn't true? And of course, I'd have to say, no, that's not what it means. But I used to say, look, when it comes to these differences, they're so important, if true, that you've got to get it right. But my suspicion is it's systematic. I don't think, mostly because it's also from a theoretical perspective, very hard.

what you'd have to do to reconcile those two numbers by putting in some new physics. Well, that has to be exceedingly hard, right? It's exceedingly hard. And moreover, it's one of those things where beauty is in the eye of the beholder. And the papers that are written to try and reconcile them are beautiful to the authors of the papers, but not beautiful to anyone who reads them as far as I'm concerned. That was my impression anyway. But it's been a major problem, and it's in on the news and everything else.

I agree with you. It's an extremely important problem. And if it's right, we need to understand it. And this is the way you would check what is now the standard model of cosmology, right? You would make a measurement at high redshift, 308,000 years after the Big Bang. You have this model, which is a predictive model that tells you today it ought to be 67. And you can measure it today.

So, you know, you can test the model. And if they don't match, it's right. Right now, they're not hitting each other. Yeah, I mean, it's incredible. Some people would say it's just incredible hubris to expect we could measure things at that time and this time and expect the match. But that's the beauty of cosmology, of observational cosmology in the last 50 years. It's gotten to the point where a mismatch is of some concern. And it's just, I think both you and I, when we start our careers, if anyone ever said, well, two different ways of extrapolating differ by, you know, two or three kilometers.

We would have said, yeah, that's great agreement. Yeah, yeah. And it's amazing that we can talk seriously about that. But obviously, as one of the people, you know, as the head of the key project, the next major tool beyond the Cmb is now the James WebSpace Telescope and the use of supernova, which I which we'll go into in a second. And so I guess it became incumbent upon you, and obviously you wanted to do this anyway, to try and check to see if perhaps.

you know, there's a problem with the local distance measurement, or at least what it was more accurately. So more recently, you've been using the James Webb State's telescope and also not just sepheids to try, because there are other ways to try and measure local distances. So let's talk about those a little bit. And then the importance of, you mentioned the importance of type 1A supernovae, which led to the observational discovery of dark energy.

But why don't we just talk about all of those things a little bit before we get to work? Yeah, so going back to the philosophy of the key project, and I think it was an important one and why the result has stood the test of time, was not to rely on one single method. And so what had evolved in sort of intervening years was the Cepheid a distance scale was the part of the determinations, the basis. of going out to larger distances and calibrating these type 1, a supernovae that we'll talk about.

And with James Wem and also with Hubble, we had a project with Hubble a few years ago. We went back to a method that we actually used at the 10% level with the key project using red giant branch stars. So these are stars that are of all stars. Our sun will become a red giant. And these are stars that we do understand the physical basis for.

unlike the sephiads, you can go and understand those from first principles. And it's well-understood nuclear physics. Yeah, yeah, absolutely. And so these stars, they've burned all the hydrogen in their core. And once that hydrogen was exhausted, the core collapsed and they have a degenerate helium core. So the hydrogen got fused into helium, and they have a degenerate helium core that's surrounded by a hydrogen shell. And that's powering the luminosity. You're still fusing hydrogen into helium in the shell.

and the star is climbing what we call the red giant branch until it reaches a point where the temperature then is high enough. It's about 100 million degrees. And it can ignite helium burning. So you're dumping this helium onto the core, but the core can't expand the way it does in a normal star because it's degenerate. And so you end up with a thermal nuclear runaway. The temperature keeps increasing in your burning thing. And very, very rapidly, the star leaves this tip of the red giant. branch and falls onto what we call the horizontal branch, much lower luminosity. And so the effect of that,

empirically, what we see is that there's this wall between, you know, the stars climbing the, many stars in the population, they climb the giant branch, and then they reach this period of core helium flash, which is well understood from nuclear physics. And then they don't ascend the giant branch anymore. And so you can measure where that discontinuity occurs. And you can measure it very accurately. And it turns out not to have very strong dependence on the metallicity, but it also can be calibrated and much more easily than for Cepheid. It's very straightforward. And it doesn't have dependence on age. And most importantly, you can measure these red giants out in the halos of galaxies. It's far away from the disc where the star formation is still going on in a spire.

There's not as much dust in the halo. There's no dust. There's no crowding or blending. And so it's a much cleaner way of measuring. And as you say, I think the difference is that unlike Cepheid's, that red giants, from a fundamental physics perspective, you can try and understand their luminosity pretty, pretty well. The fundamental physics gives it to you. And this tip of the red giant ranch,

that maximum loss is a very important point. I mean, the only time I've been a professor of astrophysics, astronomy as well as physics, but I'm a physicist. But probably the most astronomical or astrophysical things I've done was with a colleague of my Brian Chabwe, where we tried to very carefully look at all the uncertainties to try and measure the age of globular clusters by using that typically.

And the interesting thing is we looked at nuclear physics uncertainties. we did vast Monte Carlos, which is what my part of the program was. And the interesting thing in the end, as I remember, in spite of all the nuclear physics uncertainties and the atomic physics uncertainties of trying to, from fundamental first principles that relate that turnoff point to age, the uncertainty, the biggest uncertainty was still distance. The distance, that's right. It was at the end of it all, it turned out, I remember when he came, it was all still distance.

Yeah. And that's really improved. Gaia, especially in a few years, that's really going to be nailed, which is nice. But that's right. So for us, it was an opportunity to say, okay, we're not going to put all our eggs in one basket with the sepheids, but we're going to use these tip of the red giant branch stars. And then we've also developed, so Barry Maduro and I was a very close collaborator on the distance scale. And it was a pandemic project for us to look at a different kind of star, a carbon star, what we've called J-A-G-B stars,

is J-region, asymptotic giant branch stars. And these are not, you know, they're similar to the Cepheid's in a sense that we can't go. It's not from first principles to tell you what the luminosity would be. So it's another empirical relation as for the Cepheid, as for supernovae. But these stars are, they're more massive, and they're brighter than the red giant branch stars. And the most massive of these stars, so these were discovered by two astronomers, Weinberg and Nicolaev, actually around 2000.

And they were looking at stars in the large Magellanic cloud again. And it turned out that in the infrared, these stars have a nearly constant luminosity again or brightness. And so we started to compare the distances that came from measuring the luminosity function of these stars with what we had measured or what was. already published by others using the red giant branch, the tip of the red giant branch. And we discovered that there was this amazing correlation. They read really well. And so we started to pursue that with new data from Los Campanus, using the Magellan Telescope and with Hubble Space Telescope.

And that became part of our proposal to James Webb was to use the three methods, J. G, G, RGB, and Cepheds to measure distances to the same. galaxy. So the same galaxy using three. All of which, yeah, can calibrate the supernovae. I was going to ask, but I was also going to point out when it comes to the tip of the red giant, which is, I guess my knowledge is greatest there. The fact that the greatest uncertainty was the distance measurement, actually in your

point of view, from your point of view, that's the best thing, because it means all the other uncertainties don't feed into this distance. The one thing that really determines this is distance. And you don't have to worry about nuclear physics or atomic physics. physics at the level you're interested in. So, you know, that makes it really. Yeah, that's a very good way of putting it. That's absolutely right. Yeah. And so that's, yeah, that's what I was thinking. It was a bugger bear for us, but it's a boon for you. But now why, I want to ask the same question then. Why? When I asked you about Hubble,

why Hubble? Now why JWST? What's the advantage there? So Hubble turned out to be a fantastic machine for discovering sephiads. So one of the things we haven't talked about is the amplitude of the light variation is largest at blue wavelengths. And it decreases as you go to the infrared. It's a temperature sensitivity issue. And so we know that the effects of dust are much smaller at infrared wavelengths. But finding the sepheids is best done in the optical where the amplitudes are large. You can actually see the signal. And so Hubble has been fantastic for discovery. suffering cefiads, we use sepheids, let us go out farther, et cetera, et cetera. But it really reached its limit, right? Supernovae are faint. So trying to measure the sepheids in the galaxies that are,

you know, really at Hubble's limit. That's, you know, challenging. And again, you have to worry about systematics because they're going to kill you if you don't get them right. And so James Webb, which has a bigger aperture and about 10 times the sensitivity of Hubble at these infrared wavelengths and also four times the resolution for the for the detectors that were available on hSD and are available now on on on jwST so again just as in the case when we got above the earth's atmosphere with Hubble we can now go and look at the same sepheids that have been discovered already with hubble but a higher resolution with jwST in the infrared where the effects of dust are smaller and make more accurate measurements and it comes down again to systematics so you can make lots and

lots of measurements, decrease your statistical uncertainties. But in the end, it's going to be the systematics that are going to kill you if they're there. But I guess the JDIOST measures, what's better about measuring the infrared than the optical? Is there, the effects of dust or less? Okay. That's basically it. And metallicity too. Okay.

So it's not just resolution and field of view. It's also that using infrared helps you reduce some of those other systematic, uncertainties. But the resolution, so, you know, if you have stars that are near your sepheed and, you know, in some cases, not just nearby, but maybe even underneath, that you can't separate at all, or you have difficulty separated, especially if you don't have enough signal for these distant objects, you know, you're going to get the measurements wrong. So the increased resolution is vital. Okay. So some people may have wondered why we've gone through all this. And because these are the things that determine whether the heart's up, whether there's some new physics. And if you really want

to, you know, as Carl Sagan said, I guess, you know, whether it was an amazing place. Right claims require extraordinary evidence. Extraordinary claims do require extraordinary evidence. And it is an extraordinary claim that there's some new physics that none of us have ever thought about, although there is at some level, but that somehow that it's going to come into the universe of this level is an extraordinary claim, one that I've always been skeptical of, I must admit. And so now, Tadda, and I'm happy to say, just for your sake, that this will appear after your paper appears. So we're not going to preempt your firepower there. But you are now, and I have been fortunate enough, and you let me see the penultimate version of

of the results of this paper by your collaboration using three different techniques. And it's the Chicago Carnegie Hubble program. It's called CCHB. And then one thing we actually, the one thing before we get to it, you're measuring local numbers and then you want to compare it to the far number. So you want to be able to use the one thing that allows you that last step in the distance matter and ladder. And that's supernovae. we haven't yet gotten explained why supernova are of interest so why don't you at least mention that and then we'll give your result so sephiads are what we call super giant stars they're very bright

we can see them to you know impressive distances but one thing we have to worry about is that galaxies like to be around other galaxies they're located in clusters they're located near to each other, and they impact the velocities of other galaxies via their mutual gravitational interaction. And so those, what we call peculiar velocities, can be a sizable fraction of the Hubble velocity. So remember, what Hubble measured was velocity versus distance. The farther away a galaxy is the faster it's moving away from us. So the sephiids can't be observed far enough away that these peculiar velocities are a small fraction of the Hubble velocities. So we tie into objects that are brighter

that we can see to greater distances. And the type 1a supernovae represent the endpoint stellar evolution for stars, possibly that a white dwarf, very dense star that's having matter dumped on it by a companion. And the star explodes when it gets to a certain mass. And we can see that explosion to very great distances. And then the peculiar velocity that's induced by the gravitational interactions of these galaxies is just a small fraction of the Hubble velocity.

But it's particular that the supernova 1A's, it's a certain type of supernova, comes from a certain type of star, which is always more or less the same size, and therefore the explosion has always more or less, and the more or less is the hard part, more or less the same brightness. So it's relying on the fact that

while supernovae have many different brightness, is that this type of supernovae can be measured, and the way it brought, the time it's bright and everything else is, it's almost like sephiids is related to the. They have to be standardized, right? So, you know, the peak brightness relates to how fast they decline in luminosity,

not unlike sepheids. But it's possible empirically again to do that. Yeah, again, theoretically, there's been a lot of theoretical work, but empirically, it was discovered, lo and behold, that the peak brightness and the time it takes to be bright, just like Seffi, it says,

is this empirical relationship to their absolute brightness and therefore their distance. So they became the next step that would take you from the local distances. And so what your project does is... Yeah, they go at the farthest and they have the least scatter. They have a least scatter. And so what your project did was to look at the local distance measures and then tie them to some local supernovae, right? Right.

In 11 galaxies or something like that? And then use that to try and extrapolate out to measure the Hubble constants at large. Use those then supernovae to get a distance measure. And drum roll, what's the result? So we're finding the three methods agree reasonably well, given their uncertainties. There is a bit of a range. But on average, they give.

give a hubb of about 70. And what's interesting to us is that the JGB and the TRGB methods, those distances agree extremely well. They're just almost exactly on with the CMB measurements. Those ones, which at least for the, I don't know much about the Carbon Star one, but I have to say as someone who's come at this as a theorist, if I had to pick the one measure that I would trust the most,

would be the horizontal branch one because I know most about it, but also because it is the other, I know the other, I've studied the other uncertainties at great detail and I know they're small. And, and therefore I was quite pleased to see that that method gives. Lars Bilsson said something similar to me a few years ago if you were to trust any met, but again, he's modeled these stars. Yeah, yeah, yeah. And I've looked at, I spent a lot of time worrying about uncertainties. And, and, you know, because at the time, the age uncertainty was a big issue.

about whether it can constrain cosmology. What we're seeing is that there is an offset with the Cepheid. Yeah, I was going to say that when you look at your data, I was just going to, I mean, you said they agree reasonably well, but the key point is the Saffiod still give 72, right? After all of that. And the other ones are giving 69 more or less. And so there does seem to be some systematic offset with the Cephids, which I guess we just don't understand, right?

Right. And I think what's important to say about that is that those are the distances. So it's before we go to the step of bringing in the supernovae, which is another step where there might be other systematics, right? We're measuring distances here. And so what we're seeing is some systematic offset. And that's not cosmology. That's measuring distances.

And that suffades are systematically giving you a distant distance. So what it looks like is that the newspaper headlines announcing the death of the standard model of cosmology were probably a little premature and and and i think they're premature and i think you know coming back to your quote of say against extraordinary evidence claims require extraordinary evidence i still find it very intriguing to think you know maybe there is something that's different but this is hard it's it is challenging there are systematic effects that become more challenging the the lower we want to have the error bars the more challenging the systematics become come. So, yeah, when I look at these data, it's hard for me to make a, you know,

this is not a five sigma. Yeah, yeah. This is, it's no longer extraordinary. Let me put it that. I don't think it's. Some people would have claimed that it can, you know, you could still make a case that, you know, it's consistent with the higher value, but, but. Except the it's alone. It looked like it was, I want to say five sigma, which, which is almost the kind of level that in particle physics we use for real discovery. But it's, it's, you know, Sigma being the likelihood of those two things agreeing, five sigma means it's a likelihood of one in a million or something like that. Yeah, if their errors are well understood.

Yeah, assuming all the errors are well understood, assuming it's statistics and not systematics, which of course is, with systematics, it's hard to measure sigma in that case because it's, but anyway, all of that's technical issues. But now, so with 72 versus 69 at those levels, it looked like there was a real problem. when you have two other distance estimators for the same galaxies it gives 69,

it looks a lot more like it's some systematic uncertainty that we don't yet understand in Cepheid. When I looked at your paper, that was my take from it. And that's okay. Yeah, it's always great to discover, you know, there's something fundamentally wrong with our picture of the universe,

but it doesn't happen very often. And that's what I try and tell people, is that, you know, radical new discoveries don't happen very often. If they did, they'd be, you know, anyone could do them, and they'd be happening all the time. More often, it's pedestrian things. And so there may be, you know, and we still don't understand dark energy. Like, make me clear, I've been arguing for dark energy since 1984 almost, but 95, dramatically.

And we still don't understand it. So there's a lot we don't understand. But, but. And I think that's, you know, what's so interesting. interesting about this, you know, it is a very funny standard model that we have. Yeah, it is one. We don't know what the dark matter is. You know, so we keep hoping that there'll be some hints somewhere that will tell us something that will make us, you know, have progress on this. Exactly. And we keep looking for progress.

You know, it would be great to have five signal evidence. Exactly. This is one thing we know for certain. And yeah, and I think not there yet. And, but I think what's clear from what we've done is that there are obvious things, next steps that you can do to really That's what I want to do in the last half hour, so I want to talk about these steps, including some you're involved in. But at the same time, your point is well taken. And I've often said it's frustrating. Look, I've been a particle physicist.

I've been a, you know, my whole life in a way. And the frustrating thing about the standard model of particle physics is it works. And yet we know there are things that we need to understand, but we still, but when we're looking for something that disagrees with the damn predictions. And, and I've often said it, the standard, you know, even though 20 years ago, I was worried the standard model of cosmology would become exactly like that. We've got this picture that works and you keep looking for a place where it doesn't work and you don't understand some fundamental things like dark matter and dark energy, but we're looking

for something new and it keeps and it's and it's now been, well, now it's only 30 years in particle physics it's been 50 or 60 or 70, but it is frustrating. And so it's it's psychologically attractive. I have to say that it's human nature to want, you know, it's exciting to find something. And so therefore, I have to say, I think that's part of this, is that when you can look for something that doesn't agree, you hear lots of talk about it because, hey, maybe this is the great hope of something that will give us some evidence that will tell us which is the right direction to go. But as an observer, the right direction to go is always new observations. And I want to talk about them and I want to talk about the challenges to new observations. And one of the, you know, we've discussed this with malice of forethought in terms of new instruments coming on board, changing our picture or allowing us to do things we couldn't do before. And the next set of new instruments after JWST are going to be these large, very large telescopes on Earth.

And coincidentally, it turns out that you happen to be the director of the giant Magellan Telescope Project, which is, they're going to be the largest telescope in the world, right? And so it happens to be a good place to talk about that right now. You're correct. I was founding director of the giant Magellan telescope, which is a 25-meter telescope, which will be located in the Andes Mountains in Chile. And this is a telescope we hope will be operational

in the early 2030s. And it will have a resolution that's 10 times that of the Hubble Space Telescope. And so we'll be, I think, very interesting in terms of, you know, improving our resolution and measurement of, of Saffiates, tip of the red giant branch, carbon stars, et cetera. And for following up the type 1A supernovae that we're going to have, you know, order a million of these things with the Rubin Observatory, which is about to start observing, you know, within this year. So, wow, exciting. Well, let's, it's not, that makes it all sound so easy, but, um, it's been a long road.

Yeah, but let's step back because I want to give a sense. One of the reasons I love a theorist as I can come up with an idea of write a paper and then do something else. And you know, and for particle experimentalists, I can watch them spend 30 years, especially waiting for an accelerator and then building a large experiment. And these now, and when it comes to astronomy, it is somewhat similar. It's big science. And these things aren't done easily. When you're the founding director, when was the giant Magellan telescope, just give people a sense. You're right. It's going to be operational, we hope, in the 2030s. When were you, when were the first discussions that you had about building it?

So the first discussions were probably around the year 2000, and in fact, sort of just an interesting historical note. We had some discussions with Caltech at the time about possibly joining what at that time was called the California extremely large telescope or Celt. And so that was just before I became the director of the observatory. I became the director in 2003. And 2002, I had some discussions with the then president of the institution about joining Kelt. And that didn't materialize.

I wasn't director at the time, wasn't able to carry that forward. And so in 2003 was when we began to plan for what became the giant Magellan telescope. And, you know, there had been lots of discussion when it fell through with Caltech about what we would do in 2002. But, you know, when I became director, I took it upon myself to really advance the project. And that was a Carney institution that, which is separate from Caltech, although it's down the road. Yeah. And people, you know, geographically, it looks the same from outside of Pasadena, but long history there. And, yeah.

But so, you know, we cast the first mirror for the telescope. Each of these mirrors is 8.4 meters in diameter. All seven now have been cast. And we started the first one in July of 2005. And it's been a long, wrong. That's what I want to point. It's going to be 30 years more or less from the time this is, you know, talking about the kind of difficulty of doing good,

of new science at this level. And the mirrors, just to give an example, I don't know, but I mean the mirrors have to be accurately cast to what kind of precision? So the surface of these mirrors is they've been polished to 20 nanometers. 20 billionths of a meter. It's like if you took one of these mirrors and you spread it out the size of the continental U.S., the size of the Rocky Mountains would be, I think, if I'm recalling correctly, something like less than an inch.

I mean, it is smooth. And it took a long time to look. learn how to do that. The first one took seven years. These are not easy, you know, buy them off the shelf kinds of things. So it requires new technology, new science. And when I was in Arizona down the road, there were people who worried about making mirrors down at. Yeah, no, it's fantastic facility. Yeah, University of Arizona. And also, are they, are they individually deformable or not for the adaptive optics? So each one of these will have actuated. on the back that can be, you know,

push and pull on the surface at sort of seconds, 30 second or so time scales. But then they're adaptive. That's, you know, that are thinner mirrors, that are secondary mirrors. Each one, each of the primary mirrors has an adaptive secondary.

And those can be deformed on millisecond time scales. And we won't go into great detail, but that allows you to take into account the effects of the atmosphere. The mirrors can be deformed. We need the power of a hull. without going. Yeah, because the atmosphere varies, but if you can know what the variation is and you can, some using lasers and things, you can then immediately let the mirrors know how to deform appropriately to take into account. It's amazing. It's almost like a Maxwell's demon. It's amazing technology. I remember when I first talked about it. I first had a picture of it when it was really, I remember when I wrote the physics of Star Trek and it was just so, it was a laser and it was just seemed like such a, that was in early 90s.

I guess. And it was like, oh, yeah, well, maybe, but I don't believe it'll, you know, always. My attitude was always, yeah, but it won't work. I mean, it works. It is technology that has been tested and used. Yeah, no, it's amazing. And, you know, and it's all, I constantly underestimate the ability of experimentalists to do things. Or I try not do anymore, but it's, it always seems virtually impossible to imagine how this will be useful. And then it always ends up. Yeah, yeah, it's great. It's wonderful. I admire, I admire experimentalist. I admire experimental more and more and more and more jealous of that.

But anyway, our site, you know, the site has been leveled. The pier is ready for the telescope structure and the telescope to arrive and, you know, the contracts are being let. And so, yeah, we're just trying to raise the rest of the funding. Partnership is international now. There are many countries involved. How many countries? U.S. So there's Brazil, South Korea, Australia, Taiwan now.

and the Weissman Institute in Israel as a recent partner. So it's in Chile, of course. In Chile and U.S. Yeah. And a lot of U.S. partner too or no? What's that? Is Canada partner at all?

No. No. Okay. Okay. And it, yeah, I mean, it, as I say, as a theorist, I become, you know, I did a degree in math and one in physics, so I wouldn't have to take an advanced lab specifically. And now I'm more and more jealous about it.

I wish I'd done it. But anyway. How much does it cost? And by the end, I know you don't like to talk about it. People don't like to talk about it. But just think of a sense of how hard this is. Yeah. It, you know, essentially it's roughly a $2 billion project.

Two billion. And is it, and was it estimated to be $2 billion originally? I mean, these things get more expensive over time. I mean, you know, it's two things. One is inflation, of course. It brings the price. You know, 30 years since we first started talking about this. But the other is, you know, and this was a worry that I had earlier.

on leading the project was, you know, telescopes had been built, single mirror telescopes had been built. You bring the mirror to a common, the light to a common focus, but this was many pieces that had to work together. So the systems engineering, you know, is, you know, and then, you know, these mirrors also, they're off access, right? Most mirrors before, you brought the, you know, light would come in and the parabola would focus it, a single focus. But now you have to do this with these 20 ton, you know, moving mirrors that you're trying to correct. And that's not simple. And so, as I said, the first mirror took seven years to learn how to test it well and to make these, you know, the surface smooth to 20 nanometers RMS took a lot of learning. So, yeah. Wow. Okay. Well,

it's going to, okay. Now, what are you going to do with it? Well, one of the things that I think is going to be very exciting is that you know in the last several decades we of course have learned about the existence of other planets around other stars and the technology is just shy of reaching where you can measure the motions of earth mass planets so i think you know one of the most exciting potential discoveries a telescope of this nature could make is the discovery of life on an earth mass planet yeah and that you know will be feasible with you be able to measure that the atmospheres of Earth-math planets and look for biosignitions. If there are atmospheres, I have to say, you know, it's been a little depressing to look

at a lot of planets and not find that, you know, interesting atmospheres. But there may be, we may be alone. It's a question, right? Maybe a few decades ago we didn't even know whether there were other planets. Yeah, I know, I know. And the fact we know thousands and yeah. And we'll find out it'll help us know if we're alone. I'm betting we're not.

But again, you know, it's technology. Every time we build bigger and bigger telescopes, we make discoveries. And often ones we can't anticipate, right? Usually the ones. Those are the exciting ones. Yeah. I mean, that's what I always say. Every time you open a new window on the universe, you're surprised.

And I tell people every damn surprise, if I'm not surprised. No, that's, you know, I really want to see this telescope. Yeah, because one can, one look, you know, I often tell people that when we write scientific proposals, we lie. We say why we're going to do this, right? Because you've got to have a good reason to do it, especially you're going to spend $2 billion. For me, it was a few hundred thousand dollars with a sporting group. But here's why I'm doing it. But what you really hope, of course, is, you know, you tell what you're going to be doing in a theorist.

If you're an observer, maybe you talk about 20 years down the road. I was talking about three years down the road. Here's what we're going to be doing three years down the road. And my hope always was that I would be doing something completely different because of discoveries. And that's what we really hope. So the book is you're not lying. You're sticking to what you know, but, you're hopeful that you're actually going to find something completely different. Absolutely.

I mean, my most recent book is called in the U.S., in England, the known. in England, the known unknowns for after a famous quote. In the U.S. is called the Edduring Knowledge because they didn't want to have a Donald Remsen quote on the cover. It's still a good quote. But the known, it's good to know the known unknowns because that justifies why you're going to do what you're going to do.

But the most exciting thing is the unknown unknowns. And of course, that's exciting, but it would make a much shorter book, of course. But, but. But also what is the problem for the Hubblecon. Yeah, yeah. The unknown unknowns that get you. Yeah. Those are the ones that come and get you, exactly.

So planets, what else? So, you know, in terms of dark matter, dark energy, distribution of dark matter in the universe, measuring distant supernovae and trying to characterize the evolution of, you know, the rate of change of the expansion over cosmic time in conjunction with telescopes like Ruben, pretty much, you know, black holes, planets. Everything. It's just going to be one. machine, the initial mass factory.

Every area in astronomy is going to be touched by, you know, increasing capability of this magnitude. Now, just to be fair, though, of course, this isn't the only big telescopes that's being built or designed. And how does it compare with the other ones that are coming online, the European one? And I don't know if ever, I doubt anything's going to happen to Hawaii, but I don't know.

We don't know. I really hope that both telescopes, you know, US telescopes will get built. You know, there are, I believe, 16 telescopes between six and a half and 10 meter diameter and oversubscribed. So that, you know, having three telescopes in the world of these extremely large telescopes is not too many. There will be a lot of oversubscription on those. So, as you say, there's one in Europe, and they're aiming to build a 39-meter telescope, and they appear to be on track to be operational by 2030. So they're more likely come online first before GMT?

Possibly. If we had the funding issue solved, we could start tomorrow. We could just go fast. I mean, so I assume it's a big race, right? Because the first one out is going to be a lot of. It is a big race. But there's the long game, too. There's a lot of, there's a big sky out there that awaits discovery.

And in different instruments on the telescopes too. And then there's the second U.S.-led telescope is, the TMT, the 30-meter telescope, and that's a partnership of a couple of U.S. institutions and Canada and India and Japan, and they had China until recently. Okay. Well, look, let's, so, okay, so let me ask in a general sense, what's the future? What are the biggest challenges, opportunities that you can see for the field, and then I want to become more personal, and then I want to go in a, I want to talk a little bit about the sociology at the end. So just so you know where we're heading.

What's the future? What are the big questions that you think are going to be resolvable and say your lifetime in, in, in, in technology? I like to understand dark energy. I'd like to know what the dark matter is, been searching for a long time. And I think one of the real possibilities that would be disappointed. is it will never know that it only interacts gravitationally and there's no way of detecting it. But it would be really exciting if we could. That would be a big step forward. I think what's

coming on in next few years, I mean, it really is going to be an exciting time. So the Verirubin telescope, which is this six and a half meter telescope that will survey the sky every few nights, it will go very deep and follow transient objects, lots of supernovae being discovered, the distribution of dark matter with really deep fields. And so, you know, again. Is it called the Vera Rubin Telescope? Because, you know, Vera Rubin was certainly one of the first people to get evidence for dark matter and profound evidence from. I think it's nice recognition of Vera.

It's wonderful. She was wonderful. I knew her. You knew her. But, but will it be designed to look for dark matter in it? Well, you know, one of the things that it will do well is to characterize the distribution of dark matter. But again, with all these telescopes, you know, you design. it, you know, people very early on were interested in dark matter.

And then you never in the field change. That's the other thing. You design it. And I wonder, does that affect the GMT? It's like, it's like space instruments, right? You design it and it's 30 years before it comes up. And often the field has changed. Sometimes the technology has changed.

And sometimes the questions have changed. And you have to deal with that. And I assume it's happening with the GMT too, right? Oh, yeah, very much so. I mean, certainly the Hubble tension wasn't around the opportunity to look for Earth mass planets. We didn't have the capability of doing that when we were first talking about GMT. I mean, only the first planets had been discovered in 1995. So, yeah, and things really move and some things get done, right?

When you're doing your science document in 2003, some things are already done before you get there. But so, and there's going to be the Roman, Nancy Grace Roman space telescope, which is going to be launched in 2026 or so, and that's going to do an infrared survey of the sky and that's going to, I think, be really interesting. It would be great for us with tip of the red giant branch and opportunity to really do the halos well in galaxies. And, you know, it will be a survey instrument that will study the Milky Way galaxy and, you know, again, many, many different fields. And I think LIGO and Virgo and, you know, the opening of gravitational wave astronomy, I know, I'm very excited. I hope eventually there will be more of these gravitational

wave sirens, which is a completely independent way of measuring the Hubble constant. Unfortunately, nature has not been kind, and it's only delivered one neutron star, Neutron star binary to make this measurement, but you would hope with time, there will be more of these, and that I think will be important, not just for the Hubble constant, but also for dark energy. And yeah, I, so again, with new technology comes new discoveries, and there's just a lot coming online that, you know, many of these, I hope many of these big telescopes. Let me ask you another way, because people get asked this about accelerators,

as people say, the large had a cloud would be the elastic large accelerator. Will the GMT and the EELT, will they be the last large Earth-based telescopes? Or do you think there's going to be another generation? I think, you know, with ingenuity of people, space is going to become the place to do it to actually either make the mirrors in space or unfrolet. You know, it's different technology for the mirrors and interferometry. and long baselines and maybe even filling some of those baselines. So, yeah, I wouldn't be surprised if not the last,

they'll be the last for a while. Last for a while. Yeah, and they're expensive, and that always is a problem. So in space, but I think people are getting, you know, the opportunity now in space is going to get. I'm just glad there are a lot of other things to do than measure dark. I always, when people are going to somehow measure dark, determine something about dark energy, I always, I have to say,

I made a bet with a bunch of people, although they haven't ever acknowledged that they made that bet. One was Stephen Hawking and other was Frank Quilcheck. But at the time that in the 1990s, that dark energy would not, we wouldn't understand what it was. And there would be not a single measurement that would tell us it wasn't a cosmological constant in 10 years.

And I'm going to make that bet now that even after GMT comes out, we're still not going to have any observation. I wouldn't take the bet. Yeah, it's not going to be solved, I think, unfortunately. observationally we're going to have to come up with a good idea which is a lot well i don't know if it you might say it's not a lot harder but it comes up fewer you know the whole paradigm could shift yeah we're going to need so know something fundamental we don't understand i i don't think that's going to be solved by observation whereas i do think dark matter will be i think but observations could

help point you yeah maybe i'm i would love it but my hope but my fear is it's going to be constant and not observably different and and and actually that's my expectation not just my fear i would be amazed, shocked. I would have been shocked in 1995, 98. I'm going to be shocked now 30 years later. What are, what are, besides the future of the field, what about your personal goals? What do you want to do next? Yeah, I've been actually giving a lot of thought to this recently because I have another project left in me, I feel. Yeah, okay. And so I actually, you know, I have a few ideas. I'm not going to talk to you about. I may go on a read. Yeah, don't, don't.

Yeah. But I do feel I have another chapter in me and I feel, you know, we need to finish up this stuff with JWST, but there are clear steps there. And so, yeah, I'm looking forward to doing something else. Okay, good. I hope you do. I'm hoping. That would be great.

Now, I do want to end maybe with a whimper rather than a bang. That was the bang part. This is the whimper. The sociology affects science. And we talked about Monica, I alluded to it, although the listeners might not know. There was another big telescope that was set to begin a long time ago on Hawaii. And it's not begun. And it's been, 20 years that it's been delayed at least because people's concerns that somehow it affect that that building a telescope on the top of a mountain is going to somehow interfere with indigenous people's religion.

maybe, I don't know if that, maybe that's an unfair simplification, but it has affected it. And what do you, maybe you don't have to answer any of these questions, but what do you think about this? I think, you know, for a long time, people have, you know, how do I answer this? people do have sensitivities, and I think it's important to understand where people come from, including it's important for people to understand what science is about. And I think, you know, this is not just in the context of the telescope, but it's context, you know, in our society in general now, is that there's,

there are misunderstandings that arise, you know, what is science, what can science do, you know, can you just, you know, people who would look at science and say, I just don't believe it. but don't really have an understanding of what science is or how it proceeds. So I think on all sides, it's incumbent on us to communicate. And then when communication goes badly, you run into problems. And, you know, so all I can say is that I, as I said earlier, I hope both of the U.S. telescopes get built.

And if it can't be in Hawaii, then it gets built, you know, on another site. And including Las Campanis. We actually leveled the mountain while I was still a leader of the project. and made room for two telescopes. So there are other mountains. And I hope ultimately both telescopes will get built. I think it's important. Yeah, no.

And I think it's a shame. It is a shame. I always worry when religious or ideology gets in the way of science. But it happens because people do have sensibilities. And often the problem is a scientific community, not communicating well enough. And as you know, personally and professionally, that I've spent a fair amount of my professional career

on communicating science because I think it's important, and I felt it's at least as important as the science I do. Well, it's something I felt I had to do, and I believe it's essential. It's the only where we're going to get ahead. I've been a little worried when the scientific community, you know, what worries me about Hawaii situation is this rift in the scientific community

where scientists themselves are somehow arguing that science is not universal. let me put it that way, that somehow, you know, the sad thing is there are a lot of young scientists who are somehow saying that the science is somehow interfering with culture or racist or whatever and therefore it's a problem. And I think my suspicion is until the scientific community gets its act together and decides, reminds itself that science is actually, in principle, independent of any human foibles, but it's a process that brings, humanity together rather than separating it. Until we get that straight in the community, I don't think that you're going to see any progress in terms of getting the public to buy that. And then my last question in that regard is,

it's personal. We began back with your high school teacher telling you that, you know, girls didn't have to listen, which caused you to want to listen more. You know, this is another area of discussion, and I've written about it. But, I mean, you know, women, being a woman in science has been challenging. I've been arguing since I was a chair of a physics department, you know, recruiting women in science.

And the first women faculty in the department I was chair of was when I became chair, that it was a great time to be a young woman in science because there was so much interest in getting women in science. But have you, let me ask you personally. And again, don't answer if it's an issue that you don't want to answer to. have you found that being a woman has gotten in the way of your being a scientist? And do you also think in the current time that being a woman in science is harder than being a man or the other way around? So, you know, I think when I began, starting from before I became a professional astronomer, there were things that happened along the way that were not positive, you know, things that got said, people who were discouraging.

but there were also people who were encouraging. So I think it was a little bit of a mixed bag. You know, at the time that I came into the field, I ended up being a lot of first just because there were, you know, there just weren't a lot of people ahead of me at that time. And I think also that is sort of a mixed bag. I think, you know, in some instances there are people, again, who want to help you, who feel that there haven't been women.

and then there are people who don't and don't feel that you can, you know, fill the shoes of some rule because you aren't a man and a leader or what you, what people think a leader might look like. And so, you know, I do think it's both. Things have definitely gotten better since I entered the field. It was, I would almost say usually, I would go to a meeting and I would be one of a very few women. And that now can be very different. I can be in a room and there are many women in the room. And that's a new experience for me because, you know, I was used to for a long time being the only woman in the room. Yeah, it's changed tremendously in terms of demographics.

Although I would say astronomy is a field, especially internationally, where women have, there have been a lot more women in a, at least observing situation in, say, astronomy than in say particle physics or something like that early on. It's a field since Henry had 11 and before that's attracted significant and then, you know, the two telescopes he just mentioned are named after significant women. And so it's a... Nancy Gray's Roman was effectively told by the University of Chicago. She could not expect to get tenure. And that's how she ended up at NASA. And Vera Rubin was, of course, not allowed to be a graduate student at Princeton

because she was not allowed to observe at Palomar until she took a little paper outline of she made a little paper doll and put it on the restroom because their excuse had been there are no women's restroom which had problem solved and put the little doll on the door. So there were a lot of those things.

Yeah. And I guess the point is that there have been real roadblocks but we've come a long way. But I just wondered, in your sense, in your career, as you felt like it ever was a huge impediment, I've since no, but, but, but, but, um, I tend to have just, you know, worked. I, I've, I tend to try. The idea is just to do the work.

You know, show that this is what I want to do. And I don't like getting caught up in things that are, you know, and, you know, and I think people around me have changed too. I mean, there were senior scientists who made comments about women or other women early on. And, and, and, but I, they changed. They really did. They, um, which I think is great.

It's actually sometimes younger scientists who, who I think maybe don't always have the same attitude. It's interesting. Yeah, no, it's interesting being, you know, it's age demographic, because you and I have a certain age, and we can see how things have changed. And do you find out of interest, because there are now fewer men going into university, as you know,

and then women, do you find that your average meetings where there are very few men now? You know, rarely, but I do occasionally find myself in an undergraduate classroom, when I look around and think, who, I think there are more women here. There's 60-40 in general now, 60% women are 40-000. That's very different. And, you know, I think things will fluctuate and it doesn't hurt if, you know, they shouldn't always be systematically on the other side of the line.

It's been a systematic shift. It's interesting. It'll be interesting to see what happens. But anyway, I wanted to discuss that little bit of sociology because you're a wonderful scientist and the really, and the really, and, and, and, and, and, you know, The last thing, am I, I don't want to put words in your mouth. The last thing a wonderful scientist would want to be called

as a wonderful female scientist or a wonderful male scientist, it seems to me. Or a wonderful white or black or whatever color or short or tall. That's the wonderful thing about science. It should unify us all and it can be done by anyone. And it's exciting. And that's why I, I worry about identity labels. I think we should just call ourselves scientists.

And I, and that's one of the reasons. You know, I'm so happy to, well, I know you as a friend, but as a great scientist as well and why I really thank you for spending time. Explaining the important science you've been doing, it's been a real pleasure. I really appreciate it. Well, thanks very much. Thank you very much for having this conversation. Yeah, anytime.

And I look forward to seeing you. So it's been too long since we've been together except for on the screen. and so sometime in the future. Thank you very much. Okay, nice to talk to you. Nice to talk to you. Hi, it's Lawrence again. As the Origins podcast continues to reach millions of people around the world,

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