What is early dark energy?

00:00:08
Speaker 1: Hey, Jorge, is it still true that you get a lot of your work done late at night? Yeah? Yeah, you know, it's a little quieter. Nobody's calling you, nobody's bothering you. It's a little easier to, you know, get your thoughts in order. So like you do your best thinking when it's dark outside. Yes, that's right, it's like I have dark energy. Oh my gosh, I think you might have just cracked the mystery. Yeah, what do you mean? Maybe that's what's causing the universe to accelerate outwards. Cartoonists staying up too late? Oh, man, I think you're right. The universe just can't handle so many cartoons, so it has to grow to fit them all in. You are literally blowing it up. Those are some dark thoughts, man, I'll have to stay up late thinking about it. I am more handmade. Cartoonists and the co author frequently ask questions about the universe. Hi, I'm Daniel. I'm a particle physicist and a professor at U c Irvine, and I always thought of the two of us, I was more likely to blow up the universe. Well, you know, I blow people's mind. You blow up the physical universe. What can I say, it's a team effort. Together, we are unstoppable and potentially dangerous, also one of us more so than the other. And now we are here to blow your minds. Welcome to our podcast Daniel and Jorge Explain the Universe, a production of I Heart Radio, in which we do try to blow up the entire universe, but not to destroy it and make it unlivable, but so that we can zoom in and understand it in detail. We want to expand the whole universe as we expand your mind, so we can stretch it in and see right through it. We can mix our metaphors and we can pull things apart as we try to explain all of the craziness and the incredible universe to you. That's right, We try to explode your understanding of the cost moods because it is a pretty big and wonderful universe, and it's also getting bigger. It's getting bigger every year, and it's getting bigger faster every year, which means the size of the mystery, the scale of our ignorance is getting larger and larger every year. Literally, there is more stuff we don't know this year than last year. Yeah, because there is literally more universe today than there was yesterday, and tomorrow there's going to be even more universe, which means that maybe we'll never get to understand all of it. Daniel, Can we keep up with such an expanding universe? We can't think faster than the speed of light, But the universe is expanding faster than the speed of light, which I guess makes it hopeless, doesn't it? Why are we even doing this podcast? That's right, just give up, Daniel. You should become a cartoonist. Is that what giving up means? Giving up? Right? You're you know, upgrading. That's right. Stepping up to being a cartoon is stepping up, upgrading? Yes, that's what you meant to say. No, I think that even though it's unlikely we will ever understand everything about the universe, it's really all about the friends we make along the way, meaning the ideas that we captured, those bits of knowledge that we do manage to extract from the universe and digest, those moments when we pull back a layer of reality and say, oh my gosh, the universe is different from what we imagined. And one of the greatest of those moments is when we discovered that the universe was expanding, and that expansion was accelerating. Yep, the universe is just getting started. Apparently it's getting bigger, faster and faster each year. But do you think Daniel dead? You know, once you figure out the loss of the universe, you don't need to see all of the universe. You know, it's like you you once you figure out the basic rules, it's like you know, and they're done that. That's gonna be the title of my next paper. Actually it's called you know, They're done that. They're done that. Um No, Actually that's the name of a cafe my wife wants open. Being there done that. Oh nice. She's also upgrading tour from a biologist. She's in favor of everything being related. For black beans for dinner, coffee beans with breakfast, chocolate beans for dessert. It's a whole being themed, you know, being there, done that. But I think that there's never a time when we will not learn from seeing more of the universe. And the history of physics is saying, oh, we think we figured out how things work from the studies of this local neighborhood, and then we generalize me extrapoling, and we say, well, the rest of the universe probably works that way, and every time we go to check we find a surprise waiting for us. It turns out that what we learned was a special case because we were looking at only certain circumstances, only when things were really slow, or when things were really cold, or when things were really big. And every time we do turn over that stone and look at some part of the universe we've never seen before, we always do learn something new. So I think there'll never be a time when we say we're done looking around. Well. I think that's good news for this podcast, because that means we'll that we're run out of things to talk about. The stable gigsodes almost and going strong. In fact, it's it's accelerating. I feel like they're accelerating here, derated, which we're making a podcast episodes. Man, we should be doing ten a week at this point. But it is an expanding universe. It's getting bigger each year, and it's getting bigger at a faster rate each time. And it's all because of dark energy, that mysterious energy that's through's force. It's expanding space itself. It's not just making more room, it's growing room in the universe. That's right. Dark energy is the name we give to the observation that the universe is expanding, and that that expansion is somehow accelerating. The key there is the somehow. We call it dark energy because dark means we are cluelest. We don't understand what's going on. We don't even have a good idea for what could be doing it. We just have the observation that is happening and energy because it takes about two thirds of all the energy in the universe to make this happen. So dark energy is a good description for what's going on because it's also basically everything we know about it. I thought it was called dark energy because it doesn't like give off light like it we can't see it like we know it's there because we can see the universe expanding faster and faster, but we like it doesn't like give off a glow or doesn't like make sparkles. It doesn't you know, look like anything. That's what you call it dark, not because we're in the dark about it. It would be cool if it made sparkles. I think if a cartoonist head designed the universe, there would be sparkles or like, you know, action lines or something to show the universe is stretching. Right, every time you have motion in a cartoon, there's always some visual guide there to tip you off. Are you saying that cartoonists should be God's Is that what you're saying. I'm saying it's pretty clear that God is not a cartoonist. You know, conclude from that what you will. Well, maybe God needs to upgrade, just like you know, to everyone else. Step up there, dude, Yeah, grab a pencil, add some sparkles. You know, if the universe is a simulation and the engineers listen to our podcast, or if you're in the believer of a deity whoever is in charge this universe, I hope they do listen to our podcast so that they hear about all the questions that we have. But that is a name we've given it, the the expansion, the accelerating expansion of the universe, dark energy. But and it explains why the universe is it growing bigger and bigger each year. But it doesn't quite explain how we got here, right, You're right, it doesn't explain how we got here. Number one, because we don't even really have an explanation for how it works. Right, Like, we can add some energy to general relativity, and we can use that to expand the universe in our models and to accelerate that expansion. We can do that, but you know, we don't understand where that energy comes from. We have like no description of what that means and where it comes from and how it works and what its future is. And also recently we've discovered that even that picture, that simplistic picture that like maybe we just add a number to Einstein's theory of general relativity, that also doesn't quite work. It doesn't really match the story we see out there in the acceleration of the universe. This is growing problem with our understanding of how fast the universe is accelerating. Right. It's sort of related to the idea that the universe is maybe bigger than it should be, even with this idea of dark energy put into it. Yeah, like maybe the universe is bigger than it should be, or that it's not as old as we think it is. All these ideas are connected because our whole idea of how old the universe is and how vast it is, how big the portion of what we can see is depends on understanding how quickly it's been expanding. Understand how old it is. We unwind this picture, we run the clock backwards and see how long does it take to get back to the Big Bang singularity. So if our understanding of the expansion rate is wrong, that our understanding of the whole history of the universe, including its age, could be wrong. Yeah, it's a big mystery, which we'll talk about later. But there is sort of a concept that might explain all of this, and it has a very imaginative name. Right. Well, I was wondering, you know what a cartoonist called this idea. Well, we wouldn't call it anything. We just draw it. Right what it could be called sparkly dark energy. Everything should be sparkly sequence dark energy. But there is a concept that might explain the expansion of the universe in the beginning. And so today on the program, we'll be tackling the question what is early dark energy? Wait, that's the name, Like, that's the whole concept. Just put the word early in front of dark energy. That is the whole concept. And you know it's not just inspired by cartoonists staying up so late at night that it actually becomes early in the morning. I was gonna say, I'm more of a fan of late dark energy. So when I got an email from you at five am, is that when you're staying up super duper late, it doesn't cross over into early. It means I scheduled my emails. You noticed my emails always coming at five am? Exactly? Why would you schedule them for five am? That's still a bonkers time to send an email. Why don't you schedule them for eight am? Or is that for eight am on the East coast or something? You just got it, Yeah, eight am in an Eastern time, all right? But the question still stands, how late do you have to stay up before you count it as early or are you saying that no matter how late you stay up, if you're still up, it's still late from the day before. Boy, these big questions about the universe are just too confusing. I'm not sure we have a whole episode here to talk about my sleeping habits. I'm sure Aristotle had something to say on this deep philosophical question, But that is the concept that we're going to be talking about here today, this idea of early dark energy. And does that mean, Daniel, there's late dark energy. Late dark energy is the dark energy we have right now that we look around and we see because we are in late times of the universe. That doesn't mean that we think the universe is about to expire or at the end of its life. It just means like recent times of people talk about the early universe meaning the very beginning, and the late universe meaning right now. I see. So maybe we should be it should be called a current dark energy or something that's right. But one of the mysteries is how the universe accelerated in the very early days. Their disagreement about how quickly was accelerating in the very first few moments and how quickly it's accelerating now. And to solve those problems, people are working on ideas to change how quickly the universe was accelerating early on, I see, And will late dark energy at some point become early dark energy only when you go to bed right only Easter standard time. We should call it scheduled dark energy. Right there, you go just in time dark energy. But this is a new concept, early dark energy, one that physicists are thinking about and talking about, and so as usual, we were wondering how many people out there had heard of this idea of early dark energy. So Daniel went out there once again to ask people this question, and I'm eternally grateful to our cadre of volunteers to answer these questions for us so that we can get a sense for what people know and what people are thinking about. If you'd like to participate, please don't be shy right to us. Two questions at Daniel and Jorge dot com. So think about it for a second. What do you think early dark energy means? What does it do? Here's what people had to say. I am not certain, however, I guess that it is similar to the background radiation and that that it's sort of residue from the initial dark energy or the dark energy that initially expanded the universe. I mean, dark energy is probably what's responsible for expanding the universe and accelerating the expansion. I don't know what early in that context means. Was dark energy different in the early state of the universe. I don't know. Um well, as far as they know, dark energy has been increasing since the beginning of the universe. Maybe this is the dark energy that was still around, that was around at the very beginning that we might be able to find if we look far enough back with gravity or something. Probably dark energy is the dark energy from the original Big Bang or soon thereafter that it's all around today early dark energy. I have no idea what it actually is. I'm assuming it's the early stuff that comes to the party early, like maybe in the early part of the universe, kind of like the cosmic micrtive background. Definitely not my wife, that would be the late dark energy. I'm going to take a guess that early dark energy is essentially the same thing as dark energy now, just earlier in our universe, before it was the more dominant of the two between it and gravity. Well, I can think a guess that it's the dark energy right before the Is that just another word for inflation? All right, it's a pretty um straightforward I feel like everyone you said, it's it's dark energy, but early, except it's not my wife. She's late dark energy. That's my favorite answer. Wait, what was that answer from you? Because I know you're wife's days of late also working, right, that's true. Yes, she is on joge time, that's right, or a standard time. She needs to step up and become a cartoonist. Yeah, she's just she's just a pencil sketch. Away, but most people seem to be related to the early university. I guess you know, most people associate dark energy with you know, the universe itself and early I guess you know, you might think that it's dark energy that was there at the beginning of the universe. And it's confusing also because we know that there are multiple periods of expansion in our history of the universe. Like we talked about dark energy, which has been dominating the expansion of the universe for the last five billion years, although it's been around, is part of the mix since the very beginning, it's just sort of only taken over in the last five billion years. But we've also talked many times on the podcast about inflation, which is another period of rapid expansion in the universe in the first ten to the minus thirty seconds of the history of the universe. And as we said several times, we don't know if those are connect that are those different mechanisms for rapid expansion of the universe. Are they related somehow? We don't know, And so it's fair to try to lump these periods of expansion together and say, like, oh, maybe early dark energy, just inflation. That makes a lot of sense. It's kind of weird to think that the universe had basically growth spurts, right like you might think that a cold, random universe which is grow at a steady rate. But somehow the universe has had these like periods of expansion and slowing down too. And one of the key things to understand is that as the universe expands, the mixture that it has of matter and radiation and dark energy, that mixture changes as it expands, because, for example, as the universe expands, matter gets dilute. You have a certain number of particles in a box, and you make that box bigger than the density goes down. But that dilution is different for matter than it is for radiation, for example light. If you expand the box and stretch the space, light also loses energy because it gets red shifted. So radiation gets diluted more quickly, whereas dark energy doesn't get diluted at all. As you make more space and expand the box, you have the same amount of dark energy per unit space, and so the dark energy fraction of the whole universe goes up. So as the universe expands, these fractions change, and then that changes how the universe responds, and so it's this very dynamical system. Right, it's not just like the steady state thing hanging out because it's expanding. That expansion then changes how it expands. Yeah, that's wild. And it's well to think that the universe has things that it you know, conserves and keeps the same, and there's has things that keep growing, you know what I mean, Like matter seems to be constant and it's not increasing, but space needs to be, you know, growing out of the woodwork. Yeah, it's growing out of the universe work exactly. And as the universe expands, dark energy takes over and it makes it expand faster. And that's the source of this acceleration, which is crazy because it suggests the future is all dark energy dominated. Once dark energy is sort of in charge, it basically never let's go as far as we understand. And so it might be that the periods of the universe where matter we're in charge, or radiation were in charge, we're just like the first blips, you know, the first few moments of like a trillion year history. Right, the future is all cartoonists. You should join us now. It is inevitable, I see before it gets cool right as the universe gets colder and colder. But anyways, this idea of early dark energy is really interesting, and so let's break it down for people who are not familiar with this concept of even dark energy. So, Daniel, what is the regular dark energy? So the regular dark energy is a name we give to something shocking that we discovered about twenty years ago. Now, remember that, like only a hundred years ago, we discovered that there are other galaxies out there in the universe and that they are moving away from us, that the universe is expanding. This was sort of a big deal when it was discovered in like, you know, Einstein's universe that he imagined in general relativity was a static universe, no expansion, no acceleration. So the Hubble discovered that, oh, the universe is expanding. Things are floating out there. But people thought, okay, well, the universe is expanding, but probably it's gonna pull back. Probably gravity is going to be strong enough to pull everything back together and maybe make a big crunch. Or if not, then things are gonna gradually slow down due to gravity as they drift away from each other. So the question until twenty years ago was how quickly is the universe decelerating? How quickly are things slowing down? And then they went out and measured this, because, as we talked about earlier, you always got to go out and see what's really going on in the universe. And they discovered by looking at how far away stars were and looking back into the past to see how quickly they were moving away from us a long time ago, that the universe wasn't slowing down at all, that this expansion was in fact accelerating, that every year things were moving away from us faster and faster. So this blew everybody's minds. They're like, oh my gosh, not only is the universe expanding, but it's accelerating. What could be doing? And so they replaced that big question mark with a fancy phrase called dark energy, and that's about where we are today. Dark energy is just the name for this discovery for which we have no real explanation, right because without that observation, I mean, like you said, we would expect the universe to crunch back down again, right because gravity sort of doesn't give up, Like, even if things are super far away, they're still being pulled a little bit by gravity. And so eventually that acceleration that floors should bring everything back together into a little tiny dot. Right. It depends a little bit on the balance between gravity and the initial velocity. It's possible, because gravity gets weaker as distance grows, that we could have lived in the universe with no dark energy, that didn't crunch back down together, that just spread out forever, if there was sort of enough initial energy, just in the same way that you can, for example, throw a ball off the surface of the Earth at high enough velocity that it never ever comes back, even in the infinite future, has enough energy to escape Earth's potential. Well, so it was possible that things expanded out forever. But until we discovered dark energy, we never imagined that it could actually be accelerating outward, that things could be stretched out, that new space could be being made between those galaxies. Right, And so you gave that acceleration of the expanding universe that's expanding faster and faster. You give it the name dark energy because it must be some kind of energy, right, because you don't know what else it could be. We don't know what else this could be. One sort of category of explanations is something we call the cosmological constant. And this is just like a number that you stick into Einstein's general relativity equations and it reflects some sort of like potential energy of the universe. Remember we talked to the podcast about how the space is filled with quantum fields, right, all of space has fields, and it fields for the electrons, fields for photons, feels for all the different kinds of particles. And these fields can do different things. They can wiggle, which is like having kinetic energy the way like a ball rolling down the hill has kinnectic or g But the fields can also have potential energy energy stored in them because of their configuration, like a ball sitting at the top of a hill that isn't yet moving really fast. So if you have a field with a lot of potential energy, that creates this outward pressure due to this cosmological constant that comes from that field. And so we sort of like stuck this number into Einstein's theory and we gave it the right number to explain the expansion that we see. But we can't explain that number. It's just like, well, here's a number, we measured it. We don't know why it is that number or what field could be generating this potential energy. It's still a really big mystery, and I feel like that's kind of the reason you gave it a name, right, It was was sort of because it is sort of a term in your equations of the universe, in Einstein's equations, Right, like my kids are growing faster and faster, but I don't give a name to the idea or the phenomenon of them growing faster and faster. Or like if the universe he saw that it was growing faster and faster, you might just say, oh, the universe is growing faster and faster. But because you have these equations and you have you have like an actual term that kind of describes that or explain this expansion. You give it a name, and we can also calculate how much energy is necessary for this to happen, Like you find the number you have to put into Einstein's equation to explain this, and then you can calculate what sort of energy density is required in space to cause that term. Right, this cosmological constant isn't just like an abstract idea. It reflects like some potential energy stored in space itself, and then we can calculate that energy we can compare to other things and we find that, Wow, this potential energy is more than all of the energy stored in all the stars and the galaxies and the dust and all of the dark matter and all of the stuff we see in the universe. It's like more than twice as much energy as everything else in the universe. So it's a huge quantity. It's a big part of the recipe of the universe, right, And it's sort of energy because I mean, it's one thing to grow space, but it's also another thing to like push the things along that are in space. Right, Like if I was growing my house, they can just grow the house, it would also have to like move the beds on the table everything out further out towards the walls more, right, And so that that takes energy, right, It takes energy. Yeah, And what this means is that space has energy in it like this stuff because it's part of the potential energy of space itself. It doesn't get diluted when you make new space. When you stretch out the space between two galaxies, then you're creating more dark energy because you are creating more space. So it's really weird and counterintuitive. You know. It means that like energy is creating more energy. And for those of you who worry about, like, well where does this energy come from? Right, Like, how can you just do that? Doesn't that violate conservation of energy? Yes, and biolate it's conservation of energy, which it turns out, is not a deep and fundamental principle of the universe. Only a fundamental principle of universe is where space is static, where the rules are not changing as a function of time. But we don't seem to live in that kind of universe. We live in the universe where the space is changing and so energy is not served. It just like appears seemingly out of nowhere, right, But I feel like that's only true if you assume that dark energy is sort of inherent to space, you know what I mean? Like the dark energy also be you know, the result of something already in space or something that that is limited. A dark energy could be a lot of things. But what we do know is that it doesn't get diluted as space expands, so you can come up with something else that also has that property. The only thing we know about is vacuum energy is the potential energy that's the only thing that doesn't get diluted as space expand. Matter and radiation, those kind of things, they do get diluted as space expands. But then again, we don't really know what space is, right, and so maybe we need a whole new concept for how space works and what it means. Right. That's kind of what I mean is that, you know, you don't know where it's coming from, and like you said, it's changed over the years since the beginning of the universe. So even that statement that it is sort of inherent to space, we don't really quite know for sure, right, We definitely do not know for sure. We're in the part to this investigation where we're trying to just like build a general structure of an idea and ask, like what shape does this idea have to take? What can we fit into it? And we start with what we think are the simplest ideas. You know, maybe it's like this, maybe it's like that, and if that works, great, and if it doesn't work, then we need to add bells and whistles or come up with new ideas. We're definitely in the very early days. Yeah, and even though we are still only I guess inventing the general shape of the general idea. There are some problems with this general idea, ones that have to do with the beginning of the universe, and so let's get into what is the problem with dark energy, But first let's take a quick break. All right, we're talking about dark energy, and I guess there's a problem with dark energy. I mean, it's not just making the universe bigger and bigger, faster and faster, which eventually lead to the heat death of the universe, the universe dying out of basically order. There's also some problems with it being able to explain the early universe exactly. What we've tried so far is saying maybe that there's just a number we can stick in Einstein's equation, that like every unit of space has a certain amount of dark energy. And there's a few different things going on here that it's important to sort of unravel. One is that, like, we assume that dark energy is constant, but when we say constant, we mean like any chunk of space has the same amount of this dark energy, this potential energy, this cosmological constant. But as the universe expands, even if every chunk of space has the same amount of dark energy. Dark energy is fraction of the universe itself is growing, right, because everything else gets diluted away. So the dark energy fraction of the universe we don't think is constant dark energy. We thought maybe was constant like per chunk of space. That's what the cosmological constant is. That was the basic idea, like had a fixed amount of dark energy per unit of space, and then see if that evolves the universe through time in the way that we see it actually happening out there, right, And what you found is that it doesn't write like a number you have just taken now to the equation to explain the current acceleration of the expansion of the universe works. But then if you you know, you look back in time, that number doesn't work exactly. A cosmological constant doesn't quite describe the history that we see. And we can measure this in two different ways. We have late time measurements and we have early time measurements, and they don't quite agree. So the late time measurements are like, let's go out and measure the expansion of the universe. How fast are things moving and how fast are they accelerating? And these are the famous discoveries of dark energy, you know, for example, looking at type one A supernova, that kind of stuff. We can also look at the very very early universe and trying to measure how much dark energy was there back then, measure that using like the cosmic microwave background radiation. We measure that, and then we plug that into our simulations and try to predict how much acceleration there was and how fast things should be moving today. And those two numbers don't agree, right, and so there's some sort of mismatch there, Like there doesn't seem to be enough dark energy in the early universe to get us going as fast as we seem to be going today. I feel like there's two things, and maybe people are getting confused. So like, right now the universe is expanding at a velocity that's faster than it used to be. That's for sure, like the universe expanding faster than it used to expand before. But you can also talk about the acceleration of the universe, which is the idea that it's growing faster and faster each year. And so I think you're saying that even the acceleration now is faster than it used to be, Like the universe is somehow hitting the gas pedal even harder. Today. What we do is we measure this thing called the Hubble constant, you know, which is terribly named because it's not a constant. It's a measure of how fast the universe is expanding. And we can measure it like in recent times, because we can look in our neighborhood and see like how fast is stuff looking away from us? How fast was stuff moving away from us pretty recently. We can use that to measure this Hubble constant, which is like the derivative of the scale factor of the universe normalized to itself. And so that's like a measure of the expansion rate. And what we can do in the early universe is we can measure like how much dark energy was there. We can use that to predict the acceleration of the universe, which should give us like the velocity. It's sort of like saying, all right, you started your car, you were pressing on the gas pedal a certain amount, how fast should you be going after a minute? And if the rate we're measuring, like how faster car is going right now is too fast, and we're like, maybe you were pressing the gas pedal faster than you thought. You were right, right, But I wonder if maybe the folks are getting confused with because there there's also the idea in the early universe of inflation, right and the Big Bang? Now was inflation and the Big bank also due to dark energy? Or was it you know, that super rapid expansion like a second after the Uni verse was born, the universe expanded like tang to the twenties something or crazy amount. Was that also due to dark energy? Or was that something due to something else? Entirely, we just really don't know. We know that there are similar right. They both expand space very rapidly. We don't know if they're connected. There are some theories of inflation that try to connect dark energy with inflation, but there's some theories of inflation that are totally separate that say, no, it's due to this weird inflaton field that has its own potential energy which rapidly expanded space, and so we just don't know. We do notice that they are similar, right, But these questions about dark energy assume just inflation happened, and like, don't ask too many questions about it. Let's just start from the post inflation universe and try to understand the expansion history from that point. Oh, I see, when you're talking about early dark energy, you mean like post inflation dark energy exactly, not that early that I mean, not pre early, late earlier. That's right, because the early this thing that we can actually measure comes from about four thousand years after the beginning of the universe. This is the cosmic microwave background radiation. And we can look at these wiggles in the microwave spectrum in the sky, which is light from that very early plasma, and we can get a sense from what was going on back then, and so that gives us actually a measurement of how much dark energy there was back then, which let's just calculate the acceleration from that point, not all the way back to inflation. Like that's a murky mystery for another day, right, But wouldn't that sort of confuse things like at what point do you think this potential infloton field or whatever it is that costs a big bang or the inflation you know, drop off and gave way to dark energy, Like couldn't there have been overlapped? How do you know? Like what cause inflation isn't like affecting us today. Yeah, we don't write it could be connected. The basic ideas are the same. That you have some field with a lot of potential energy, you put that into Einstein's equations. It causes this repulsive effect on the universe to stretch it and expand. It could be that inflation has a different kind of potential energy from a different field, this in photon field, or it could be related. Also, we don't understand either of them. We don't understand why inflation happened. We don't understand why it stopped. We don't understand if it did in fact stop, or if dark energy is an extension of it. Yeah, we're clueless about all that stuff. So basically we don't know anything. Daniel. We're in the dark about the whole dark energy thing. We're mostly in the dark. Until very recently, though, it seemed like if you assume that there was a fixed amount of dark energy after inflation, after somehow the in photon field died away or whatever, that if you assume there was a fixed amount of dark energy, that it mostly described the universe like our rough calculations and early measurements suggested things were banging on that the measurements we had of this cosmic microwave background radiation told us a certain amount of dark energy. You put that into the equations and ran them forward in time. You've got a universe that looked a lot like the one we see out there right now. But as these measurements got more and more precise than a problem emerged and they didn't quite work. Oh, I see, all right, So I'm trying to put my head in the place of a physicist. So you're saying that the Big Bang happened, inflation happened, done, like we're going to put a market there and ignore everything that happened before. Like after information, we're going to reset our thinking. We're going to assume that there's something called dark energy that's continuing to keep the universe expanding at a faster rate. And you're saying that before we used to be able to just plug one number in and it sort of worked like it it mostly described things, But now now it doesn't quite work. Now it doesn't quite work exactly. Now the universe seems to be expanding at late times more quickly than we can explain with the amount of dark energy that we measure from the early times. Like whoever was driving the universe back then with dark energy, they put their foot on the gas pedal at a certain level. But now when we look at the speed of the universe, it seems to be going faster than can be explained by that amount of gas. So that's the question. Is the cosmological constant actually a constant or do we need to make it more complicated? Right that number used to plug in that used to work before now doesn't work before, which means that basically it's not a number. Kind of yeah, now it's a function. Right now we need a new little bit, meaning like the amount of dark energy per block of space is or it has been changing. That's the question exactly, And so early dark energy is an attempt to try to fix this problem is to say, well, maybe there's dark energy, which is just a number and it's constant. But now let's add a new thing. Let's not just change dark energy. Let's add a new kind of expansion to the universe, like a new unexplained expansion that's going to only take place in the early universe, Like there was dark energy, which is accelerating the expansion the universe. Plus let's add a new bit which only turned on an early universe, got things going extra fast, and then disappeared, which is why we don't see it today. So that's the idea of early dark energy, is to try to like patch up this problem with dark energy, not by saying dark energy changes with time, but by saying there was a new thing, yet another way to expanding universe, but this one only lasted from like zero to four years after the Big Bang? Oh? I see wait? So um, so you I think you're saying that early dark energy is not dark energy. So early dark energy is not dark energy exactly. It's a new thing that is like dark energy and also kind of like inflation, right, and that it expands the universe due to some potential energy field, but it's somehow also disappeared before the CNB, right, so that it doesn't like mess up everything we see later. Right. I guess an early hooge would be so foreign to regular hooge that you would have to call it a different whoree. I think that's what you're thinking, is a physicist, what gives the sense of the scale here? How different is this post inflation, post Big Bang dark energy expansion than the expansion we have now due to dark energy. It's turned out to be pretty significant. You know, early on when we were measuring these things for the first time, we really didn't know how fast the universe was expanding, and we had big error bars on all of our measurements, and so things seemed to kind of disagree a little bit, but nobody was worried about it. Now these measurements have gotten really really precise, and the difference between the predictions for how fast we are expanding and how fast we should be expanding if dark energy was constant and there was no early dark energy are different by about ten per cent. So this is measurements of the Hubble constant, and like the late measurements, the ones using like supernova and sephids and other kinds of stuff, they measured this Hubble constant to be about seventy three, where the units are kilometers per second per mega PARSEEK and the early ones, the ones that come from the CNB where we measure what was going on in the early universe, how much dark energy there was, and then predict how fast we should be accelerating. They predict sixty seven, So that's seventy three versus sixty seven, and the uncertainties are getting pretty pretty small, Like, these are precise measurements now. The Plank satellite gave us really precise measurements of the CMB, and we have lots and lots of supernova now and lots of other kind of ways to measure the leration today. So the difference between these two has become pretty significant. It's like five or six sigma, which means that it's like, doesn't seem to be just a statistical error. It seems like it might be real. Well, it's it's it's ten percent different, but you're saying that your confidence in that ten percent difference is getting stronger. Exactly. People were sort of thinking like, okay, these numbers are far apart, but the errors are big. And then as we get more and more data, sometimes what happens is that the things sort of drift together. You're like, oh, two measurements of the same thing using different ways as they get more precise, if you're really measuring the same thing, they should eventually agree and if you've understood all of your uncertainties and biases. But these two things, as they've gotten more precise, have stayed far apart. And so now yeah, they seem to be far apart, and the confidence in both measurements is pretty high. All right, So there's about a ten percent difference in what we thought was the universe's expansion and what it actually was before. And so just regular dark energy on time late dark energy doesn't planet, and so you need this concept of early dark energy. Early dark energy is sort of in the same spirit of dark energy. It's like, hold on the universe seems to be doing something we can't explain. Let's just add another piece to the mix and see if we can use that to explain what's going on. Later we'll come in and figure out why is this thing here, how does it work, what exactly is creating this dark energy. The first step is just like, you know, rough out an idea for what pieces you need at what time to explain the history that we see right right. Basically, it's sort of like admitting that dark energy is changing. Right Like, the original idea for dark energy was that it was some sort of like constant inherent property of space and never changes. But now you're sort of being forced to say, hey, dark energy kind of is changing, So maybe it's not some constant of the universem inherent property of space. Maybe it's there are multiple dark energies, or maybe it's the dark energy that's changing, yes, exactly. And there's a whole category of ideas there. There's ones where there's multiple dark energies, like a new one that appears early in the universe and then disappears, right, which is why we don't seem to see it today. Others where dark energy itself changes and then stabilizes to a certain number and then is fixed for the rest of the time of the universe. There's a whole ridiculous spectrum of ideas. A lot of particle theories I talked to say that most of them are kind of ridiculous. They call them theoretical shenanigans. But you know, it's just an effort to like try to cook up a recipe that explains what we see and then later figure out the details of that recipe right, right, And in the meantime, let's confuse everybody with shortcuts here to call things. Why not just call it dark shenanigans. That would be valid. Oh man, that would have been awesome. I would love if we had called it dark cinnanigans. That's right, DS for because I have dark matter, dark energy, and dark shenanigans. That seems like a really nice trifecta. Sounds like what all physicists, and you know, the theorists are going crazy with these ideas because every time you give them an opening to be creative, then they come up with all sort of silly ideas for like what might be creating this. Lisa Randall, who's a famous theorist who came up with the idea of extra dimensions. She has a paper about rock and roll solutions to the early universe that involves like new weird fields in space, some of which rock back and forth to explain like why they disappear, and some of them would you like, roll away gradually and fade out before we could see them. Mhm. So she called them rock and roll solutions literally in her paper exactly. Her paper really is called rock and Roll Solutions. It's pretty fun paper, but it's definitely in the theoretical shenanigans category. Yeah, because I guess you, I mean, before you saw dark energy was this constant thing, but now knowing that it's changing, I mean, you're sort of blowing the whole thing open, right, Like you're saying, now you could pretty much imagine anything that could explain that change. Yeah, although it's a bit more complicated, right, It's simpler to say, oh, the universe has this new property, and that property is a constant and it just is. It's more complicated to come up with a new piece which you have to explain and then also explain why it's no longer around. It's just as tricky as inflation. Like with inflation, we have this very rapid expansion of the early universe which you have to explain. You also have to explain why that stopped. And so early dark energy is a special puzzle for theorists because they have to explain why it exists and also why it no longer seems to exist. Right, and they also have to get up early, which is a big problem. Just stay up late enough, or drink a lot of dark coffee. All right, Well, let's get into whether this early dark energy is real and what we're doing to measure it and or confirm all of these dark Shenanigan ideas. But first let's take a quick break. All right, we're talking about the mystery of dark energy, which apparently, um, it's not this constant thing. It's sort of this seems to be this kind of fickle saying almost like it was pretty lazy at the beginning of the universe after inflation, right, that's the idea. But now it's sort of getting steam, and it's maybe getting darker and later, and so it's getting more energy exactly. And it's amazing to me that dark energy sort of ever worked. You know, we look back now the history of the universe's expansion and it's incredible that you can add this simple term to just say, well, space seems to have this weird potential energy in it, which Einstein's equations predict cause this expansion, this repulsive force to stretch out the universe, and that that mostly describes what we see after inflation. We see this period of very slow expansion then sort of picked up steam around eight or nine billion years after the universe and is now accelerating very very quickly. It's sort of amazing that that complex history can in fact be described almost by a constant term. I think that's pretty cool. I mean, the fact that it doesn't quite work means, you know, it needs some new ideas, but I think it's sort of awesome that it works at all. Well, it's the only sort of sort of works right, Like it could just be like totally wrong, Like our whole sort of formulation of the universe could be totally wrong, and this could be sort of like the thing that makes us see that it's wrong, right exactly. And that's why we do these studies in more and more detail. You know, when you're first looking at something, if you have like a simple explanation that sort of mostly describes it, that's kind of cool. Like if you're plotting to quantities that you've measured and they follow along a straight line, you're like, that's interesting, and then as you get more points, you're like, oh, it turns out it's not quite a straight line, it's almost. Then you know you need to refine your understanding, your theory. But the fact that like mostly a straight line describes it, it is already pretty powerful. Well, I think a big question that you also rais is whether or not this changing dark energy effect is actually real, because I think, as you said, it's it's sort of based on two very different kinds of measurements, right, Like we measure dark energy now using one thing, but then early in the universe we measure doing using something else. So like, you know, like a ten percent difference using to totally different measurement systems seems sort of reasonable. It does, depending on those uncertainties, right, Each of these things are very different styles and measurement and we do this on purpose because we're interested in something deep and true in the universe, so we try to get a handle on it in different ways to make sure that we're not getting biased by one of our measurement techniques. And these really are very very different measurements. It's important to understand, like the early measurements are very indirect. You know, what we do when we look at the cosmic microwave background radiation is we don't actually see dark energy directly. What we do is we measure like how much dark matter was there, how much normal matter was there. We can tell that because we can see those two things sloshing around in the early universe plasma and creating like weird sound waves. It's called bury on acoustic oscillation that we talked about once on the podcast. And we can also measure how much energy there was overall in the universe, a measurement of the curvature of space back then. So the measurement of dark energy from the early universe is actually just a a traction. It says how much energy was there in total? How much can we account for. We're using dark matter and normal matter, the rest of which we call dark energy, So it's sort of like, just by subtraction we assume how much dark energy there was back then. It's very indirect. Well, I guess, because it's so indirect, it makes me kind of feel like it's just kind of a big guess. I mean, you're looking at this picture from the early universe and you're trying to, you know, throw it out these little tiny nuances in the ripples of it. In in you're sort of assuming all these proportions about the universe. Like how confident are physicists about these measurements. There's a lot of debate about that because in order to extract from that the measurement of dark energy, you've got to make some assumptions, like this data comes from the Plank satellite, which is an exceptional instrument, very high precision measurement of the cosmic microwave background radiation like revolutionized cosmology. But in order to interpret this stuff, you need to have some models in your head that you're using to interpret this data. So what people have been doing in the last two years as this discrepancy has arisen between this measurement from Plank, from the early data from the CNB and the late measurements from the supernova is to try to revisit those things and ask questions. It's like, is there another way to interpret this data? Can we see it another way? Are there assumptions that we're making that we can change And they've tried like changing those assumptions and starting from scratch and using other ideas, and the measurement is persistently too small, like they've banged on it and banged on it and banged on it some more, and it just seems to be very consistent. So that's the early expansion of the universe. What about the late expansion of the universe? How are we measuring that one? So that we have to find some way to measure distance in the current universe, that's the key. We want to know how far away things are and how fast are they moving away from us. So in order to measure the velocity of a function of time in the universe, we need to measure velocity of the function of apparent distance. Because things that are far away from us, we see them in the past. Right, something that's a billion light years away, we're not seeing light from it today. We're seeing life from a billion years ago. So in order to understand the expansion the universe over time, we need to understand the velocity of the universe as a function of distance from us today. It's really the same thing. And to measure those distances is always very very tricky, because how can you tell something that's really close by and dim or really far away and bright. Right, You can't tell unless you know how bright it's supposed to be. So the key to all of these late time measurements are calibrations of how far away things are. So we have things like type one A supernova which blow up in a very predictable way. We know basically how bright they should be. And they have other ways to do this, like these sephids, these variable stars where their pulsation is very closely connected to how bright they are at their source. And people have been really creative and finding other ways to measure this expansion using other calibratable objects neutron star, gravitational waves, and quasars, and this recent measurement about red giants, and all of these things are uncertain, uncertain different ways, Like we don't quite know our measurements of the sefids reliable. Are we accounting for the amount of dust between us and those stars correctly? That's why we try to do it in several different ways. The amazing thing is that all these different measurements using ways ours or supernova or sephids or red giant stars that have a helium flash in a very predictable way, they all mostly agree with each other, and they all disagree with this early measurement. So it's really compelling and fascinating. Right, But is there another way I guess to kind of link the two measurements other than this dark energy discrepancy? Do you know what I mean? Like, if I measure something with one ruler and I measure another thing with a different, totally different ruler way of measuring things, and they disagree, I wouldn't necessarily say that that's a whole new constpant in the universe. I like try to find other ways to see that the two rulers or the two measurement systems agree in other things, right, Yeah, And that's what they haven't done by trying to come up with essentially other rulers. They got their Type one a supernova ruler, and their SEFID ruler and their tip of the red giant branch ruler, and all these different rulers all basically seem to agree with each other, and they should all make sort of different kinds of mistakes if they do make mistakes. So that's a clue that like, maybe this is real. Maybe the universe really is expanding in this surprising way, because we haven't found a problem with any of these measurements, right, but have you tied the early universe CMB measurements to things like the say, facifits or the red giant measurements. The only way we can tie them is by taking that measurement from the early universe and using it to predict how fast things should be accelerating today, and that's where the disagreement is. So what we're trying to do is make other measurements from the early universe. And there's some pretty exciting, very recent results from a telescope in Chile called the Atacama Cosmology Telescope that also looks at the CNB, but in a way that's different from the Plank satellite, right, And that sounds super exciting, But I guess maybe my question is, and we like confirm these measurements or confirm our ability to measure the expansion of the universe using the CMB with other ways or other ways of measuring the expansion. Now, like you're saying, the early universe expansion is measured by the cosmic microwave background radiation CNB, but the current expansion is measured in a totally different way, and they come up with different numbers, which may be something about dark energy that's changing, or it could just be maybe that our two measurements are not quite linked together or calibrating. Yeah, it could be an issue with calibration. We can't measure the dark energy today the same way we can from the CMB. Right, That measurement we could make because we had this special window where we get this life from the cosmic microwave background radiation. We can't make that same kind of measurement today. That would be really awesome if we had a late time CMB that we could use to measure and compared to the early time CMB. That's just not available. Right. The universe doesn't give you everything you want. You just got to look around for opportunities and and your understanding must be wrong right now. Yeah, I mean, it's amazing that we can do any of these measurements at all, or that we have these amazing instruments to measure these expansion of the universe. Now, and before I guess, I'm just saying, I'm just trying to understand what it is that the mystery is. And it seems like there's a discrepancy, a difference between measuring the early expansion of the universe using the CMB and measuring the current expansion of the universe using you know, other methods like supernovas and and seffords and red giants and things like that. And you know, one possibility for this difference is that there is something weird about dark energy that's changing or I wonder another possibility, just to you know, give us some context, is that maybe these two ways of measuring the expansion of the universe are not quite calibrated, right, Yeah, exactly one of them could be wrong. They could both be wrong, exactly. And people are digging into the details of how these measurements were made to look for potential sources of error or bias that's why it's so valuable to measure these things in multiple different ways, because it checks your understanding and because they have different potential sources of uncertainty, right, you know, all affected by the same bias. And that's one reason why they built this telescope in Chile, because it has different sensitivity than plank and so it probes the cosmic microway background radiation in a slightly different way, which gives a different power to answer this question. Right, Yeah, I guess it's an ongoing work. But um, I guess assuming then that our early measurement of expansion and our lading measurement of expansion are both correct and the same and have the same sort of basis in reality, then that's a big deal. That means dark energy is changing, right, So it's not a constant thing, and it may even change in the future. It may change in the future. Yeah, exactly. It means that the story is more complicated than just you have one field which has some potential in it and it has this constant contribution per unit of space, which tells you this story of the expansion has to be more complicated than that. You need some like extra bitted gas in the early universe to get us going so that we are expanding as fast as we see today. And you're right, we don't understand what that is. We don't know if it will come back, like and some of these raw can roll theories like that early dark energy rolls away, but hey, it could rock back, you know. It could be that the future has more accelerated expansion, or that it could go the opposite directions somehow. What it really means is that we just don't know very much about the future of the universe. Right. We could be headed towards a heavy metal rock universe or a easy listening universe. One of the things that is really interesting for me is that these new measurements from this Chilean telescope, they support this idea of early dark energy. They measure the cosmic microwave background radiation in a different way than plunk like. The details are kind of technical, but you know, ground based telescopes always have different sensitivities than space satellites, and when they analyze their data they see something consistent with theories of early dark energy, Like their data are more consistent with their being early dark energy than with their not being early dark energy. So they sort of disagree with Plank a little bit. Wait, what do you mean? Did they disagree Plank? When they look at their data, they don't see any specific evidence for early dark energy. Early dark energy, if it existed, would be this like extra expansion of the universe just before the CNB plasma was formed, and it would change a little bit how those ripples look. And Plank doesn't see that. But this new telescope, the ACT in Chili, it does see some weird ripples in the CNB that seemed consistent with their being an extra bit of expansion just before that plasma recombined, and so it's sort of more consistent with their being early dark energy than with their not. It's like a three sigma effect in their data. M I see. So again, just kind of maybe confirming that dark energy has changed and may change in the future. Right, It's more consistent with this complicated story of dark energy than with the simple idea of dark energy just being a number. Right, But wait, I thought the same B also confirmed that, Yeah, the Plank measurements don't agree with the late time, but they also don't see evidence for early dark energy. These measurements agree with Plank about how much dark energy there was, but they also see this extra effect which seems consistent with early dark energy. So it's new telescope disagrees with Plank a little bit. Maybe you need earlier dark energy or mid mid to late morning dark energy, sparkly dark energy. Well, and I think all this again means that the future is unpredictable, right, I think I think that's kind of the bigger deal is that, you know, if dark energy was constant and just the number that wasn't changing, then we sort of know how what's gonna happen in the future, and we know how the universe is going to end. But if we don't, if it's something totally complicated and different, who knows what's gonna happen, right, it could turn off for example, Yeah, we don't know what's going to happen in the future. It also, I think it's super fascinating affects our vision of the past. You know, if the universe has been expanding more rapidly than we thought, that means that it's younger than we thought because it didn't take as much time to get this expansion happening as we thought it did. Because the foot was on the gas pedal more strongly than we thought it was. So concretely, it might mean that the universe is only twelve and a half billion years old, not thirteen point eight, right, So we might have just like deleted a billion and a half years of history. Boom, you're all younger, everybody. Yeah, well, we're not younger. We're definitely not getting younger. But maybe the universe is younger than we thought exactly. And so that just goes to show you how will we know about the future of the universe, and how will we really understand about its past? How this picture of the universe and its expansion is still a big question mark. Yeah, I mean it's still dark basically, right, dark energy is that it's dark. It's a mystery. It's definitely probably not a constant. Who knows what it could be, right, that's right. Dark history even more fun than drunk history. Yeah, it could be cartoonist um drawing crazy things in the universe. I hope that's our future as well. Yeah, and I think it just confirms my theory that, you know, staying up late keeps you young, right, it seems to work so far exactly. Let's keep taking measurements all right, We'll stay tuned as we learn more about dark energy and what it could mean for our history of the universe and also the future. Maybe somebody out there listening to this could be the person who discovers what's really going on with early or late or mid to late morning dark energy. There's certainly a whole lot left to discover, a lot of time for a dark energy cook to come in. Basically, you might not even have to stay up late to figure it out. Well, thanks for joining us. We hope you enjoyed that. See you next time. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of I Heart Radio. For more podcast For my heart Radio, visit the i heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows. Yeah