Science Friday - Is It Time For A New Model Of The Universe?

Episode Date: July 16, 2025

For decades, astronomers have been trying to nail down the value of the Hubble constant—a measure of how fast the universe is expanding. But some cosmologists say there’s evidence that the universe is expanding faster than physics can explain, and our current models of it might be broken. Hosts Flora Lichtman and Ira Flatow talk with Wendy Freedman and Dan Scolnic, two cosmologists with different takes on this constant controversy.Guests: Wendy Freedman, a former team leader of the Hubble Key Project, is a professor of astronomy and astrophysics at the University of Chicago in Chicago, Illinois.Dr. Dan Scolnic is a cosmologist and associate professor of physics at Duke University in North Carolina.Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

I'm Flora Lichten. And I'm Ira Flato, and you're listening to Science Friday. Today on the podcast is our model of the universe broken. So it could be very broken or a little broken. All options right now are on the table. We're going to start with some simple questions like, how old is the universe? Simple, right? How did it form?

Astronomers and physicists starting way back with Einstein have filled up lots of chalkboards trying to model the universe tweaking the model with each new discovery. Expanding universe? That's no problem. We'll put in the cosmological constant to take care of that. Wait, the expansion is accelerating? The constant is not constant? And if it's accelerating even faster than we predicted? Uh-oh. Houston, we may have a problem. But there's more. It seems that dark matter may be stranger than we thought. Dark energy is behaving strangely. Strange radio signals have been detected coming through,

the ice in Antarctica, and wait, they found the missing matter, the normal stuff, not all that dark stuff, the stuff that's been missing all these years. What the heck is going on here? We start the hour by talking about why some cosmologists are saying it may be time for a standard model overhaul. And here to comb the cosmos and massage the matter are my guests. Wendy Friedman, professor of astronomy and astrophysics at the University of Chicago, and Dan Skullnick, Associate Professor of Physics at Duke University in Durham, North Carolina. Welcome to Science Friday. Thanks so much for having us.

Nice to have you. Okay, let's start at the beginning. Let's talk about the standard model of the universe. Wendy, what is the standard model? Our standard model is a model in which the universe is expanding, as Edwin Hubble taught us, and a model that contains about one-third matter and two-thirds in this form that we call dark energy. And the matter that we know in love is only about one-sixth of the overall matter mass in the universe, mass density. And so it's a rather strange model in the sense that we don't yet know what the dark matter is,

and we don't know what's causing this acceleration. But nevertheless, for 25 years now, this standard model has held up with the best available data. But now there are these exciting indications that perhaps there are things missing from our standard model, and that's what we're actively trying to understand. In fact, Dan, in a story about your work, your quoted is saying,

to some respect, our model of cosmology might be, I love this word, broken. What do you mean? How broken is it? Right. It's a great question. Dan, go ahead. So it could be very broken or a little broken.

It could be that we need to completely rethink what dark energy is or what dark matter is. Or it could be just actually a very small tweak. Maybe there's another species of neutrinos or maybe how dark energy used to behave was a little bit different. So all options right now are on the table. Wendy, you agree? Yes, I agree. I think this is what is so exciting about the time we're living in is, you know, as you push the forefront of any field, you don't know where things are going to land, but there are these exciting prospects that maybe there's something new to be discovered. And we'll see how it all turns out. But this is the exciting time, hints of tantalizing hints of something new. Are there places where you disagree? Disagree on the model. I think in terms of interpretation now, how significant are these new effects.

that we're seeing, and I think only time is going to tell us how that will turn out. It's always difficult when you're making very challenging observations. We're trying to measure things that are hundreds of millions of light years away from us at enormous precision and accuracy, and that's challenging. And so we'll see where it ends up. Let's start then at some of these questions I raise, for example. Let's talk about the Hubble constant, Wendy. Why is it so important. There's disagreement about what that constant should be. The disagreement is called Hubble tension. Tell us, so I'm getting excited about listening about that, Wendy. So the Hubble constant is a measure of how fast the universe is expanding at the present time today in our own local

neighborhood of the universe. And it's been something that we've been trying to measure now for almost a century since Edwin Hubble discovered the expansion. And even until recently, a few decades ago, astronomers were arguing about whether the universe had a Hubble constant of 50 or a Hubble constant of 100. And those had very different implications, the universe that was either 20 billion years old or 10 billion years old, a very large difference. And so in the early 2000s, we were able to make measurements with Hubble and determine that the age, you was about 13.7 billion years old with a Hubble constant of 72 or 73. And now, as the precision has continued to improve, we're getting to a few percent accuracy. And measurements of the

Hubble constant have centered between values of 67 and 73 or 74 or so. And although that now sounds like they agree compared to where we came from between 50 and 100. If the uncertainties are small enough in those measurements, that would indicate that there's something really fundamentally wrong with our standard model. So the standard model predicts, and these are measurements based on the cosmic microwave background fitting to the standard model, predicts what the expansion rate would be today, and that value is 67 or so. And yet the local measurements, many of them, are giving a higher value.

Now, what's new is that these measurements from the microwave background have been made at a precision of better than 1%, which is extraordinarily accurate and unusual for cosmology. And so what we need to ascertain is how significant is this difference? And the reason that it's exciting is that we live in this universe and whatever we predict should, match up with what we're measuring. And right now they don't. Dan has been calling it a Hubble crisis, Dan. Right. So astronomers or physicists, we use the word tension when something is getting significant, but you're not really sure, is it going to stay or could it kind of disappear? And what's been amazing over the last decade or so is the community has really worked on this.

And now there's a number of techniques of using supernovae or pulsating stars or strong lens measurements, all these different ways. And kind of on the local side, on the nearby side, everyone seems to be in one direction compared to the prediction. So this is kind of happening now so much. I think we're transitioning from tension to crisis. Okay. Let's move on to some other topics I brought up.

And that is, Dan, the universe is expanding faster than predicted by theoretical models, expanding faster than can be explained by physics? Right. So there's a couple pieces there. So we have this model. that there is dark energy that's making the universe expand faster and faster. It accelerates an expansion. But just this last year, various teams have started measuring that maybe this dark energy that we typically

describe with a cosmological constant, supposed to be constant in space and time, maybe it's getting weaker. And that, I think, no one really expected and has caused a ton of excitement in the cosmology community. Do we have any idea why this is happening? there's a lot of ideas of what could be going on, but kind of too many ideas, and nothing as beautiful and as simple as what Einstein first came up with is cosmological constant idea. So astronomers and physicists really love the idea of simplicity and beauty, and there's not going to have been a second great idea that's matched the one that we have that feels itself pretty not satisfactory. But Wendy, this sort of kicked in not at the, you know, not at the beginning.

of the universe, it's sort of kicked in in our measurements a little later. Yes, well, that's the beauty now, is I think we're living at this exciting time where we can make measurements not only locally, but we can look back farther in time and in distance. And these differences are starting to poke up out of the data. And the question is, you know, as you make new measurements, will they survive as you make more and more accurate measurements? And so there's these tantalizing hints right now, and it will likely take several years before, you know, and these are ongoing programs. There's much more data coming, which is what makes this so exciting. And we'll see either the signal will get stronger or it won't.

And right now we don't know where it will end up. Yeah, my question to both of you, do we need new physics that we, you know, just don't know what's happening? I mean, if you don't know with dark energy and dark matter, if you don't know what's, 96% of the universe is made out of? What do you know? Yeah, it's a really great question. The thing that's always kind of surprised me about our model is we say, okay, there's 95% that we don't understand. And that 95% can be explained by two things, dark matter and dark energy. And each of those things can just be explained by one thing. It's super simple. Whereas us people living on our planet in this galaxy, we have this world of

complexity and periodic tables and all these different things that make us up. But the rest of the universe, that's super simple. And maybe it'll just turn out that we're going to learn sometime soon that the rest of the universe can be complicated too. What do you mean super simple? Why don't we know then if it's so super simple? This is why we have to do the measurements. I mean, physicists always start out with the simplest theory possible and then wait for something to prove that wrong. I think that's kind of where we're seeing. When do you agree it? Super simple? We might have lost money. Okay.

Well, do we need new instrumentation then, Dan? New tools to figure this out. Absolutely, and we are getting it. Just this year, the Verruben Observatory is starting. I heard you talk about that earlier. We have new space telescopes launching. This is kind of really the golden age of astronomy instrumentation. Don't go away because after the break,

we're going to hear about some of these new ones. instruments. The opportunities now to make these measurements have never been greater. So yes, it's an enormously exciting time. We're talking this hour about the mysteries of the universe. And on the line where this is Dan Skolnick, Associate Professor of Physics at Duke University. And let's go right to the phones to Raymond in Leymour, California. Hi, Raymond. Hey, how you doing? Hey there. Go ahead. Yeah, my question was, I'm endlessly fascinated with you. this subject, but is there a, does a scientific community have like a best guess on what the dark matter and dark energy actually is? Because I've read so many things and, you know, possibly

antimatter, moving backwards through time and all kinds of stuff. And also, is there a possibility that, you know, with the multiverse theory, that it could be the universe next door, you know, gravitational pull, pulling and causing different stuff? Yeah, Dan, everybody wants to know. Great. Yeah. So for best guess, that's the things that we put into our standard model. So for dark energy, our best guess is that it's what we call cosmological constant. It's like the vacuum energy of space. The problem is that our theories of quantum mechanics don't really match up to the magnitude of the dark energy we see. So we're pretty kind of stuck on a theory there. For dark matter, we know that it gravitationally attracts and that it doesn't interact. with light. So the best idea is that it's just the kind of cold dark matter. And there's a number of ideas of kind of what is the particle that does it. And in places like Sir and the Collider in Europe, they're trying to actually find this in laboratories. But so far, no one's been able to see it. In terms of the multiverse, I'd say that it's probably not that there's like

a neighboring universe that's bumping that's kind of causing this. This feels very internal to our universe. The kind of weird thing in terms of the multiverse and kind of why that gets raised in terms of dark energy and dark matter is that for some reason the amount of dark energy and the amount of dark matter that we have seems kind of finely tuned so that eventually there'll be life in this universe. So people say, well, maybe there are other universes, other parts of the universe where dark matter and dark energy are different amounts.

And we just happen to be in the good part of the universe or the good universe. Isn't one of the mysteries of dark energy, is that even though we have found it, there should be a whole lot more of it? Right, yes. So if the energy of space itself, empty space, it has energy. It has energy. Right. And it kind of if we kind of put our theories to kind of how big, how strong that should be, it just does not match what we see. So what's kind of so frustrating now with where we are is that our best model, even our kind of theory, isn't consistent with it.

You know, I love it. We don't know what it is, but we should have a lot more of it. I remember hearing Stephen Weinberg talk about this years ago and how much more there was. Okay, let's go to the phone. So Pat and St. Louis. Hi, Pat. I have a question about black holes. I was thinking that when a black hole starts sucking out all the energy and light and squeating everything like it does and pushes everything out, that when that happens, that's their big bang. That's like a whole new universe opening up.

And I pose that question to cosmologists at Washington University, a month or so ago, and he said that there are equations that would show that that's possible, but that he said it couldn't be because the equations don't go the other way. Well, my idea was that not everything in the universe goes only one way. I mean, both ways. So why not? But I didn't get a chance to ask a follow-up question. But it's been on my mind ever since.

All right. Now we've got Dan in captivity here for you to answer that. Great. Yeah. It's a really great question. So black holes have kind of always been a really important area of study because a lot of the different scales of physics mix. You have general relativity, which explains gravity. That's the big scales.

And then you have quantum mechanics, which explains all the small scales. And a black hole is this kind of big thing and small thing coming together. And people think if you're ever going to kind of solve all of physics together, it's through understanding black holes. Now, at the center of a black hole, we say that there's some singularity. And we also say kind of at the beginning of the universe, kind of before the universe started, maybe there's like a singularity. So there is absolutely kind of common themes, whether a black hole itself can kind of start another universe. I'm not so sure. But I think the answers kind of how to understand the universe can be reasonably found possibly by understanding black holes themselves.

All right. Thank you. I hope that explains something. I want to move on to something quite similar but quite different, and that is that I've been reading that scientists at Caltech are reporting they have found all the missing visible matter in the universe. They said that 76% of the universe's normal matter lies in the space between galaxies. I mean, is that important that they found that? It is great that they found it. I'd still say that that visible matter is part of the 4%. So definitely kudos to them, but we still have 96% or 95% we're working on. But good to shore up the stuff that we can see. Okay. Dan, tell me about the Vera Rubin Observatory. It's in Chile.

It's up and running. What is it measuring? What are you looking forward to? Right. So Verra Rubin Observatory is the instrument that our whole. community rallied around in what we call the decatal process. Basically, the instrument that we all said, if there could be one instrument, astronomy, this would be it. And it is a huge telescope, eight meters with this huge field of view. It's about a camera that's like a thousand

times bigger than a normal camera that we're used to. It's the most number of pixels of any camera that's ever been built. And it will do all sorts of different science and astronomy. So it's trying to find planet nine. A lot of scientists think that there's another planet in our solar system. That's not Pluto. It's this other planet. It will be able to find asteroids coming from other solar systems. So some people, when we've seen one before, thought maybe there's aliens on that asteroid, but what you're going to start finding a lot of these asteroids? And then it will find or really try to figure out what are this dark energy problem and dark matter problem. You think it will help with some of these big questions that we've been talking about?

Absolutely. And it's optimized to try to solve all these things. Tell us a little bit about who Vera Rubin was and what she's known for. Right. So Verra Rubin is often credited with discovering dark matter. So what she was looking at is how light goes around galaxies. And she noticed that kind of unlike with our solar system where something, far away moves around, its center slower. She noticed things further from the center of the galaxy actually are moving pretty fast. And she was able

to deduce that there's this extra matter throughout the galaxy that we're not seeing. And this is a pretty profound discovery that we now know is about 25% of the universe. And one thing that the observatory in her

namesake is trying to do is figure out what exactly this dark matter is. She was really the kind of pioneer of both the observing side and figuring out, hey, there's something else here in the universe. Got Wendy Friedman back on the phone with us, Professor of Astronomy and Astrophysics, University of Chicago. We were talking about the new very urban telescope. Are you excited by that also? Enormously excited. I think it's going to give us a view of the universe we haven't had before in the sense that not only will it go deep and cover a large area of the sky, but it will also tell us over time,

how things are changing. So it's a unique new instrument. Very excited. And it opened to everyone, open access. So I think we're going to learn a lot. I understand there's a global watch party, first watch party on Monday, and people can look for that. Here is how you go to see it. Ruben Observatory. Or you can look at for their YouTube channel. We'll put it in a link on our website so everybody can see it. Let's go to the phones to Leesburg, Florida. Hi, Logan. Hi, Logan. Are you there? Logan, no, sorry. Maybe we'll get him back. How about to Ted in Harrisburg, Pennsylvania.

Hi, Ted. Hello, how are you? Hey there. Go ahead. Hey, I love your show most of the time. Oh. This kind of stuff just absolutely fries my brain because I've never heard any of this stuff

discussed sort of under the umbrella or in the context of infinity and infinite space. So if everything just keeps going on and on and on as many times as you want to say and on, then what the heck are you guys talking about? Dan, infinity is that what most people think it is, is it? Right. So in astronomy, typically, we put some ground rules on. When we talk about the universe, we say that's really just the observable universe.

There's limits to how far we can see based on how old the universe is and how the distance that light could travel in the time of the universe. Now, what's beyond the universe, it may go on infinitely. We do not know. But we say that's kind of outside the realm of what we can measure. And our work ends before there. Well, I'm glad to frying somebody's brain here because my hair hurts all the time when I talk about it and I enjoy talking about.

So we go to the phones again. Let's see if we can go. Let's go to Blackwell and Chattanooga. Hi, welcome to Science Friday. Hello, good to hear you. Is your hair being, is your hair gone fire today? Not at all, really. So my question is, if the Big Bang

represents one unit of energy, okay, and one unit of mass, where is centripe? Point being, if the conscience that we perceive as Newtonian and Einsteinian physics are constant, then the acceleration of the universe, regardless of dark matter, light matter, whatever, the acceleration of the universe would seem to be maintaining that constant that we observe in the organization of planets, galaxy,

stars, what have you, because it's losing energy. You're talking about the... You're saying about the conservation of energy going on here? Yeah, that's entropy, the loss of energy. So what's your question? If we start with one unit

of energy and one unit of mass, okay, well, that energy dissipates, right? That's entropy. Okay. So, would you consider, does it make sense that the continued acceleration

of the perceived universe accomplishes the constant observation of Newtonian and Einsteinian physics? Okay, let me get that question. Wendy, do you have an answer there?

So what general relativity allows, unlike Newtonian gravity, for example, is this repulsive form of gravity that we term dark energy. And the simplest explanation for this dark energy is that it's a cosmological constant. And that energy in the form of dark energy is a constant in time. And now, so, you know, these different models allow for different values, including we had to explain a universe where the cosmological constant was zero early on. So we don't have a theoretical explanation right now for what could be causing this change as a function of time or the function of redshift in the dark energy. So what I would say at this point is we don't have a good theory for what could be causing this. but right now it's an empirical measurement. We are seeing evidence

at some level of precision that the dark energy is evolving as a function of time. And so what will account for that ultimately we would need a fundamental physical theory to do that and we don't have that at the current time. But what I would come back to is that this is allowed within general relativity all consistent and it's up to us now

to improve our measurement. and actually determine what is happening in the evolution of the universe. Dan, Wendy, I mean, hearing you talk, you both said it's an exciting time. I mean, does it really feel like, wow, what a time to be alive for you in your world? Absolutely. We have these marvelous facilities now. I mean, the James Webb Space Telescope with this incredible resolution and sensitivity to allow us to make measurements of distances where we've never had this opportunity before,

starting with the Rubin telescope now that's going to do this survey of unprecedented depth and sensitivity, a new NASA satellite, the Roman Space Telescope satellite, and these measurements of the barionic oscillations that are telling us about the potential change in dark energy along with supernovae. So the opportunities now to make these measurements have never been greater. So yes, it's an enormously exciting time. Dan, you too? Yeah, and I'll just add that for all the instruments that Wendy discussed, they've been planned and worked on for almost 20 years, a number of them.

So we've been waiting and working on these for a really long time, and it just so happens that a bunch of these, kind of all the things we've been waiting for are coming online last year, this year, next year. Are they threatened by federal funding cuts? We hope not. Yeah, I'd say, like for the, the, Roman space telescope, it's possible that it will not get as much funding as needed to launch next year. That's still being figured out. But I mean, I would just say that we've worked

kind of really hard. We're at the near the finish line for a number of these things. So I think wouldn't it be worth it to kind of not push across the finish line right now. Would you think that we're going to have a unification of quantum mechanics and general relativity any time soon? Fear is dream and, you know, whether this is 22nd century physics dropped into the 20th and 21st centuries. I think we can't predict that, but certainly the hope is there. Dan, do we need the new discovery of something that we unite the two?

Yeah, yeah, or a someone that could come and you just kind of figure this all out. So for now, I think what we can do is just kind of keep measuring the universe, seeing what fits someone doesn't until someone can kind of figure out how it all makes sense. Are there up-and-coming geniuses on the horizon? Maybe they're listening. Yeah, they're listening right now. Well, we hope they are, and I want to thank both of you for taking time to be with us today. Wendy Friedman, Professor of Astronomy and Astrophysics at the University of Chicago.

Dan Skolnik, Associate Professor of Physics at Duke University in Durham, North Carolina. Good luck to both of you in your quest. Thank you so much. Thank you very much. And that's about all the time we have. A lot of people help make this show happen. Santiago Flores. D. Peterson Smith.

Floresamaz. Emma Gomez. Jackie Hirschfeld. I'm Ira Flato. We'll see you soon.