What did the early Universe sound like?

00:00:08
Speaker 1: Hey or he. Are you a fan of boba tea?

00:00:11
Speaker 2: You mean like bubble tea?

00:00:12
Speaker 1: Yeah? Is that what the kids are calling it?

00:00:14
Speaker 2: I think they call it boba, but I think maybe most people might not know what that is. But yeah, I'm a fan. My kids are really into it, so I've grown to like it.

00:00:22
Speaker 1: Well, my daughter loves it, but personally, I can't get over the fear of being choked by a floating blob every time they take a sip.

00:00:28
Speaker 2: Oh. Well, first of all, they don't float, which tells me maybe you don't drink boba very often, Busty. The other thing is they're not that big. I think they're pretty much smaller than your throat.

00:00:39
Speaker 1: I think maybe these new trends are just not for the faint of heart.

00:00:41
Speaker 2: So do you not like bubbles in general?

00:00:44
Speaker 1: No, I'm pro bubbles in the universe, just not in my tea, not as a choking hazard.

00:00:49
Speaker 2: I see what about economic bubbles? Those are bad news?

00:00:53
Speaker 3: Mmm.

00:00:54
Speaker 1: I'm hoping to ride the podcast bubble until it pops.

00:00:56
Speaker 2: Yeah, there you go. Not all bubbles are bad. Hi am Jorge Mack, cartoonist and the author of All Ours Great Big Universe.

00:01:17
Speaker 1: Hi, I am Daniel, I'm a particle physicist and a professor at UC Irvine, and I'm actually fascinated by the mathematics of bubbles.

00:01:24
Speaker 2: Oh yeah, it is, and it just a sphere.

00:01:26
Speaker 1: Some bubbles are spheres, but you can also get bubbles of all sorts of different shapes, which solve really complicated sets of mathematical equations to like minimize surface area.

00:01:35
Speaker 2: Now is that math or is that physics.

00:01:37
Speaker 1: It's using physics to solve math. It's like using the universe as a computer. Whoa bubble computers?

00:01:43
Speaker 4: Man?

00:01:43
Speaker 2: That sounds like an awesome topic for a podcast episode, but I'm guessing that's not what we're talking about today.

00:01:49
Speaker 1: We are not talking about my new Bubble Computers startup, which will be serving bubble tea in the lobby, but we are talking about bubbles.

00:01:56
Speaker 2: You're going to ride that tech bubble. But anyways, welcome to a podcast Daniel and Jorge Explain the Universe, a production of iHeartRadio in.

00:02:04
Speaker 1: Which we want you to ride the bubble of understanding. As human thought expands further and further into the universe, we understand more and more about this incredible and crazy cosmos. We decode the messages that come to us from all these distant places and try to piece them together into a fragile bubble of understanding.

00:02:22
Speaker 2: Because it is an awesome universe inflating every day with more and more awesomeness. We like to inflate your brain here on the podcast until it pops with an epiphany about how the universe works.

00:02:32
Speaker 1: I don't want anybody to bring the pop man. We want to very gently inflate it.

00:02:38
Speaker 2: I said, with an epiphany.

00:02:40
Speaker 1: Oh, that sounds terrifying, but you're right. We do want listeners to have that moment of understanding where suddenly things click into place and you go, oh, I get it. This thing I used to hear in that bit, I thought I understood. Those actually fit together into a holistic idea about how the universe works, and that, in the end, is the goal of this podcast.

00:02:58
Speaker 2: Yeah, we like to taggle the bit mysteries about the universe from its large scale and what kinds of amazing things you can find out there in the reaches of space, but also in the smallest of scales done at the atomic and particle sizes, from the beginning of the universe to the end of the universe.

00:03:12
Speaker 1: Exactly, and those little bubbles at the particle level in the early universe turn out to have a ripple effect that create bubbles in our universe billions of years later and millions of light years across. It turns out that bubbles are not just the core idea between my future billion dollar bubble computing startup or the drinks that my daughter enjoys. They are also fundamental to understanding the early universe and the structure of the universe today.

00:03:37
Speaker 2: But you mean you can tie the Big Band to boba.

00:03:40
Speaker 1: The Big Bang was basically just a bobba bubble.

00:03:42
Speaker 2: Big bubba bubble bang boom. Yes, Daniel said, a lot of things that happened at the beginning of the universe, even small, microscopic things that were going on, have a huge impact in what the universe looks like today, and maybe even might have tip things in our favor for to be creative and for our galaxy to be the way it is now.

00:04:02
Speaker 1: One of my favorite things in physics is figuring out a way to sift through the clues that are left to us about what happened in deep time, what happened in the very early universe. If we can figure out the signs and signals, but those early events left for us. We can actually reconstruct a complete history of what happened in the universe. It's like the biggest detective game ever.

00:04:23
Speaker 2: I don't think they can even trace how boba was invented, so that would be an amazing feed if we can, you know, trace our origin back to the Big Bang.

00:04:31
Speaker 1: Are you saying it just organically bubbled up from nothing?

00:04:33
Speaker 2: I don't know, that's the mystery. Maybe it was aliens. I knew that NASA had secret alien technology. I just didn't realize it was Boba technology. Well not NASA, and NASA's is from Earth. Am saying the conspiracy runs deeper?

00:04:48
Speaker 1: Oh my gosh, wow, intergalactic Boba bubble conspiracy.

00:04:52
Speaker 2: There you go. That's what the world needs, more conspiracy theories.

00:04:57
Speaker 1: But we are here on this podcast breaking down conspiratorial nons sense and telling you the truth about what we do and do not know, how we trace back to the history of the early universe and how it affects our lives today.

00:05:07
Speaker 2: So today on the podcast, we'll be tackling the question what did the early universe sound like? Interesting questions about the sound of the early universe.

00:05:21
Speaker 1: Yeah, did it sound like somebody choking onbah?

00:05:25
Speaker 2: Maybe that is the origin of the universe. Maybe they're all here because some intergalactic god choking a giant boba.

00:05:35
Speaker 1: Exactly. He took his daughter out for intergalactic boba and the rest is history.

00:05:39
Speaker 2: Yeah, maybe black holes are like the boba of you know, the higher beings.

00:05:45
Speaker 1: Black hole boba. I will definitely sell that in the cafe of my bubble computing startup.

00:05:50
Speaker 4: Yeah.

00:05:50
Speaker 2: They are pretty dense right in drink, They're like thing in the drink.

00:05:54
Speaker 1: They're terrifying.

00:05:55
Speaker 2: Oh my gosh, you're really afraid of boba.

00:05:58
Speaker 1: When I take sip of a I want to enjoy a fresh liquid and not worry that something is going to shoot down my throat. Yeah.

00:06:05
Speaker 2: I think we've established the bow is not for you.

00:06:09
Speaker 1: But I am a big fan of bubbles, including sound bubbles in the early universe. People don't usually think about what the universe sounds like because they think about space is being mostly empty and so diffuse that sound waves can't effectively travel through it. But that wasn't always the case.

00:06:25
Speaker 2: Yeah, so this is an interesting question, and it sounds like the early universe sounds are related to bubbles, like bubbles popping or bubbles forming, bubbles.

00:06:33
Speaker 1: Forming and slashing around, and even oscillating. In physics, this whole field goes by the fancy name of baryon acoustic oscillation b AO or bow.

00:06:44
Speaker 2: Mm oh, well we should be talking about bows then, buba.

00:06:49
Speaker 1: Bubble bow.

00:06:50
Speaker 2: Those you can't probably choke on if you try to eat a whole one at once.

00:06:54
Speaker 1: Don't put those in your drinks, folks.

00:06:55
Speaker 2: Yeah, that that's an interesting idea, Daniel. You might have invented the newest trend.

00:07:01
Speaker 1: Is that going to start a whole new universe?

00:07:03
Speaker 2: Maybe? Yeah, maybe somebody will struggle to swelling one and originate a whole new universe of lawsuits, I'm guessing. But anyways, it's an interesting question. What did the early universe sound like? And it sounds like it's related to something called the baryon acoustic oscillation, and so, as usual, we were wondering how many people had heard of this concept? Do they know what it is? Do they want to know.

00:07:26
Speaker 1: What it is? How could they live so long without hearing about it? So thanks very much to everybody who answers these questions for the podcast. If you'd like to join the crew. Please don't be shy. Write to me two questions at Danielandjorge dot com, or contact us on Twitter or join our discord. We'd be happy to send you these questions.

00:07:44
Speaker 2: So think about it for a second. Do you know what baryon acoustic oscillations are? Here's what people have to say.

00:07:51
Speaker 3: I have no idea, but it reminds me of something in a video game I used to play. Oscillation is kind of like a ring of something I because in my video game there's like a big circle ring the boss called the ausciliator. Acoustic is like kind of like antiqueish.

00:08:10
Speaker 1: I think, I'm sorry, I'll pass. Never heard about it.

00:08:15
Speaker 4: I believe this is pressure waves in the cosmic background radiation that is caused. We can see the slight changes in temperature caused by pressure waves, and because the pressure waves cannot propagate at greater than the speed of light, the size of the acoustic variations gives an excellent estimate of the distance to the cosmic background radiation source, and therefore that, in conjunction with the red shift that we observe, gives us a very good indication of the hubble constant at that one point in time approximately four hundred thousand years after the Big Bang.

00:09:03
Speaker 5: Based purely on the name, I would guess that baryon acoustic castellation has something to do with either using sound to cause beryons to bump each other or using sound like properties to study how they behave.

00:09:19
Speaker 6: I have it in my head that baryon acoustic ostellations has something to do with the beginning of the universe and tower. The original quantum fluctuations prior to inflation taking place could be seen as being like waves through the kind of plasmi y stuff at the beginning, and then when that gets blown out by inflation, you can still detect and see those acoustic ostellations today.

00:09:41
Speaker 3: So Buryon's some atomic particle, and probably.

00:09:49
Speaker 1: It has acoustic.

00:09:51
Speaker 3: Oscillation to it, like having a pattern repeating over a period of time.

00:09:57
Speaker 6: Or something like that.

00:09:59
Speaker 2: All right, and a lot of answers here. I like the person who said, I'm sorry, I'll pass. What do you say to that, okay next? Or do you try to convince them that they want to know what burying acoustic oscillations are.

00:10:12
Speaker 1: I don't want to pressure anybody. I'm just impressed that they decided to record their passing and send it in rather than just not responding.

00:10:19
Speaker 2: Oh, they actually like took the time to record this exactly.

00:10:22
Speaker 1: They sat down and record their answers and sent it in even though they were passing.

00:10:26
Speaker 2: I love that, I see. Do you think what do you think happened? Do you think they heard the question? Were like, I don't want to say anything about burying acoustic oscillation.

00:10:32
Speaker 4: Yeah.

00:10:33
Speaker 1: Well, the rules are no googling, no looking at the questions ahead of time. I want people's real, spontaneous ideas about what these topics are, because we want to get a sense for what people out there know before they look things up. And so this person was just reading through the questions in real time and recording themselves, and maybe their brain just had a bubble which popped and they decided I got nothing.

00:10:51
Speaker 2: Okay, I see, it's more like I got nothing not so much A.

00:10:55
Speaker 1: No, thanks, next question please.

00:10:59
Speaker 2: Well, people seem to sort of into it or know that it's somehow related to the beginning of the universe. And also I guess to something related to waves, right and sound and oscillations. Nobody gets bows or boba.

00:11:13
Speaker 1: Nobody made the boba connection. That's just me, all.

00:11:15
Speaker 2: Right, well, let's jump into it, Daniel. What are baryon acoustic oscillations.

00:11:19
Speaker 1: Yeah, baryon acoustic oscillations are really fascinating, sort of like fossilized sound waves from very very early universe. You know, like if somebody's playing an acoustic guitar or like an acoustic recording. It refers to the quality and the fabrication of the sound waves that you're hearing. So acoustic there tells you that you're hearing sound waves, and the word buryon tells you what you're hearing those sound waves in that you're hearing it as baryons bump against each other.

00:11:45
Speaker 2: But I guess maybe not to confuse folks in this case, acoustic doesn't mean necessarily sounds your hearers at the air. They can also mean like sound waves and you hear in the ocean or maybe through even a solid right.

00:11:55
Speaker 1: Yeah, exactly. Sound Waves can travel through air, but they can also travel through water, or they can travel to steal, they can travel through your body, They can travel through any kind of gas or plasma. Sound Waves are just pressure waves. If you have a bunch of molecules that can interact with each other, that can push against each other, then if you push on one side of that blog, then it's going to push on the next layer, which pushes on the next layer, which it pushes on the next layer. That's what sound waves are. You're hearing us right now because the speaker in your ear is making sound waves that push on layers of air, which push on the next layer of air, etc.

00:12:26
Speaker 2: I see so an acoustic wave or acoustic oscillations. They're just like when things propagate through material, because things are bumping into each other basically through electromagnetic forces, or can it be other force.

00:12:38
Speaker 1: It's almost always electromagnetic forces. The crucial thing is that they bump against each other. If they pass right through each other, then they don't cause pressure waves. The crucial thing is that they're bumping up against each other. That one layer pushes the next layer, which pushes the next layer. The microphysics of how that pushing happens is electromagnetic. You have electrons in one atom are pushing up against the electrons in another atom. They don't like to overlap, they resist each other. It's the same reason why you don't pass through your chair, or when you're leaning against the wall, the wall pushes back or the Earth is pushing up on you. Basically, anything structural is built with electromagnetic forces, because that's the bond of chemistry.

00:13:13
Speaker 2: Now, for those of us who are not particle physicists, can you remind us what a baryon is.

00:13:18
Speaker 1: Yeah, Baryons are anything made out of quarks, basically baryon the shorthand for our kind of matter, stuff like protons and neutrons, these are baryons. We call them baryons mostly to distinguish them from the other kind of matter in the universe, dark matter, which is some other kind of stuff that's out there. It feels gravity, it has masks. We think it's made of stuff. We don't know if it's made of particles, but we're very sure that it's not made of our kind of particles. And so when we talk about the very early universe, we have a few components to sort of like that very early universe smoothie. There's baryons, there's photons, there's dark matter, and so we talk about baryon acoustic oscillations because it's the sound waves in those early universe protons mostly that we're thinking about.

00:14:01
Speaker 2: Does that include electrons as well or electrons something else?

00:14:04
Speaker 1: So electrons are not technically baryons because they're not made out of quarks. Baryons are particles that are made out of three quarks. Quarks are these incredible particles that feel a strong force in order to have a neutral particle and the strong force and that it doesn't have an overall strong force charge the way, for example, a proton, an electron can make a neutral atom with no overall electric charge. In or for quarks to come together to make an object that doesn't feel a strong force, it's overall neutral. You need either three of them or two of them, and if you put three of them together you get a baryon like a proton or a neutron, or there are other more exotic baryons. So technically an electron is not a baryon, but it is included when you talk about baryonic matter, which is like atoms made out of a baryon and an electron.

00:14:50
Speaker 2: I see that makes not a lot of sense.

00:14:55
Speaker 1: Yeah. The short answer is you can lump electrons in with baryonic matter even though technically they're not buryons.

00:15:01
Speaker 2: Okay, I see, so it's really just regular matter. You're using that shorthand for regular matter or at least the matter that we're made out.

00:15:08
Speaker 1: Of, exactly the matter that really matters.

00:15:10
Speaker 2: So then we're talking about the Big Bang. This is the early moments of the universe, and now what was going on there.

00:15:15
Speaker 1: When we talk about the Big Bang. It's also important to clarify what we really mean by the Big Bang. If you say that to a lot of people, they imagine some very dense dot in space which then exploded to make our universe. But when physicists talk about the Big Bang, they really have a different idea in mind. First of all, we don't go all the way back to the creation of the universe. We don't know how the universe was created, if it was created, if it existed forever, how everything came to be. We only go back as far as our theories can describe, which is some moment around fourteen billion years ago when the universe was filled with a very very hot and dense material. Our theories go back that far, and our observations verify that that happened. Where that stuff came from and how it got there and all that stuff is all very speculative, and we have theories about that inflation, et cetera. But really the Big Bang, when physicists describe it, starts from that very hot, dense state and then watches it expand and form our universe. So the Big Bang is not like a singularity at some point. It's a moment in time when the universe was very hot and dense and filled with plasma.

00:16:16
Speaker 2: Well, part of it was that there was a lot less space back then in that those early moments of the universe, or at least what we call the early moments of the universe. Like space expanded a lot since then, from then to now, and so basically maybe a way to think about it is just like all space was more compressed, but it had the same amount of stuff and it so everything was hot and dense.

00:16:34
Speaker 1: Yeah, it's tricky if you think about size and use words like smaller, because we don't know the size of the universe. It might always have been infinite and might still be infinite today. What we do know is about the density. So as you say, it's more compressed, so you should think about a universe, whether it's infinite or not, just as filled with really hot, dense stuff and then space expands. That's the Big Bang as we think about it today, and makes everything more dilute. So things are cooling down and getting more dilute. There's more space per bit of stuff. That doesn't really tell you anything about whether the universe was infinite or not. We obviously don't know.

00:17:07
Speaker 2: And so I think the early universe went through a lot of different phases, right Like, at some point there weren't even maybe quantum fields or the quantum fields. We're still trying to figure it out. And then things started to change. But as you said, at some point in that history, everything was basically a hot plasma exactly.

00:17:21
Speaker 1: Things started out so hot and dense that we can't even really use the physics of today to describe it. You can't even really talk about particles because the fields were so filled with energy. But eventually things cooled down and particles formed, and you got quarks, and you've got electrons. Those quarks then cooled down to make protons. And it's really that moment that we want to zero in on today, the moment when we had protons and electrons and photons and also dark matter in this big hot plasma. But that hot plasma is not uniform. It's not like everywhere in space has exactly the same hot plasma, there's little ripples in it. Some parts are denser than others, and the baryon acoustic oscillation describes how the bears in that hot plasma, we're sloshing around and ringing with sound waves.

00:18:05
Speaker 2: Well, I think maybe a good way to think about plasma is that it's basically just a gas. The only difference between the plasma and a regular gas is that the atoms are broken up, right Like in a regular gas, like the air we're breathing, the electrons are tied together with the protons and neutrons into atoms. But in the plasma, it things are so heated up that they break apart. But it's still basically a gas, right Like it's just things flying around a space.

00:18:29
Speaker 1: Yeah, exactly. It's a gas of charged particles, and it's sort of a natural evolution of matter. You know, as things get colder, they form more structure because they don't have the energy to escape the power of those bonds. So you think about an individual electron, if it has a lot of energy, in other words, if it's in a really hot gas, then it's going to have too much energy to be captured by a proton. But as things cool down, then those electrons are susceptible to being captured by the proton, and then you get neutral hydrogen. So as the universe cools, you go from having charged plasma like you say, a charged gas, to having neutral gas. And so, yeah, plasma is just a charged version of a normal gas.

00:19:05
Speaker 2: Right, It's a gas made out of ions, right, electrons and programs lying around on their own, and so like any gas, it would have sound waves.

00:19:12
Speaker 1: In it exactly, so that hot plasma was not a quiet place, right. It was also super dup or dense, which means that sound propagated through it at shockingly high speeds.

00:19:22
Speaker 2: All right, well, let's get a little bit more into this hot plasma, how it works, and how those early sound waves in that plasma led to the universe we see today, Boba included. So let's dig into that. But first let's take a quick break. All right, we're talking about the early sounds of the universe. Now, Daniel, what genre of music do you think the universe sounded like at the beginning? Was it like elevator music? Was it like rocking, banging music? What do you think k pop?

00:20:01
Speaker 1: I think it sounded mostly like white noise and screams.

00:20:04
Speaker 2: I see, that's right, that's right, because it was some deity choking on a giant black hole wave exactly. It was not a pleasant sound. But yeah, you were saying that the universe was basically at some point it evolved into basically all hot plasma in it, and there were sound waves in it and ripples in it, because I guess it's just the gas. And so even the air we see around us is not perfectly totally completely uniform, right.

00:20:30
Speaker 1: That's right. There are pressure waves everywhere as you talk, and you're making pressure waves. As the wind blows and makes pressure waves. As there are temperature variations, you've got pressure waves, and so nothing around you is really totally uniform.

00:20:42
Speaker 2: So then what made those waves in the early universe? Like, if I just have a room here and I leave it alone, the gas is gonna basically all equalize, isn't it.

00:20:50
Speaker 1: Mm hmm exactly. So to get sound in the early universe, you need a couple of things. First of all, you need some initial over densities. You need some spots be a little hot and a little denser than others. Then you need a way for it to propagate or for it to ring So what are those initial over densities come from. Because if we're imagining the early universe just this big hot plasma, and we say everywhere in the universe is the same, there's no special location to the universe. There's no reason why the universe would put more stuff here and than there. Then it's hard to imagine like where any sort of really initial ripple might come from. And that comes just from quantum fluctuations in the very very early universe. So way back before the plasma even formed, much earlier on, you had just some quantum fluctuations, particles popping in and out of the vacuum, just true quantum randomness. It is true that everywhere in the universe follows the same laws of physics, But if quantum mechanics really is random, then it can do different things in different spots, and that's how you get little, tiny fluctuations. But then inflation or whatever caused the universe to expand dramatically blue those tiny little quantum ripples up to tiny little macroscopic ripples big enough that gravity could do something with them.

00:22:00
Speaker 2: Well, you call them tiny microscopic quantum fluctuations. But I wonder if back then, when the universe was a lot smaller, basically, like all of the quantum particles and fields were basically more on top of each other. And for example, the size of an electron today it does seem huge back then. Is that a good way to look at it.

00:22:18
Speaker 1: It's definitely true that everything was much more compressed back then, like you had the same amount of stuff with less space between them. But those electrons probably weren't even born yet when these ripples that were talking about were made. Eventually that same energy did cool down and spread out into specific particles. But the ripples we're talking about are probably pre particle. They're just like ripples in the froth and quantum fields before you can even really identify them as particles.

00:22:42
Speaker 2: Right right. I didn't mean to say that there were electrons back then, but I just mean like the scale things was very different back then. Like what we might ignore today is a quantum fluctuation because it's so small. Back then, maybe a quantum fluctuation was huge.

00:22:53
Speaker 1: Right, Yeah, it's a really interesting comparison. I guess. Really the only meterstick we have to compare today with back then is the speed of light, And so you do have this sense of like the horizon that an electron could see, like what fraction of the universe an electron could interact with, and then that did later get blown up. And so back then the electron was sort of in a smaller pond of the universe of sort of a bigger deal.

00:23:14
Speaker 2: Then, as you said, these small ripples kind of god stretched out as the universe expanded. So maybe take us through a little bit of what was happening as the universe started to expand, like what was going on with dark matter.

00:23:26
Speaker 1: Yeah, so you have these initial ripples which create over densities, mostly in the dark matter. Remember that there's more dark matter than anything else. And so if your mental image you're imagining some like hot bright plasma, add a layer to that, an invisible layer of dark matter, which has most of the mass of the matter in the universe at the time, not most of the energy. Most of the energy in the universe at this time is still in photons. It's mostly radiation dominated. But most of the stuff in the universe is dark matter. So now you have these little ripples. You have like a little bit more dark matter here and a little bit more dark matter there, and dark matter has gravity of course, it so it starts to pull things in. Because you have a little bit more dark matter, it means it has more gravity than everything around it. It's going to start to pull stuff in, which gives it more density, which gives it more gravity. So dark matter is starting to form clusters, it's starting to amplify those initial quantum fluctuations.

00:24:16
Speaker 2: Well, I guess the big question is what do we know about dark matter in those early moments, Like we know that regular matter, it started to dissociate into protons and electrons, and before that they dissociated even more. Did dark matter break down too, or did it also have quantum fluctuations or does it even have quantumness to it?

00:24:36
Speaker 1: Yeah? Wow, I wish I knew the answer to any of those questions. We don't know, right because we don't know what particles dark matter is made out of, if it's even made out of particles in this theory. Instead, we treat dark matter sort of as like a collisionless fluid, some that has no interactions other than gravity. We think just about its mass density and the gravitational impact of that. We don't try to break it down into the microphysics because we don't have that story at all. We don't know if dark matter is ten different kinds of dark particles that are all turning into each other and back or not. But because it doesn't interact with the baryons except for gravity, we don't really need to know those details. I mean, we'd love to know, who wouldn't want to know, But it doesn't change the story of the baryon acoustic oscillations that we're focused on today.

00:25:19
Speaker 2: I see at this point we're just squinting at dark matter. We're sort of waving our hands. We're like, well, I don't care what's happening at the small microscopic level of dark matter. It could be anything, but you just sort of treat it as, like you said, like a cloud or liquid of stuff.

00:25:35
Speaker 1: Yeah, it's not that we don't care. We deeply care, and we'd love to know. But the game of physics is trying to make progress even when you don't know things. And so here's a question we can focus on even without knowing what's going on with the dark matter. We can still think clearly about what's going on with the baryons because we think we do understand their interactions.

00:25:54
Speaker 2: Right, So then you're saying that the dark matter was influenced by the quantum fluctuations of the regular matter. But could dark matter itself have had its own quantum fluctuations.

00:26:03
Speaker 1: Now, they had their own quantum fluctuations for sure. Dark matter and regular matter both come out of these initial quantum fluctuations. So one spot in the universe we have like an over density of energy that turns into more dark matter and more normal matter. And it's mostly the quantum fluctuations in the dark matter itself that spur everything we're talking about, because it's the gravity of the dark matter that triggers everything.

00:26:24
Speaker 2: Right, because there's more dark matter than regular matter. But then are you assuming that, like the dark matter fluctuations and the regular matter fluctuation, we're somehow in sync in the early universe.

00:26:34
Speaker 1: The quantum fluctuations we're talking about again predate the formation of the particles themselves, and this division of energy into dark matter and normal matter, which frankly, we don't understand, and to understand it, we'd have to have a better idea of what particles there are and how the quantum fields sort of filter out into the dark matter. So we just say that there's an initial quantum fluctuation and then at each point, if you have more stuff or less stuff, you get about eighty percent of it into dark matter and twenty percent of it into normal matter. So from that point of view, they are correlated because they come from the same initial quantum fluctuations, which are independent from the dark matter or the normal matter nature.

00:27:08
Speaker 2: I see, you are sort of imagining a point in the universe when even dark matter was maybe dissociated or didn't exist exactly.

00:27:17
Speaker 1: Those are where the quantum fluctuations are happening before we even have dark matter or normal matter, and then down the road tiny fractions of a second later, when we do have matter, some of that energy has gotten into dark matter and some of it into normal matter.

00:27:29
Speaker 2: Okay, So then both dark matter and regular matter are kind of have these expanding fluctuations ripples, which, as you said, create pockets of the higher density dark matter and regular matter, which then I guess is what creates the sound ways, right, because when you have something more dense in one side, it tends to try to go to the other side.

00:27:50
Speaker 1: Exactly, and sort of a push and a push back here. So dark matter is creating these over densities. It's like gravitationally collapsing things, and that's fine for dark matter. Dark matter doesn't really care happy it gets pulled in by gravity and overlap with itself whatever. But baryons are different. Baryons and photons interact with each other, and so if you squeeze them down, then they're going to push back. Like you squeeze a bunch of baryons together, they push against each other and they push back out. And remember that there's a huge number of baryons but also an enormous number of photons. So as you squeeze these protons together, then they're effectively squeezing on the photons, which push back out. So it's sort of like a mini version of what happens in a star where you collapse it gravitationally and then it creates fusion, and that radiation pressure from the fusion keeps the star from collapsing. Here you have dark matter pulling blobs of baryons and photons together, and then those photons and baryons interacting when they get squeezed to push back out, and that's what creates these ripples in the baryons.

00:28:52
Speaker 2: Like it's like the dark matter collects all of the other the regular matter tries to squeeze it down, but then it bounces back exactly.

00:29:00
Speaker 1: It bounces back sort of like a mini weaker version of a supernova, you know, gravitational collapse, which then bounces back out in impollusion, which leads to an explosion. Some way I want to get clearing people's minds, which is sort of crazy to imagine, is the ratio of different particles, Like there's about a billion photons for every proton and every electron at this point at the universe, Like the universe is mostly light, so there's a huge number of photons pushing against these baryons.

00:29:27
Speaker 2: Now, are you sweeping electrons and protons into radiation here or do you actually mean real photons that later got transformed into electrons.

00:29:37
Speaker 1: Totally fair question, because you're right that if things are moving near the speed of light, we just call it radiation. But here we're talking about real radiation. We're just talking about photons. We're treating electrons, protons, and photons separately, and it really is mostly photons. But those photons they push on the baryons, they push on the protons, they push on the electrons in a way that they of course don't push on the dark matter. So the dark matter is collapsing into the center and the baryons get pushed back out because they have this electric interaction that dark matter doesn't have.

00:30:09
Speaker 2: But the photons are not being pulled together by gravity, are they?

00:30:12
Speaker 1: Photons are affected by gravity? Right, Photons bend around the Sun, or can bend around a black hole. So as dark matter curved space, photons are also gathered into that well together with the protons. But then they push back and there's so many protons, so many photons that you get a sound, right, This is the sound of the early universe. Is this pressure wave in the buryons created by the baryons and the photons being squeezed down by dark matter.

00:30:37
Speaker 2: M it's the sound of regular matter being uncomfortable, like whoa, whoa, I don't want to be so close to my neighbors.

00:30:45
Speaker 1: Exactly, it's here, Exactly. It's the sound on the subway when another ten people get on and squeeze you into the back and you're like, hell, I can't breathe back here.

00:30:54
Speaker 2: It's the groan of a million introverts.

00:30:58
Speaker 1: What you're saying, Yeah, Another one rides. That was the sound of the early universe.

00:31:03
Speaker 2: Another one gets gathered by dark matter against its well.

00:31:07
Speaker 1: Exactly, and the density of the universe is really really high, and the density controls the speed of sound. Like sound travels faster through water than does through air because those molecules are more tightly packed together, so the soundwave propagates more quickly, and their bonds are more rigid because they're denser. So the soundwave propagates faster through denser materials like steel than it does through water, than does through air, than it does through really diffuse gases like the upper atmosphere. And in the early universe, things are super duper crazy dense, So the speed of sound in the early universe is like half the speed of light.

00:31:42
Speaker 2: Whoa wouldn't you have to call it radiation wave?

00:31:48
Speaker 1: Fair point? Fair point?

00:31:50
Speaker 2: All right? So then there were these waves from the material sort of bouncing back, and that means that like those waves propagated at which made things more dents in some places than others, right, because that's what a wave.

00:32:03
Speaker 1: Is, yeah, exactly, So you have this dark matter core and then you have this density wave of baryons propagating out. But this doesn't last forever, right, Things in the universe are happening fast, and the universe is expanding and it's cooling, and at some point, around three hundred and eighty thousand years after this first moment, we can describe what we call the beginning of the universe, or at least the Big Bang. Things cool down enough that the protons and the electrons did bond together to make neutral hydrogen. The electrons no longer had enough energy to escape the pull of the protons, so the universe became transparent to photons instead of opaque. So now when photons are flying through the universe, instead of interacting with all the protons and the electrons, they see now they just see neutral hydrogen. So they no longer push on it. Now they just fly through it. And so the universe can expand and cool, and these photons can dissipate, and so the sound wave basically got frozen.

00:32:54
Speaker 2: It's sort of like if you suddenly frozy ocean, you would see all these water molecules frozen in the shape of a wave.

00:33:01
Speaker 1: Yeah, that's right. Or say you slap your hand in your bathtub and it creates a wave and then you suddenly cool it to freeze it. You can come back later. You can still see that water wave. Otherwise it would have kept propagating and slashing around. But now it's frozen because your bathtub, the water has cooled so it can no longer propagate. And the same thing happened in the universe. The universe became transparent, it became cooler, became less dense, and the photons passed through this wave overcame it. So now that single ring of sound is like frozen in the structure of the early universe.

00:33:32
Speaker 2: Right, But I guess maybe the confusing thing is is that it's like a sound wave in the density of photons. Right. It's like there were sound waves propagating because the regular matter was interacting with photons and with itself. There were waves in that slash. But then it's almost like you took away the regular matter, you took all the protons and electrons out of it, and now suddenly the light was kind of stuck in these like oscillations of density. And that's what we see today.

00:33:58
Speaker 1: The light was really powering these oscillations. It's the thing that was pushing the baryons and the electrons along. Once the electrons and baryons cooled so they became neutral, they're no longer like riding this wave of the light, so they sort of jump off the train. They get frozen where they are, and the light continues on and it just passes right through and it diffuses around, and that becomes the cosmic microwave background light that we still see today. So we see the echoes and the ripples of that light today and we can measure it. But the baryons electrons got left behind after that moment when they could no longer ride the light train, because they became neutral.

00:34:31
Speaker 2: Right, Yeah, that's kind of what I mean is that it's not like the photons continue to ripple with this sound. It's more like you took out the regular matter and so the photons that were creating those waves stayed in those different layers of density, and.

00:34:45
Speaker 1: The photons can keep propagating out and rippling, and they did. In truth, it's a little bit more complicated, it's like sloshing back and forth. But basically the picture you should have in your head is like a core of dark matter and then these rings of frozen sound waves. At the time, we're talking about like five hundred thousand light years across, where you should have like more baryons, like a higher density of baryons, this baryons frozen sound wave like five hundred thousand light years across, and then the light continuing on and slashing through the whole universe.

00:35:14
Speaker 2: Right, It's almost like the light the photons were holding the regular matter in these wave patterns, and then you took away the wave the water basically, and so you have this light kind of stuck in that pattern.

00:35:25
Speaker 1: And we think that basically this seated the structure of the whole universe. After this point, gravity takes over. In places that you have more dark matter and more baryons, things are going to get clustered together more and more and more, and that's where you're going to end up getting galaxies, and that's where you're going to end up getting gas clouds, and then stars and planets and people and podcasts and eventually boba.

00:35:46
Speaker 2: And bows as well. All right, well, let's dig into how we can see this cosmic microwave background. Well, we know about it and also what it means about how we ended up here today, So let's dig into that. But first let's take another quick break. All Right, we're talking about the sound of the early universe, and it sounds like. It sounded kind of uncomfortable. It was really hot and crowded, and the regular matter in the universe did not like it exactly.

00:36:23
Speaker 1: Very loud but very short lived early universe scream.

00:36:26
Speaker 2: And so you were saying that you had these ripples of matter kind of bouncing back from being compressed. Things were slashing around, things had sound waves in it. But then at some point the regular matter kind of froze into place. They got together into atoms, which then let the light continue on. Does that mean that at that point the universe went silent?

00:36:46
Speaker 1: Yeah, basically that's when the universe quieted down and the speed of sound dropped really really fast. Right, So things couldn't propagate nearly as fast.

00:36:55
Speaker 2: Why not, Like we wouldn't regular atoms carry those waves.

00:36:58
Speaker 1: Well, regular atoms can still carry waves the way they do today, Like the sound we hear today is mostly in neutral atoms in the air. Right, So neutral atoms certainly can bump into each other and can certainly carry sound waves. But the pressure was just a lot lower because the photons had decoupled, and so the density was a lot lower, and so the speed of sound just dropped very quickly, and so there still was sound, it was just much slower moving. It's no longer anywhere close to the speed of light, and so it's effectively frozen because soundwaves can still propagate, but just very very slowly. So things are not going to change very fast the way they had initially.

00:37:31
Speaker 2: I wonder if you can still measure those waves in the regular matter, you know what I mean? Like, I wonder if like collectively all the galaxies in the universe still has ours kind of slashing around or being moved around by those sound waves.

00:37:44
Speaker 1: Absolutely you can, and we have looked for this and we have actually seen it. We can see these waves in two different ways. One, we can look back at those photons from the early universe and see these ripples, like there were more photons in some places than in others. We can look back at those photons ones that we're created when the universe just became transparent, that's the cosmic microwave background radiation, and we see these ripples and we see exactly what we expect. But we can also see it in the structure of the universe today. Those rings that were five hundred thousand light years across, they expanded as the universe expands and now we expect them to be about five hundred million light years across. So what people have done is they've looked at the distribution of galaxies and they say, hm, our galaxies just like sprinkled randomly everywhere, or is there a typical distance between the galaxies? How are they clustered? So they gathered a bunch of galaxies together, they did these redshift measurements to see how far away they are, so we can have a three D map of the galaxies in the universe. And then they just like counted up what is the distance between all the pairs of galaxies? Is there any preferred distance?

00:38:49
Speaker 2: And what did they find? Did they find that there's so even or did they find that this distance varied according to leg a soundwave?

00:38:55
Speaker 1: So they found that it was not smooth, that there was a bump there, that you were more like you have galaxies about five hundred million light years apart than you were other distances. And this is exactly what they expected to see because those rings the sound horizon from the early universe was five hundred thousand light years across at that time. But the universe has expanded since right We've had deceleration and acceleration. We know the expansion history of the universe, and we expect those rings to now be five hundred million light years across. And when you look at the distribution of galaxies, you see many more at that distance apart than you do at like four hundred million or six hundred million. So this is like twenty years ago. In two thousand and five, they saw this statistical evidence for the Bury on acoustic ostellations that when you add up all these galaxies and compare their distances, you tend to see the more at exactly the size of this sound ring.

00:39:46
Speaker 2: No wait, are you saying that somehow this early sound wave got frozen and the distances between galaxies and the structure of the universe, or are you saying this sound wave is still rippling through the structure of the universe.

00:39:58
Speaker 1: They got frozen in the early universe and then gravity took over it like seeded the structure. It's like if somebody sprinkled a bunch of seeds in a circle and you came back one hundred years later and you found a bunch of oak trees and you wondered, like, why are there oak trees in a circle? It comes from the initial distribution of seeds. And so here we're talking about slashing around the very early universe when things were still very chaotic, left this over density of baryons in these sound rings which no longer were able to ripple as fast because the photons the decoupled and weren't pushing them anymore, and things got cooler and less dense. And those are likely initial seeds which formed galaxies, which grew up to be galaxies.

00:40:37
Speaker 2: I see. So we also see these frozen sound waves out there exactly.

00:40:41
Speaker 1: And so about twenty years ago people saw the statistical evidence. They're like, oh, the galaxies tend to be more far apart at this particular distance than other distances. And that was evidence that the baryon acoustic ostellations were real, that we were seeing them in the universe. But very excitingly, just a few weeks ago, people see an actual single bubble. When you look out into the unit, you can actually see like a ring, a huge structure, ring of galaxies and superclusters lined up into a massive bubble. How big this thing is, a ring structure about two hundred and fifty mega parsecs around, And we're sort of near the center of it, and at the actual center of it is this huge supercluster called the Buches supercluster, which we think was gathered together because there's a huge dark matter blob at the center of this ripple. And then along the edges are other superclusters that we found, like the Slow and Great Wall and other pieces that we've been discovering of structure here and there in the universe. Turns out they assemble themselves into this incredible, enormous ring two hundred and fifty mega parsex across.

00:41:43
Speaker 2: Now it's a bubble because, as you said, the early universe dark matter brought together this barren matter, the barrion matter bounce back, and when it bounced back, I guess it looked like a bubble, right, that's what you're saying. And then the universe expanded, things froze, and we still see that bubble today exactly.

00:41:58
Speaker 1: And you can look at this paper. You can see in this distribution of galaxies this sort of faint ring. It's not crisp and clear.

00:42:05
Speaker 2: It's not like there're no legs a ring or a bubble.

00:42:07
Speaker 1: It's definitely a bubble. It's a sphere. But you know this is a physics paper, which means it's two dimensional slices. So if you look at the slices, you know, we don't publish in three D yet, we're not three D printing our papers. But actually, if you look online, they have a really cool animation of which you can see the three D version, So it definitely has a three D structure. But in two D slices you see rings.

00:42:25
Speaker 2: I see. But was the analysis done in rings or was it done in a bubble? Or were you saying ring because that's how you read it in the paper.

00:42:31
Speaker 1: Well, originally they spotted it as a ring. They were just like, hold on, is that a huge ring? And then they started looking in three D They're like, wow, look at that. It really is kind of a bubble. And then they calculated the size of it and they were like, huh, this is exactly the size you would expect from a single baryon acoustic ostellation bubble, which nobody had ever seen before. And these folks, they weren't looking for this. They were doing some other studies of galaxies and their distributions, and they just like spotted this visual and they were like, hold on a second, this is literally a frozen scream from the early universe.

00:43:01
Speaker 2: Whoa They were like, that's a big boba.

00:43:04
Speaker 1: It's a big that's when you would choke on for sure.

00:43:07
Speaker 2: Yeah, they choke Maybe they were drinking boba at the time. They're like, well, what what is that?

00:43:12
Speaker 1: And I think it's super cool because it gives us a way to understand not just how our universe was formed and why we have galaxies over here and why we have galaxies over there, but also how the universe expanded, Like we know how big that sound wave when it was created, because it just comes down to like the physics of protons and photons and dark matter, how they push on each other, and we know how big they are. Now we can measure them, and so that gives us like an independent way to measure the expansion of the universe, which of course is a big question and a deep mystery, like the source of dark energy and how that all works.

00:43:44
Speaker 2: I guess maybe a question is why do we see more of these bubbles, Like wasn't the universe filled with these sound ways and these screams of the early universe. We aren't these bubbles more obvious?

00:43:54
Speaker 1: Yeah, great question. We haven't seen that much of the universe. You know, our precision maps of the locations of galaxies basically are just big enough to include one of these. If you look online and check this thing out, you see that this one bubble occupies a huge fraction of the known galaxies we've seen. We just haven't looked out far enough to see one of these things before.

00:44:14
Speaker 2: Oh wow, it's that big of a bubble, Like it's almost the size of the observable universe.

00:44:18
Speaker 1: You're saying, it's almost the size of the set of galaxies that we have mapped. Well, yeah, as things get further out, it's harder and harder to map these things. You need more and more precise measurements. Like if we could use a James Webspace telescope and pointed in every direction for a month, we would get an awesome map of the galaxies in the universe. But the map we have is really sporadic, and in some places it goes really far. In some places it doesn't because we just don't have enough telescopes and enough telescope time to do these careful surveys.

00:44:46
Speaker 2: Well, as you said, it sort of gives us sort of like a marker in the history of the universe and how it expanded. And now what's the connection to dark energy.

00:44:53
Speaker 1: Well, dark energy is our word for how the universe expanded and how that expansion has accelerated. The picture we have is that the early universe was dominated by matter and radiation early on and expanded and things cooled. But then that matter radiation starts to decelerate the expansion of the universe, start to slow it down, because that's what energy density does. It curves space and pulls things back together. But at the same time, some new force was waking up, something we call dark energy, was pushing the other direction and accelerated the expansion of the universe. And this is something we'd like to understand the detail because we don't understand the mechanism for it, but we want to understand the history so we can get a better sense for what might have been causing this. So measuring the precise rate of the expansion and how the universe has grown over time is very, very valuable.

00:45:38
Speaker 2: I see, because I guess these bubbles can't just come up randomly, right.

00:45:41
Speaker 1: Yeah, these bubbles have a fixed size in the early universe, just determined by like the physics of acoustic oscillations which we think we understand, and then they're stretched by dark energy to a new size which we can measure. So measuring the size of these bubbles now and comparing them to the size we knew they had in the early universe gives us a way to say how much has the universe been stretched, which, of course is something we're very interested in.

00:46:04
Speaker 2: All right, Well, another interesting exploration into our origins and how much we can and how much we still don't know about what was happening.

00:46:12
Speaker 1: To me, it's amazing how cosmology has gone from a field where it's like mostly hand wavy stories with rough numbers to a field where we can measure things and do precise calculations and compare this and that and know things about the early universe from these calculations. We have filtered through crazy data to get these stories of the universe, to find these clues to build back this history of what happened and how we all got here.

00:46:37
Speaker 2: I see it's now precision handwaiting.

00:46:42
Speaker 1: Baby steps, man, baby steps, Boba steps.

00:46:44
Speaker 2: You need a thicker straw.

00:46:47
Speaker 1: Well, we need are more smart people thinking hard about how the universe works and asking questions and listening to podcasts.

00:46:53
Speaker 2: All right, Well, the next time you're in a crowded subway, think about how the universe felt back then, how it's screamed out in a discomfort, and how we still see those screams today in the shape and the distribution of galaxies and also light.

00:47:07
Speaker 1: And please continue to enjoy your boba at your own risk.

00:47:10
Speaker 2: Well, we hope you enjoyed that. Thanks for joining us, see you next time.

00:47:22
Speaker 1: Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio Apple Apple Podcasts, or wherever you listen to your favorite shows.