Daniel and Kelly’s Extraordinary Universe - Is dark energy a fudge factor?

Episode Date: September 8, 2026

Daniel and Kelly talk about whether physicists are trying to understand the expansion of the Universe, or pull the cosmic wool over everyone's eyes. Also, horse milk comes up.See omnystudio.com/listener for privacy information.

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I'm Emmanuel Jochi. And Kai Wright. And this is Big Lives. Listen to Big Lives on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. What's going on with the universe? First, we thought it was static, single galaxy, floating in space forever. Then we learned it's actually expanding. Everything out there is running away from us.

More recently, we figured out it's actually accelerating. What's happening? This is one of the biggest questions in modern physics, and the whole area of studies loosely called dark energy, which is also confusingly sometimes the name used to refer to the leading hypothesis for what's happening. But do physicists know what they're doing here? Is it physics or is it just a bit of wonky math they stick in to make their precious equations work? out. We're going to dive deep into all of that and hear my rants on the subject. Plus, we'll have the usual hilarious digressions all on today's episode of Daniel and Kelly's extraordinarily dark universe.

I study parasites and space, and this is the second Fudge Factor episode we've done. Hi, I'm Daniel. I'm a particle physicist, and I love thinking about how aliens imagine the universe. And I suspect that there are alien physicists out there like me who don't like fudge. Oh, wait, we've talked about why you don't like fudge before. So two things. First of all, when I saw the outline you sent me, I thought, oh, Daniel didn't realize he sent me the same outline twice. And then I was like, oh, wait, this is different. Where do you think the phrase fudge factor came from? Oh, that's a great question. I'm going to speculate without a chance to research about the history of this word. I don't know. I mean, in general, like to fudge something means to mess it up or to fake it, right? And I wonder if that came from somebody like me who doesn't like fudge or there's another sort of scatological implication of fudge. Yep, that occurred to me. That occurred to me. And so maybe that's a connection. Like, oh, you fudged it. I don't know. That's a great question. What do you think? I have no idea. I also, you know, as a biologist, the scatological connection was made in my brain. But I didn't bother looking it up. I just thought, I'll, you know, I'll ask Daniel. Are you looking it up? I'm asking the robot. Let's see. So, based on my internet research, the Oxford Dictionary of Word Origins says that the term fudge expresses annoyance in this trace to the 18th century.

originating from another word fadge was sort of meant to fit. So then where did the word fudge as a delicious treat come from? And apparently fudge, the not so delicious treat, is characterized as like an accident. And so somebody was trying to make something else, and they fudged it, and they made fudge. Oh, interesting. We learned something new today.

Yes. And we unnecessarily went for the scatelized. reference. Well, on brand for us. Exactly. Yes. Yes. Well, we've previously talked about how dark matter is not a fudge factor. Or would you still say dark matter is a fudge factor? Oh my God. Did you listen to the episode, Kelly? The whole point of the episode was dark matter is not a fudge factor. Okay. And so, I mean, let me give some context here. You know, there's a lot we know about the universe and we do our best to communicate, but there's also a lot of misinformation out there about what we know and how it works.

And a lot of folks out there under the impression that dark matter specifically is just like a number we plop in the equations to make the math work because we're stuck on some dogma and we'll never give it up, even if we have to insert ridiculous fudge factors in it to get it to describe the universe,

which is not a fair characterization of dark matter and also avoids the very rich, fascinating field that has surfaced like 10 independent lines. minds of evidence for this thing we call dark matter. So I was inspired to do an episode contrasting this popular science misinformation with the reality of the research of dark matter. I just like riling you up. Well, button pushed successfully. And in that episode, I made a contrast between dark matter, which is a thing, and we have a lot of evidence for it and definitely not a fudge factor. And I used the cosmological constant, which we'll talk about in more detail today, as an

evidence of something which kind of is a fudge factor. And a bunch of people rode in and they were like, wait, are you saying dark energy is a fudge factor? And so I thought, all right, we need to do an episode on, is dark energy a fudge factor? Not as a repeat, not as a rehash, but does an even deeper dive into the subject of cosmic fudge factors. Well, I love fudge and I love learning about whether or not something is a fudge factor. So I'm sure we're going to have a lot of of fun today. All right. I hope we don't fadge it up. Oh, watch it, Daniel. We don't want Matt to have to start dropping a bunch of bleeps into this episode. So fadge is my new favorite archaic term. I'm going to use it everywhere. I'm going to work on incorporating it into my life as well.

I'm a little worried about what might happen if I use it in front of my seventh grader and then she uses it at school and slips up a little bit. So, all right. Well, we did what we always do here at D-K-E-U. And the first thing we do when we have an interesting question is we share it with the extraordinaries. That's right. Research step number one is ask our audience. That's right. And so we asked our audience, is dark energy a fudge factor? And if you want to be on the list of people who get our questions that are our first step of research, write to us at questions at danielandkelly.org. And you can get added to the list of folks who get our emails. So think about it for a moment before you hear these answers. Do you think dark energy is a fudge factor?

Here's what folks had to say. It doesn't mean that it's not real. Just we don't know what it is or how it is yet. Well, I still don't really know what dark energy is. So I don't know. It seems maybe it's more of a thing than dark matter. I don't know. I have no idea.

I think yes, until we know what it is. So I'm also a barista in my other job. And dark energy definitely sounds like a full. fudgy sort of latte. I would totally drink that. I don't think so. Something is creating space. Yes, we don't know what it is. We don't even know if it's a constant, so. Dark energy is not a fudge factor. Yes, I think it is. Your Honor, I refer you to the previous case of dark matter and whether it's a fudge or not, in which I responded, it isn't. So therefore, I must also respond that dark energy is not a fudge either.

We don't know the underlying mechanism with why it happens. So, I mean, to me, that seems a lot like dark energy. Like, it could be one forces behind there, or it could be three forces. I do not think it is a fudge factor, but I have no idea how to classify it. Dark matter is not a fudge factor.

I think dark energy might be. All right. So if I remember correctly from the dark matter episode, there was a lot of, like, oh, I don't know. I think people were a little bit less willing to call it a fudge factor. It looks like there's a little bit more willingness to lean into fudge factoriness when it comes to dark energy. Some people here standing up for dark energy, you know, an empty space has energy, et cetera. But yeah, definitely a lot

more skepticism here. And maybe we'll circle back at the end of the episode and see if that was justified. Yeah, well, I think it shows that they've been listening to what you've been saying. So, all right, let's, you know, we talked a little bit about the etymology of the phrase fudge factor. But let's go ahead and dig into like, what does a scientist mean when they say fudge factor? What is the scientific definition of this term? Yeah, well, I think fudge factor is used in two ways. One, it's used as an insult or a smear to suggest you have a theory that's not really serious. Or you're making the theory work even though it's not right.

You've fudged it. And I think that's the essential implication or accusation in this. popular misinformation to say that scientists have an idea and it doesn't really work. And it's not even very convincing because all they've done is tweak the numbers to make it work or they've added something to the equation, something mathematical. That's the other implication that this is purely a math trick and not something physical, not something real, not something that reflects how the universe actually works because the equations, if you left them alone, would predict something else, and you had to add a fudge factor, you know, like if I had some mathematical

equation that predicted, you know, Kelly's height and it predicted three feet. And then we were like, well, that's wrong. And I'm just going to add a number to it. I've fudged it so that it gets the right answer. You'd say like, hey, your Kelly height prediction equation is garbage because it's got a big fudge factor. And it needs a fudge factor to work. And therefore, it's not any good. But would you learn something if like, you know, if you said, okay, I'm, going to predict that Kelly is three feet tall and then I'm going to add a fudge factor, but over time, every time the fudge factor ended up being the same value. Would that like hint at the realness of the importance of that mathematical constant or something? Like are fudge factors a way of like

place holding? There's something we don't understand. But if it's always the same value, then maybe we're on to something important. Maybe. I think that you have your finger on something useful, though, which is that science is iterative. And often we can have fudge factors on the first attempt. You know, sometimes a fudge factor isn't a way to lie to people and mislead and to try to pull the wool over your eyes and get you to believe our theory. It's just a way to say, we don't know how this works. Let's try something. And if it doesn't quite work the first time, we'll insert some factors, some fudge factors, which will then intend to come back and revisit and improve on. Like, as you say, say if my prediction for Kelly's height is three feet too short and for Zach's height is three feet too short and for Katrina's height is three feet too short, I'm going to go back and look at my prediction. I'm like, why am I systematically three feet too short? What is wrong with my theory? And then I'm going to come back with another theory that I hope is more accurate, right? And so in that sense, a fudge factor is not always bad. It just sometimes means you're in the early days. You know, like say, for example, you're walking through your backyard and you see like elephant footprints.

Right? And so you're like, okay, well, I'm going to assume there probably some elephants. I don't have direct evidence yet, but I have a theory that there are elephants walking through my backyard. If you come back and you find like elephant poop and tusk marks and you smell elephants, you're like, okay, yeah, the theory is probably right. But if you don't, or if you come back and you see like cow poop and you smell horses and the evidence doesn't really come together, then, you know, you've got to adapt your ideas. And so it's okay to have initial guesses that aren't totally solid, even sometimes just not. numbers that you put in there that you don't yet know how they work until you later do figure out why they work. It just reflects that you're not done sometimes. I find myself trying to figure out how many people there are on this planet that could sniff the difference between an elephant, a cow, and a horse. There must be someone who works at a zoo who can do that. I think I can do that. I spend enough time around horses with my daughter. This is a very definite horsey smell for sure. But is it different than a cow poop smell?

Because they're all eating grass. Okay. Oh, man. Well, I mean, you know how different animals all have cheeses that taste different, right? Like goat cheese tastes different from cow cheese. You ever notice how goats smell different from cows? And how goats smell is sort of related to the way goat cheese tastes. I love it.

There's just like a goadiness. Yeah. And the goat milk has a bit of a goadiness too. I love it. I love it all. All right. Point made. I've never had horse cheese, but I'm pretty sure if somebody made horse cheese, I could be like,

this is pretty horsey. You know what? I shouldn't have said, because I do think there are cultures who do a lot with horse milk. Just I don't happen to have come from one. I see. You've never had horse milk yourself. I have never had horse milk myself. I have a friend who has had fermented horse milk.

It was not his bag. But, you know, it's the bag of many other people. Well, rather than pulling us back from this decoration, I'm going to take us even. deeper and ask you what is the most interesting kind of milk you've ever had interesting i mean i don't think i haven't had that much milk variety i have had uh never had like beaver milk have you no but you like animals you have like 65 000 different kind of things in your farm who knows doesn't mean i'm suckling all the animals i come across we we do joke about uh you know should we try pig milk and get pigs or something. But no, I've had sheep milk, goat milk and cow milk.

And I think that's, those are all the milks I've had. Wait, I have had pony milk, actually. Oh, when? Not horse milk. Yeah, there's a little organic store near where we used to live when we were at CERN and they sell pony milk. And I was always like, what is this? We have to try in these tiny little bottles. And so yeah, yeah, I actually have had pony milk. I forgot about that. Wait, so wait, so what's the difference between pony milk and horse milk? I just think of ponies as young horses. Oh my gosh.

Kelly, what? What's, okay, you tell me, horse guy. What's the difference? How far off topic are we now? Really far. All right, we're going to have to bring this back and to tell a story about like making fudge with pony milk or something. Ponies.

Okay. So ponies are not baby horses. That's a foal, right? A baby horse is a fool. A pony is a distinct. fully grown, smaller equine, right? So, like, a mature pony is, like, five feet tall at the shoulder, while a mature horse is much taller than that.

So it's sort of like a, you know, it's like a short version of the horse. But they're the same species, right? Yeah, they can breed and their offspring are fertile, but it's sort of like, you know, the difference between a chihuahua and a Great Dane. Okay. Right. It's like a chihuahua is not a baby version of a Great Dane, right? It's a smaller kind of dog.

All right, all right. Well, thank you for this biology lesson, physicist. All right, all because of my elephant example. My biology knowledge is a fudge factor, and let's move back to physics. The point I was trying to make is that often we don't understand something or we understand things partially. And what you want to do the first time is like make simplifying assumptions and fill in numbers you don't know and then come back and revisit it. and so you can dig in and understand it better. And so, you know, maybe you want to call that a fudge factor while you're still in the process of figuring it out, but not in a negative sense, not in a misleading sense, right? We're open, we're honest about what we're doing and the status of our understanding.

I think fudge factor has too much of a negative connotation for me to say that a first draft of a theory, even with numbers you don't understand, has fudge factors in it. What would you call it instead? You know, I'd say unexplained constants. Okay. You know, for example, we measure the coefficient of friction. When two surfaces rubbed together, how much force is there on those surfaces because of their friction? And if you didn't understand how friction works, that it's due to the microscopic grabbing and dragging of the little features of those surfaces, you just measure it as a number and say, well, that's what it is. And then if you have a microscope, you can zoom in.

And you can see, oh, Teflon is a tiny coefficient of friction because it has almost no surfaces. And sandpaper has a high coefficient of friction because look at all these features. And so then you can reveal it. You can even calculate it from those images. And so that's an example of like you start with a fudge factor and then you go off and you explain it. You understand it. And so it's not always a bad thing. Sometimes it's just a first step towards a deeper understanding.

So before we take a break, I guess I want to like jump the gun a little bit and know is dark energy a like this is a first step this is the constant we're using to hold in place or is it a lot are we farther than that oh it's so much more nuance than that kelly you're going have to listen to the rest of the episodes sorry oh man all right well let's take a break so I can work up the energy and we'll be right back behind every night spent making the kids lunches there's a lot of suspense will they finally eat the veggies. Will the pesto survive the first bite? Will the lunchbox come home empty, half full, or come home at all? And will you finish episode eight before you're done slicing the

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I wait for him to mess up. Listen to lots to say with Bobby Bones and Matt Castle on the IHeartRadio app, Apple Podcasts, or wherever you get your podcasts. All right, we're back and we're discussing whether or not dark energy is a fudge factor. So, Daniel, catch us up. What is, what do we think dark energy is? Yeah, it's important to separate dark energy into two different categories, which are very closely related, but also really importantly different. one is our observation that the universe is expanding and that that expansion has been accelerating for the last five to six billion years. So that's number one. Observation experimentally,

and we'll talk about in more detail, that the universe's expansion is accelerating. Sometimes that's called dark energy. The other side of it is our attempt theoretically to describe that, to make sense of it, to either incorporate it into our theories or to extend our theories to describe it. And that involves something called the cosmological constant. And so those are two big pieces of this concept of dark energy, but it's important to keep them separate, because one is something we know, we measure, we see, and the other are theoretical ideas that we're putting together to understand it. Okay, so tell me more about how we know the universe is expanding and how we like measure this. Yeah, so we've known that the universe is

expanding for more than 100 years. Before Hubble and Henrietta Levitt, people thought the universe was static. They thought, here we have a galaxy and we're just sitting out here in space and stars are stars and that's it. And they didn't even know that there were other galaxies. And it wasn't until Henrietta Levitt came up with a way to measure the distance to things in the sky, right? Otherwise, it's hard to know. Are you looking at something that's big and bright and far away or something that's small and dim and close. Those two things look similarly. Until Levit figured out how to use sephids, these variable stars, so you can measure how their brightness changes. And by measuring how their brightness changes, you can tell what their brightness actually is. And then you

can from that determine how far away they are. So she came up with this method and then Hubble used it and discovered that, oh my gosh, there's a bunch of stuff in the sky that's not even in our galaxy. It looks like it is because it's sort of mixed up there in the sky, but it's actually really, really far away. That was the first observation that there's stuff outside our galaxy. There's more than one galaxy in the universe, right? Mind-blowing realization. At the same time, he discovered that those galaxies are redshifted, that they're moving away from us. So that's when we went from the universe is static to the universe is expanding, and everything is moving away from us.

That sounds like an exciting time to be alive and be in the world. science. Yeah. And for like 80 years or so, that's how we thought the universe worked. And the big question in cosmology at the time was, is that expansion going to turn around? Is there enough gravity in the universe to pull everything back together to make a big crunch? Or is there not enough gravity in the universe and things will slow down, but keep drifting forever? Those are the two options. Like keep drifting forever and expanding or slow down, turn around and come back to a big crunch. Oh, boy.

Existential dread. Yeah. And so to know whether it was going to be like option A, drift forever, or option B, big crunch, they had to make more precise measurements of the really far away stuff to look like further back into the history of the universe and see how those distances have been changing with time to see, is it look like it's going to turn over or does it look like it's just going to keep spreading out? So they developed a new way to measure distances, which are type 1A supernova.

These are stars that go supernova. they implode, but they're a special kind of supernova. There are stars that didn't initially go supernova. They just became white dwarfs. They didn't have enough mass in them to go supernova. But then later, they get fed extra stuff from their partners in a binary system. And then they go over the threshold and become supernova. And because of this particular way that they do it, it's very easy to measure from how their

light curve goes, how bright they actually are, and therefore how far away they are. So then we got this extra can. handle that we could use to measure really, really far away stuff. And then we had much more lever arm on our fits to understand the whole history of expansion. Because Cephids, these variable stars, tell you about closer up stuff. And type 1 supernova are bright enough to tell you about really far away, really far back in time, longer history. Okay. I think you lost me on the lever arm thing, but then you summarized the point and I'm with you now. Yeah, well, the point of the lever arm is just like you're trying to understand the trend and you want to be able to predict it.

say, for example, you're looking at like Apple stock and you want to know, is it going to go up or is it going to go down? If you only have three days of data, it could go anywhere. But if you have like 50 years of data, you can make a more confident prediction. And so you have a better lever arm. Your like theories are more constrained. And so in order to predict the future of the universe, we had to look deeper into the past. And this was one of my favorite moments in science. It was around 2001.

They made this measurement and they asked the universe, okay, is it scenario A that we're going to expand forever, or is it scenario B, we're going to come back and do a big crunch? And the universe said, no. It's secret option C. It's something you never considered, which is that neither of those things are going to happen. And instead, what's happening is that the expansion is accelerating. Both option A and option B suggest that the expansion was decelerating. Things were slowing down. And the difference was like, is it going to slow down enough to actually go to negative expansion to collapse or just sort of drift out towards slower and slower expansion? But secret option C, which is reality, says no, the expansion is increasing.

It's getting faster and faster every year. And this was mind-blowing. This was a crazy idea. Where were you when you learned this? Because this was in our lifetime, right? Yeah, absolutely. I was in grad school, and I was actually at Berkeley in grad school. And one of the big teams to do this was at Berkeley.

Saul Perlmutter was leading that team. And he was racing against another team in Australia. It was very exciting. New grad students working on that. project up at LBL was a big deal. And then of course they won the Nobel Prize like five minutes later. So yeah, it was a big deal at Berkeley at the time. It was exciting. You know, anytime you find a surprise, those are the best moments, you know, because it shocks you. It forces you to upend your understanding, you know, to get rid of the dogma and change the mainstream narrative. The kind of

thing that like, you know, pop-sign misinformation artists are always saying we never want to do. This is our dream come true. See, I'm imagining grad student Daniel, like taking the whole weekend off, going out into the woods, and just staring at the stars with his mind blown for a whole weekend. Yeah. And also being like, why didn't I work on that? Oh, my gosh. I could have won a Nobel Prize in grad school. Whoops.

That was the second time I missed a Nobel Prize. I also had the opportunity to go to Caltech and to work on LIGO, the gravitational wave observatory. And I remember visiting and they were like, you should join this really exciting time. And I remember thinking to myself, these guys are crazy. They're never going to see this. Oh, my gosh. They're going to work forever and see nothing. And boy, was I wrong.

Oh, man. All right. So if you want to know which projects are going to win the Nobel Prize, check out projects Daniel turns his nose up at. Exactly. They have a great track record. And so this is a big surprise. This is shocking.

Already invites a lot of wonder and speculation about the cause of it. Because we're talking about accelerating the whole. whole universe, the amounts of energy required and involved are staggering, right? Absolutely staggering. You need some kind of thing which is going to overwhelm gravity and overwhelm all the other forces. So this is a big new opportunity to learn about the universe. And before people were like, well, you're really sure you want to like up and our understanding the universe. They're like, let's figure out some other ways to see if this is right. Because what if you're like misunderstood how supernova go or misunderstood something else, and then that's misleading you, it's confusing you, right?

So what we always love to do in experimental science is find other independent lines of evidence that will either contradict the first ones that tell you like, no, you messed up that the story's not coherent or tell you the same story. Because if you hear the same story from four different ways that aren't related to each other, then it's much more likely that you're hearing the true story. You know, it's like you see elephant poop and you hear elephants and you smell elephants and you see elephant footprints. You probably got an elephant problem. Yeah, it would be nice to also see the elephant.

But sometimes you don't get to do that. And so we've done that. We have other ways to know the universe is expanding and accelerating. One of those is something we've talked about in the show a few times. It's Berion acoustic oscillations. This comes from seeing the universe used to be a hot, glowing soup of matter and light. And it was really dense. And so you think about sound in space, you think, oh, nobody can hear you scream.

But back in the early universe, the universe was dense enough that sound could travel through that soup and actually traveled like nearly half the speed of light. It was really, really dense. And so sound traveled really, really fast. And then you had these oscillations where like light pushed matter out and then gravity pulled it back in and then it bounces back out. So you get these oscillations and those create sound waves. can see those sound waves propagating through the early universe plasma because where things were

denser, you later on, like billions of years later, you get more galaxies. And where things were less dense, you get fewer galaxies. And when they go out there and they look at the pattern of galaxies in the universe, they can see these rings. These like literal rings in the sky. Those rings are still there because they got frozen in when the universe expanded and the speed of sound dropped. So those sound waves were propagating out, and then they got frozen into the universe, into its structure, when the universe cooled and expanded enough that the protons and electrons combined to become neutral. So then they were transparent and they weren't getting pushed by the photons anymore, which changed the whole dynamics and then left this imprint. And so we know when that happened. We know how big the universe was when they got frozen in.

that's like an absolute measure of something. Like here's how big it was when it was created. And then we can see how big it is now. And that gives us a standard candle, like a metric, a ruler. So we can use that to see the history of the expansion of the universe because we can see it now. We can see it in early days. We can see it six billion years ago. If we look out into the universe, it's like somebody put a meter stick in the early universe.

And then we watch it grow through time. And you can just use that to literally measure the universe. expansion of the universe and see it accelerate. That's amazing. Yeah. So at first I thought it was going to be like you, you get two points of time. You can get that point and then where we are now, but you'd need multiple points of time to be able to say it's accelerating. Exactly. Yeah. But you've got that. So that's amazing. It's very cool. And then there's another one which is super cool, which relies on time. These are like cosmic clocks. The idea is to find galaxies who formed stars a long time ago

because stars kind of evolve in a predictable way. Older stars look different from younger stars, and we know how long stars last. And so by looking at, like, how blue are the stars and how red are the stars? Because blue stars tend to be hotter and brighter and not live as long, and red stars are cooler and dimmer and live longer.

So as a population of stars age, they tend to go from having some blue stars to having fewer blue stars. So by measuring, like, how red is this population of stars, you can get a sense for how old it is. So just by looking at a galaxy, you can age it by measuring the redness and the blueness. Now, if you look at two different galaxies, at different red shifts, ones that are further away and ones that are closer, then we're seeing clocks at different points in cosmic history.

Awesome. Their stellar populations have different ages. And so you don't need to know the absolute age of either galaxy. You just need to know the change of age, which comes from their change in redshift. and that tells you the rate at which the cosmic redshift is changing with time. So the galaxies sort of act as clocks that let us measure how fast the universe was expanding when their light was emitted. And you repeat this at like many different red shifts, as you were saying, and you can

trace the expansion history. So neither of these are perfect, and there's questions about both of them. One relies on understanding early universe dynamics. Another one relies on understanding stellar populations. and there are uncertainties, but the cool thing is that they are different uncertainties. There are questions and things we don't understand that are different in each of those three measurements, but they all paint the same story about the universe's accelerating expansion. So in the same way that dark matter is not a fudge factor because we have lots of

independent evidence for it in all sorts of different ways. People thought of so many different ways to check dark matter, and it almost always comes out with the same story. Like, yes, there's a lot of invisible matter out. there. In this way, we've also checked the expansion of the universe, and it tells the same story over and over again, that it's been expanding the whole time. In the very early universe, it was decelerating. And about six billion years ago, it turned around and started accelerating, which is amazing, absolutely amazing. Maybe this is too much to ask, given the uncertainties

associated with each of the different methods that we use that you mentioned earlier. But do these various methods sort of clock the accelerations and the decelerations at the same rate or similar rates, or is there a lot of variability there? Yeah, great question. And this is connected to a big puzzle in cosmology right now, which is the Hubble tension, which tells us about the rate of expansion. The Hubble constant, which is not a constant, it's the number that changes in time. Thanks, physicist. And to be fair to the physicist and naming, the Hubble parameter, which tells you the rate of expansion is changing. But technically, the Hubble constant is just its value now, which is a single number.

number. The Hubble constant tells you about the rate of expansion, and that changes in time. And so we can measure this in lots of different ways, and we don't see total agreement. There are things we still have to figure out there. And I think that's a sign of a healthy field because people are constantly coming up with new ways to measure this. And the overall story is the same. We know this expansion. We know that expansion is accelerating. There are differences in those expansion rate measurements in early universe and in late universe, and it requires more common. complicated theories than some of the simple theories that we'll talk about in a minute. And so, you know, whether you expect that expansion to be constant in time or not is a subtlety that you

have to address when you build your theory of why this is happening. But yeah, there's also some fuzziness and disagreement about the measurements themselves. Well, I still think we're very clever apes that we figured out any of these methods of trying to explore what's happening in our universe. I know from this tiny little rock that we basically never left just by gathering photons that happen to come our way, we figured out this incredible cosmic story that's so much bigger than us. It's amazing what people can do. As Hazel says, you know, science is just like looking around and figuring out what happened. I mean, but that's a profound question. What happened? What happened? I know. Exactly. I love the way she minimizes it. She's like,

you know, you're just figuring out what happened. I'm like, yes, what happened to the whole universe. That's right. Or, you know, through the evolution of our species, there's lots of what happened questions that we want to answer. Okay, so you mentioned that there's sort of two different things people are talking about when they mention dark energy. And the first is the fact that the universe expansion is accelerating. So we've talked about that. So let's take a break. And when we come back, we'll dig into this cosmological constant idea.

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Hey, it's Emily Tish Sussman, and this season on She Pivots, the podcast where women share bold career moves and personal turning points that changed everything. I sit down with culture-shifting women who share their pivots like Paraly Olympian Alley Truitt. I was like, I'm a month out from a shark attack. There's no way in 11 months I'm making the Paralympics. Advocate Elizabeth Smart. I still feel that drive that I can still make a difference, so I'm going to keep going.

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Listen to No Safe Words with October and Tony Gonzalez, part of My Culture Podcast Network. Available on the IHeart Radio app, Apple Podcast, or wherever you get your podcast. Hey, it's Bobby Bones. Join me in former NFL quarterback Matt Castle every Wednesday on our podcast, lots to say with me Bobby Bones and Matt Castle. We also bring in friends, they're mostly Matt's friends,

who are current or former NFL players, some of the biggest names in me, music and entertainment, and then we have some really great conversations that can pretty much go anywhere. The great Pac-Man Jones. After the game, the whole national task force for police officers was at the stadium. Like, hey, buddy, yeah, you, Adam. Come up. And one of my favorite segments we do is called situational awareness, where Bobby and I give

each other real-life situations and figure out what we'd actually do. You're in training camp, and the rookie quarterback has one good throwing session in front of the media. Suddenly, everybody online says he should start over you. week one. How do you handle this? You just go back out to practice the next day and wait for him to mess up. Listen to lots to say with Bobby Bones and Matt Castle on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. And we're back and we are talking about whether or not dark energy is a fudge factor. And the second thing that people are usually referring to when they say dark energy is this idea of a cosmological constant. Yeah. So Daniel, what's the cosmological constant? Yeah. So we want to, understand how could the universe's expansion be accelerating? What's doing that? Right. And there's a

really interesting history here, which goes all the way back to the beginning of the story we told about when people thought the universe was static. So Einstein came up with his theory of general relativity, which tells us about space and time and how things pull away from each other or expand also. And this theory, general relativity, it predates Hubble and Leavitt. Their discovery that the universe was expanding, that wouldn't come for a long time, many years after Einstein developed his theory of general relativity. So Einstein was dealing with the assumption that the universe was static. And when he put his theory together, he was like, all right, so mass bends space, the universe is filled with mass, so he should curve space and pull stuff together. And then he thought, hmm,

well, why hasn't the universe collapsed, right? Like if space is filled with all this mass, why hasn't it all crunched down? So his theory is, essentially predicted the universe should have collapsed already. But he looked out and he said, no, the universe is static. So what's going on? Well, there's another knob in his theory. General relativity tells us that space is bent by mass. And the sort of pop-side level understanding of general relativity tells you, like, you can think of the force of gravity as the bending of space. But general relativity is much more than that. Different kinds of energy enter into general relativity in a different way.

So, for example, if you have internal stored energy, what we call mass, that does bend space and does pull things together effectively, although they're actually in free fall. But if you have another kind of energy, stored potential energy that just fills space, that actually makes space expand. So general relativity broadly

doesn't just pull things together. It can also create expansion. Okay. And he recognized that at the time. Question mark. He knew that at the time. And so he thought, all right, what if I just stick a number into my theory, if I add potential energy to the universe, no explanation, no idea what it's referring to, just a fudge factor, put that in to balance gravity so that the universe is then static. Because that's what he wanted to achieve.

You wanted to describe the universe. You wanted to see if his theory was compatible with the universe. And like, is that a bogus thing to do? No. is just essentially asking, well, what would I have to put into my theory to make it describe the universe as I think it works? And remember, he was working on a false premise. He was trying to describe a static universe because that's what they thought we lived in at the time, but they were wrong. The OG Fudge Factor. Yeah, exactly. So the OG Fudge Factor was put in to make

general relativity agree with the misunderstanding at the time that the universe was static. And the other problem with this fudge factor is that the universe, as he described it, had these two competing things. The cosmological constant pushing out and gravity pushing in. And for that to give you a static universe, they had to be perfectly balanced. So the universe is like on a knife's edge. It's not stable at all. Somebody like drops an extra raisin in the universe, boom, it collapses. You get a tiny smidge more potential energy, boom, it expands.

So it doesn't really make sense as a model of the universe. One more reason to hate raisins. I know. Wait, hold on a second. I love raisins. What are you talking about? You're wrong is what I'm talking about. Yeah, no, raisins and cookies are why I have trust issues.

That's only because you think they're chocolate chips and then you're disappointed. But if you're expecting raisins, then you're like, yum, raisins. No, then I'm mad at whoever thought I would like raisin cookies. They don't know me at all. Well, the answer is to put raisins and chocolate chips into your cookies. Oh, no. Okay, no los dos. Because then you've ruined.

You've ruined the cookie with the raisins. All right, we've had milk digressions, raisin digressions. We should start a food podcast. So then Hubble and Levit Discover, the universe is actually expanding and this whole point of putting in a cosmological constant to describe a static universe. Yeah, that was a mistake. And Einstein said, okay, I'm going to get rid of the cosmological constant. And instead, I'm just going to assume that the universe is decelerating.

It's currently expanding, but it's decelerating. So here we're distinguishing between. essentially velocity and acceleration. So think about how your car operates. You can have forward velocity. You're going to 50 miles an hour and you're hitting the brakes. You're decelerating. So you're slowing down. You're still moving forwards, but you're slowing down. That's what Einstein assumed was happening. He said, okay, the university isn't static. The car is not just sitting on the highway. It's moving forward and gravity is slowing it down. And so he thought eventually it will crunch, right? That was his supposition, but he didn't need cosmological constant anymore, and he got

rid of it. Okay. And this was in response to information he learned in his lifetime. He went from the universe, was stable to decelerating. Okay. Yeah. And that's the origin of his comment, I don't know, the exact quote, that the cosmological constant was his greatest blunder. You know, that he put it in and then he had to take it out. We all make mistakes. Yeah. Now, 80 years later, we discover, okay, the universe isn't just expanding. It's expanding and accelerating, right? And so now people are searching for a theoretical reason that might happen. What could accommodate that?

Not just that it's accelerating, but that it started accelerating recently, right? That the history is that we were always expanding, but that expansion was decelerating for the first nine billion years. Gravity was winning. And then around five billion years ago, nine billion years into the history of the universe, it turned around and it went from decelerating to accelerating. So like the car was always moving forward. It was slowing down the first nine billion years.

And then we went from brake to gas. And now we're accelerating. We're still moving forward, still expanding. But now that expansion is faster and faster. Somebody was like, would you like a cookie with raisins in it? And you put your foot on the gas to get out of there as fast as you can. The universe is trying to escape raisin cookies. I am driving as fast as possible to the oatmeal cookie factory.

No, huge mistake. So I can consume them as quickly as possible. Huge mistake. And so the cosmological content can actually describe this. It's amazing because the cosmological constant says, look, you got some potential energy that fills space. We don't know what it is yet, but say you have something that fills space with potential energy, that will cause some expansion.

And the amazing thing is that as the universe expands, matter gets more dilute, radiation gets more dilute, right? By which I just mean, like, you got more space and the same amount of stuff. So the density of matter goes down, which means essentially, gravity is decreasing, but potential energy doesn't. It's just built into space. So if you get twice as much space, you get the same density of potential energy everywhere, which means more energy. So as the universe expands, the cosmological constant becomes more and more important, because everything else has energy density that's decreasing with expansion, but it's staying

constant, which means that it gets a larger and larger fraction. So expansion leads to more expansion because as time goes on as the universe expands, the cosmological constant becomes more and more important, which according to the equations of general relativity means more expansion, until at some point it starts to win and it's overwhelming everything. And that's what happened five billion years ago. So it actually goes to accelerating expansion or runaway effect. And this predicts that as time goes on, it's just going to get more and more important. We're in a dark energy dominated universe. So my brain is trying to make a connection. to Nother and her theory, did she play a role in helping us understand this?

So she didn't help us understand it, but she helps us understand the implications. Because you're probably remembering that Nother has her amazing theory about conservation laws and symmetries. And there's a connection between whether the laws of physics and space are constant in time and conservation of energy. And she says, if space is not expanding and the laws of physics don't change, actually if the action is invariant over time, then energy is conserved. Check out our whole podcast episode about action if you want to learn more about that. But now we know that the action is not invariant. That as time goes on, space is expanding and accelerating. And that's how we know that energy can't actually be

conserved in our universe. It shouldn't be conserved in our universe because the symmetry that you would need to get energy to be conserved is broken, right? Space is expanding. The action is not invariant. So that's the connection to Emmy Nuther. Is that what you're asking about? Yeah. And so I guess now I'm trying to connect that to, so as the universe is expanding, you're getting more dark energy being made. Yeah. Is that the same thing as the cosmological constant? Or is that like a part of understanding the cosmological constant? It's a great question because that's the essential piece of this theory that helps us make it work. That as the universe expands, cosmological constant is constant. It's a constant density. It does not get

diluted. That's the reason that it works. And it's sort of amazing that our theory of general relativity already has a knob in it, which if you crank it up to a certain number, explains perfectly, well, not perfectly, but very, very well, this accelerating expansion of the universe. You don't have to reach for totally crazy new theory of physics, but what you do need is potential energy that fills all of space and doesn't get diluted. And that's the big mystery. So we've made sort of a step forward by saying like, oh, there's a feature of general relativity, which can create this thing which we've observed, which we know is happening. But it also kicks the can down the road and invites more questions like, well, what is this potential energy? You just like put it in there. You haven't said

what it is. Like potential energy is not just some abstract thing. You know, you can have potential energy stored in a field, for example. Like, we know that the Higgs boson is a particle that's an oscillation in a field that's out there. And one of the most interesting things about the Higgs field, the reason it does what it does, is because it's filled with potential energy, unlike other fields. The photon field and the electron field don't have this kind of potential energy, but the Higgs does. It's potential energy that fills all of space.

And you might think, oh, wow, is the Higgs boson field providing the potential energy you need to explain the cosmological constant? Wouldn't that be incredible? If you came out from one direction, you're like, hey, the universe is expanding, and that suggests maybe there's potential energy in all of space. And you came out from the other direction. You're like, hey, particle physicists have discovered a field that fills all the space with potential energy. And you might be like, oh, my gosh, am I about to make the best understanding in the history of the universe?

And you put these two numbers together and say, do they agree? And the answer is no. And not even to within 10% or 50%. They're different by 10 to the 120. And so let me guess. This is a project you decided you wanted to be part of because it was destined to not get a Nobel Prize. Oh my gosh. Understanding theoretically the source of dark energy would definitely get somebody the Nobel Prize. And so we have no explanation. We have this mechanism, the cosmological constant, which if you put into general relativity, can explain the accelerating expansion of the universe, but we have no idea what the cosmological constant really is. It's just like a first idea. It's a number we put in to make things work. And then we come back and say, well, what could be causing it? Our first idea, all the quantum fields that we know about, definitely is not explaining that.

That does not work. No. We don't have another better idea yet. So is that a fudge factor? I mean, I think it's a little bit of a fuzzy question because it's not a bad fudge factor. It's not like, hey, we discover the universe is expanding and accelerating and we really want to stick with general relativity. So we're going to do anything to make it work and we're just going to stick this number in here. We're going to pretend it makes sense even though it doesn't, right?

And that's the cartoon version of conspiratorial physicists trying to defend general relativity or something, I mean, which makes no sense because in reality, everybody, especially the conspiracy folks out there, are trying to disprove Einstein. I get Einstein was wrong emails 10 times a day. So I don't know why they think that like physicists don't also want to disprove Einstein. Because then your face would be on all the posters. What would your quote be, Daniel? My quote would be, this is never going to work.

Dana was wrong again. Oh, I'd put that poster up. Good to always be skeptical. And the issues go deeper, right? It's not just that we don't know where this number, the cosmological constant, comes from. Like, we'd love to be able to derive it from first principles to say, oh, the Higgs field, and you calculated it, here's the potential energy, and here's the number you put in, and it works. We're not there yet. We're nowhere close to there.

The other issue is that, as I was saying earlier, even just having a single number, the cosmological constant doesn't quite work because we see the number you would need changing over time a little bit, like the early universe measurements, the late universe measurements. So it might be that there's two different kinds of expansion happening there. So there's a theory of like early dark energy that is like another kind of dark energy that turned on early, kick things off and then turned off, right? Which maybe explains why we're seeing inconsistent measurements. So there's like complexity to this fudge factor. And there's, you know, even other people out there with totally different theories. Like a couple of years ago, there was all this excitement because people noticed

a correlation between supermassive black holes at the hearts of galaxies and the expansion of the universe and the accelerating expansion. And so there was thought for a while, like maybe supermassive black holes in the hearts of galaxies are the dark energy. Maybe somehow they are doing this to the universe. And there were some problems with that theory and it sort of went away. But, you know, what it shows you is not a field that's out there to convince you of one idea of whether or not it works, but it shows you a group of people, being curious about the universe, scratching their heads, being open about what works and what doesn't work, like the big mysteries that, you know, we write 10,000 Popsie articles about a year, the Hubble Constant and all this kind of stuff,

we're open about what's not working, and it's a work in progress, and it's a work in progress that's out there in public, right? You can see people making a progress, people disagreeing about it, people suggesting new ideas. There's no, like, defense of the dogma here. This is just like you're watching it in action, you know, welcome to the forefront of ignorance. This is how science works, right? We're confused about how stuff is happening, and we're trying to figure it out. There are some older questions that we're much less confused about, and we're not very excited about revisiting, like, is the speed of light constant? You know, maybe we're wrong about that. I don't know, but it doesn't seem as interesting as other questions that we know we're wrong about,

and we have no explanation for, like, you know, what is the source of dark energy theoretically? So that's why some questions get more attention and other questions get less attention because, hey, we're humans and we're driven by our curiosity. And curiosity is frankly subjective and personal and, you know, driven by what gets you excited to get out of bed and spend your day doing science. Yeah. All right. End rant.

Well, and this is the process. You know, you do what you can with the information you have. You try to collect more information so that you can ask more specific and better questions. And sometimes you're going to be wrong. You just always need to be open to that. And you move forward the way you can. Yeah. And, you know, let's also acknowledge that there are structural incentives in science and in every human endeavor for people to defend an idea even after we're clear it's not working. You know, there are people who, have an idea and stake their reputation on it and it was their baby and when it doesn't look like it's working and they try to make it work and they stick to it and they're definitely structural incentives for people to not do the right thing but you know people have lots of incentives and they're not always necessarily just going to follow one set of incentives they're also incentives to figure out how things work and to be a reasonable human being and in reality people's actions are complex and this is not a perfect system and I don't want to suggest

that like science is a pure meritocracy and every idea rises to the top perfectly. And we always work on the most useful thing. Absolutely not. It's a messy political human endeavor. But it's also the best system we've ever had for building knowledge about the universe. So anyway, I was supposed to end my rant a minute ago, but this is the real end. That's rant part too. But I mean, there's like, you know, different people have different incentives.

So if you've got somebody whose incentive is to like not back down even when it's clear that their theory isn't explaining things as well as they thought. There's an incentive for the, like, lab at the other university across the pond to prove you wrong. Or there's the incentive of your grad students who, like, maybe, you know, when they start their own lab, realize, I'd really like to be remembered as someone who was right. And so, you know, maybe they slowly push back against, you know, the ideas of their advisor because they don't want to spend their whole careers on an idea that's wrong. And so, you know, it's different people have different motivations. And hopefully in the end, science pushes everybody towards the right answer.

Yeah, and another note for people who have ideas that they're excited about and they can't get the community to engage. You've written grant proposals and they've all been rejected or nobody will read your theory. Like, remember that resources are limited, right? In a perfect world, we could give everybody money to investigate the universe no matter whether their theories were popular or not. But we have limited funding. We have limited time. We have limited attention. We have to make decisions about where to put those resources. And so rather than having like one random YouTuber decide, this is what's most important and everybody else is wasting their time, we have panels, we have consensus. We argue over it. We do it in public so that we try to get as many perspectives as possible. But in the end, yes, we do want to put resources towards the things we think are going to lead to discoveries. And that means investing in the mainstream ideas, not always giving money to everybody out there or the crazy new idea, which may be promising and may even be. be correct, right? But in the same way that, like, the best screenplays aren't always turned into movies in Hollywood, right? Like, the system is imperfect because of limited resources. And that's just the unfortunate reality. And the way to fix that is not to burn academia to the ground, but to spend more money to fund more of these ideas further from the mainstream, right? If we instead invest in academia, then we get more crazy blue sky. We can take more risks.

Anyway, for the third time, I'll say end rant then. Yeah, yeah, sure. We'll see. We'll see. I should stop saying that. That's right. It's like when you title a document or, you know, a grant or a manuscript, final version, you are making it certain that there will be a final version two and a final version three.

But, okay, to try to bottom line, we have this observation that we feel pretty darn confident about because we've seen it using a bunch of different methods. Yes. We're doing our best to explain it. We realize that there's shortcomings in our explanations. We are trying to figure it out. It is an open conversation. And this is one of the big exciting questions in physics that, you know,

hopefully we'll figure out in the next generation or two or this generation. And if Daniel thinks the lab is probably not on the right path, those guys and gals are getting a Nobel Prize. That's my backward seal of approval, exactly. That's right. All right. Well, thank you, everybody, for going on this journey with us back into history to our understanding of the universe and how the universe evolves and how our understanding of it has evolved

to try to predict the future and how our understanding will eventually, we hope, coalesce into a crystal clear picture of the history and future of the universe. Good luck, physicist. Thanks everybody for listening. Please go and do us a favor and rate the show on whatever podcast app you're using. It really helps people find us. Daniel and Kelly's extraordinary universe is edited by the amazing Matt Kesselman. He really is a wizard. You can also find us online on Blue Sky, Instagram, and X, D&K Universe.

Come engage with us. You can email us at Questions at Daniel and Kelly.org. We really do want to hear from you. And you can find our website, www. www. danielandkelly.org, where you'll also find an invitation to join our Discord, where everybody comes and talks about the amazing universe. And we also have the most amazing moderators. This is an I-Heart podcast.

Thanks for joining us. It's the U.S. Open, which means the tennis world is about to lose its mind, and I'm here for every second of it. Join me, Renee Stubbs, on the Renee Stubbs' tennis podcast, as I break it all down. The matches that matter, the drama that doesn't, and the stories everyone's talking about. No cliches, no sugar-coding, just host analysts

from someone who's actually. been there. Coco looking real good. Her serve looks so much smoother than it has in the past. I was really happy to see Eager back to what she looked like a couple of years ago when she was winning dominantly. Listen to the Renee Stubbs tennis podcast on the IHart Radio app, Apple Podcasts, or wherever you get your podcasts. We've all heard the phrase, all things in moderation, but that's clearly not us. That's why we're celebrating 300 episodes of That's Messed up, our Law & Order SVU podcast, where we cover the real crimes behind the stories and interview actors from the show. We're doing it big for the 300th episode.

We've got Icon, Heartthrob, and SVU guest star, John Stamos, and we're best friends now. In waiting for Guffman, Chris... Are you serious? It's like her favorite movie. That's like my favorite movie of all the time. I've seen it like 150 times.

Me too. Oh my God. New episodes of That's Messed Up, drop Tuesdays. Listen to That's Messed Up on the IHeart Radio app, Apple Podcast, or wherever you get your podcasts. Hi, I'm Michael and Perioli. I want to tell you about my new podcast, Underground Zero.

We're looking at the opportunity of a life Time for us and the other families. It's about the aftermath of the 9-11 attacks. What's in the tunnels? Gold bars, Silver Inge. It could be worth half a billion. Starring Steve Sharipa, Rea Abruzzo, and me, Michael Imperioli. How far you're prepared to go?

Depends how far you are. Listen to Underground Zero on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. In February of 2024, celebrated illustrator and children's book author, Petra Mathers, and her husband Michael, mailed letters to dozens of friends and family members to let them know they had died by suicide. Dear honies, this is goodbye. By the time you get this letter, both Petra and I will be dead. Next stop, Wonderland. Why did they do it? And what did it do to the people they left behind? Listen to death of an artist on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Welcome to Decoding Women's Health. I'm Dr. Elizabeth Pointer, chair of Women's Health and Guidance.

at the Atria Health Institute in New York City. I'll be talking to top researchers and clinicians and bringing vital information about midlife women's health directly to you. A hundred percent of women go through menopause. Even if it's natural, why should we suffer through it? Listen to decoding women's help with Dr. Elizabeth Pointer on the Iheart Radio app, Apple Podcasts, or wherever you get your podcasts. This is an IHeart podcast.

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