Did the James Webb Space Telescope just solve the biggest mystery in cosmology? The discrepancy between different measurements of the Hubble constant has been causing a lot of ajda in the astronomical community for the past few decades. But Wendy Freedman, a renowned astronomer and professor at the University of Chicago, is at the forefront of efforts to alleviate and solve this cosmic conundrum.

Data are convincing. I wanna be convinced by data.

Known for her pioneering work on the Hubble Key project, the very reason the

Hubble Space Telescope was launched

in part, and her significant contributions to measuring the Hubble constant and properties of stars throughout the universe. Friedman is now leveraging the cutting edge capabilities

of the Webb Space Telescope

to tackle the Hubble tension head on.

The only way that we will understand how we're limited by systematics is to make the measurements very precisely in each case.

With decades of experience and a deep understanding of the intricacies of cosmology, of measurements, of accuracy and precision, there's simply no one better to shed light, if you will, on this issue. So join us as we take a deep dive and perhaps resolve the tension, the frustration, and the anxiety plaguing astronomy today, courtesy of the brilliant Wendy Freedman. Let's go.

Wendy Freedman, thank you so much for coming back on the podcast, your second time on the podcast.

Glad to do so.

Yeah. And thanks for hosting me in your beautiful office here at the University of Chicago. It's, it's always a pleasure to come here. I get to experience humidity whenever I come from Southern California, where you used to live for many theory.

I did.

Director at Carnegie. So we're gonna talk about your your career and your current research and this really cool looking model in the background over there that you're so intimately connected with. But the theorists thing I think I would be interested in talking about here are these recent results that you have, been participating in and leading to large part with the James Webb Space Telescope. So how did that come about? Because I understand from my friend, Adam Rees, who's been on many times, it's very difficult to get time on the James Webb Space Telescope. They don't even know of all your accomplishments because everything is blind. The the the pros walk us through the process. How did you come up with the idea? What is it telling us, and how is it perhaps resolving the Hubble tension without the need for a psychotherapist?

You're you're right. It's very difficult to get time on JWSTs, highly oversubscribed Brian very competitive. At this juncture, we're trying to measure the Hubble constant more accurately than it's ever been possible to do before. And as you know, there's this possibility that there's a discrepancy between the nearby values of the Hubble constant that we measure locally and what you get infer from measurements of fluctuations in the cosmic microwave background. So we need higher accuracy than we've ever had before in the local distance scale, because the microwave background observations now are so precise that we need to make sure that, locally, we can have a a comparable competitive decision to see if the discrepancy is real. So our, focus with JWST, and we wrote a proposal that depends not only on Cepheid variables, which we use, for example, with the Hubble Key Project, which the CHUs team also uses, but including also 2 other methods, the, tip of the red giant branch and a new fossil carbon stars we're calling JAGB stars. Yeah. And the premise is that because there are systematic uncertainties in any method, they all have their own.

Yep. Yep.

The only way that we will understand how we're limited by systematics is to make the measurements very precisely in each case. So you need distance indicators that are very precise internally and compare those. So that was our proposal to measure the distances to nearby galaxies that have hosted type 1 a supernovae. None of the methods I've just described goes out far enough that you can get into the smooth Hubble flow. The cure your motions induced by gravitational interactions are too large to measure it accurately at the 1% level that is now a goal.

Mhmm.

And it's a very challenging goal. Let me ask you. I'm sure. Yeah. And and so what we're doing is measuring the distances to the same C using these theory techniques. So we didn't know, are they all gonna agree? Will there be 3 different answers? And will there be an outlier? Let's see. And and if there's systematics, we'll try and cover that.

Any and so do the tip of the red giant branch, the carbon stars, the cepheids, do they have kind of orthogonal systematic effects? Do they share common effect systematic effects, you know, that that have to be mitigated and you learn something from one branch and apply it to another one, or are they distinct?

Well, the nice thing about them is they're computers different populations. So Cepheids are young stars. We find them in regions nearby to where they actually form because they haven't had time to, move away, diffuse away from the locations where they were formed. And so we can only find Cepheids in the disk, and these are young stars. The tip of the red giant branch, we can find them in the disk and the halo, but the advantage of the tip of the red giant branch is that you don't have to work in the disk because in the disk, you have lots of dust, You have high surface density of stars, so you have potential crowding and blending of the objects, and that means you can't measure the velocities very accurately. And when we get out in the halo, you don't have those problems. And then the JAGB stars are an intermediate population. We find them in the outer disk, so not as much of an issue of crowding or reddening bang, and a very different evolutionary stage again.

So that's the advantage. They have completely different systematic. In common, what they have is the calibration. So with JWST right now, our galaxy that provides the geometric distance to to anchor all the galaxies is NGC 4258. It's a galaxy that has a black hole in its center, and it has, water mega lasers that are orbiting the black hole, and you can use those objects to, estimate a distance, which is a geometric distance. So all of them have that zero point calibration, and that's true even if you had more calibrators. You know, we can use in the same ones for for the target galaxies. So there are nice things.

They're all independent in terms of their physics, and, and then the relative comparison is very straightforward.

You can Now when you say tip of the red, John, how do you know something's truly in the tip? How how important is it to be? And maybe say a little bit about HR diagrams and how you know this for some of the folks that might not be as as much of, astronomy mavens. Well, theory can't be as much as you, but but even as me. Can you describe what does it mean? What is exactly going on with that tip of the red giant branch?

Yeah. So giant branch stars are phase of stellar evolution. Our sun will eventually become a red giant, and they have a core that has already exhausted the hydrogen at its centre. Right? Most stars spend most of their lives burning hydrogen into helium. It's called the main sequence, and they spend most of their lives there. So these stars have used up their hydrogen. It's now got a core that's made of helium, a very dense state of helium, it's a degenerate core, and they're surrounded by, an atmosphere of of hydrogen and or a shell of hydrogen and then then an atmosphere. And the hydrogen is burning again, still burning hydrogen into helium, and it keeps the so that's what's powering its luminosity, and it's dumping the helium that it's forming onto the core.

So it's getting hotter and hotter, more and more luminous, and then it reaches at the point where the temperature is about a 100,000,000 degrees, it can start to burn, helium Mhmm. Via this triple alpha process science a nondegenerate way.

Yeah.

And so there's a thermal runaway. So the the star can't expand because it's degenerate. The temperature is now enough. You can start burning the helium, and it can't expand. So it just the temperature gets higher and higher and higher, runs away, and the star very rapidly then falls onto the horizontal brains. It does it. So it's no longer ascending the red giant branch. It's reached the tip, And we call that the core helium flash, and the position at which this core helium flash occurs is what we observationally refer to as the tip of the red giant branch.

And it's very well defined. You can actually see it by eye. You don't have to do any sort of, you know, data processing to you you see it. The theorists climb, and then there's a a small population of asymptotic giant branch stars. They provide a sort of pedestal, but you can still see this discontinuity. You measure the first derivative of the luminosity function, and we do lots of tests of injecting artificial storage, etcetera, etcetera, to see how well we're doing that, but, it's very well measured. It's very simple, and and that's a nice aspect of the method because cepheids, are more Keating, and you have to measure periods for them. You have to, have colors so that you can correct for dust.

You have to worry about crowding and blending because they're in high surface brightness areas. You have to worry about metallicities. There are a lot more factors that go into measurement No. Cepheid distance.

Do the, TRGBs, do they appear in globular clusters? Because those are also they have

They do, indeed. And WEMO's measurements in the Milky Way and globular clusters and use the Gaia satellite where they're at parallax. Isn't those She

gets very accurate.

So the number, you don't have as many of them. Yes. We have to do it in a a way that we form a composite column magnitude diagram. But, yes, absolutely.

Well, you brought up the complexities of Cepheids, and I wonder if you could recount this, you know, kinda wonderful story in the history of astronomy or Henrietta Leavitt's, law, which well, I want you to describe it. Yeah. I can't read this. When I had on, I had on, like, famous philosopher of of the mind, his name is David Chalmers, and he came up with this this this concept called the hard problem of consciousness. You may have heard about it.

Like Yeah. But it's Clarke. Absolutely.

Yeah. So he's he's he's an amazing I had him on, and he's from Australia. And I said, you know, David, if I had on ACDC, also from Australia, and I didn't ask them to play Back in Black, I'm not doing a good job as a podcaster. So I have to ask you as the, you know, one of the foremost astronomers of our of our generation, describe what Henrietta Levitt did. And my question for you is, how could they make so much progress and so much, you know, reliance on cepheid properties before they even knew anything about nuclear fusion. I mean, when she came up with this law, which you'll describe, they didn't know nuclear fusion even existed or how how it took place in in the core of stars or even elsewhere. So, please, could you describe what is Levitt's law, and how could we make the progress that we made so rapidly in the early part of 1900 without knowing any nuclear physics?

Henrietta Lemitt was studying stars in our nearby galaxy that we now know are galaxies. We didn't know that at the time. It's in the living. The Clarke Magellanic Cloud and the small Magellanic Cloud. And so, you know, as you know and many of your listeners probably know, there was a debate at that time about whether or not our Milky Way galaxy was the entire extent of the universe or whether it was a a C, as we now call them, similar to the Milky Way. I mean, that there were other galaxies similar to the Milky Way. And there was no way of of telling because you couldn't measure the distances to these objects. There was no way of doing that other than for the nearest stars.

And what Henrietta Leavitt discovered was that there were stars in the Magellanic Clouds that were changing their brightness over time. And so she had access to harder plates, photographic plates, which was the detector at that time, And these are glass, big glass photographic plates. And what she found was that the Brian of these stars, they would get greater and greater and then at full more slowly. So rapidly rise in brightness and then fall off more more slowly. And that was similar to stars that have been known since the 1700, Cepheid variables that were known in our own Milky Way C. And so she made the further observation that the brightness of these stars was related to how fast they were changing in their brightness, the period of variation of the stars. So she plotted the brightness versus the period in a logarithmic form and discovered there was this really tight correlation. And so the implication of that is that if you could measure the distances to, some Cepheids by some means, geometric perhaps means, then if you could measure the brightnesses of these stars, if you could find them in other objects, then you could determine just by the inverse square law of light how far away.

Right? Light falls off as one of the over the distance squared. You could determine the distance to that galaxy. So it was an empirical Mhmm. Discovery. So it was an empirical

Mhmm.

Discovery. And again, nobody knew what the implications were, what the fundamental underlying physics was, but it was an empirical relation. People didn't even realize that the stars were pulsating at the time. Now that came a few years later, but her results had enormous implications. And, sadly, she died before she was ever to see what the implications and results were.

There's a new book coming out in the fall from MIT Press about her that I'm, I'm gonna have the author on. She's coming out in September.

She's great. I've spoken to her.

Oh, you have? Okay. Great. Well, for any of this conversation, oh, funny talking there. So another thing that I've wondered about, you know, kind of inverting my question is now that we know so much about nuclear fusion, has that impacted, you know, does that have an effect on the cosmic distance ladder? Does it does it affect things or have we improved it? Not merely knowing there's a correlation and understanding that causation. In fact, in great detail through simulations of nuclear fusion, understand the quantum mechanics of it, and so forth.

Yeah. It's interesting. I mean, in procepheds, we don't yet. We cannot start from first principles and of all the cepheds and say, this is what, we should find for the next crisis. They're too computers. And the atmospheres are complex, so we don't there's still really a lot of uncertainty about how metallicity might affect the luminosities of these stars. So it's not just the theory understanding the stellar evolution part of the interiors, but there's also an atmosphere and the atmosphere is in motion. So, no, we we cannot do that.

The probably closest, method would be the tip of the red giant branch. The stellar evolution of those stars is very well understood, and it's been understood for decades. We understand the core helium flash. That's been really well modeled, and and we do understand the nuclear physics of of those objects.

And some of the these carbon stars, you

what do they call JABE or JABE. Yes.

JABE. That has to do with their, spectrum Clarke. Okay. I like that. The bands in which measure them, the j band or

It's the region. So it turns out that there's a, in the infrared in the j band, which is about 1.2 brains, the luminosity of these stars is is essentially constant. Lower. And so it's it's a subset of carbon stars. And the reason, apparently, that they show this constant luminosity is that these stars at their phase of evolution are actually dredging up material from the interior interior regions to the atmosphere. And some of that is carbon, and that's what makes them look red.

Yeah.

And so, the stars that are more massive, they are burning the carbon before it gets to the surface. The stars that are less massive never actually dredge up the theory. So there's this intermediate mass range where they are dredging up the Brian, and they so they're they're bounded by these two limits after physics.

These aren't like white dwarves that are pure carbon. Right? No. No. No. Understood. Okay. So Mhmm. The physics of the carbon stores, are they also fairly straightforward compared to this modeling Cepheids from first principle that you said is

I theory, no. Again, understanding their atmospheres is just it's not simple. It's it's much more complex than the than the giant branch stars.

For those of us who aren't, you know, as familiar as you are with with, you know, even main sequence in stellar evolution. So my understanding was that, you know, the sun's nuclear fusion, you know, processes really occur in a relatively small volume, less than 10% of the volume of the star. Maybe I'm wrong about that, but you were you were saying that for some of these objects that they, tip of the red giants, they're fusing in the outer in the outer atmosphere. Is that is that No. It's still in this core. Okay. Still in this core.

And what fraction? Is a atmosphere above that. Yes. And And the shell surrounding the core. No. No. No. It's it's it's very much the interior. But I think it's important to say that these methods, all of them, we don't understand type 1 a supernovae to modernize

That's yeah.

These are empirical relations that we're using. And so, you know, they appear to work, but that that I think is, as I said, other than the tip of the red giant branch, probably a disadvantage than that we don't understand them well enough to to, start from first principles and say what the what you thought to be.

And would you have wanted to use the Webb telescope regardless of, you know, the the targets that you were looking at, and it just happens to be the most advanced space telescope with understood systematics at least? Or could Hubble still play a role in in measuring these these tip of the red

So, you know, Hubble has been the work force for the extra lack of decusson scale now for decades. Right? And it's what we use for the key project. It was, the Cepheids have been the gold standard. And, the reason that Hubble is so useful for Cepheids is that Cepheids, their amplitudes are larger in the optical part of the spectrum than they are in the infrared. And the temperature sensitivity is smaller in the infrared. So it's a very good machine for finding these variable stars. And and so I think that will remain the case to actual actually discover Cepheid. Hubble is a tool that you want or something that has, sensitivity in the optical.

In terms of getting at the systematics, which is what we really have to be focused on now, JWST has a huge advantage in that, it is sensitive in the infrared part of the spectrum. So the dust that we've vaguely talked about is, the effects of dust are much smaller in the infrared. The resolution is about 4 times higher, for JWST than it is in the infrared with Hubble. And so the crowding resolution issue is much less, and, the effects of dust are less. And, those are the types of things now that we need to improve upon to get a more accurate measure. So find them with level, following them off with JWST is fairly the way to go at the moment.

Have they I talked to Bob Kirschner not too long ago and and his late graduate student, Andy Friedman, was my post doc for a while in San Diego. They used to talk about the differences in in type 1 a supernova and the infrared sort of had this new luminosity peak that sort of comes on delayed and so forth. Are there other peculiarities that come in in when you have an infrared machine like Webb that you wouldn't have noticed or maybe couldn't have measured as accurately? Or is it primarily because it mitigates dust and the systematic effects that you speak about for

I think for the objects we're studying right now, do you that bang it's really the tool that we need. Where we're limited right now is just a small number of objects that are available to actually measure. And so the statistics are still better with how old than they are with JWST, but that will improve, with time. And the other is that, because the samples are small, we're, you know, looking potentially that there may be systematic effects in the calibrators themselves. And that's something that is, you know, it's we're starting to see hints of in the data, and and we need to follow-up and make sure that that these aren't problems. So the the objects that have been measured with Hubble that are the most distant objects in that sample, and it's a small sample. It's a few dozen. And, to show that they don't have resolution issues, which so if you get, let's say, your corrections for for crowding wrong, then you're gonna get your colors wrong.

Then you're gonna get the dust, corrections wrong. Then you're gonna get your C correction wrong. So it's not a simple matter as, you know, maybe there's one systematic, but they are degenerate. And so that could have a lot bigger effect than you think if you get them wrong. And and the farther you push out in distance, the harder those measurements become. So I think within a few years, we will understand if if now either there are no problems or then there are problems with the more distant ones that we'll see. We just don't know yet because the distant ones haven't been tested.

Right. And, if, like, one of your grandkids comes to you and says, you know, nano or whatever they call you, grandma be right. You're not good. You know, I really want you to to help me on my, you know, 1st grade science fair project. I wanna develop, measure the Hubble constant using this other tool, and it could be anything. What would be most exciting for you? And I'm kinda interested in your thoughts on things like standard sirens and all sorts of exotic phenomena. But what would make you just so excited to move into

I am very excited about the standard sirens.

I'm talking about them. Maybe explain them first to the audience.

Yeah. So the idea with standard sirens is these are objects that now have been detected, with LIGO and other gravitational wave detectors. So there are 2 neutron stars that start spiraling and coalesce, and, you can use the measurement of the gravitational wave interaction If you have also a measurement of the, the velocity, you need a a spectrum of the object. So you need an optical or some sort of telescope to also get a velocity, because you need, distance and velocity to to, to measure the Hubble constant. And and so it's a very nice technique. It's independent completely of all the kinds of systematics that we're talking about for the local distance scale, and, conceivably, it could be measured at larger distances where you don't have to worry about peculiar velocities, etcetera, etcetera. But they turned out to be unbelievably rare. So there was this beautiful object that was discovered in 2017, almost as soon as they turned on the detectors found this object, and hundreds of people chased it in the optical and knew it was just an amazing

messengers astronomy.

Multi messenger astronomy and the the you were given how fast that happened, you would predict there would be a lot more objects that have been there. And I looked at a single I don't like it science. So, theory shouldn't be a more statistics, so that that's not gonna happen anytime soon. But I really like the method because it's, you know, again, based on fundamental physics, doesn't have a lot of the systematics that we have to worry about, like dust and calibration. And it will have its own uncertainties, and that will become clearer when you have a larger sample too. I do. But it would it would allow us, I think, to test what we measure. And it's my strong belief that we won't have a Hubble constant to 1% until we have several methods that are precise at the 1 to 2% level.

And during the key project that we did with Hubble when it was first launched, we had 5 different methods. And the whole philosophy, and coming back to why did we do what you did with JWST, was let measure several different, ways of of of measuring distances, and then each of those will have its own set of systematics, and some of them will be unknown systematics. You know, I can tell you dust will always make something look fainter. That's a systematic effect, But there you discover other types of systematics as you increase your precision and sort of other effects will pop up out of the noise, and you may not be aware of those until you get better measurements. So until we have several methods at the 1 to 2% level and then can compare them and get a robust estimate of the overall uncertainty, then I don't think the local distance scale is gonna be providing the type of, you know, extraordinary claims required extraordinary evidence. Yeah. You need extraordinary evidence, and I don't think it comes from one method. You take it.

Not gonna get it from the associates alone, and that's why we we developed these other two methods.

Hey theory, students of the impossible. The Hubble tension might be one of the biggest mysteries in the cosmos, but a mystery to me is why so many of you that watch, engage, and love this podcast are not yet subscribed. It's only about 50% of you that are subscribed according to YouTube's physics or podcast app metrics. So let's resolve this great tension for me. Just subscribe. Click wherever you're watching on video. Click subscribe And on audio, make sure you follow the show. And don't be afraid to leave an asterism, a small number of stars as a review no matter where you're listening to this podcast.

Or if you're watching it, leave a comment and a thumbs up. Now, back to the episode.

Oh, yeah. I was always curious just about why wasn't the key project led out of a, a Space Telescope Science Institute? How did it come to be led by you and and, and Carnegie and and the role that you played in it? And maybe you can take us back. How did it what was the nucleus of it? I should say one of my close friends is Nick Spitzer at UCSD, and his dad, of course, is Weidman. And, we've played a big role in the nucleation of Hubble intelligence when did that when did it become clear that this was worthy of a key project? People think about Hubble. They think about, oh, the pillars of creation, the deep field. They don't know because it's maybe not as pretty in the visuals as as those objects Clarke, but, our subjects were. But, how did it come to Brian? And and and how did you come to be the leader?

Yeah. The the genesis of it was actually Riccardo Giacconi, who was the director of Space Telescope Science Institute, even before Hubble was launched. And and Riccardo's concern was that if you left the decision of how to allocate the time on the telescope to, you know, a group of astronomy, our time allocation computers, or tax, there would be a natural tendency to divide up the time into tiny little pieces. Right? Because it they everybody knew this was gonna be oversubscribed. We waited for decades, get above the 1st atmosphere, and it was pretty exciting.

Yeah.

So he put together a panel of gray beards as it was called at the time and and asked them to consider, what were big projects that only Hubble could do. And if you said, you know, Hubble were to fall into the ocean a year after it was launched, you know, what would never get done from the ground that so what was unique to Hubble? And so they came up with this idea of key projects that got competed. We wrote a proposal for the key project, and we had to, in fact, apply for time every year. We were not guaranteed the time all the way through. Mhmm. And and one of the projects that was recommended was the extra galactic distance scale. And at that time, there was this debate about whether the Hubble concept was 50 or a 100, so a factor of 2 debate. And and so we were invited to compete for that, and we did.

And the original leader of the group was Mark Aronson. Yeah. I was his deputy. And so, Mark was tragically, killed in an accident at Kitt Peak in 1987. Hubble was supposed to be launched in 1986 right after what became a Challenger, the Challenger accident. And so the the whole project was delayed. And and then, in the 19 nineties, we, reproposed and also, of course, then the spirit of collaboration happened. And, so our proposal will go through when we got our first data in December 1993.

And so, Rob Kermit, Kennecott, Jeremy Mould, and I became co PIs, and I was the PI in charge of the science end of the project. Is it? So Jeremy was management, and Rob was budgeted as we put funnel together and but we worked closely together.

For a long time, Alan Sandich, or you know, obviously theory, very, conversant with his his work and his famous claim that cosmology was a search for two numbers. Was there a concomitant search for q naught, the deceleration parameter? Or did we think about it, or was that even on people's minds as part of a key project that Hubble would eventually play a huge role in deciphering?

It was certainly on people's minds, and, you know, Sandidge certainly was interested in q not his second number. And, it really wasn't until, it became possible to find large numbers of supernovae, and CCDs became available. And And then the accuracy that with which, again, you could measure luminosities of supernovae and actually discover that there was this decline rate dependence and that you could standardize them. Right? They had a much bigger scatter. So you needed something that would take you out again far enough that you could actually see what was expected to be deceleration at that time and, large enough samples of them to to actually, again, be able to make experiment. So and, you know, unlike cepheids, we can come back. We use some of the same cepheids that were discovered by Hubble and Scentage and right? They they're still there. Yeah.

It's It's a real advantage. Supernovae, you have them once, and you're done. And so, you know, as people like Saul Perlmutter, who discovered or can't you have funny idea that you could do this in a batch way, observe them at one time, then follow-up another time, you know, new moon, and then find them in large numbers. And and this discovery of this relationship between the peak brightness and how fast the supernova declines that really set it going. And then again, CCDs were sensitive at more than one wavelength. That's what helped us with the Cepheid so that we could correct for rendering. That was a big part of what we did with the key project. You couldn't do that with photographic plates.

So it was both it was technology, really, that allowed us to start addressing these questions.

If you could order, you know, God to produce a supernova at bridge shift of 2 or something, like, how would it impact cosmology and your research or and even, you know, upcoming future research claims about dark energy evolution? How would a supernova I don't know. What what would be kind of the dream you're you're talking to God now, not just your what would you what would it do, if anything? Take it take it, take note of the fact I'm not an astronomer. So Sure.

I think, you know, what we need now are, methods that so cosmology, again, playing a role. Right? Infrared helps us enormously. We get, you know, rid of the dust, high resolution. Calibration is really important. It's not exciting, but nature isn't giving us a lot of geometric anchors. There are 3, maybe 4, nearby where we have precise percent or so distances to galaxies. Now we have this mid range where there's, you know, maybe 3 dozen C, and then you have this problem that the bright ones actually seem to be closer. Is that a systematic effect or, you know, what are we seeing there? So we need should we do nature to supply us with a larger sample so that we can be comparing apples with apples with enough statistics along the way that we can make these measurements really accurately? Because what we're doing, you know, we're starting from, say, our galaxy, Cepheids or tip of the red giant branch stars in the galaxy, and then we move out to galaxies where they've had type 1 a supernovae, and we can measure Cepheids in tip of the red giant branch with Hubble, and then we're stepping out to the realm of the type 1 a supernovae.

And all of the theory along the way have to be, you know, at the percent or less level if we're gonna claim we've measured h not to 1%, and we can compare with microwave background. That's really hard. We're not dealing with things that you can measure with a ruler or a meter stick. Right? It's these are astrophysical objects, and, it it's a challenge. It's that way. Yeah. So, you know, what I'd like is a a distance indicator. That's why I like this the gravitational wave sirens that are independent of the things like dust and metallicity and crowding.

Yeah. But then you just don't have very many of them. So we don't have a perfect distance indicator. And, you know, amazing, they're beautiful. It's beautiful. It's beautiful technique, but there's exactly one galaxy in the local neighborhood where it's edge on, and you can make this experiment. And the next nearest one is 50 kiloparsecs 50 megaparsecs, sorry, away. So there's one.

Right? So if there's systematics in that, we have a way yet of Detail. Getting to that. The Large Magellanic Cloud and Milky Way have different metallicities. With Cepheids, you're gonna have to correct for that. Some people say there's no metallicity effect, but it has a huge effect on the ultimate zero point. So it it, you know, systematic matter.

Yeah. Experimentalist real quickly for the audience. Somebody that be for another.

Yeah. So we have within stars where you have the inter nuclear furnaces and you're building up the heavy experiment, eventually when these stars die, either a supernovae or red giants, they they put these heavier elements out into the interstellar medium, and then the next generation of stars is formed with a a greater metal abundance. Now in astronomy, we refer to anything that's heavier than hydrogen and helium as a metal, so that

can get trolling. Trolling to the system.

That that's slightly confusing. But the metals have this effect that in the atmosphere, they're scattering radiation that's coming out from, you know, these nuclear processes in the core, and they can make the star change the star's luminosity. So it's expected that there is an effect, on the luminosities of Cepheons due to differences in metallicity. And, it's not something that people worried about in the days of the factor of 2 debate. 1, we're arguing about 50a100. And so empirically, Theory Madora and I did the first test of metallicity in 1990 in the Andromeda Galaxy. And we said, okay, we know that cepheids are all at the same distance. And, with CCDs, we're able to correct for reddening for the first time.

Was there another, you know, effect on the zero point due to metallicity? If we made the measurements at different radial distances in the galaxy, we knew ahead of metallicity gradient. Could we see an effect? And we measured there was 0.2 magnitudes per logarithmic unit of metallicity, but the uncertainty was Clarke. And people have been arguing about it as soon as it you know, same value, but some of the studies on the most nearby objects say there's no effect at all. Some recent measurements with Gaia say it's twice as large as so it's it's a systematic.

And the gradient rises because there's more concentration of higher star formation rate near the core and C

you could Some more processing of, you know, nuclear elements than, in the inner regions. Just more star formation, more gets spewed out again until you get a gradient in in the galaxy as you go up from the center.

You've talked a lot about in recent talks I've heard you give about accurate cosmology versus precision cosmology. You know, when we when we describe the kind of desirables of of what an astronomer or physicist do, I always say, you know, a systematic is an effect that decreases your accuracy. You can measure something really precisely, tight groupings, you've talked about that. But I also I often will say, at least for us in cosmology and CMB experiment particular, we know we have a systematic from dust. The dust is everywhere. You know, it's I always joke when they, you see these studies and, like, this new compound cures, you know, baldness, and it's always at the end of it, it's like in mice. You know? So I always say, like, you know, the version of it The funny thing. Exactly.

Yeah. So there's a meme online where people say, well, just say C. You know? Because it's like, who knows? But as if you drink 10,000 cups of coffee, you know, in your in your morning, you're gonna die. Well, how do you know? Because we did this in mice. Well, how do you know? But I always say for astronomers, we just say dust. Like, dust is basically the the pernicious brains Dust or magnetic fields. Magnetic fields that sometimes they're linked together, right, as they were for us with our Clarke measurement of inflationary gravitational wind. But I would say to my students, if you have a systematic, that means you have to now build another experiment, and that another experiment is not gonna do the science you wanna do.

It's just gonna measure this annoying thing that you didn't wanna measure. Is that same true in astronomy? I mean, you really kinda get to almost throw away the the data that you collect all in an effort to remove this pernicious effect.

I mean, it's true that a lot of what you have to spend your time thinking about are of the systematics. And and if you make your measurement in the optical, you know, which is what Hubble, he didn't know. Mhmm. The photographic plates were only sensitive to blue wavelengths. Right? So you make it over and over and over and that, you know, the effects of dust are huge in in in blue wavelengths. So, yeah, then it had to be done all over. And so, yeah, what we were able to show, with CCDs is that if you made measurements at multiple wavelengths, you could actually see the effect. You know, there's an interstellar extinction law that depends inversely on the wavelength.

And if you could make measurements at many wavelengths, you could actually measure that and correct for it, and and that's what we do. Then you have to worry about. It's and then as I said, it's all wrapped up in can you get accurate photometry if your stars are crowded and they're blended with other theory? Oh, there's another star right under your Cepheid. You can't measure that. Right? It's underneath, so you have to do a statistical correction. That's

Yeah. So I have the old office that once was occupied by Jeff Burbage at UCSD, and he and Margaret, were alive and kicking for many years when I started at UCSD. And she left a lot of her old photographic plates in his office, which I inherited, so no one's getting them, you know, without the fight for me. But I would see things, including, you know, galaxies and spectra. And, you know, I'm an amateur astronomer, so I'd look them up. And I realized that she and Theory Rubin interacted very closely at UCSD, for a period about 2 years, when Vera was on leave from another Carnegie Institute of Magnetism, which I don't even know if that existed. Terrestriant. Does it still exist? Still exist.

Oh, wow. Talk about Vera Rubin in the context of, the her new tell the telescope that bears her name. What is that likely to do for your research and transformative for for astronomy and cosmology? What are its impacts likely to be?

Yeah. It's gonna be amazing. I mean, it's gonna be surveying the sky. It could be, you know, covering the southern sky every few nights, that means it will be able to, over time, build up an incredibly deep image in the sky, but also look for transients, things that are that are changing. And and so, for example, with supernovae, it likely to find something like a million or so supernovae. And and one of the things right now, people have surveys. They do them with different telescopes, different instruments, try and calibrate them. They put them all together, and there's, again, systematic calibration.

I see.

So to have a homogeneous sample so just, you know, for that one example, that's gonna be amazing. It'll all be done with the same telescope, and then you have to follow-up with spectroscopy to get the redshifts and but that will be done, and that will be, I think, very important. So, you know, we have our Carnegie Supernova project. It's a smaller project, but it was done in a homogeneous way. Then, you know, the SHOES group has another project. They put together 18 surveys. Right? So you walk big and homogeneous, and that

That's gonna come from it. Yeah. And then speaking of other technology that you're in involved with behind us, and maybe I'll this over, I'll

drop it. No. Can you stop to drop it?

Yeah. And so one of the kind of so it's a scale model of the Giant Magellan Telescope Observatory in Chile in La Serena, not too far away from,

honored to 7 mirrors.

Yeah. Gold plated and a mock up mirror. So, I did witness the construction of it, in the early in the middle of 2019. I was in Chile, and it was very impressive to me. Talk to me about what this instrument's gonna do and, how it fits in with this other, you know, kind of portfolio of enormous telescopes, extremely large telescopes. There was an overwhelmingly large telescope at one point. Talk about what GMTO is gonna mean for astronomy.

Yeah. So there are 3 of these extremely large telescopes. Giant Magellan Telescope is is one of them, and and the overwhelmingly large telescope became the EELT, which is the European version. So this one is a 80 foot diameter mirror. You can't tell that from this model, but Yeah. Each of the mirrors here is 8.4 meters in diameter or 27 feet in diameter, so it's it's quite a structure. And it will be located in the Andes mountains in in Chile, and, that the time for first light that we're estimating, it's sort of early 20 thirties. The whole mountain has been now leveled, and the structure for the pier is in place.

The, 7 mirrors, all of them now have been cast at the University of Arizona. And, one of the first light experimentalist going to be an instrument with very high resolution spectral resolution for detection of Earth's mass planets, which will be extremely exciting. It's a very wide field of view on the sky. So it's, it's, not as large as the European telescope, but it has this multiplexing advantage that you can do. You can cover a very wide area simultaneously. And it's now an international partnership. We started off, when I became chair where we had 3 partners. We now have, well, 13 now and growing, and, international partners with Australia, South Korea, Brazil, Weizmann Institute in Israel, and then universities and other scientific institutions across the US.

So it's it's, as it's moving. We're ready to go if we have the science. And, we're we're looking to the National Science Foundation for the the rest of the construction funding.

Yeah. They have to make a decision They where they're gonna go. But it's an incredibly impressive facility. I was joking. They built 2 option for 2. You know, it's like when you when you buy a Brian they say, well, you can buy the undercoating, you know, for free. I guess you need that here in the Midwest.

Yeah. So we leveled them out in 2012, I think. And and we left enough room so that if if another telescope project comes along and wants a really good site, which lost upon a ship, there's room for that.

That's spectacular. I use the there is a or currently a a large Magellan intelligence. There's a Magellan telescope there, which is about the size of one of these, sub mirrors. Right? It's, like, maybe maybe slight. It's small. Let's know. Than that. Yeah.

Oh, that's right. 6 meter 6

and a half meter telescope, and these are each 8.4 meters.

Yeah. Coolest thing about that is that they have an eyepiece, the I think the Nascent focus. I've already got a focus theory use. It is. And, they let us look at it, and we saw Ada Carina. And I felt like, you know, a kid in the candy store. I mean, looking through a mirror bigger than the than the Webb telescopes prior.

No. It's a big name. You actually see howler at the side when you're looking at an eyepiece. And and it actually is exactly my height. So

That's great. And there were people there, you know, they were trying to court donors. They didn't appreciate it. I'm like, from looking at all these theory, like a smudge, you know, and an eyepiece when you're looking at, you know, m 31 for the first time through a 2 inch refractor, Then looking through I'm just like, I just want to monopolize it

for Monopolize spoiled all of us. Right? You see the beautiful color images and people look through the telescope and expect that you're gonna see C beautiful color. Yep.

You just see this much. Yeah. Unless you're, unless you're looking through a 6 meter telescope. So, Wendy, as we wrap up, this podcast, you know, it's originally named after a a saying by the great late sir Arthur c Clarke who said that the only way of knowing the limits of the possible are to go beyond them into the impossible. So that's the name of the podcast. But he said many other things like, you know, for every expert, there's an equal and opposite expert. I like to drop that on my department chair when he's getting a little too out of control. But the one I wanna have you react to, and I'm not calling you elderly, but he he did say the following.

He said, when an elderly but distinguished I'm not calling you that. You've only been a professor for, you know, you're a young a new professor. He said, when an elderly but distinguished scientist says something is possible, she is very likely to be right. But when she says something is impossible, she is very much likely to be wrong. I wanna ask you, what have you changed your mind about over your Clarke? Or what have you been wrong about, if Keating, that you'd pick out to kinda substantiate or even refute that effort.

I think I was fairly skeptical about dark energy at the beginning, and that stemmed from seeing what happened with the Cepheids and ignoring reddening. And at at the point when people started to talk about it, they hadn't really measured reddening for supernovae, and there weren't very many objects. And until that happened, you know, data are convincing. I wanna be convinced by data. And so, but I think, you know, it sharpened things. People then did start to get hopeful filters and and crack for dust. That's, you know, the data speak for themselves.

Yeah. It's wonderful. Well, Wendy, thank you so much for your second appearance on the Into the Impossible podcast. I hope you'll

We should I just say before they would be happy out now, but but I was open to the fact that there could be a cosmological constant. And there were actually 2 directors at Carnegie who said to me, we know what the Hubble constant is. Why are you wasting your time? I mean, literally, that was because, we knew what the ages of globular clusters were. That was Sandage's experiment, so the Hubble constant was gonna be 50 to be consistent with the ages of globular computers. And and my strong feeling was we have to measure it. So, I mean, that's been just what I feel, and it's true today.

There is still yeah. It's margin in the head.

And let's measure it well, and and it's tough.

But Currently, that's right. Don't only focus on precision. Wendy Freiberg, thank you so much. It's been a great joy.

Nice to talk to you. You are. Happy to be here.

You made

it to

the end.

I know you're gonna love this interview with Bob Kirschner. And click here for a playlist of the best episodes from the past few weeks. See you next week on Into the Impossible.