Pod Insights Daniel and Kelly's Extraordinary Universe Is the standard model of dark energy in trouble? About this episode Daniel and Kelly talk about recent measurements that leave us confused about the expansion of the Universe.
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Transcript 10,788 words · 8 speakers
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1:43 Daniel Whiteson? Physics lets us ask big, fat, juicy questions about the universe. I would claim that these are the biggest, the fattest, the juiciest questions, though chemists and biologists may disagree, of course. But physics lets us discover new elements of the universe, like dark matter, that tell us our whole picture of what the universe is made of was wrong. It lets us think about the size of the entire universe and whether it's expanding. It gives us the tools that will let us discover the forces that are capable of squeezing and stretching galaxies and clusters and structures beyond even our capacity to hold in our minds. Probably the most powerful and mysterious discovery in physics, maybe ever, is dark
2:28 Daniel Whiteson? energy. The accelerating expansion of the universe has changed our picture of where the energy in the cosmos is, what it does, and how it shapes the present and future of the entire universe. Will things be torn apart, left to huddle together with a few other bits of matter among a vast, empty cosmos? Will things rush back together to collapse everything and everyone into incredible density, mimicking the conditions of the early universe? Only dark energy has the sheer scale and scope to determine the fate of the universe, which is why we are all so desperate to understand it. And dark energy is recent, a concept from the last couple of decades, which means we have a lot left to learn and that surprises deliciously await us.
3:13 Daniel Whiteson? Recently, you might have heard that our understanding of dark energy was thrown into new disarray by some recent measurements. What does that mean about dark energy? What does it mean for our understanding of the universe? What does it mean about the future of the universe? Should you still save for retirement or just fly to Vegas tomorrow and blow it all in a weekend of excess and debauchery? We'll dig into the history and the latest mystery and let physics do its best to answer these questions. Welcome to Daniel and Kelly's Extraordinary Expanding Universe.
3:57 Kelly Weinersmith? Hello, I'm Kelly Wiener-Smith, and I study space and parasites, and I am so glad we have finally figured out dark energy.
4:06 Kelly Weinersmith? Right, Daniel? That's what we're talking about today, isn't it?
4:09 Daniel Whiteson? Hmm, what a setup, what a setup. Hi, I'm Daniel. I'm a particle physicist, and I wish I was as cool as dark energy sounds.
4:19 Kelly Weinersmith? Oh, it does sound... Pretty cool. You all are really hit and miss on terms of naming stuff, but with dark energy and dark matter, the Goth in me really digs it.
4:28 Daniel Whiteson? Yeah, I don't know if they're appropriate in the sense that they describe it well, but they do sound very cool. And, you know, that's one angle for sure.
4:35 Kelly Weinersmith? Yeah, no, for sure. So what percent of the articles that you see online about dark matter and dark energy, do you feel are like completely accurate?
4:46 Daniel Whiteson? Completely accurate? Wow, that is a small fraction.
4:49 Kelly Weinersmith? Oh, man.
4:50 Daniel Whiteson? Unfortunately, there is so much clickbait nonsense out there. And that really disappoints me because I imagine if I wasn't an expert and I was curious, like lots of our listeners are, who want to understand the universe in good faith, they read these things and they don't know whether it's nonsense or not. So if you're out there and you're reading an article and it tickles your nonsense meter, feel free to send it to us. I will let you know if you should believe it or not. But unfortunately, on average, the answer is no, because science journalism is hard and getting all the nuances right is very, very hard.
5:23 Kelly Weinersmith? Yeah, that's true. But I gotta say, you know, Ed Yong, for example, so he's our biology king of science journalism. And he does a really good job of like running stuff by people to check. So like, you can't possibly know everything as a science journalist if you're covering a broad range of topics. But I do feel like if you run it by experts, like, why aren't you getting an email every single day from like, people writing about this to be like, hey, can I run it by an expert before I put it online?
5:49 Daniel Whiteson? Yeah, well, there's a lot to dig into there. I think that a lot of folks don't run things by experts. They read other popular science coverage and get their information from there because not that many experts are accessible. You know, professors don't usually answer their emails, though some of us do. And they don't respond on time and you have a deadline. So instead of talking to somebody who is a professor about dark matter, you read a bunch of other articles about dark matter, especially if you're not a physicist yourself, so you can't read the primary literature and decode those weird scientific squiggles. So yeah, I think a lot of us are inaccessible. But hey, that's what this podcast is about, right? Is making all this science accessible without dumbing it down, digging deep.
6:30 Daniel Whiteson? Do what's actually going on and giving everybody out there an understanding and an opportunity to ask questions. So if you're a science journalist and you want to write an article about dark matter and dark energy and you have a question or there's something you don't think you fully understand, don't write about it. If you don't understand it, ask me and I will explain it to you.
6:47 Kelly Weinersmith? There you go. Well, today we are tackling a topic that is really making the rounds that is not quite accurate. Is that safe to say?
6:56 Daniel Whiteson? I think there's a lot of coverage of this issue, which misses a lot of the important details. Yeah.
7:01 Kelly Weinersmith? Okay. All right. So the question we're asking today is, is the standard model of dark energy in trouble? And I'll be honest, I thought the standard model just referred to a thing and I didn't think that it included dark energy. So are there multiple standard models? Or does the standard model I'm thinking of also encompass dark energy?
7:20 Daniel Whiteson? So the standard model you're probably thinking of is the standard model of particle physics. And you're right, that does not encompass dark energy. In fact, it doesn't include anything about gravity. It's just the quantum forces and the particles and that kind of stuff. So that's of particle physics. There's another standard model of cosmology, sometimes called Lambda CDM. And that's like our picture of the history of the universe and how it expands and dark matter and dark energy and all those things. It's sort of the standard model of cosmology.
7:49 Kelly Weinersmith? Oh, okay. That sounds super interesting.
7:52 Daniel Whiteson? Yeah. And this is sort of standard model in a lowercase sense. Standard model of particle physics, often written with uppercase sense and just refers to as the standard model. But you know, you could have a standard model of, I don't know, parasite interactions or something I imagine, right? And it's sort of just like the normal usage of the words.
8:08 Kelly Weinersmith? And for that one, it would be capital letters, the best standard model or the most interesting standard model. But anyway, moving on, we asked our audience, is the standard model of dark energy in trouble? And here's what they had to say.
8:24 Speaker 05 Yes, because recent research has indicated that dark energy weakens over time and therefore is not constant as astrophysicists first believe. I would say yes, and I wouldn't be surprised about it. So quantum dark energy, that disagrees with the standard model. So.
8:40 Daniel Whiteson? Yes. Well, what would you expect if all you do is hide out in shadowy places and make yourself unknown? Of course, you'd get into trouble. The cosmological constant is not a constant. So yeah, I'd say it could be in trouble.
8:55 Speaker 06 This can only be a good thing as we are becoming one step closer to understanding the truth. Given how little we understand today of dark energy, I really hope the latest findings do put the model in trouble. Because otherwise, what's it gonna do? Run away from us at an accelerating rate forever? I didn't think the standard model included dark matter or dark energy. So I'm gonna surmise that the standard model is not in trouble. Judging from the headlines, I would say yes. So I'm just curious, would the standard model of dark energy have to go to the principal's office? Have to hire a lawyer? Have to appear before Congress?
9:35 Speaker 06 It may well be in trouble from some of the headlines you read, but I'd like to delve further and see what Dr. Dan's got to say. Dark energy may be changing over time and may not at all be constant. The standard model of dark energy is no longer in line with data that's coming in. So yeah, I think it needs to go to the principal's office. I seem to remember hearing something about the expansion of space being variable. Is that something to do with it?
10:15 Speaker 06 Wasn't aware that there was a standard model for dark energy, but I suppose the question being asked assumes that it is. So I'm going with yes.
10:27 Kelly Weinersmith? As always, I love you all. The answers are either deeply insightful or absolutely hilarious. And so thank you for writing in.
10:40 Daniel Whiteson? You too can participate. Send us your response to the questions of the day. Just write to us to questions at danielandkelly.org and we will hook you up.
10:50 Kelly Weinersmith? What did you think of these responses?
10:51 Daniel Whiteson? I thought they were great. Some very insightful, some totally hilarious, some echoing your question about like, hold on, if you're talking about the standard model, that already doesn't include dark matter and dark energy. So yeah, great stuff.
11:03 Kelly Weinersmith? Yeah, I bet there were a bunch of people who are like, this is another trick question from Daniel because he loves those.
11:08 Daniel Whiteson? They're not trick questions.
11:12 Daniel Whiteson? It's just that, you know, if I'm asking you about something, then we expect it's probably something interesting we learned recently in the news. So yeah.
11:21 Kelly Weinersmith? All right. Well, let's start from the beginning. What is dark energy? I mean, I know, because it's physics, you're going to be like, oh, we don't know.
11:30 Daniel Whiteson? It's great to define these terms because like just in philosophy, you can avoid a lot of talking past each other if you're clear about what you mean. And so the best way to understand dark energy is to understand that it's just our observation of how the universe is expanding and accelerating. And that's a very recent observation and it's important, I think, to put that in historical context and remember like what we used to think was happening to the universe. What we used to think was natural or made sense. Only like a hundred years ago, before Hubble and Levitt and those folks, we thought the universe was static. We thought there was just a single galaxy, just a bunch of stars hanging out in space.
12:11 Daniel Whiteson? And that was it. That was the universe and it wasn't shrinking. It wasn't expanding. There weren't stars that went on forever. It's just like one, our little blob of stars hanging out in space. And that was people's mental picture of the universe. Your mental picture, probably influenced by wonderful programs like Nova and pictures from James Webb, is of a universe filled with galaxies upon galaxies upon galaxies. It's a very different context. And for me, the context of our lives, that's what physics is about, right? So this like is a complete shift in the universe we think we're living in.
12:41 Kelly Weinersmith? I mean, if there were more parasites in space, I'd probably be way more excited, but that is an inspirational thing to think about.
12:47 Daniel Whiteson? The day the James Webb Space Telescope discovers space parasites, I'm also going to be excited about parasites.
12:55 Kelly Weinersmith? That's what it would take, huh?
12:56 Daniel Whiteson? At a minimum. All right, so before Hubble, we think there's just one galaxy hanging in space. And that's natural. People imagine like, oh, it's probably been like that forever. Because why wouldn't it be? You know, the idea of an infinite past was sort of fine, not a big deal. But then Hubble and Henry at 11 figured out a way to measure the distance to stuff. This is really important because when you're looking out into space and you're looking at a star, you can't tell, hey, is that star incredibly bright but really far away? So it looks kind of dim or actually kind of dim and like not terribly far away. Cosmologically speaking, you can't tell the difference if you're just looking at the intensity of the star. Because you don't know how bright it actually is.
13:38 Daniel Whiteson? And if you want to make a 3D map of the universe, knowing how far away things are is crucial, right? Because otherwise you're just looking at like the equivalent of a 2D screen that surrounds the earth.
13:48 Kelly Weinersmith? And so how do you calibrate so that you can figure that stuff out?
13:52 Daniel Whiteson? Yeah, so we can actually tell how far away stuff is if it's pretty close by using parallax. It's the same principle that you can use to like tell whether a baseball is close to you or far away from you. It's the fact that you have two eyes, binocular vision, and they give a different view of the same object. And if something is close by, your eyes have a very different perspective on them. And if something is far away, they have basically the same perspective. Like if you hold up your coffee cup in front of your face and you close one eye and open the other one and go back and forth, you can see that the picture looks very different. And your brain uses that information to say, okay, the coffee cup is close. But if it's farther away and you do the same thing, there's hardly a difference. Your brain does all the math and tells you where the baseball or the coffee cup is. So that's parallax.
14:36 Daniel Whiteson? We can do the same thing in space because the earth goes around the sun. And so over the year, we have different views of the cosmos. So if something is pretty close to us, then it looks pretty different from one side of the sun and the other side of the sun. Whereas if it's like a gazillion light years away, that hardly matters.
14:54 Kelly Weinersmith? Wow.
14:54 Daniel Whiteson? So we can use that to measure the distance to nearby stuff.
14:57 Kelly Weinersmith? So you've got to be pretty patient to figure this stuff out then? Because it's like six months or something in between data collection moments.
15:03 Daniel Whiteson? Yeah, and this is actually the reason that the Greeks got the solar system wrong. The Greeks knew about this method and they used this method and they tried to see how far away the stars were and they couldn't see any wiggles at all because the stars are pretty far away. They assumed they were pretty close. And so because they didn't see the stars wiggling, they assumed the earth doesn't move around the sun.
15:26 Kelly Weinersmith? Oh, that's fascinating, but we have better instruments so we can calculate that. Is that the difference?
15:31 Daniel Whiteson? Yeah, exactly. It wasn't until like the 1800s that we could see the stars wiggle because the wiggle is small. Right? And so without those instruments and without the assumption that the stars are close, you can actually tell how much the stars are wiggling and then you can tell the distance to them. So the Greeks assumed the stars were close and they couldn't see them wiggling. So they assumed the flaw was that the earth wasn't moving. Of course, the earth was moving. It's just that the stars were further than they thought. Sort of fascinating history of like what people think is natural.
15:58 Kelly Weinersmith? That is a pretty clever way to try to figure out if the earth is moving or not. Like, good on them.
16:02 Daniel Whiteson? Yeah, exactly. People are always dumping on the Greeks, including me for many years, for not doing empirical science, but like this is a good example of coming up with a clever technique, taking some data, using it to draw conclusions. And, you know, they got the wrong conclusion because they had one mistaken assumption, but it's pretty solid work.
16:21 Kelly Weinersmith? Yeah.
16:21 Daniel Whiteson? Anyway, we can do that for nearby stuff. More distant stuff is hard because if it's not wiggling, you can't tell is it a zillion miles away or two zillion or half a zillion. And so what Levitt and Hubble figured out was a way to measure the distant to more distant stars by looking for a particular kind of star called cepids. Cepids are stars that have variable brightness. Like they're not just like the sun that mostly burns the same. They go brighter and dimmer and brighter and dimmer. And that's because of stuff that's happening on the inside. There's like hot layers that rise to the surface and then collapse. There's like, you know, crazy stellar chemistry going on there.
16:57 Kelly Weinersmith? Are they made out of different things than our sun?
17:00 Daniel Whiteson? It's definitely connected to what they're made out of because part of the star is opaque to its own radiation and so the glow from the inner part of the star then pushes that part of the star out and so you have to have some sort of opaque layers and they think this might be involved with helium, but it's not totally understood. There's some similar stuff going on inside our star with layers of opacity and transparency. We can talk about that another time. We talked about the solar magnetic field. But the cool thing about these stars is that the variability in their brightness is very closely connected to their actual brightness. So if you, for example, watch one of these stars and you say, oh, it goes bright and dim in a period of a day, or five days, or 10 days, you can use that
17:43 Daniel Whiteson? to determine how bright it actually is. So you can measure the actual brightness of it just by measuring its variability.
17:50 Kelly Weinersmith? Wow. Okay. All right. So now we know about variability of brightness and we know how it appears to change as you move locations. And what do those two pieces of information tell us?
18:01 Daniel Whiteson? So they allow us to weave together what we call a distance ladder. For nearby stuff, we use parallax. And then near the edge of our ability to do parallax, there are some cepids, some of these variable stars, and we can use parallax to calibrate the cepids. And then we can use cepids to go even further. Because if you know how bright a star is actually, its true brightness, and then you measure its brightness here on Earth, you can tell how far away it is. That was the missing piece, right? We didn't know if this star is bright and far away, or dim and close. But now because we know whether it's actually bright or actually dim, we can tell if it's close or far by its apparent brightness here on Earth. And the crucial thing is that these two ladders, the parallax and the cepids overlap. So there's a period where we can use parallax to calibrate the cepids and then extrapolate
18:44 Daniel Whiteson? further for stuff that we couldn't use parallax for. And this gave Hubble this incredible 3D view of the universe we didn't have before. He could tell the distance to all kinds of stuff that nobody knew. Hey, is that actually a smudge in the sky but pretty close? Or is it something incredible and massive but deeply, deeply distant?
19:03 Kelly Weinersmith? And is this how he figured out that the universe was expanding because things changed their location over time?
19:09 Daniel Whiteson? Well, number one, he discovered that there are other galaxies. We used to see these things in the sky and we called them nebulae because they were sort of like smudgy in our telescopes. And people are like, hmm, maybe they're just big clouds of gas. Turns out, no, they're actually entire galaxies that are much bigger in many cases than our galaxy, like Andromeda, so much bigger than our galaxy. He was able to unravel this puzzle because he could measure the distance to them. He found cepids in those nebulae and he was able to tell, oh my gosh, that thing is so much further away than anything else, any of the other stars. It's its own galaxy.
19:42 Kelly Weinersmith? Oh my gosh.
19:43 Daniel Whiteson? So instantly this like mental picture of the universe just expands from we have one galaxy with some fuzzy clouds in it, to take all those fuzzy clouds and promote them to their own super distant, incredibly large galaxies. And the universe now filled with galaxies. That's often overlooked in the story about the expanding universe because what Hubble actually discovered is that the universe is filled with galaxies, not just our own.
20:07 Kelly Weinersmith? And around when was that happening?
20:08 Daniel Whiteson? This is the 1920s. So it's just like 100 years ago, right? Most of humanity had no idea that this was the case. But you're right, the other crucial thing that Hubble discovered is he was able to measure the apparent velocity of these galaxies. By looking at the light that came from the galaxies and how it was shifted by velocity, if something is moving away from you, its wavelengths are lengthened. If it's moving towards you, its wavelengths are shrunk. And so if something is redshifted, it means it's moving away from you. And all the galaxies were redshifted and the ones that were further were redshifted more. And so this is Hubble's discovery that there's a close relationship between the distance to a galaxy and its apparent velocity. Things that are close by are moving away from us slowly.
20:51 Daniel Whiteson? Things that are further away are moving away from us faster. Things that are very, very distant are moving away from us even faster. So this is Hubble's view of the expanding universe. And this was like second mind blowing revelation. Not only is the universe filled with galaxies, they're all running away from us.
21:07 Kelly Weinersmith? Yeah, right. So if everything is running away from us, does that mean that we're the stinky galaxy? Or does that mean that we're the center of everything? Or like, why is everything moving away from us?
21:18 Daniel Whiteson? Everything is moving away from us, but everything is moving away from everything. The whole universe is expanding. The mental picture in your mind, actually, velocity is not the best way to think about it. Because when we get to acceleration in a minute, somebody's gonna be like, hold on, Daniel said you could measure acceleration. Acceleration's absolute. And they're gonna be right. The right way to think about it is in terms of expansion. Space is growing. So everywhere, space is just stretching, which is why the raisin bread analogy is actually kind of perfect. Like, all the raisins in the raisin bread are getting further away from all the other raisins. No matter where you look in the universe, everything is moving away from you. It's not because we're in the center. We're not special. There is no center. Everywhere in the universe sees everything moving away from it.
22:03 Kelly Weinersmith? Well, I also try to move away from raisins as much as I can. But, all right, so where do we go from here?
22:09 Daniel Whiteson? So Hubble measured this relationship between the velocity of distant galaxies and their distance from us. If you plot those on a graph, you get a straight line. The slope of that line is the Hubble constant, is this relationship between them. And this we know now is a number that's something like 70 kilometers per second per megaparsec, which essentially is a measurement of the expansion rate of the universe. It's not a velocity. It's a velocity per distance. Because as distances grow, the apparent velocity the galaxies are moving away from each other with grows also. So you can't like compare this to the speed of light, has the wrong units. But this is the crucial number. It tells us how fast is the universe expanding. And it's called the Hubble constant, even though it's not a constant.
22:55 Kelly Weinersmith? Thanks, physics.
22:58 Daniel Whiteson? But we have now this new view of the universe that everything is expanding. And for decades and decades, that's what people thought was happening. The universe was expanding. And they were wondering about the future, like, hmm, is it gonna continue to expand? Or is gravity gonna eventually win and pull all these galaxies back together into some sort of big crunch? That was the big question being asked in like the 1990s, when people tried to look even further into space.
23:24 Kelly Weinersmith? That sounds existential, but don't be scared. We're gonna give you some more information when we get back from the break.
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24:09 Speaker 03 Hey, it's Nile Rodgers, host of Before the Moment, a Lincoln original series. I'm driving across the country, meeting some of today's most fascinating artists. We're exploring the stories before the moment that changed everything. This week, I'm joined by breakout singer-songwriter Alex Warren. I'm like a kid when I play music and I make music and I'm constantly asking questions. You know, whether it's an instrument I haven't played or it's a melody that seems foreign. You know, there's every night you play a show and it's the same set, but it's not the same show. At a certain point, your brain kind of goes on autopilot. When I'm in the studio or if I'm making music, that's where that curiosity kind of comes back, you know? I haven't lost it yet, but I'm also on my sophomore album.
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25:44 Kelly Weinersmith? All right, we're back. Daniel just made us all a little bit nervous that the universe might have a big crunch at some point, where everything sort of smooshes back in on itself. So, Daniel, help me sleep tonight. Give me some more information.
25:56 Daniel Whiteson? Well, I can sell you some big crunch insurance if you like. It's a really big payout. If the universe collapses into a mega black hole, you will get a big deposit in your bank account. How about that?
26:06 Kelly Weinersmith? Doesn't make any sense. I'm not convinced.
26:11 Daniel Whiteson? Some people just buy insurance to feel better. Remember those folks who were buying like Y2K insurance? Like they say, if society collapses, we will helicopter you a bunker with weapons and food and whiskey. And I was like, they're never getting that delivery.
26:24 Kelly Weinersmith? No, no. I've started researching having pigs and I've been watching videos about pigs. And one of the videos, you know, nonchalantly, they said, oh no, it was geese. It was geese. I've been researching a lot of animals lately. Anyway.
26:38 Daniel Whiteson? It's not that easy to confuse pigs and geese.
26:40 Kelly Weinersmith? I want both of them.
26:42 Daniel Whiteson? Make sure you click the right button when you order because, and I hate to be the one to bring up cannibalism. Pigs will eat your children and geese will not.
26:49 Kelly Weinersmith? That's true. And pigs will eat each other, as we learned in the Triconella episode. Anyway, one of the videos that I was watching in a totally like nonchalant voice was like, well, and from a prepper perspective. And I was like, oh, this is the community that I'm associated with now. My interests align with this community. All right, anyway, back to dark energy.
27:07 Daniel Whiteson? So in the 1990s, we wanted to look even further into space to understand the broader scope of this expansion. Because the further we look into space, the further back we look in time, right? Because remember, light traveled at finite speed. And so if something is happening across the universe right now, we can't see it for billions of years. But when we do look out deep into the universe and we see light that has taken billions of years to get here, we're seeing things that happened billions of years ago. So if you want a timeline of the expansion of the universe, so you can better predict the future, for example, like what is the trajectory? It's useful to look further into the universe and to know how far away is that stuff? And how quickly is it moving away from us? That gives you like a deeper picture and lets you extrapolate to the future.
27:50 Daniel Whiteson? And cepids are cool, but only if you can pick them out, right? Only if you can like find an isolated cepid in that distant galaxy. So you can identify it. That works great for stars and for regions of our galaxy. And for nearby galaxies, it's okay. But thinking about really distant galaxies in the early universe, cepids basically peter out. So we needed a new technique, a new element of our distance ladder to look further into the universe and into the history of the expansion.
28:17 Kelly Weinersmith? Okay, and I keep asking, tell us about dark energy, but we don't really get to it. So is dark energy gonna be the answer now?
28:24 Daniel Whiteson? We're just about to get to the dark energy because in the 90s, people figured out another standard candle. These are type 1a supernova. And these are incredible collisions when a neutron star which failed to go nova or failed to become a black hole, gobbles up a little extra bit of matter and goes supernova in this very precise, very predictable way. The light curve rises and then it drops. And it's not that all type 1a supernova are the same, but from the shape of the light curve, you can determine the true brightness, just like with the cepids. There's something you can measure without knowing the distance that tells you the true brightness, and then from the true brightness and the apparent brightness, you can tell the actual distance. And type 1a supernova are perfect for looking deep into the universe because they're incredibly mind-bogglingly bright.
29:10 Daniel Whiteson? Like a type 1a supernova can be brighter than its entire galaxy for a moment.
29:14 Kelly Weinersmith? Oh, my gosh.
29:15 Daniel Whiteson? So people realized this, and then there was a huge race. There was a team in Australia and a team at Berkeley that realized if they gathered enough of these type 1a supernova and watched them, they could make a 3D map of the universe that dwarfed Hubble's map and went deeper in the history of the universe, which would allow us to make a fit and project and predict what's gonna happen for the future of the universe. Like, what an exciting moment when you realize you have an ability, just on this tiny little planet in the corner of the universe, to understand the fate of the entire cosmos. Like, oh, my god, drunk with power, right?
29:46 Kelly Weinersmith? Were you at Berkeley at this time? This was before you.
29:49 Daniel Whiteson? All this stuff was happening while I was at Berkeley. I was not involved in it at all, but yeah, I knew it was happening at the time. It was really exciting, and they were like really racing. Both teams knew, like, okay, we know how to do this, and it's gonna take a few months. Let's not be scooped.
30:02 Kelly Weinersmith? Yeah.
30:03 Daniel Whiteson? In the end, it was kind of a big tie, which is great, but they discovered something shocking. You know, they discovered that the expansion of the universe is not slowing down, and it's not just continuing smoothly, it's speeding up. Right? That's dark energy. Dark energy is the observation that the expansion of the universe is accelerating, that year after year, this quantity we call the Hubble constant is gonna be growing, that the expansion of the universe is increasing.
30:29 Kelly Weinersmith? And I remember that you told us in a prior episode, what is energy? Or where does energy come from? That as the universe expands, it seems like more dark energy is made. Is that right? But I'm still having trouble thinking of dark energy as the expansion of the universe. Is it like the battery that's fueling the motion outward?
30:50 Kelly Weinersmith? How do I think of this?
30:51 Daniel Whiteson? Yeah, so the phrase dark energy is sometimes used to describe this observation. We know the universe is expanding and that expansion is accelerating. And then naturally you're like, well, what's doing it? What's an explanation? Daniel, what's the theory of it? And there are a few possible explanations for what could be creating this accelerating expansion. And sometimes they are described as dark energy, but really they're all very loose placeholders. And the sort of leading candidate until recently was what you described. The idea that space might have some inherent energy within it, a potential energy. And energy in general relativity is complicated. A lot of people think, oh, general relativity tells us energy makes curvature, you know, just like mass and anything causes things to collapse.
31:36 Daniel Whiteson? But general relativity is complicated. As we talked about in our potato episode, different kinds of energies contribute differently. And if you have potential energy in all of space, it actually creates repulsive gravity, pushes things apart. It accelerates the expansion of the universe. So on one hand, we observe the accelerating expansion of the universe. On the other hand, we have a way within general relativity to create accelerating expansion of the universe. There's a knob there. It's called the cosmological constant. You just put this number into the equations and say, if space has this energy inherently in it, then that would cause the accelerating expansion. Now, we haven't observed that energy directly. It's not like we found that energy, not like we identified where it comes from or what quantum field it would be or whatever.
32:19 Daniel Whiteson? It's just like, okay, this number would work if we could figure out if that's really happening. But that number, as you say, assumes dark energy is constant. You make more space, you get more dark energy. And that predicts a very specific way in which dark energy grows over time. Because it actually starts to take over as the universe expands, matter gets diluted, right? You have the same number of protons, more space, its density goes down. Radiation gets diluted even more because it also gets red. shifted, but dark energy doesn't. But a cosmological constant, dark energy, see, I just used it to describe the explanation, not the observation. It doesn't get diluted. It's universe expands, more space. It's a constant in space. And so therefore it goes up in its fraction.
33:04 Daniel Whiteson? And the naive prediction then of the future is that dark energy just is a runaway effect. Because as the universe expands, its density is the same, which means its overall fraction keeps rising. So that's sort of the standard model of cosmology until very recently. People were like, well, look, this works pretty well. We have some dark energy. We have some dark matter. We have this history of stuff. How matter used to dominate, how radiation used to dominate, how as the universe expands, those things change. And that described things pretty well. And people are like, okay, let's really nail it down. Let's measure things super duper precisely in lots of different ways and make sure we always get a consistent answer. Because if so, that tells us we know what we're doing. Over the last few years, though, there's been some tension in this concept, this standard model of cosmology.
33:48 Daniel Whiteson? And it goes by the name of the Hubble tension. And it's old news, but it's important to keep in your mind when we talk about the new news in dark energy, because the two tell very different stories about the future. So the Hubble tension is like a great piece of science. You know, anytime you see something new in the universe, you want to check your assumptions. You want to measure three different ways with different assumptions, because that could reveal that, oh, actually, we don't know what we're doing. Or no, we really do understand this. Like, you know, we saw the top core of the Tabatron, we measured its properties. Then we measured it in completely different collisions and a different collider and a different country with different detectors and different people. We got the same numbers. We were like, okay, probably was there. We got that right. If not, we'd be upset, right? Or we'd be confused and we'd think there's a hint.
34:32 Daniel Whiteson? So they did the same thing with measuring the Hubble constant. The measurement I told you about is what we call the late universe measurement. We're looking at the expansion of recent times. We're using parallax, we're using cepids, we're using type one A supernova. That's like, you know, in the last 10 billion years of the expansion of the universe. It can't penetrate all the way to the very beginning, because you need type one A supernova, and they do eventually get dim. But that's what we call the late universe. Measurement. So you measure that you get one number for the Hubble constant. It's like 74 kilometers per second per mega parsec. And a lot of careful work has been done there because like, what if you don't understand supernova? What if there are different kinds of supernova? What if the light is passing through different stuff?
35:14 Daniel Whiteson? And there's like entire batteries of people whose entire PhD was like thinking about a way. Maybe this went wrong and finding some clever way to check it. It's incredible what folks are doing in astrophysics.
35:26 Kelly Weinersmith? So you had mentioned that the Hubble constant isn't actually a constant. It's a number that's changing. So that number that you gave us, is that a value for Friday, March 23rd? Or, okay.
35:36 Daniel Whiteson? Yeah, it's the recent measurement, right? It's the late time expansion number. But you can also measure this in the early universe. Because if we look at the most ancient light in the universe, the cosmic microwave background light. It's light from when the universe was very dense. It was a hot plasma and it was glowing and suddenly it became transparent. So instead of that light continuously being emitted and absorbed, it was just emitted, not absorbed, and it's still around. And if you look at that light, you can learn so much about the density of the universe and what it was made out of. You can tell how much dark matter there was. You can tell the expansion rate. You can tell how much matter there was. It's really incredibly rich. And we can have a whole other episode digging into that science. But essentially what you can do is from the wiggles in the cosmic microwave background radiation,
36:22 Daniel Whiteson? you can measure the expansion rate of the universe. And then you can propagate that forward and say, if we measure the number from the early universe, what number should we measure today? So now you can compare these two numbers apples to apples. And the CMB measurement, what we call the early time measurement propagated forward to compare, gives us a different number, gives us a number like 67. And when these two numbers came out, people were like, hmm, that's weird. But you know, the uncertainties were kind of big, like 67 isn't that different from 74? If they both have like a plus or minus 10 on them. But as people spent more time and more energy whittling down these uncertainties and measuring these more precisely, the numbers didn't change, just the uncertainties shrank. And now we're like, yeah, these are two different numbers.
37:06 Daniel Whiteson? And that's a concern because. If it's right, it means dark energy can't be constant. It means like maybe dark energy is growing with time, right? Dark energy was weaker and now it's stronger in late times. So this is the sort of context for these recent measurements by Daisy and dark energy. We're going to talk about in a minute. Dark energy, this incredible story of discovering the accelerating expansion of the universe, an attempt to describe it using a simple model, the cosmological constant, which requires dark energy to be constant in space and therefore grow in a certain way. But then our observation, this Hubble tension, that it doesn't quite work. That our measurements from the early universe and our measurements from the late universe don't jive with the way we think dark energy operated. That maybe it wasn't a constant in space, right?
37:51 Daniel Whiteson? Maybe that whole assumption was wrong. Maybe it's not a cosmological constant. Maybe it's some other weird thing that's sort of similar, but not quite. So that was sort of the context for this recent study.
38:00 Kelly Weinersmith? Okay, so the tension is that 74 is not 67.
38:05 Daniel Whiteson? That's right. And remember, to explain this tension, you need some theory where dark energy is getting stronger recently.
38:11 Kelly Weinersmith? All right, well, let's take a break. And when we come back, we're going to find out what this new experiment tells us about this tension. And also, I hope one day there'll be a Wiener Smith conflict to match the Hubble tension.
38:25 Daniel Whiteson? Well, it's good to have goals in science.
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39:06 Speaker 03 Hey, it's Niall Rogers, host of Before the Moment, a Lincoln original series. I'm driving across the country, meeting some of today's most fascinating artists. We're exploring the stories before the moment that changed everything. This week, I'm joined by breakout singer-songwriter Alex Warren. I'm like a kid when I play music and I make music and I'm constantly asking questions. You know, whether it's an instrument I haven't played, or it's a melody that seems foreign, you know, that's every night you play a show and it's the same set, but it's not the same show. At a certain point, your brain kind of goes on autopilot. When I'm in the studio, or if I'm making music, that's where that curiosity kind of comes back, you know? I haven't lost it yet, but I'm also on my sophomore album. So, and
39:50 Speaker 03 you, you obviously are an expert here. Experience Before the Moment, a Lincoln original series on the iHeartRadio app, YouTube, or wherever you get your podcasts.
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40:42 Kelly Weinersmith? All right, we're back. So, Daniel, you were telling us about the Hubble tension, and that labs have been working hard to try to figure out what is causing this difference between the two different measures for the expansion of the universe. What have we learned recently?
40:54 Daniel Whiteson? Yeah, so there's been a lot of really interesting stuff happening with dark energy, including new theories, like, I want to spend one minute talking about this time-scape theory a lot of people heard about. Huge PR echo chamber from this one paper, where somebody suggested that, wait, maybe the universe isn't actually accelerating in its expansion. Maybe we've messed it all up. And his theory was that time is slowing. Down near massive objects. We know that to be true. So maybe the universe is act lumpier than we thought, and there are parts of it that are denser. And that's affecting the way we're interpreting this stuff. And so this theory went by the very cool name of timescape. And it pinged around the internet and I got a bunch of emails and a bunch of listeners heard about it and thought, oh, my gosh, are we getting rid of dark energy? And that would be very cool and we're open to that.
41:39 Daniel Whiteson? But this theory requires the universe to be a lot lumpier than we see it to be. Like it would require a lot more mass in areas to create this kind of effect. So the data just are not consistent with the timescape theory. Nobody I know in cosmology is taking that seriously at all, even though we got a lot of press. So there's not often a lot of correlation between like how excited the science journalism corps is about an idea? And how excited the actual scientists are about an idea? But there is something that happened very recently. Measurements from the Daisy experiment. They used a completely different method to measure the expansion of the universe. One that fits sort of very nicely between the very early universe measurements from the cosmic microwave background. And sort of later time measurements from the supernova crew.
42:24 Daniel Whiteson? They use something called baryon acoustic oscillations. Where they see these rings of matter from oscillations in the early universe.
42:33 Kelly Weinersmith? And by looking at these rings of matter. It sounds like you just said that it agreed with both of the values. So it agrees with 67 and 74.
42:41 Daniel Whiteson? So this is a new technique. And it's cool that it's complementary. By complementary, I just mean like they're using another approach. When I say it fits nicely in between, I don't mean that their measurement is in agreement with the other folks. I mean that the time period they could study is deeper into the past than the supernova folks. And not all the way back to the beginning of the universe. So in the universe timeline is sort of a nice like probe of the sort of middle section.
43:07 Kelly Weinersmith? Okay. All right. Cool.
43:08 Daniel Whiteson? And it's very cool study. What they're looking for are these incredible echoes in hot plasma from the early universe. And so, you know, you imagine the early universe, it's very hot, it's very dense. You got like protons, you got electrons, you got photons, you got dark matter in there. It's incredible. Mostly radiation dominated, like the early universe, mostly photons, like a billion photons for every electron. It's hard to wrap your mind around because we're definitely not photon dominated now. And that's because the very early universe made matter and antimatter, which then mostly annihilated into photons. So that's why it's bathed in photons. Anyway, these photons flying around and they're pushing on the electrons
43:50 Daniel Whiteson? and on the protons, right? Because that's what photons do. They interact with charged particles. And so this plasma made of protons and electrons is opaque to the photons, pushing on them, just like the light inside the variable stars, creates this outward pressure, creates these density waves, right? But dark matter, on the other hand, is pulling the stuff in because of gravity. Dark matter ignores all this pressure, photons fly right through it. So you have these two different effects. The photons pushing out and the dark matter pulling in and you get these density ripples.
44:21 Kelly Weinersmith? So we know that dark matter does act on electrons and protons. Is that what you were saying?
44:25 Daniel Whiteson? It acts on electrons and protons only through its gravity, but there's a lot of it in the early universe. It's very dense. So this does have an effect.
44:32 Kelly Weinersmith? Okay.
44:32 Daniel Whiteson? And so these density ripples are moving through the early universe at a really incredible rate. You know, the speed of sound depends on the density of something. So for example, speed of sound through air is pretty high, but it's tiny compared to the speed of light. But the speed of sounds through water or through the earth is faster, right? The speed of sound through steel, that's why you could like feel a herd of buffalo coming through the ground before you heard them salivating or crushing you, for example.
44:59 Kelly Weinersmith? They're herbivores, I think, Daniel.
45:02 Daniel Whiteson? They would still crush you.
45:03 Kelly Weinersmith? Yeah, that's true.
45:04 Daniel Whiteson? Anyway, in the early universe, it's so dense that the speed of sound is about half of the speed of light.
45:10 Kelly Weinersmith? Wow.
45:10 Daniel Whiteson? Incredible density. So these pressure waves are shooting out at half the speed of light. But then at some moment, the universe is expanding and it's cooling. And so protons and electrons come together to make neutral hydrogen, right? And now they are transparent to those photons. So photons are flying through them instead of pushing on them. And that's a little bit of quantum mechanics right there, you know, because a free electron, when it's just flying around. Not an hydrogen atom, same with the free proton, can absorb a photon of any energy. It's just like, no big deal. It's only when you can find them that you get the energy levels, this quantization of the energy levels, that the electrons become so picky and say, I'm only going to absorb a photon of this energy or of that energy.
45:53 Kelly Weinersmith? Posh electrons.
45:54 Daniel Whiteson? I know, they become all snobby in their fancy houses. And now the universe is transparent, so the pressure stops. So you get these rings that were created, and then at some moment they get like frozen in, because they can no longer change, there's no more outward pressure, right? So you get these high density regions in the universe. You might think, why do I care about early universe high density regions? Well, dense regions in the early universe lead to structure in the later universe. Like, why do we have a galaxy here and not over there? Because 14 billion years ago, there was a little bit higher density of stuff. And then gravity took over and gathered that together into a big blob of stuff that became a galaxy. So what they do in this experiment is they don't look at the early universe plasma, they
46:37 Daniel Whiteson? look at the distribution of galaxies in the universe. They just like make a huge map of galaxies and they look for rings. And they find these bubbles, these same literal bubbles, you can see them in the universe. It's incredible, I know. Do you have this like foam of galaxies where the galaxies are mostly on the edges? And then you also have galaxies in the middle, but there's a higher density along these bubbles. So incredible. So much cleverness here to see this early history of the universe. So around 20 years ago, we saw these first evidence that like we could actually see the baryon acoustic oscillations from the early universe as it transforms into the structure of the universe today.
47:17 Daniel Whiteson? And the recent measurement by Daisy is like a super big camera that looked at more galaxies than anybody ever had, measured them deep into time, deep into the history of the universe, all the way back to like 300 million years. So you can see this structure early on, you can see it later. And because it was sort of a fixed yardstick, you can measure the expansion of the universe by seeing the size of these bubbles over time. It's sort of like, you know, a standard candle again. Recently came out with the results and this fills in a gap, right? We saw the early universe measurement, we saw the pretty late universe measurement. This sort of like the middle age universe measurement. People wondering like, what's it gonna say? And is it consistent with the dark energy density that's like constant in space? Or what's the behavior of it?
48:02 Daniel Whiteson? And so the results, very confusing.
48:04 Kelly Weinersmith? Oh no, I thought you were gonna give me a clear answer, Daniel.
48:07 Daniel Whiteson? No, unfortunately, or excitingly, it's in chaos. So they see something that doesn't look like dark energy is constant in space. The expansion of the universe that they measure is not consistent with a dark energy that just has constant density and therefore grows at a certain rate as the universe is expanding. And you might think, okay, that's cool. The Hubble tension already told us that, right? Yes, but the Hubble tension told us it was different from constant in a different way. The two are inconsistent. This new measurement from the Baryon Acoustic Oscillations is inconsistent with constant dark energy and inconsistent with the Hubble tension explanation.
48:44 Kelly Weinersmith? Oh no!
48:45 Daniel Whiteson? Yeah, so this sees dark energy as weakening. According to Daisy, dark energy is getting weaker over time. The Hubble tension needs something that's getting stronger at the end of the late universe. I mean, there are other ways to explain it as well, but it's not like, oh, this nicely clicks together with this other story we already were getting hints of from these other experiments. So I talked to a cosmologist in my department here, Kev Abazajin, and he calls this chaos cosmology. Something basic is wrong, you know? And that's fine, it's early days. We've only been studying dark energy for a couple of decades, right? So like, of course, we're still learning really basic things about how it works. And what we've done is apply a very simple model.
49:26 Daniel Whiteson? Take the cosmological constant, use it to describe dark energy, see if that works. Amazingly, it kind of worked for 20 years until we made more and more precise measurements. Now we're saying that doesn't really work and we don't have a better framework right now. We don't have an explanation that can explain these recent results from Daisy, the Baryon Acoustic Oscillations, and the Hubble tension that we've been puzzling over for the last 10 years or so. So there's a lot of exciting discoveries to be made.
49:53 Kelly Weinersmith? Does this mean that the universe is not? Not necessarily expanding anymore, or just that our understanding of the mechanism is not good?
50:03 Daniel Whiteson? The universe is definitely expanding.
50:05 Kelly Weinersmith? Okay.
50:05 Daniel Whiteson? We don't really know if it's accelerating at the same level as we thought it was.
50:10 Kelly Weinersmith? Okay.
50:11 Daniel Whiteson? And we don't know how that acceleration is changing, right? So it's sort of like another derivative there, the jerk of the universe, if you will. And that says a lot of important stuff about the future, right? The previous view, the simple view, the cosmological constant, that dark energy was constant in space and therefore increasing as a fraction of the universe over time. That painted a picture of a universe torn apart by dark energy. Because as things get further apart and dark energy takes over, then they just move apart faster and faster and faster and faster. And the future of that universe is a bunch of galaxies that collapse into black holes that are just like incredibly far away from all the other black holes.
50:51 Daniel Whiteson? That would be the future of the universe if the cosmological constant was the thing causing the accelerating expansion. But if Daisy is right and dark energy is somehow weakening, then that acceleration is not necessarily going to keep ramping up. And it could be that it slows down and it could even turn around. We could be heading for a time when you wish you had purchased Daniel's extra premium Dark Crunch insurance policy.
51:16 Kelly Weinersmith? You con man.
51:18 Daniel Whiteson? That's what this whole episode is just a grift for me to sell you my insurance. Look, I promise you, Kelly, if the universe collapses into a black hole, I will be there with like a cabin and goats and whiskey and whatever you need to survive the end times.
51:32 Kelly Weinersmith? And pigs and geese.
51:33 Daniel Whiteson? Exactly. We'll put that on your policy, no extra charge.
51:36 Kelly Weinersmith? Oh, thanks, Daniel. I love a good deal.
51:43 Kelly Weinersmith? A lot of information came at me today, so forgive me if I'm totally getting this wrong. But I thought the explanation for why the galaxy has rings depended on our understanding of dark energy.
51:53 Daniel Whiteson? Dark matter.
51:54 Kelly Weinersmith? Dark matter. Oh, I'm always mixing those two up.
51:58 Daniel Whiteson? Dark matter is providing the gravity to pull it back and to make these things oscillate. Exactly. That's why it's called baryon acoustic oscillation. And then it's frozen in time, one certain moment in those oscillations, because the photons can no longer push on that stuff because it becomes neutral. Yeah.
52:14 Kelly Weinersmith? Got it.
52:14 Daniel Whiteson? Dark matter, dark energy. I know, it's also dark. It's a dark universe out there.
52:18 Kelly Weinersmith? Yep, yep. Okay, so now what do we do? We've got this third way of measuring it and we're even more confused. Do we just look for a fourth way to measure it?
52:27 Daniel Whiteson? Yes, and of course, we're gonna work on that. But also, we have some theoretical work to do. We need to understand how you could describe what we see in a way that's consistent and makes sense across these different experiments. That's why it's so important to do these measurements in so many different ways, because we need to unravel like, well, what assumption are we making in this one? The same way like the Greeks made the wrong conclusion about the solar system because they had one wrong assumption that the stars were pretty close. Maybe there's a basic assumption about the universe we're all making that's leading us to miss the obvious explanation for what's going on here. And podcast in 5,000 years can be like, ha, ha, ha, they didn't realize, I don't know what it is that makes it so obvious why they're seeing what they saw.
53:10 Kelly Weinersmith? Well, I hope we figure out immortality so that you and I could be having that discussion. We'll look back and be like, oh, we were so silly when we were young.
53:19 Daniel Whiteson? Life insurance policy is basically like immortality insurance. Anyway, I'm not selling any insurance. I'm just hoping that everybody out there enjoys the mysteries of the universe. We live in a very turbulent time when we don't understand the universe and we're constantly getting these updates that remind us that there are huge discoveries to be made. So aspiring young cosmologists out there, don't worry, there are lots of things for you to figure out.
53:50 Kelly Weinersmith? Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you.
53:55 Daniel Whiteson? We really would. We want to know what questions you have about this extraordinary universe.
54:01 Kelly Weinersmith? We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you.
54:08 Daniel Whiteson? We really mean it. We answer every message. Email us at [email protected] .
54:14 Kelly Weinersmith? Or you can find us on social media. We have accounts on X, Instagram, Blue Sky, and on all of those platforms, you can find us at D and K Universe.
54:23 Daniel Whiteson? Don't be shy, write to us.
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55:01 Speaker 01 That sound? That's pro squash. Some of the fastest rallies in sport played inches from the crowd. The National Squash League is back in full swing for season four. The sport is heading to the 2028 Olympics for the first time, and you can see it up close right now in a city near you. Watch every match for free on YouTube. Search National Squash League and hit subscribe or follow live updates on Instagram at N S L dot sport.
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