Solving the Crisis in Cosmology with Wendy Freedman

Can we resolve the crisis in cosmology? Neil deGrasse Tyson and co-host Matt Kirshen take on Hubble Tension, the difference between the estimated ages of the universe, and how to solve it with legendary astronomer Wendy Freedman.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Neil deGrasse Tyson?

  So Matt, we do every now and then have to check in on the universe.

• 0:03Matt Kirshen?

  We do. How's it doing, Neil?

• 0:05Neil deGrasse Tyson?

  You know, it has issues. You know, there's a crisis, a family crisis with the galaxy.

• 0:10Matt Kirshen?

  I hate it when crises happen. Can we resolve it? Is there any way to resolve it?

• 0:13Neil deGrasse Tyson?

  Maybe. I think we did, actually, coming up on StarTalk.

• 0:20Neil deGrasse Tyson?

  Welcome to StarTalk, your place in the universe where science and pop culture collide. StarTalk begins right now.

• 0:33Neil deGrasse Tyson?

  This is StarTalk. Neil deGrasse Tyson, your personal astrophysicist. Today I've got as my co-host, Matt Kirshen. Matt, welcome back.

• 0:42Matt Kirshen?

  Thank you, Neil. It's nice to be back.

• 0:43Neil deGrasse Tyson?

  We caught up with you. You're on a cruise. Man, you comedians go every... You got the cushiest jobs.

• 0:49Matt Kirshen?

  I don't know if it's cushy. It's cushy to be on a cruise, I think. Telling jokes on a cruise, that's a little more work. But yeah, we were chatting about this last time. I'm on a boat right now and then I'm on a big land tour. I'm touring with Sarah Millican, who's a great UK comic. And then off the back of that, I'm doing some headline shows of my own in clubs. So, mattkirschen.com, you can stalk me. If you go to mattkirschen.com, you can find everywhere.

• 1:09Neil deGrasse Tyson?

  We'll stalk you there. All right. Well, take us on a boat next time.

• 1:13Matt Kirshen?

  Absolutely. Absolutely. There's space in this tiny cabin.

• 1:15Neil deGrasse Tyson?

  And you're also a host of Probably Science. Did I finally get that right?

• 1:19Matt Kirshen?

  You did. You did. I feel like you've always known the title. I feel like...

• 1:24Neil deGrasse Tyson?

  I haven't been on in like eight years, so I'm waiting for my call.

• 1:27Matt Kirshen?

  We are. We are. We are talking to your people right now, because let's get...

• 1:32Neil deGrasse Tyson?

  Well, today, we're going back into the universe, deep into the universe. And you know, there are theorists who run around, think they know what's going on, but they have to ultimately answer to the observer who's getting the actual data. And we have someone back on StarTalk who was here just two years ago. That's how fast this field is moving. Wendy Freedman. Wendy, welcome back to StarTalk.

• 1:57Speaker 02

  Thanks very much. Yes, data is the ultimate arbiter.

• 2:02Speaker 02

  We have lots of ideas, but if it doesn't, if they don't fit the universe, we throw them up.

• 2:07Neil deGrasse Tyson?

  You are the judge, the jury, and the executioner of theorists.

• 2:12Neil deGrasse Tyson?

  Is that how much power you wield, Wendy?

• 2:15Speaker 02

  We need them both. The data without theory is not very useful. So it's when you have an interplay between the two that it becomes interesting.

• 2:23Neil deGrasse Tyson?

  She's being nice now because she's got to meet theorists later at the conference.

• 2:27Matt Kirshen?

  You don't have the theorists ever going like, no, no, no, I think the stars are wrong. I think our equations are right.

• 2:33Neil deGrasse Tyson?

  Yeah, I spent some time at Princeton, which has a very strong history of theorists. And there's a, there's a motto there, never trust an observation unless it's confirmed by a good theory.

• 2:45Neil deGrasse Tyson?

  That's their mindset. They know they're like full of shit, but they want to say it.

• 2:53Neil deGrasse Tyson?

  So Wendy, you are a professor of astronomy, University of Chicago, in an endowed chair. Let me get that right, the John and Marion Sullivan, university professor. That's a whole other level of professorship and in astronomy and astrophysics at U Chicago. And now since we last had you on, you've been busy. Oh, my gosh.

• 3:15Speaker 02

  That is true. That is very true.

• 3:17Neil deGrasse Tyson?

  The National Medal of Science. Oh, my gosh. This is the highest award the country gives, the United States gives to scientists. There's also a National Medal of Engineering and so this is.

• 3:30Speaker 02

  And medicine.

• 3:31Neil deGrasse Tyson?

  And medicine, yes. Thanks for reminding me of that. And I was once on a committee to select the National Medal of Science. That goes through the National Science Foundation. Does it still do that?

• 3:41Speaker 02

  Yes, it does.

• 3:42Neil deGrasse Tyson?

  Okay, cool. So that was, so it depoliticizes it enough so that, you know, you can really trust that who's in there for that award earned it in all the ways one would expect.

• 3:55Speaker 02

  Science is not political. It has no political affiliation. That's one of the meanings of science.

• 4:02Neil deGrasse Tyson?

  Distinguishing it from basically everything, yes. And you were cited for your pioneering work in measuring the expansion rate of the universe.

• 4:12Neil deGrasse Tyson?

  Is that all?

• 4:15Neil deGrasse Tyson?

  Yeah, you were, I remember you were right out of the box with the Hubble telescope. Hubble, that telescope was in part named after Edwin Hubble on the expectation it would do exactly what you did with it, was to settle the arguments, right? Could you just remind us what that was?

• 4:32Speaker 02

  Yeah, so when I started in pre-Hubble, the argument at the time, there was a big debate about the size and the age of the universe. And people were arguing about whether the universe was 10 or 20 billion years old, which is a big difference. And so Hubble was built, in fact, the size of the primary mirror of the telescope was set to allow, they didn't let it go any smaller, because they wanted to be able to have Hubble measure Cepheid, the stars that we use to measure distances, with that telescope. And so there was an effort, of course, because you could save costs, to cut the size of the primary mirror even further.

• 5:12Speaker 02

  And it was set by that, to resolve this debate between a Hubble constant of 50 and 100 at that time.

• 5:20Neil deGrasse Tyson?

  Yeah, and we came up at the same time, and I just remember that being, you know, the biggest argument anyone would ever have in the coffee lounge. You know, people, they'd split in the coffee lounge, who was the old, old universe camp and who was the young universe camp. And of course, the actual answer landed nicely in between those two numbers, as one might have predicted with hindsight.

• 5:41Matt Kirshen?

  Also, they had to split the bets, whatever they were betting against each other with.

• 5:44Neil deGrasse Tyson?

  Yeah, it didn't land in either camp. It was like, right, it was, it was, yeah, like, right in the middle, right?

• 5:51Speaker 02

  It's interesting, because, you know, there were two groups, competing groups, that, you know, Sandage and Tyson and de Vauclure, who were making these measurements. And so the arguments between 50 and 100 centered on their argument. But if you look at the published values at the time, there were plenty in the middle.

• 6:08Neil deGrasse Tyson?

  Yeah, I would have never known that, because while that was going on, I was at the University of Texas, which was home base for Gerard de Vauclure. He was Mr. Young Universe, right? And so, did I get that right? He had the Hubble constant of 100.

• 6:23Speaker 02

  He had a high Hubble constant, yeah.

• 6:24Neil deGrasse Tyson?

  Yeah, so it was, so I, we had no, we were not allowed to think outside of his box there.

• 6:31Speaker 02

  That's okay, I was at the Carnegie Institution with Alan Sandage. That was a very interesting time. He, he kind of... Disagreed.

• 6:41Neil deGrasse Tyson?

  Yeah, yeah. So thanks for solving that. And also, just this year, named Time Magazine's 100 Most Influential People in the World. Congratulations on that. When was that announced?

• 6:55Speaker 02

  Oh, I'm trying to think of when it was announced.

• 6:58Neil deGrasse Tyson?

  Oh, so it's already happened. You already had the celebration?

• 7:01Speaker 02

  Oh, yes. Yes, we did. We had a nice.

• 7:03Neil deGrasse Tyson?

  So you came through New York and you didn't, you, you came through, you.

• 7:07Speaker 02

  Came through New York. In fact, your people checked with me, was there a date where I could do this?

• 7:12Neil deGrasse Tyson?

  My people were on the case. Okay.

• 7:14Speaker 02

  Yes, but it didn't work out.

• 7:17Neil deGrasse Tyson?

  All right.

• 7:17Speaker 02

  Yes, it was a lovely event, not what your usual scientific conferences are like.

• 7:23Neil deGrasse Tyson?

  Yeah, it's a celebration. Yeah, yeah, it's good. And this paper that you hinted at when I had you in New York for the Asimov Panel Debate, your paper solved what so, I think, you're going to give me details here, but, you know, if you just believed the newspaper headlines or the clickbait in news websites, you would think that all of cosmology was in crisis and we're all ready to just cry in our, you know, cry in each other's laps about how to solve this. And you landed at a place that seemed like, yeah, we got this and we don't have to give up the Big Bang to do it.

• 8:07Neil deGrasse Tyson?

  So the Hubble, remind us what people are calling Hubble tension. Just tell me about that.

• 8:13Speaker 02

  The Hubble tension is what's arisen in the last decade or so. We make measurements of the Hubble constant, the current expansion rate locally using stars like Cepheids. We also use red giant branch stars and other ways of doing these measurements tied into Type 1A supernovae, these bright.

• 8:31Neil deGrasse Tyson?

  Right. So all of these, in this list that you mentioned, these are yardsticks, standard candles, right? So, because not every object serves as a way to know how far away it is, right? So there's only a handful and they're cherished, right?

• 8:46Speaker 02

  Yeah, they're rare stars. They're, for example, Cepheids, when we go and try and discover them, like we did with Hubble, maybe one in a thousand stars that we measure turns out to be a Cepheid. So they're rare, but they also have a signature, and in the case of Cepheids, discovered by Henrietta Leavitt. Uh, that the brightness of the star correlates with how fast it's varying in its brightness, so-called period luminosity relation. And we can use that relationship to determine the distance.

• 9:16Neil deGrasse Tyson?

  So, how many, that was a full hundred years ago or more, right?

• 9:19Speaker 02

  That's right. Everything that we have done since then rests on her work.

• 9:23Neil deGrasse Tyson?

  And, and what's, what's kind of cool is she gets to make those discoveries because the men wouldn't allow the women to do any, to, to work, but you know, where's, what's the most tedious work that is possible in my, in the field? And it's like classifying stars and measuring the brightness in their spectra. And that's where all the discoveries happen.

• 9:43Speaker 02

  So, that's right. And she was, you know, astute enough to notice, you know, not only were these stars varying, that she was finding in the large Magellanic Cloud and the small Magellanic Cloud, but there was this correlation, the brighter stars were taking longer to go through their cycle of variation. And this is the basis of what Hubble's discovery, that there are other galaxies outside the Milky Way, that the universe is expanding, we use it for the key project, we use it today. And she fell into obscurity. Uh, she was kind of lost in the dustbins of history for a long time, but we're, we're recognizing her now. And I think that's, you know, the New York Times actually, uh, wrote an obituary about her in 2024.

• 10:24Speaker 02

  Died, I think in 2021.

• 10:26Neil deGrasse Tyson?

  It was a catch-up obituary.

• 10:27Speaker 02

  Catch-up obituary.

• 10:28Matt Kirshen?

  Did they write it fresh or did they just find it in an old drawer from?

• 10:31Speaker 02

  No, no, they wrote it fresh. But they're making an effort to try and they're recognizing that it wasn't just men who did things in those days.

• 10:40Neil deGrasse Tyson?

  Yeah.

• 10:41Speaker 02

  There were other.

• 10:42Neil deGrasse Tyson?

  Reconciliation project.

• 10:43Speaker 02

  Yeah. Yeah.

• 10:45Neil deGrasse Tyson?

  Very good. And, and I'm happy to report that at least in our era, uh, when the, the textbooks that we taught from and, and learned from, uh, there was good mention of the women of Harvard at the time.

• 10:57Speaker 02

  Um, and we now refer to the Levitt Law and, and that, there was a meeting at Harvard in 2008, which was the centennial of her first publication on the PL relation. And we decided that it would be appropriate to rename it the Levitt Law. I had actually been doing that for a while.

• 11:14Neil deGrasse Tyson?

  Instead of the period luminosity relation, right.

• 11:15Speaker 02

  Yeah, yeah, there's a Hubble law, there's a Hubble constant, there are Hubble galaxies and different cosmologies and classifications and so on, and everything rests on the on the PL relation.

• 11:25Neil deGrasse Tyson?

  I'm all in. Take us back to the Hubble tension. And how much of a tense, how tense was it in the room? Or how, were you in the room when it happened? What is, I don't like that word tension. I mean, in science, if things don't agree, that's kind of fun. You know, I just got a sense that it was it was more a marketing ploy to get clicks on a website. And but maybe you have a different view as one who's in the middle of fixing the tension. What, where did you come from there?

• 11:56Matt Kirshen?

  So, sorry, just to be clear, just for me to answer, so it is actually meaning tension in the common English sense. It's not using tension in some kind of physics sense. You're actually using it like awkwardness or discomfort.

• 12:08Neil deGrasse Tyson?

  Yeah.

• 12:09Speaker 02

  Well, it's signaling a discrepancy between what we're measuring locally when we use these stars like Cepheus or red giant branch stars and supernovae to to measure the Hubble constant, the current expansion rate today. And when we compare that method with what you infer from the cosmic microwave background, the background radiation from the big bang, you can measure these very small fluctuations in the temperature and also the polarization of the background radiation and fit those with the the spectrum with what we call the standard cosmological model. And that's been now in place for a quarter of a century.

• 12:53Speaker 02

  And when you do that, this is a predictive model, it tells you how the universe will evolve, and it tells you that the expansion rate today would have a value of 67 with a very small uncertainty of less than 1%.

• 13:09Speaker 02

  And when we use CFIIDS with HST, we get values more like 73. And so that's a rather small difference compared to 50 and 100 where we started off when Hubble.

• 13:22Neil deGrasse Tyson?

  Yeah, I would have been, I would have said, "Let's go have a beer. We're good here." That's what I would have said.

• 13:27Speaker 02

  I think it would have been appropriate to relax a little bit and have at least a day to celebrate the things that have gotten closer. And there are always crises in cosmology, and I think it was a very rare time around 2001, 2003. So our HST key project results came out in 2001. We got a value of 72 with an uncertainty of 10%. And then WMAP, the Wilkinson Microwave Antisotropy Probe, first measurements of all sky in space for the microwave background, got a value of 71. So it looked pretty good. And the acceleration of the universe had been discovered. The age was something like 13.7 or 13.8 billion years.

• 14:11Speaker 02

  And wow, here you are measuring locally using stars and you're using the redshift of 1,100, 380,000 years after the Big Bang, you're making these tiny measurements of the temperature differences. And away, they agree pretty well.

• 14:29Neil deGrasse Tyson?

  So the two puzzle pieces fit by making local measurements and distant measurements. So that pretty much tells you you're on to something there.

• 14:52Speaker 03

  This is Ken the Nerd Neck Zabera from Michigan, and I support StarTalk on Patreon. This is StarTalk Radio with Neil deGrasse Tyson.

• 15:10Neil deGrasse Tyson?

  So Wendy, if you can remind people, when you just say the number 73, or 67, or 70, or 50, or 100, that is a measurement of precisely what?

• 15:20Speaker 02

  That is a measurement of how fast the universe is expanding at the current time. It has units of inverse time, so it is also a way of getting at the age of the universe. It, in detail, is kilometers per second per megaparsec. So when we talk about a Hubble constant of 70, we mean 70 kilometers per second per megaparsec.

• 15:46Neil deGrasse Tyson?

  So for every megaparsec, an object is away from Earth, from our galaxy, it'll have a recession velocity of 70 kilometers per second?

• 15:58Speaker 02

  Yes, and so, you know...

• 16:00Neil deGrasse Tyson?

  And you add another megaparsec, millions of parsecs, then there's another 70, and this just keeps adding?

• 16:06Matt Kirshen?

  So the further away you are, the quicker it's moving away from you?

• 16:09Neil deGrasse Tyson?

  Exactly.

• 16:10Speaker 02

  That was Edwin Hubble's original discovery. What he showed is the farther away a galaxy is, the faster it's moving away from us.

• 16:17Neil deGrasse Tyson?

  And so those units, he first plotted that, and so we named the unit of that... So it's basically the slope of that line, I think, right?

• 16:26Speaker 02

  That's exactly what it is.

• 16:27Neil deGrasse Tyson?

  Yeah, yeah, yeah.

• 16:28Speaker 02

  The lower that correlation is the Hubble constant.

• 16:30Neil deGrasse Tyson?

  Okay, cool.

• 16:31Speaker 02

  That is the expansion rate at time t equals 0 now.

• 16:35Neil deGrasse Tyson?

  Okay. We're fitting these measurements into a base model of the universe, right? I mean, right now, we say, well, the universe had a beginning, it was long ago, it was hot, it was gonna have a future, and it's expansion. Could there be some... is this like epicycles? Could it be like epicycles, where, oh, the planets, they're on these epicycles, and they do this, and now we can explain everything? Could we be missing something so fundamental that it's hidden in plain sight?

• 17:05Speaker 02

  We could be missing things. I think that's what is exciting about these particular, you know, the time at which we're making these measurements is that we're pushing the boundaries of what is possible to do with these measurements and to test the framework. So we talk about a standard model of cosmology. That's a model that has, it's a universe expanding. It has dark matter where you have the ordinary matter that we're made out of is only about one-sixth of the total matter in the universe.

• 17:42Neil deGrasse Tyson?

  So we don't matter is what you're saying?

• 17:45Speaker 02

  We do matter. A sixth of you matters.

• 17:48Neil deGrasse Tyson?

  A sixth of you matters.

• 17:51Speaker 02

  And the other part is not telling us what we're about. But anyway, we do matter. We're the luminous stuff.

• 17:58Neil deGrasse Tyson?

  Oh, yeah, there you go.

• 17:59Speaker 02

  And the luminous stuff shines a light on the dark stuff. That's how we learn about it. You don't just matter, you glow.

• 18:06Neil deGrasse Tyson?

  Oh, very nice, Matt. I like that.

• 18:09Speaker 02

  Two-thirds a form we call dark energy, which is causing the universe to accelerate. So there's plenty of room, to get back to your question, for things that we don't yet understand. Because we don't yet know what the dark matter is, despite decades of trying to detect it. We know it's what it does to the luminous matter. We know that it's there because of its effects on the luminous matter, but we don't know what it is. It's most probably a particle left over from the Big Bang, and there are lots of efforts to try and discover it, but that has not occurred. And the dark energy, we have no physical understanding. There's no physical theory that can explain what the dark energy is. So, yes, there's lots of room for...

• 18:54Speaker 02

  Us understanding this standard model. So there could be cracks in the model and maybe the this discrepancy is one of the things that's pointing to something missing from the standard model.

• 19:06Neil deGrasse Tyson?

  And let me emphasize something that you just briefly mentioned. So we started our our careers with a Hubble constant ranging from fifty to a hundred. And we resolved that we sort of and we met somewhere in the middle. And we knew that was a problem, but the uncertainties were pretty high back then. So the might one might even say the uncertainty bars overlapped. So that you say if there's the answer is anywhere, it's going to be somewhere in the middle. Now you're saying we have two results that are much closer to each other. Yet the uncertainties are so small, there is no chance of them overlapping. So something has to give.

• 19:49Speaker 02

  So either this is really interesting and we're learning about some fundamental problem fundamental property of the universe. Or we've underestimated our uncertainties.

• 20:01Neil deGrasse Tyson?

  Okay. I'm going to bet on the second one.

• 20:05Speaker 02

  That could mean that we'll learn something about astrophysics, about the properties of stars, different, we're going to learn something about supernovae or cepheid's or something interesting astrophysically, but not necessarily telling us about cosmology.

• 20:19Neil deGrasse Tyson?

  Would you have used the word crisis?

• 20:21Speaker 02

  Would I have used the word crisis? No, I don't believe it's a crisis, not in my opinion.

• 20:26Neil deGrasse Tyson?

  Yeah, that's my opinion as well, but your opinion is way more valid than mine in this space. So we're going with your opinion on this for sure.

• 20:35Matt Kirshen?

  I would have used crisis for that if anyone was to survey me. I'm.

• 20:38Neil deGrasse Tyson?

  You're a crisis camp?

• 20:40Matt Kirshen?

  Yeah, is my opinion valid? Where's my opinion rank amongst?

• 20:44Speaker 02

  Yeah, fortunately, we don't vote on these things.

• 20:48Speaker 02

  The data thing again. It's important.

• 20:52Matt Kirshen?

  Why doesn't someone with almost no science training weigh as much in this as a leading scientist? It's just not fair.

• 20:59Neil deGrasse Tyson?

  Yeah, it's not a democracy. That's the problem. It's not a democracy. So then Wendy, you step in once again. And come up with some sensible understanding of what's going on. Could you update us on that recent research paper of yours? And who are some of your colleagues on that?

• 21:17Speaker 02

  Yeah, so we have a small group, which a really nice group. It's small and efficient.

• 21:23Neil deGrasse Tyson?

  Excellent. As they, that means you get more, more done. And, correct me if I'm wrong, any good collaboration, everyone on the team brings their own special awareness and understanding and specialty to it. Otherwise, you just have redundancies and there's no point for that.

• 21:40Speaker 02

  Yeah, and everybody is working really hard. It's, our proposal, the proposal that we put into James Webb Space Telescope, so this is where we're focused now, was to use three different distance indicators. The Cepheid's that we know and love, the tip of the red giant branch, which is a method that Barry and I and collaborators have been working on for many years, and in the last decade or so have really refined it in terms of improving its precision.

• 22:09Neil deGrasse Tyson?

  So this is a section on the Hertz-Sprung-Russell diagram, which, which, which, which doesn't really clarify. If you say, "The tip of the red giant branch is a special place on the Hertz-Sprung-Russell diagram." Now we're all clear. So, what you're saying is, as stars age, they change in properties, but there's a certain property that they take on, that has a good, that an ensemble of them will have a consistency that you can rely on, that you can see at great distances. Is that a fair way to characterize?

• 22:47Speaker 02

  Yeah, that's fair. These stars, so our sun, our own sun, will become a red giant later in its evolution. And so these are stars that have masses comparable to...

• 22:56Neil deGrasse Tyson?

  Matt, it's in five billion years, so don't worry about this one, okay?

• 22:59Speaker 02

  Yeah.

• 22:59Matt Kirshen?

  I'm busy that day.

• 23:03Neil deGrasse Tyson?

  You got a gig that day, alright.

• 23:05Matt Kirshen?

  Yeah, I can move it. If it's important, I can move it, but I'd rather not.

• 23:10Speaker 02

  These stars have a degenerate core, which packed very, very densely. And they've exhausted all the hydrogen in the core. So that most of the of a star's lifetime is spent burning hydrogen into helium in its in its core, fusing hydrogen into helium. And then when the star contracts, it's not hot enough to start burning helium. And that would happen in a more massive star. So it's burning hydrogen in a shell and and putting more helium onto the surface of this core. And when the core reaches a certain mass, a certain temperature, then there's a thermonuclear runaway. So suddenly you can start helium burning and it releases a lot of energy very, very quickly.

• 23:55Speaker 02

  And then the star settles down onto another obscure term in the Hertz-Brown-Brussel diagram, what we call the horizontal branch. But the point is that these are now fainter stars. And the position at which this, what's called core helium flash occurs, occurs at a very well-known luminosity. And so what that means is we can use, we will observe stars in different galaxies, see how bright they are.

• 24:20Neil deGrasse Tyson?

  Another standard candle for you.

• 24:21Speaker 02

  It's another standard candle. Calibrate them locally and then use the inverse square law to get the distance. So it's a very clean method.

• 24:30Neil deGrasse Tyson?

  And allow me to offer an apologia to our fellow chemists. When astro folk say things burn, we don't mean what you mean, okay? We buy that.

• 24:44Neil deGrasse Tyson?

  Yeah, yeah. Hydrogen burning, you said it, you said it briefly in there, but then you went back to burning. Yeah, it's hydrogen fusion, fusion. But we just, we're very sloppy there and I apologize to chemists. It's your word.

• 24:57Matt Kirshen?

  If you need to put that out, do you?

• 24:59Speaker 02

  I think the bigger insult is that we consider pretty much everything heavier than hydrogen and helium to be a metal.

• 25:08Neil deGrasse Tyson?

  Oh yeah, we call them metals. Yeah, we're bad with our chemistry, but we're sticking with it. We're stubborn in this regard.

• 25:14Matt Kirshen?

  If you've got the hydrogen burning in a star, do you, like, if you need to put that out, do you, do you, is it a water hose or do you use a blanket or foam? Which of the three extinguishers are we talking about?

• 25:23Speaker 02

  Don't do this at home. Don't get close to this.

• 25:27Neil deGrasse Tyson?

  Well, go the other way.

• 25:30Neil deGrasse Tyson?

  So you're working on, I interrupted you quite on purpose, but you were working on several methods of distance determination, yes?

• 25:43Speaker 02

  So we're using the James Webb Space Telescope to measure distances to galaxies using these three different methods: the carbon stars, the red giant stars, and Cepheid's. And that will allow us ultimately, and we're partway through this project, to determine how well we've measured the distances, right? Do all three methods agree really well? Is there a large spread in the values? Do two agree, one's an outlier? This will give us a chance to say, what are the overall uncertainties?

• 26:16Neil deGrasse Tyson?

  Got it.

• 26:17Speaker 02

  And those nearby galaxies that we're observing with JWST, those galaxies then tie into the distant universe where we can see type 1a supernovae well out into what we call the Hubble flow.

• 26:30Neil deGrasse Tyson?

  So you're the base of that pyramid? I mean, they don't know the distance to the supernova any better than you would know what its foundation is, is that?

• 26:43Speaker 02

  That's right. We can measure the relative distances of supernovae. We can see which ones are farther away, but we don't know what the absolute distance is.

• 26:51Neil deGrasse Tyson?

  To calibrate them, okay. So you, not to put words in your mouth, but you think in the results of your work, you will show perhaps that people were overzealous in their, in their small uncertainties that they were reporting. And maybe the uncertainties are a little wider, where they would then overlap, and then it's not a tension, and there's not a Hubble crisis, and a cosmological crisis, and we can all go out and have a beer.

• 27:18Speaker 02

  Yeah, I think, you know, to quote the late Carl Sagan, "Extraordinary claims require extraordinary evidence." And I'm not yet seeing extraordinary evidence. So our result, we're getting a value of about 70, and that agrees very well with what we got from Hubble using these red giant branch stars. And, and I think the uncertainties still, they're, they're, they're not at the level that, that come out of the cosmic microwave background measurements. The cosmic microwave background measurements have a precision of better than 1%. So they've really set the bar very, very high.

• 28:00Speaker 02

  And that's just not possible yet to make measurements at that level of accuracy. When you're trying to use stars that are millions out to hundreds of millions of light years away. That's

• 28:12Neil deGrasse Tyson?

  So I didn't, I didn't appreciate that. So you're saying that the cosmic microwave background measurement determination of the Hubble constant is the gold standard against which other measurements have to match. No one thinks that there's a problem with those measurements at all. Is that correct?

• 28:27Speaker 02

  So far, there is no indication that the measurements themselves are an issue. So there are the measurements from the Planck satellite, this European satellite, which is still the gold standard in the field, it's all sky. And there are two groups on the ground, one in the Atacama Desert and one at the South Pole, that in fact came out with very recent measurements, and they're very much in agreement. The issue is, in order to get the Hubble constant from those measurements, you have to have a model to fit the data. So this is the beauty of this. Given the model, you predict what the Hubble constant today should be. How do you test the model? You measure the Hubble constant today.

• 29:09Speaker 02

  So if you can measure it with enough accuracy, not just precision, but accuracy.

• 29:14Neil deGrasse Tyson?

  Tell us the difference between those two.

• 29:17Speaker 02

  So, you know, if you have a coin and you flip it, um, you know, if you do it a few times, you might get more heads than tails. If you do it enough times and your coin isn't weighted in some funny way, it's going to come out 50/50. And the more times you make the measurement, the more accurate your, your measurement is going to be. But then there are other kinds of errors that no matter how many times you make your measurement, you're still going to have what we call systematic errors. And an example of a systematic error would be We know that stars like Cepheid's form in the disk of galaxies where there's astrophysical dust. So dust, just like here, if you're looking at a mountain far away and a dust storm blows

• 29:58Speaker 02

  up, you look at the sun, or you look at the mountain, the sun's gonna get redder and fainter. You know, same thing happens if you're, if there's a fire, right, if you've seen a red sun. That's what happens when we're looking at these Cepheid's through the dust. They get redder and dimmer. And so if they, they look dimmer, you're gonna say, oh, this is farther away. If you haven't corrected for it, no matter how many times you make the measurement, you're still gonna have an error. So there, there's this distinction between precision and accuracy. And if you only use one method, if you're only using the Cepheid's, you're not gonna be able to tell what the systematics are. So you have to use, you know, that, that's my strong feeling, that's what drives my research, is you have to do this in more than one way.

• 30:43Neil deGrasse Tyson?

  So Wendy, you're thinking that systematic errors are prevalent within these measurements? Because they, they sound all precise and everything, but they could be, they're precise yet wrong.

• 30:55Speaker 02

  That's exactly right. Yes. And, and I think certainly historically, that's what we've seen in these measurements. It's always the systematics that come back to bite you. And often, they're unknown systematics. We know about the dust now, we can correct for it, but what are the things that we don't yet know about? And could there be errors in the calibration and in the calibration of the dust loss? So there, there are lots of potential gotchas.

• 31:24Neil deGrasse Tyson?

  And you've got an advantage there, because people who come to this as cosmologists, they don't know anything about stars, as far as I can tell. You have a huge background in sort of traditional astronomy, where stars in a galaxy, the dust, the reddening, the magnitude, all of this. And so that makes you particularly potent on that frontier.

• 31:50Speaker 02

  Well, I think astronomy is different than physics. Astrophysics is different than physics. We don't have a laboratory where we can go in and we can work with the equipment and we understand the equipment and do tests that we set. We're working with these stars that are far away, that have metals in their atmospheres, pulsating stars, exploding stars. If we look at the supernovae, we don't understand yet, although there's some interesting hints that maybe we understand one of the mechanisms for exploding supernovae. But there's scatter in the relation for supernovae. And the supernova magnitude, supernova luminosity depends on the color of the star, how fast the supernova,

• 32:34Speaker 02

  how fast it's declining. The mass of the galaxy, which truly has nothing to do with the supernova itself. It's a proxy for something else. And then there's additional leftover scatter. And different groups have different calibrations of the supernovae. And so when we're comparing our local observations with the cosmic microwave background, where it's clean and what is referred to as linear physics, and there are different groups that are getting the same answers, and with a precision, again, of better than 1%, the onus is on us, I believe, locally, to really show that we have overcome the systematics in using these stars.

• 33:18Neil deGrasse Tyson?

  You kept referring to today's value of the Hubble constant. That implies Hubble constant had a different value in the past. So then why are you calling it a constant?

• 33:28Speaker 02

  The Hubble constant refers to a Hubble parameter at the current time, t equals 0, H 0. And actually, the Hubble parameter, the parameter that describes, governs the evolution of the universe, changes with redshift or with time.

• 33:45Neil deGrasse Tyson?

  So a little bit of a misnomer to call it a Hubble constant.

• 33:50Speaker 02

  Yeah. It's confusing. It is the value of the Hubble parameter at the current time.

• 33:56Neil deGrasse Tyson?

  So you're messing with people, again, just like when you talk about hydrogen burning and all the metals on the periodic table. So Wendy, if people are looking at different parts of the universe, at different objects, and getting different Hubble constants, why can't the universe just be different in these different sections? Why must the whole universe... Be giving you the same answer to that question.

• 34:17Speaker 02

  So several different things to unpack in your question. You could ask, is there a concentration of mass locally? So that, or maybe we live in a giant bubble, say. And that

• 34:31Neil deGrasse Tyson?

  Matt lives in a bubble, I'm trying to get him out of the bubble.

• 34:35Speaker 02

  So maybe the expansion rate locally is higher because they were being pulled to this mass concentration. And that was talked about a lot at the time when we were arguing about 50 and 100. Maybe the mass distribution wasn't well mapped out. But now there are literally thousands of supernovae that have been measured. You can measure really well across the sky. And there's no evidence that it is varying locally from region to region, to the percent level. As I said, the universe does evolve with time. And we don't know, as I said, also, we don't understand what the dark matter is yet.

• 35:19Speaker 02

  We don't know what the dark energy is. So there's lots of, I think, the tension is a tantalizing idea that maybe this is additional physics because we don't yet understand the nature of the dark energy. But it's very interesting because in the last decade, there have been probably 1,500 papers that have been written and posted to the astronomical archive that have tried to explain the Hubble tension. And none of them has succeeded. And the reason is, in large part, because there's so many other observations that can constrain what a model can do. And the effect that it would have, that we would be able to measure

• 36:03Speaker 02

  with measurements today, or with the microwave background, or so on, and so on. Um, this is where we are at the forefront. We're trying to push the limits. We're trying to understand what, what is governing? You know, what is the, what are the constituents of the universe? How is it evolving? Um, but we don't yet have all the answers and we need really accurate data to do that. And so I would say it's not that this is, is completely solved. I, I, I think, you know, we need to do a better job showing that there is a, a significant tension. Um, and, and as the data improve in future, this is going to go one way or the other, right? Either the signal is going to improve or it's going to fade away.

• 36:44Speaker 02

  And one of the examples is recently with measurements of the, of the microwave background, there was a tiny little hint in the measurements from the Atacama Desert, the cosmology telescope, uh, in an early release of theirs that maybe in the polarization, there was a hint of what might be due to evolving dark energy that could explain the Hubble tension. But they just come out with a release with much more data and the signal just disappeared. It was noise. If it had been real, it would have been really apparent, but it went away. So that's what happens. You, you see things at a, a level of significance that, you know, we call two or three sigma. Five sigma is supposed to be the gold standard.

• 37:26Speaker 02

  It would be a one in 1.7 million chance that it, that isn't correct. I just don't think we're at that level yet. We have more work to do.

• 37:35Neil deGrasse Tyson?

  So this, this bit about the change in the, in the dark energy, uh, properties, that, that made serious headlines when that came out.

• 37:46Speaker 02

  Well, this is different. This is, this is an early dark energy that would have explained the Hubble tension.

• 37:50Neil deGrasse Tyson?

  Early dark.

• 37:51Speaker 02

  All the dark energy, it still could be evolving. And, and again, this is early data. There's going to be a lot more coming, um, in the next.

• 37:59Neil deGrasse Tyson?

  And just to, to, to recast something you said a moment ago, but, uh, tell me if I haven't oversimplified it. The, these, this, these 1,500 papers of people trying to explain the Hubble tension, they'll come up with an accounting for it, but then it breaks something else that we know very well would not. Be the way it is. That's right. If their idea were correct. So it's so it's quite the Rubik's Cube. You can't just explain one thing without affecting 100 other things that we know very well. Yeah, that's right. As it fun. So this is this is part of what gives us confidence in the overall Big Bang scenario for the origin of the universe because it's supported in so many ways with so many different

• 38:44Neil deGrasse Tyson?

  branches of astrophysics. And so, yeah.

• 39:12Neil deGrasse Tyson?

  This is actually a cosmic queries. You know, Matt, you didn't tell me this was a cosmic queries. Why didn't you tell me that?

• 39:19Matt Kirshen?

  Oh, because I don't feel like I have any seniority on this show.

• 39:28Neil deGrasse Tyson?

  Okay, I grant the astrophysics powers in the flow of content.

• 39:35Matt Kirshen?

  I wanted to be your your knave. Does this make me a knave? I don't know.

• 39:39Neil deGrasse Tyson?

  Squire, your squire, yes.

• 39:41Matt Kirshen?

  Squire, squire, I'll tell you that. I have to put your mortarboard and your gown on you and just said you have to battle. So, yeah, there's some great questions, as always, sent in by your Patreon subscribers. So Hannah Cantley from the City of Roses, Portland, Oregon, says, could the effects of dark energy on spacetime geometry potentially arising from entropic forces complicate our ability to measure distances to distant galaxies, especially considering that their apparent recession speeds may exceed the speed of light due to the expansion of space, and this might necessitate new models and techniques to account for these influences?

• 40:15Neil deGrasse Tyson?

  Yeah, Wendy, is Wendy, is dark energy messing with you? Could it be messing with you? If you don't know what it is, you can't say that it's not messing with you. How about that?

• 40:25Speaker 02

  Well, I think that's fair. I think we know very little about what the future evolution of the universe is going to be. You know, will dark energy decrease with time? Is it constant? Is it Einstein's cosmological constant? And I think these are empirical questions right now, because we don't have a good theory. And ultimately, we do hope that there will be a fundamental theory. But right now, we're being guided by observations. And the observations that it's decreasing with time, evolving with time, and getting less, there's less of it now, that's new. And there are many more experiments on the drawing board that will, that will test that. And we'll see how it lands.

• 41:09Neil deGrasse Tyson?

  Now, isn't there a dark matter telescope coming online?

• 41:13Speaker 02

  It just did. That's the Vera Rubin Telescope.

• 41:16Neil deGrasse Tyson?

  Oh, wait, what's the Nancy Grace Roman Telescope? What's that one?

• 41:19Speaker 02

  That's a survey telescope. It's a NASA telescope in space.

• 41:23Neil deGrasse Tyson?

  But is that, that's, didn't they call that the dark matter telescope or not?

• 41:27Speaker 02

  Originally, the Vera Rubin Telescope. It started out probably 30 years ago, almost.

• 41:32Neil deGrasse Tyson?

  Oh, yeah, okay. Well, we just did two shows on the Rubin Telescope. So we're up on that. Matt, what do you got next?

• 41:37Matt Kirshen?

  Alyssa says, there is still a gap between how fast the universe is expanding based on nearby measurements versus predictions from the early universe. Based on your work, do you think this means we're missing something in how we measure it? Or could it mean our current model of the universe needs to change? And Alyssa also says, thank you for being a badass woman of science.

• 41:54Speaker 02

  I think that's precisely the question we want to answer. And I think, I personally am open to it coming out either way. But I have to be convinced by the data. And at the moment, I am not convinced by the data that there is this crisis and that there's something broken in our standard model. So time will tell.

• 42:15Neil deGrasse Tyson?

  Yeah, and if I can add to that, I think most occasions in the history of science, where there's been some discrepancy, just a better data, better or more data resolved it. And every now and then, it requires new physics. So I see what you did there, Wendy. You're saying you're not, you're not ready to have to require new physics because the data to be obtained still needs to be refined. So Wendy, there's great precedent for people such as yourself to take that view of the world, but you don't want to miss new physics.

• 42:51Speaker 02

  That's right. That would be very exciting. I'd love to see it, but I want to be convinced. And I'm just not at a point where I could be convinced.

• 42:58Neil deGrasse Tyson?

  Good, good. All right. Matt, what's next?

• 43:01Matt Kirshen?

  All right. Jamie and Sabrina from Transylvania ask, in the future, when we're all zipping around the universe on starships, how will we keep track of the expansion of the universe? How will we find our way home when home isn't where we left it?

• 43:15Neil deGrasse Tyson?

  Oh, I love it. You got a coordinate system for us in the future? A GPS for the cosmos? Wendy?

• 43:25Speaker 02

  I don't think I'm going to be around that time we can do that. But I think, you know, the what, what we're measuring in our own Milky Way galaxy, if we were to go to Andromeda or other galaxies in our local group or beyond, we would be measuring the same thing. I think the frightening thing to think about is in 60 billion years, if you're worried about future, the acceleration of the universe, if it continues, if it doesn't, the dark energy doesn't decay, then we won't see other galaxies and we won't have the chance to make the measurements that we're making today. So I think that we're living in an interesting time, but we don't have to worry in the same way Matt doesn't have to worry about the sun.

• 44:14Neil deGrasse Tyson?

  Or if I could add a sort of more obscure but possibly relevant example. In the old days, when they had their first generation of seaworthy chronometers, very important in navigation and finding your longitude around the world. Davos Sobel's famous book Longitude really blew open that field for the public. Yeah. And she is one of our, I think she was the very first StarTalk interview. Oh, my gosh. Deep in our archives, find Davos Sobel, the author of that bestselling book. Anyhow, what I understood they did, they would make these chronometers and finally close the back and all the springs and the thing, they'll all be in there.

• 44:56Neil deGrasse Tyson?

  And then they check it and it would either gain time or lose time to the standard. Rather than reopening the clock to try to, quote, "fix it", they accurately measured the rate at which it was increasing time or decreasing time. And that became an equation to correct during, to correct the time they read during their voyage. So something similar to the question is, if you know the expansion rate and you know how long you've been gone, then you can backstrapulate through where you know we should have been. And then you still can find there's no place like home.

• 45:40Neil deGrasse Tyson?

  So bring an equation with you for the expansion of the universe and then go backwards along that path and you should be able to get home.

• 45:47Matt Kirshen?

  Well, while we are talking about distances, I think we have time to squeeze in this question, hopefully from Chris from Marlborough, New Jersey. Says, "Dearest Dr. Tyson, Dr. Friedman, and any esteemed guests", I'm going to count myself as esteemed in that case. Chris asks, "Would the way you conduct your work change if we found out definitively our universe is infinite or finite in size? Also, which option do you find more plausible? Thank you very much, both of you, for your stewardship of cosmic curiosity."

• 46:16Neil deGrasse Tyson?

  Oh, I love that sentence. Beautiful.

• 46:19Speaker 02

  I'll say it wouldn't change how we would do our work. I think we would continue to observe the universe, measure it, and see what's out there. So that would not change. You can answer the other part, Neil.

• 46:35Neil deGrasse Tyson?

  No, how would you feel emotionally if the universe were finite versus infinite? How about that?

• 46:40Matt Kirshen?

  And which do you believe is true?

• 46:41Speaker 02

  I find this interesting, these kinds of conversations. So I, I think I'm not emotionally attached to one kind of universe or another. I, I really have no emotional.

• 46:52Neil deGrasse Tyson?

  That's a healthy posture in science, for sure.

• 46:55Speaker 02

  And I, and so I don't have a feeling about it.

• 47:01Speaker 02

  But I, but I, I remain in plenty curious about what it is. And I love the process of science that allows us to ask these questions and then go out and, and make measurements and, and try and answer some of these questions. But I, but I have no particular favorite child of a universe.

• 47:19Neil deGrasse Tyson?

  Okay, yeah, I'm leaning infinite. I'm all infinite, just because that's more fun. That's gotta be more fun in an infinite universe, yeah. And Wendy, do you remember there's a scene in the film 2001: A Space Odyssey, where towards the end where it gets kind of psychedelic? Uh, one of the, one of the captions of the scene is, uh, to Jupiter and beyond the infinite, or something, there's, they get infinity in there.

• 47:46Speaker 02

  That's where it's a bell, yeah.

• 47:48Neil deGrasse Tyson?

  Yeah, I forgot the exact quote. And just, I think it's fun to get people thinking about infinity because we know we can't wrap our head around it. And so it keeps you nimble.

• 47:59Speaker 02

  Yeah, you know, real time in these things, answering things like this, I just don't want to get tangled up in, yeah.

• 48:07Neil deGrasse Tyson?

  But it is fun when you first learn calculus. You know, you have to really cozy up to the concept of infinity.

• 48:12Speaker 02

  Sure, sure.

• 48:13Neil deGrasse Tyson?

  And infinitesimals, like the opposite of infinity. So.

• 48:16Speaker 02

  But where they're going in these questions, yeah, I just never.

• 48:19Matt Kirshen?

  I'm, I'm still on Zeno's side and I refuse to believe motion is possible.

• 48:24Neil deGrasse Tyson?

  Oh, Zeno's paradox? Yeah, yeah, yeah.

• 48:27Matt Kirshen?

  I'm firmly.

• 48:27Neil deGrasse Tyson?

  You do get to where you're going.

• 48:29Speaker 02

  Yeah.

• 48:29Neil deGrasse Tyson?

  You do get to where you're going. So Wendy, this has been a delight to have you back. Oh my gosh. Congratulations on the National Medal of Science.

• 48:38Speaker 02

  Thank you.

• 48:38Neil deGrasse Tyson?

  And I think I get to tell people it comes with no money.

• 48:42Speaker 02

  Success. That's correct.

• 48:44Neil deGrasse Tyson?

  But another visit to the White House. We've met there a couple of times.

• 48:48Speaker 02

  Yeah, we have.

• 48:49Neil deGrasse Tyson?

  And it's always good to bring some science into the White House. The country's better off anytime that happens. So our health, our wealth, and our security are enhanced.

• 48:59Speaker 02

  All of the above.

• 49:00Neil deGrasse Tyson?

  All of the above. All right. And Matt, good to have you on again.

• 49:05Matt Kirshen?

  Thank you.

• 49:06Neil deGrasse Tyson?

  Enjoy your cruise.

• 49:08Matt Kirshen?

  I will. I will. It's so far so good. We're getting away with it so far.

• 49:12Neil deGrasse Tyson?

  Okay, and we'll find you online at Probably Science. Once again, this has been Starko, a Cosmic Queries edition, but filled with updates on observational cosmology, giving us our understanding of the universe that we so desperately seek. I'm Neil deGrasse Tyson, your personal astrophysicist, as always, bidding you to keep looking up.

• 49:36Speaker 02

  Am I allowed to say anything else? No. That's the last word.