How fast is the universe blowing up?

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Speaker 1: Hey, Danny, does the universe just blow your mind? Oh? My god. Constantly I feel like I can't crame enough universe into my brain. Is just excluding what's the most mind blowing thing about the universe in your opinion? I think the thing that drives me the craziest is that the universe itself seems to be blowing up. The universe is blowing up. You mean like it's going viral on social media? Probably hashtag universe, hashtag everything in the universe Twitter account is lit. No, I mean that it's getting bigger, and the speed at which the universe is getting bigger is also getting bigger. Hi am pre hammack Tunis and the creator of PhD Comics. Hi, I'm Daniel Whitson. I'm a particle physicist, but my brain is filled with crazy ideas about space, time and particles. So welcome to our podcast Daniel and Jorge explain The Exploding Universe, a production of My Heart Radio, in which we try to take the entire universe, everything in it, and squeeze it down into this audio connection to you, downloading it into your brain so that it blows up your gray matter. And so are we here to try to blow your mind a little bit of physics at a time, you know, just a little bit of physics each week, twice a week, and hopefully your mind is getting maybe bigger, may be more connected to this giant universe we have out there. That's right, The universe is out there, and we think it's for everybody understanding. This incredible place we live in shouldn't just be the province of cutting edge scientists. It belongs to everybody, and that wonderment, that amazement should be accessible to everybody out there. And so our goal is to make sure that you actually understand the way the universe works and what science does and does not know. Is the universe for everybody? Daniel, I don't know about that. I mean, all solar systems matter, man, All solar systems are made of matter. Yeah, that matter matters, no, But I think that the universe is for everybody. You know, you don't have to be a scientist to look up at the stars and wonder how does this whole crazy universe work? Or to look down at your feet and wonder about the particles that you're made out of. And everybody deserves an explanation. And you know, science is mostly something that's publicly funded, it's put on by governments, it's of the people, by the people, and for the people. And so this is the for the people part where we try to disseminate what science has learned to everybody out there. Yeah, for sure, I think definitely the universe itself is definitely of people and with people in it, and for the people. Yeah, there's a lot of prepositions you can go for there. I just hope it's not through people. You know, we don't don't want to go around people are over people, what do you mean? I think the universe is all those things, the universe going through me right now. Well, the goal of this podcast is to get the universe into people. Well, we're trying to um talk about the universe and just kind of help everyone wrapped their heads around this incredible and complex and really big universe, right and possibly getting bigger. Yeah, And this whole idea of the size of the universe is something which is very very modern and people have been looking at the stars for a long time. People have known about other planets, people had the idea that there are other stars out there, But it's only been a hundred years that we've known that there are other galaxies, and that those galaxies are moving away from us, and that the size of the universe itself might be expanding. So it's very recent in human history that we really have any understanding at all of the entire context of our lives. Yeah, that's wild, Like a hundred only a hundred years ago, we thought it was just us right, like us and the stars around us, and maybe that's it. Yeah, hundred years ago, people thought it was a bunch of stars hanging in space, and it's just sort of been like that forever. So most humans who ever lived had the raw long understanding of the entire universe. Only the people who were awake, alive and listening to this podcast have any sense for the actual context of their lives. Yeah, it's just a small error, you know, just a few Brazilian whatever. And the thing I love about that is it suggests that there might be other enormous contextual errors that we're making, you know, basic assumptions we have about the way the universe works that are just wrong. That in a hundred years some future podcast will be smirking at and twortling at our ignorance, being sarcastic about while eating bananas precisely. Do you think people back then a hundred years ago I thought the universe was a finite size, or did they think it was infinite, or did they think it had a size but just not as big as we know to be right now. I think about a hundred years ago, before Hubble, for example, people thought it was just a bunch of stars and it was finite, and it just sort of a bunch of stars hanging in space. You know, imagine like a single galaxy, whether or not space itself went on beyond the edge of that galaxy. I think there was a lot of debate there, but I don't think people ever imagine that there could be like an infinite number of stars. Well, and so that's the topic for today's podcast is it's about the size of the universe and more specifically, how that size is changing, because the size of the universe is changing, right, that's right, And this is something that Hubble himself began. Hubble is famous not because of the Hubble space telescope, which was named after him, but because he's the guy who discovered that the universe is expanding. The things that are far away from us are moving away from us. Really quickly, where like one raise in an expanding loof of raisin bread. And the thing that's amazing is that we're still learning about that. We're still learning about how fast the universe is expanding, and we're still not sure of it. We still don't really know the answer. Really. We think it's expanding faster and faster, but we are not quite sure how fast it's it's it's growing, or kind of what's causing it. Right, that's exactly right. And people measure this stuff and there's lots of different ways to do it, and those measurements they make don't quite agree. And so that's what we're gonna be talking about in today's podcast. Yeah, So today on the podcast, we'll be tackling the question how fast is the universe blowing up? Yeah? And this is a really fun question and what I've been tracking for a while because different teams of scientists are trying to measure this expansion of the universe in totally different ways, and for a while they sort of agreed until recently their measurements be getting more and more precise, and now they're not agreeing that well, And then I got a question from a listener somebody who wrote in to ask about it, and I thought, all right, it's time to do a podcast on it. So here's a question from Mike and Madison who wanted to know. Hi, Daniel and Jorge. My name is Mike. I'm an engineer from Madison, Wisconsin. Could you guys please explain what we're doing to try and solve the unmatching hubble constant mystery? Also, why does it have to be a single constant? Couldn't the universe be expanding asymmetrically or at changing or different rates depending on where you are in the universe. Also, I'd like to give a shout out to my uncle Jim McClain for introducing me to this amazing podcast. All right, thank you Mike and Madison in medisine. I like how it's alliterative. Yeah, and in a moment we will dig into what the hubble constant is and how it's connected to the expansion the universe and is it a constant after all and all that kind of stuff. Yeah, because it's kind of a very technical question. Like at first I heard this question, but I didn't really even know what he was asking about. Yeah, And the best way to think about it is that the Hubble constant is just one way to understand how fast the universe is accelerating. It sort of helps determine it. But of course it's confusing because it turns out the Hubble constant not actually a constant, so it's an un constant constant. We are constantly messing up the names of things in physics. We are constantly throwing out the dictionary, it seems. You know, if we just redefine the meaning of the word constant, then it's a constant, right, and then we'll redefine the meaning of the word redefine, in which case we'll be do this all the time in physics right where we have particles that spin but it's not really spin, you know, we have particles with flavor, but they don't really taste like anything. And now we have constants that are not really constant. It's like a whole new language. It's like, I feel like you're doing on a purpose, Daniel, just to confuse us, and um makes us, UM makes us wonder about this crazy No, no no, no, we I'm going to use the Donald Trump defense. It's out of pure incompetence. We're not trying to confuse anybody. We're just not capable of doing any better. I see that's a defense name after him, but not necessarily something he does. That's right, But I was wondering are people paying attention to this? Does everybody know what the Hubble constant? Is? Are they aware this tempest and a teapot about how fast the universe is expanding? Or is that something just scientists are thinking about? Yeah, how many people out there even know what the Hubble constant is? So, as usual, Daniel went out there into the streets and as random strangers if they knew what the Hubble constant is, think about it for a second. Do you know what the Hubble constant is? And if somebody asked you on the street to define it, would you be able to give an answer? Here's what people had to say. Something to do with the way things around its space. I guess I don't know. Something do the Hubble telescope? I don't know. It's the only thing that I know that is Hubble esque, like a mathematical equation or something. I feel like, something about how the stretching of the universe that's do with gravity or something. I mean, does it have to do with that one Hubble and like a red blue shift or anything or not. That's that w the extent that I would know, no idea. Have you heard of Hubble? No? I guess it has something to do with lights and the stars and space. The scale you're getting there. Yeah, it's a scale constant of lights through three dimensional space, and it's like a cost bar jegal constant um. Doesn't have to do with the size of the universe. I think that's all I can get from my memory right now. I've heard of Hubble. It's like the telescope, right, And I'm not sure what the Hubble constant is, all right. I feel like some people knew a lot about it, but a lot of people didn't know anything about it or had heard of it. Yeah, and some people were totally wrong. But I love these answers. You know, some people think it has to do with the Hubble space Telescope, which I guess indirectly it does, because you know, the space telescope was named for Hubble, who discovered this thing and and quantified. They've done a lot of really good branding on the Hubble telescope, you know, like it's a thing. People know what it is, and because when most people associated with the name Hubble. Yeah, the Hubble pr team has done a good job. Hey, you know they produced these Instagram ready images all the time. They're beautiful. You know, you just google Hubble and you've got a lot of really gorgeous stuff to look at. Let a Hubble bubble up. Yeah, you know, particle physics service Umble doesn't produce as much like pretty pictures that you can look at and say, oh wow, look at that amazing thing out there in the universe, because it's harder to visualize tiny particles. So from that point of view, astronomy definitely has the lead over particle physics. Well, I I am definitely in league with all of these people on the street. I don't really know or have a good idea of what the Hubble constant. Maybe. Up until a few years ago, I never even heard it. I mean I heard of the Hubble telescope, not the Hubble constant. Really, do you remember the moment you learned about the Hubble constant, probably like five minutes after meeting you. Then I do bring it up pretty quickly in conversation. Hi, how's it going, how's the weather. Let's talk about the Hubble constant. So it's it's not related to the telescope. This telescope was named after Edwin Hubble Um. But Hubble in his time did a lot of amazing discoveries, and one of them was this idea of a constant in the universe. Yeah, precisely. The Hubble constant is related to the Hubble telescope, and we actually do use the Hubble telescope now to help nail down the Hubble constant, which is sort of a fun little loop there. Yeah. Of course Hubble died well before the space telescope launched, But you're right, he was the one who figured out that the universe is expanding. Right. Do you think he named the constant after himself or was it named for him? Oh that's a great question. Um, I have to go back and look at the paper. Now we refer to it as H zero, you know, H for Hubble and zero for constant. But I don't know how the Santa Claus constant is. What if you should have called it? Um No, But I don't know if he called it H in his paper or if he just observed this. The breakthrough the heat provided is that he figured out a way to measure the distance to really far away objects. You remember, we had a whole podcast about how we measured the distance to stars. It's tricky because you don't know when you look at a star if it's really bright and far away or really close and kind of dim. So you have to know the distance um in other way. And he was the first one to figure out a way to measure the distance to far away stars because, as we talked about in that podcast, it's really hard to tell that distance. I mean from here from Earth, things just look like little pinpoints of light and they can be really far, they can be really closed. You don't really know, right, So Hobbi used these really cool stars called Cepid's now another astronomer. Hen we had a Levitt had earlier discovered that there's a way to relate how fast these stars pulsate to how bright they are. And we want to say thank you very much to Marcus Pustle for raising this issue and reminding us of Henrietta Levitt's work. Apologies that we neglected to mention her contributions in an earlier version. So if you know how bright these stars are supposed to be because you can tell how fast they're pulsating, then you know how far away they actually are by measuring their brightness here on Earth. So building on Levitt's discovery, this gave Hubble a way to estimate the distance to those ours. And then that's a moment he made an incredible discovery that some of these things were super duper far away. He's like, Okay, now I have a way to measure the distance to these stars. What are the numbers? The calculation? That's what it sounded like. And they had calculators back there were mechanically and it's probably turned cranker. Maybe somebody was shoveling coal in the side of his calculator. Um no, I guess I probably had a room full of people doing math paper. Here's the sound of it. There's my dramatic recreation of his calculation. But he had this moment of discovery, developed this new tool, a way to understand how far away things are. And the numbers he got were crazy, Like the numbers he got were like these can't even meet inside the galaxy. And that's what made him realize that some of the little dust he was seeing the guy weren't in our galaxy. There were other galaxies far away. So he gave us this ability to understand how far away things were and gave us the first view outside of our galaxy into deep deep space. That's the first he did, was he expanded our idea of how big the universe was and how far away things were. But at that time, I think a lot of people, most people thought that the universe was kind of fixed, right, like it wasn't. Maybe he figured out how big it was, but at the time, most people, I think thought the universe wasn't changing, like it was fixed. Yeah, precisely. Once he was able to know how far away stuff was, he could also measure how fast it was moving away from us, and then he made this plot. He's like, only maybe just plot everything in terms of how far away it is versus how fast it's moving away from us, And it just sort of fell in a line. So the farther away something is the faster it's moving away from us, and that the slope of that line is the hubble constants the ratio between how far something is from us and how fast it's moving away from us. Because that's a weird concept. I think. I think that can you imagine a universe getting bigger? It's kind of not intuitive to think that how fast things are moving and way from us would change. Right, Like if you think of when a grenade explodes out in space, you know, all the bits are moving away from each other, but they're sort of moving at the same rate. They're not moving faster and faster the further out you go in the explosion, right precisely. And the reason you shouldn't think of the universe as a grenade is because of grenade, the explosion comes just from the center. Is that one explosion, and then everything is just getting pushed from there. But the universe's expansion is totally different. It's much more like raisin bread than like a grenade. When you cook a loaf of raisin bread, it doesn't just expand from the center. Every part of the bread is expanding, So all the raisins are moving away from each other. Everything is stretching at the same time. Even the stuff that's way out there is also stretching. Yeah, if you wanted to mix the metaphors, you'd have to, like have a grenade bread loaf of bread that's grenades, you know, that's expanding all the time, A bunch of tiny little grenades. I guess in the end bread is expanding because of all the east. You can think of the east is like microbial grenades. It's like it's always exploding everywhere, and oh, the stuff that's really far away has a lot of yeast between here and there. And so it's the stretching and the expanding compounds, you know, like it's it's getting bigger and bigger and bigger, bigger and bigger and faster and faster. The further away from you you go, precisely between us and things that are far away, there's more space, and so there's more space to be expanding, and so the velocities are larger. And then you're saying that the Hubble constant, it's is what tells us just how fast that's happening, Like, is the raising bread crazy with it's some kind of crazy yeast or was this some kind of you know, dull, old, kind of mild, timid east which is expanding our raising universe a little bit slower. Yeah, And so the Hubble constant is expressed in terms of velocity per distance. For every light year you go, how much faster are things accelerating away from us? All right, let's get into the details of this constant, and let's get into um these apparent controversy about what that constant actually is and how it's changing and why it's changing. But first let's take a quick break, al right, Daniel. So the universe is getting bigger, and the rate at which it is getting bigger is getting bigger itself. And so this idea of the Hubble constant, it's something that tells us how fast it's happening. And the thing that would have blown Hubble's mind is that this expansion is not constant. You know, Hubble, imagine, oh, things are moving away from us at a certain rate, and if you want more expansion, you just need a larger space. And that's cool. But it's only twenty years ago we realized that something else was happening as well, that this expansion wasn't just continuing, but it was actually accelerating. So the Hubble constant is not constant. In time. As the universe is getting older and older, this expansion is sort of picking up speed. Yeah, It's like Yeese is going into overdrive. Yeah. And so that's why the Hubble constant turns out to not be a constant. He thought of a constant. He was just measuring it in one snapshot, but it turns out that it's actually changing. Do you think at some point maybe you'll consider changing the name of it so that you don't have to caveat it is the constant that's not a constant. There's a movement now to call it the Hubble parameter, and I think in most of general relativity they call it the Hubble parameter. But there's also this, there's a Hubble constant which has historical value to it. And so it's going to take a while. You know, we're a hundred years in. Give us another hundred years. Maybe you'll find the right name for you. But it's maybe it's it's more like the Hubble rate. Maybe would that be a better name, like the Hubble rate of expansion of the universe. Yeah. Well, in the end, really I think it's best to connect it to the dark energy fraction of the universe because the thing that's causing the expansion is this thing that's dark energy. Right, it's only twenty years ago we discovered that the universe is not expanding constantly. It's a spending at an accelerating rate, which means that every year it's getting bigger. Faster and faster. And we did this by making another breakthrough by looking even further into the past and into the far universe, by finding these supernove but that we could use sort of in the same way that Hubbo use those sthords to extend our distance ladder even farther. And that told us that this acceleration of the universe started about five billion years ago. And that's what we call dark energy. We say dark energy is some weird, mysterious thing which started dominating about five million years ago, and it's causing the universe's expansion to accelerate. You're saying that, um, the Hubble constant is not a good name for it, and so the solution is not to change the name, but to call it something else mysterious. Well, I think it's to dig into the source of it to understand why is the universe expanding? And I see, let's not worry about the name. Let's focus on what's making the universe get bigger and substance over style, right, that's my motto, because I certainly don't have much style, so I gotta go over a substance. There's I think there's a physics style like it's a thing, isn't it. You're either digging for compliments or you're baiting me into a trap here. I can't tell which one maybe, But the hubble constant, I think it's it's interesting to dig into the units it has, because you were saying earlier the hubble constant, which is not a constant, but I guess that's it has a value right now, which is, you know, kind of around seventy kilometers per second per million light years, seventy kilometers per second per mega parsek mega parsic, which is sort of like a distance, right, Yeah, parsek is a distance, even though in Star Wars they use it as a time like didn't Han Solo do the kettle run in eleven parsecs or something, which makes absolutely no sense. He ran tens and ten or something like that. UM, and those units are sort of hard to understand, so I transformed it to another set of units that makes more sense to me. It's forty six thou thousand miles per hour for every million light years, so stuff around us is moving away from us at forty six thousand miles per hour. For example, UM, if you move a million light years away, things are moving away from us another forty six thousand miles per hour. Things around this are moving away from us at forty six thousand miles per hour, but things a million light years from here are moving at what is it, ninety two thousand miles per hour, And if you go another million years further out, you add another forty six thousand miles per hour that it's moving away from us exactly. And this is changing because as the universe expands, matter and radiation and all that stuff gets diluted, right, it gets thinned out. It's like fewer stars per cubic light year. But dark energy, dark energy does and dark energy is like a property of space. Every new chunk of space that's made has its own dark energy. So dark energy, we think, is probably constant in time while everything else is getting diluted. And that's why the universe started excel rating about five billion years ago, because it's about five billion years ago that dark energy became the dominant thing. It became of the energy density of any chunk of space. The emptier space is, the I guess, the easier it is for dark energy to expand. It is that kind of what you're saying, that like, as it gets emptier and emptier, it's easier for its expand and so it expands fast. Precisely, there's some complicated general relativity there. The expansion of the universe is controlled by how much matter there is and how much radiation there is, which tends to pull it together, and then also how much dark energy there is, which tends to push it apart. And so as matter and radiation get diluted away, dark energy takes over. And and that's assuming that dark energy is constant. That when you create new space, you get more dark energy, and so that's what's causing this acceleration. There's less gravity, I guess right, precisely. And so really what we're doing, we're measuring the Hubble constants. We're trying to get a handle on the dark energy. Like what fraction in the universe is dark energy? We'd like to know about that now. We'd also like you know about that in the future, like is dark energy going to tear our universe apart? And we're curious about it in the past, like the very early universe, what fraction of the universe was dark energy? How do these things all work? Because we just don't understand dark energy like at all and so we know that the Hubble constant or this kind of proportion of dark energy is getting bigger, which means the universe is getting bigger at a faster rate every second right now, which is a little alarming. But I think what you were you were saying is that there's some kind of controversy about just how much dark energy there is because we measure different ways, but they don't come out the same number. That's kind of what Mike was asking about, right. These two physical quantities, the amount of dark energy and the Hubble parameter, they're connected, and so we measure them together and lost to different ways. And when we do that, we measure these using different techniques, we get different answers for the Hubble constant. So that's what he calls the unmatching Hubble constant mystery, precisely precisely, And we do this a lot in science. We say, here's something we think we understand. Let's measure it three different ways and see if it agrees. If it doesn't agree, then that we have to go back and question one of our assumptions. It's like a clue that's something new is going on. So it's a really valuable way to do things to measure something in independent ways and look for a mistake, because one of those ways could be like flawed, right, And so you want to make sure that if you look at it from different angles, it all looks the same. Yeah, one of the techniques could have a problem with it, right, and you don't want that bias to change the way you look at the universe. But also your assumptions that you make when you say, like, these two different techniques should give the same answer, maybe one of those assumptions is wrong. If you're watching a thunderstorm and you say, hey, well, you know how far away was that flash? I'm going to make an assumption about how far away it is based on how long the difference between when the light comes here and the sound comes here. You know, and somebody else makes the same measurement somewhere else, do they get the same answer? If not, that, you know, there's something wrong with your base assumptions. And so you want to make multiple measurements and that helps you check those basic assumptions. You kind of want to double check if you're going to make claims about the universe and the future and how big it is. Yeah, I mean these are grandios results. Yeah, absolutely, you definitely want to get this stuff right. Okay, So there's two ways to measure the Hubble constant or I guess the amount of dark energy in the universe, and they don't agree. So so what are these two ways? Well, the first one is just looking at the distance sladders, like how far away is stuff and what is its velocity? And we can measure the velocity by looking at how much the light from it is red shifted, meaning that if something is moving away from you at a certain speed, it changes the frequency of the light, like stretches the wavelength. And so we can tell how fast something is moving away from us by measuring its velocity directly, and we know how far stuff away is. So this is a natural extension of what Hubble did, and so we can use that basically just to measure directly how fast is stuff moving away from us? And that's I guess this is the most straightforward. I mean, I know it's not simple, but it's it's kind of the most direct way to measure the expansion of the universe. Is you just look at something really far away and you see how fast it's moving, and you look at something really close by and you measure how fast that move that's moving, and so that gives you the whole picture of how the the raising bread is expanded precisely. And the wrinkle there. The thing that makes it not trivial is that the stuff that's far away, we don't see what it's doing right now. We see what it was doing a billion years ago, for example, So we have to do some back calculation to account for the fact that some of the information we're getting is old. On the other hand, that's also a cool clue because it tells you how the expansion is changing over time. That's how we discovered that it was accelerating. We saw stuff really far away moving at a different speed than we expected. But isn't it easy to confuse the two, Like, if something far away is moving really fast, how do you know that it's a factor of the time that's passed in between or the factor of the distance it's from you. Well, we measure those two things separately, right, We measured the distance and the velocity totally separately. And once you know the distance, then you can calculate how long the information took to get here, and so we can sort of triangulate all that stuff. I mean, the best thing would be if we could get a complete snapshot of the universe at every time, then we could get all this crazy information and really triangulates stuff. You don't just get to wish for the data you want, You work with the data you have. I think the real triumph hear of physics here is is the acronym for this project. That's like such a great acronym. Yeah, this is called the Shoes Experiment Supernova each zero for equations of state. And I wish I'd been in that meeting where they were like coming up with acronyms of the white board to to explain this thing. I always wonder about that. They're like did It's like do they try really hard? Do they? You know what sacrifices must you make the science to get the perfect acronym? I don't know what kind of grammatical sacrifices must you mean? That's not even the best slash worst acronym we're gonna talk about today. Hang on for later on we'll be talking about all right. So that's one way to measure the universe. It's just measure things and how fast they're moving and how far our way they are but we can also um do something more interesting, right, Yeah, we can look back at the very early universe. And we've talked about this on the podcast about the surface of last scattering, the moment that the universe went from a hot, opaque plasma and cooled down and ionized and formed atoms that light could fly through, and the light from that plasma, it's called the cosmic microwave background, still flying around through the universe because after that moment, the universe became transparent. And so we get this light from the cosmic microwave background, and we look at it and we look at all the wiggles in it, and we can extract an incredible amount of information from these wiggles, and the most important thing that we pull out of that is we get the fraction of the universe that is matter and the fraction of that universe that was dark energy. But that's really far back in time, at the time of the Big Bank basically, right, Yeah, relatively speaking, it's three hundred thousand years after the Big Bang, and we're getting a sense for what was going on back then. And if you look at it online, look for the cosmic microwave background, it looks just like a massive it looks like a giant soup, and and so and so. But you're saying, you guys, have you know, special formulas that really look into the what the soup looks like. And you can firm that you can tell a lot of things like how much dark energy that there was at the big after the Big band, But how do you how did you extrapolate that to now? Because didn't you tell me that it's changing. Yeah, so we look at this bubbling soup and precisely the arrangement of bubbles and the size of the bubbles tells you a lot about the competing forces on the soup. And some of those forces aren't matter. They're pulling it together, and some of its dark energy that's pushing it apart. And the important thing to understand is we're not measuring the hubble constant itself back then there weren't even stars back then. Or measuring is how much dark energy there was. And you're right, we measured dark energy how much there was back then. And then what we do is we just assume that dark energy hasn't changed. The dark energy is constant, that that every unit of space has the same amount of dark energy now as it did back then. Wasn't there less space back there? So that means there's more dark energy now, there's more dark energy now. Um, it's a bigger fraction in the universe. Right now dark energy dominates the universe. But in the early days it was a tiny irrelevant bit player because most of the energy density was in the form of matter and radiation. But then as the universe expands, that dilutes and now matters like really spread thin. Oh wait, you're saying that, like a cube of like a cubic meter of space always has the same amount of dark energy, no matter if it was now or before when the universe was smaller. That is the key assumption. We are assuming that we think that might be the case. That's sort of the simplest idea. And what we're doing by measuring the Hubble constant or the expansion of the universe at different times, is trying to probe whether that idea is wrecked. And so, assuming that dark energy is constant, you measure what it was back in the time of the Big Bang, you propagate that forward. You can get a number for the Hubble constant, assuming of course, that dark energy is constant and that radiation and matter have just diluted. We don't know that dark energy is constant. We're assuming that. And if you assume that, then you get a number. And this number is different than the number you get when you measure the velocity of the stars. Yeah, if you look at the velocity of stars, you get a number like seventy four with an uncertainty like one and a half units of kilometers per second per megaparsic. But if you look at the early universe, you get a number like sixty seven point three with a smaller uncertainty like half. And so those two numbers, you know, they're different by you know, seven, and the uncertainty item is pretty small. And both of those teams have been working really hard to make their measurements more and more precise. And as the measurements get more and more precise, the numbers have not been getting close together. The errors have been getting smaller, but the numbers have not been changing because you know, as a as someone who's not a physicist, I would look at these numbers and think, oh, that's pretty good. Seventy four sixty seven, what's ten percent different? Government work, you're not good enough to make for engineering. But the key thing here is understanding your uncertainties, like how well do you know these things? And people spend a lot of time, like many many peach DTCs, understanding what are the uncertainties on our distance measurements to supernova or coming up with other ways to make these distance measurements to cross check, or understanding the uncertainties in the cosmic microwave background. And you've got to know those uncertainties so you know how well do I know this thing? Because if you don't know how well you know it, you can't answer the question are these two numbers in agreement or not? So a lot of the work goes into nailing down the size of these uncertainties to knowing how well you know something. So that's the mystery, then, is that we're trying to measure how much dark energy there is in the universe, which is making it grow bigger. And if we measure look at it one way, it's it says there should be seventy four of this dark energy. If you look at it another way, it says it should be sixty seven. And that's that bothers a lot. It bothers them because it seems really unlikely to be an accident. Like, if there really is one Hubble constant and both of these things are measuring the same number, then what are the chances of getting two numbers that are this far apart. It's like we've done into calculation, we do the statistics, and it's like one in ten thousand. So it seems really unlikely. A much more likely explanation is that there's something wrong, either something wrong with our assumptions or something wrong with one of these measurements. All right, well, let's dig into what could explain this mystery and what it means for the future of the universe and for you and for me, and but and for the people for whom the universe is for, but not the people for for whom the universe is not for. But first, let's take a quick break. All right, we have a disagreement in physics in the measurement of how much dark energy there is in the universe. And so how do you guys decide? Do you just fight it out? Do you get into a boxing ring or cage or something and you throw a couple of pencils in and see what happens? Are Yeah, it's too physicist grappling with whiteboard markers and calling each other's faces and stuff. Um, well, maybe one side as a chalk on the other side as a marker. Now, it's all actually very friendly, incongenial, and everybody wants to understand it, and it's sort of a good situation. You know, when you make one of these discoveries that two measurements you make of the same thing don't agree, it's a clue, it's a sign. And that's what we're looking for. We are trying to understand the universe, not just confirm what we thought. And so when the universe tells you that you're understanding is wrong, that's it's the first clue to getting new understanding. And so they're getting room and they try to think, like, well, what could explain this? Is one of us doing it wrong or is one of our assumptions wrong, And that's I think is the most exciting explanation. Right. Well, I have a favorite, Daniel, I don't know if you have a favorite. I like seventy four more than I like. I mean, like one of one of these measurements seems more direct to me, Like if you're measuring the speed of the stars directly through my telescopes. That seems a lot more direct than like looking at a picture of the universe fourteen billion years ago, and then it's replating it. Like, do you guys have a favorite? Do you think one of them in particular is probably wrong? Or what's the general feeling? Well, I like the one from the early universe because it's just so clean and precise, Like you don't need to know how far away anything is, or make some extrapolation from this kind of star and the other kind of star and sort of walk up the ladder. There's a lot of assumptions involved in those distance measurements. Whereas the cosmic microwave background so pure and clean and so much about it works. It's predicted and been confirmed in so many other ways that we have this model of the universe that just really holds together. It's hard to imagine how that is wrong. And so I like that measurement. I'm not sure why. It's maybe just an aesthetic thing. You do have a favorite. I do have a favor. I've just admitted on areas well. So there's some possible explanations that for what could be wrong, right, that's because something must be wrong if these measurements are not matching up. So what's what's a possible explanation. I think one of the favorite explanations of cosmologists is this thing called dynamical dark energy, the idea that dark energy isn't maybe just like a property of space and constant that for every cubic meter is basically the same dark energy, but maybe it's changing in time as the age of the universe. I have to say, holy cow, that's amazing, and that would help resolve because remember, we have this measurement from the early universe that's measuring one dark energy for action that gives you a Hubble constant, and these more recent measurements from nearby stars and supernovas that give you a more recent measurement. So one way to make those two things agrees to say you're not actually measuring the same thing. The thing you're measuring, it's is itself changing. So you're saying, um, one possible explanation is that the Hubble constant, which is not a constant, is actually measuring something that is not a constant. You constantly amaze with your understanding. That's kind of what you say. I feel like that's kind of what you're saying it's not only not a constant, but what you're it's measuring is not a constant. That's right. And to shroud our previous mistakes, we slap a cool label on it and call it dynamical, right, yeah, d D. And then, of course, you know another thing we do is to try to like get an unbiased third estimate, Like, let's come up with a third way to measure this and see if it agrees with one or the other two. Let's do this democratically, let's take a vote. You're saying, there's a third way now to measure this dark energy in the universe, and I think it deserves a Nobel prize right away, just in its awesome acronym. Well, it's an acronym that contains acronyms. So they call it the Holy Cow experiment, uh aged zero lenses in Cosmo Girl well spring, right, and so Cosmo Girl is the name of another experiment that stands for something else. And so these guys have used data from the Cosmo Girl experiment to try to measure the expansion rid of the universe totally independently. Wow, I mean that's just genius in acronym. It's like not only are you betting an acronym in an acronym, that you're betting a whole different project in this project acronym. And then if they discover something awesome, they get to shout, holy cow, we discovered it, Holy cow, holy cow did it. And this is another way essentially to measure how far things are away, and it uses gravitational lenses and says, let's say you have a really bright source of light, like a quasar, and then between you and that source of light is a is a big lens, like a big galaxy. Because remember, galaxies have a lot of gravity, and gravity bend space, so it can act like a lens. And what happens is then that quasar gets distorted and you get multiple versions of it arriving here at Earth because the galaxy between you and the quasar has lensed it. You know, sometimes you get like weird and duplication effects and a lens and so somehow that tells you something about how it's expanding. The university expanding, Yeah, because the different images take different amounts of time to get here, and these quasars flicker as you can watch these different images flicker, and by how much time there is between the flickering in one image and the other image, you can tell essentially how much space it's gone through. And so the delay between the two different images gives you a sense for how far away the original quasar was. Is it kind of like lightning and thunder like precisely you see it and you hear it, and you use those two things to figure out how far away the lightning was and how bright it was. Precisely, that's exactly the way we do it. And so this is a totally different way because it doesn't rely on supernova, doesn't rely on sephids or the other stuff. It's another way to make the distance measurement. And their measurement agrees with the supernova measurement, really with my favorite measurement, not you. I should have said it agrees with you. That was their announcement. Actually, holy cow agrees with cartoon. Holy Cow cartoon is nailed it. The only person surprised was the cartoonist. So so it's agreeing with one of the measurements, which is measuring measuring the stars themselves. And so then, um, doesn't that close the argument doesn't that you know, and the mystery it doesn't because remember they're measuring things at different times in the universe, and so this would have been problematic for the supernova measurement if it had disagreed, because they're measuring the same thing and sort of the same epic of the universe, and they really should This is like confirmation of the supernova measument. And but the early universe one from the cosmic microwave background is measuring something older, and so it could still be that they're both right. And the explanation is that dark energy is changing. Oh, I see, it's it's like there you could say that it's not wrong. It's just that a change between when I measured it and now. Yeah, it's like I didn't get the answer wrong on the test. I was just answering a different question. Maybe this tells us that this dark energy constant is changing or has changed since the beginning of time. Yeah, it could be. Um, there's a lot of things we can do to check the cosmic microwave background radiation measurement, and they've done all those checks and it all works out and it really seems very convincing. It's hard to imagine how they would get that number wrong. On the other hand, the supernova measurement now has independent verification from a completely different way to measure these distances. So it's hard to understand how that one could be wrong. So I think we're going to have to rethink our fundamental understanding of what's going on with dark energy. Right, Maybe dark energy not a constant after all. Maybe it's cool. Maybe we should never be assuming things are constant. You know, that's just sort of like the physics things like, don't call constants constance. We're constantly making that mistake. Well, um, it sounds then, though, that this mystery is getting resolved as we speak right now, So Mike and Madison stay tuned. It sounds like, as we speak, we're resolving this mystery. Yeah, and other stuff is gonna come online to sort of give us more pictures of this. We can use things like gravitational waves from neutron stars collisions to try to measure the distance to things. So that's gonna give us another measurement and hopefully that can peer further back in time than the quasars are. The supernova so we really need to do is get another measurement of dark energy in the very early universe. And so people have ideas for how we might do that, and gravitational waves might help, and so stay tuned. This cosmic mystery might eventually get resolved, and it might get resolved in a way that totally up ends our understanding of the entire universe. But I think one thing is here, which is the mind blowing part, which is that it's pretty clear. And now I guess three measurements that the universe is expanding, and it's expanding faster and faster, like this is not a theory anymore. No, that's for sure. Nobody, no reasonable scientists disagrees with that. It's even more well understood than climate change. Of scientists, that's right. All the scientists except the ones that go on Fox News, believe the universe is expanding and that expansion is accelerating. So I guess, yeah, the next time you can go out there and look at the night sky, just think about maybe the future. You know, in the future, things are going to be even bigger. The future is big. It's looking big, and it's also uncertain because if dark energy is changing, we don't know what's changing it, why it's changing, and how it's planning to change in the future. Is dark energy gonna get stronger and stronger. Is it gonna stop dissipate? Turn around, go the other direction. We really just can't predict the future because we have no understanding of this dominant source of energy in the universe. Alright, so stay two and thank you Mike for sending us this question. If you have a question about the universe or about something that you've always wondered about or read about, send it to us and we will try to answer it. Thanks to everybody who sends in their questions. Remember questions at Daniel and Jorge dot com is your fastest route to an answer about the universe. I hope you enjoyed that. Thanks for joining us. See you next time. Before you still have a question after listening to all these explanations, please drop us a line. We'd love to hear from you. You can find us at Facebook, Twitter, and Instagram at Daniel and Jorge that's one word, or email us at Feedback at Daniel and Jorge dot com. Thanks for listening, and remember that Daniel and Jorge explained the Universe is a production of I Heart Radio. 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