Daniel and Jorge Explain the UniverseDaniel and Jorge dig into the history of matter, dark matter and dark energy, explaining how they used to have very different ratios.
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Hey Daniel, what's your favorite kind of pie? Oh?
Boy? What's not my favorite? I mean they're all so good.
Are you a fan of chocolate pie?
You know?
I am?
I mean, who isn't?
What about white chocolate pie?
Hold on? Is that a thing? I mean? Who would do that to a pie.
I'm sure there's a whole universe of pie out there. I imagine there's a you know, someone in this infinite universe that thinks white chocolate pie is the best.
If so, then that's definitely proof that aliens exist.
But I thought your favorite might be the number pie pie are squared. Give you the area of the pie, the actual pie you want to eat.
I guess pie are around pie for everyone.
Pie am Jorhem, cartoonists and the author of Oliver's Great Big Universe.
Hi. I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I'll try almost any kind of.
Pie, really, any kind of pie. What about Durian pie? Have you tried that one?
I might have to hold my nose, but I hear Durians delicious.
Yeah, I've come across Durian pie, and and you kind of do have to hold your nose if it's not something you're used to. It's pretty powerful stuff.
Sounds like an exotic adventure.
But anyways, Welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which we take you on an exotic tour through the craziness and mysteries of our universe. We hope to explain to you everything that makes sense to us and everything that doesn't make sense to us about the way this amazing, beautiful, sometimes tasty, and sometimes stinky universe works.
That's right. It is a delicious universe and fragrant as well, full of amazing facts and incredible processes going on, all of which we take together, mix it up, put it inside of a crust, and bake it for you here on the podcast.
One of the deepest but most concrete mysteries of the universe is what's it all made out of? Anyway? What's out there in the universe? How much of it is this stuff? How much of it is that stuff?
And why Daniel, would you eat mystery pie? Like, if somebody gave you mystery pie, would you still eat it?
Depends on the somebody. I gotta say, Yeah.
I guess if it's like a ten year old you don't want to try a mystery pie. But if it's like a famous chef, maybe yeah, yah.
Sure, absolutely yeah, I'll eat a mystery pie and no problem.
What if it's like a world famous ten year old chef?
Is this some reality prank show. Then probably no.
That sounds like a great idea for the next YouTube pit mystery pie.
But one reason you might take a bite of mystery pie is just your curiosity. You want to know what kind of pies are out there in the universe, what's possible to make out of the basic building blocks? What kinds of stuff are there in the universe. In the end, that provides some crucial context to the nature of our existence. Are we men out of the most common stuff? What are the kinds of stuff is out there? How does it all come together to make this glorious cosmos?
I guess the universe is kind of like a deep dish pie of mystery, isn't it, Because there's so much we don't know about it. Ninety five percent of the universe is a total mystery to us exactly.
And we can take this pie metaphor one step further and describe what the universe is made out of in terms of a pie chart. Basically, some fraction of the universe is made out of our kind of matter, Another fraction is made out of dark matter, A much bigger fraction is dark energy, mysterious stuff we don't even know so understanding the universe pie is a basic goal of physics.
Yeah, that's a very basic and deep question about the universe that you can ask, what is the universe made out of? What is it all about? And so we've discovered some pretty eye opening facts about the universe, and one of those facts is the idea that the kind of stuff we're made out of, you and I, apples and chocolate and white chocolate. Also they're all made out of a kind of stuff that only represents five percent of the entire universe.
Some of the most delicious moments in physics are those times when you realize that everything you've understood is just a tiny fraction of reality, that you've been looking at it like an unrepresentative sample of everything, and that the wider context is very different from what you imagined. It's like you're opening a third eye to understanding what the universe is really like. And in the last few decades that's been our experience discovering that most of the universe is not the kind of stuff that makes up me and you and hamsters and lamas and pies.
And stars and galaxies. It must have been pretty incredible to think that all of these thousands of years of science and knowledge seeking as humans have all been just for the five percent of the universe that we now know we represent.
Yeah, and it's sort of a special moment in science when we know very well how little we know. We're always prepared for the fact that there's stuff we don't know out there, But now we can measure very precisely what fraction of the universe is the stuff we know anything about, which tells us very accurately how little we know about the universe.
Yeah, And so today on the podcast, we'll be asking the question what's the matter with the matter in the universe? Something wrong with the universe? Or is it more like, what's the matter with you? Like it's doing something it shouldn't be doing.
Well, there's lots of really interesting questions you could ask about this universe, pie the fractions of the universe that are made up of the various components. One of them is like why is it this way? Another another way? But you can also ask questions like has it always been this way? Has the universe had different distributions of matter and dark matter and dark energy at other points in its history? And what does the future hold.
Like how long has this pie been sitting out there? And has it changed or decayed or rotted or changed to another kind of pie in the meantime?
Does it matter? When you get a slice of the Universe pie, You might get a very different serving. It might be more chocolatey or less chocolity, depending on when you got your slice.
Whoa like not just a mystery pie, but like an ever changing mystery.
Pie exactly because the universe is a dynamic place. It's not static. It's expanding and it's cooling, and it's sloshing around and frothing, it's making galaxies and black holes. All sorts of stuff is happening, and so we have no reason to believe that the Universe pie today is the Universe pie that's always been.
Do you think the Universe pie is getting more delicious or less delicious?
I'm an optimist, so I like to think that the Universe pies most delicious days are ahead of us.
Oh wait, so that means you're never going to eat some, are you until the day before you die or something?
No, it means I'll continuously eat it and every bite will be more tasty than the last.
Oh, but what if it's not infinite. What if it's finite, then you got to play some sort of like you know, optimization game here, how much of the pie should you eat every day?
You think there's some scenario where I eat the last bite of the universe and use it up.
I don't know how hungry are you for knowledge about the universe?
Famished, But you know what they say that last bite is always the most delicious, like the last cookie in the bag.
Yeah yeah, but although if you eat it right before you talk, you're not going to have a very long lasting memory of it.
But I guess if I eat the entire universe, then I'm the last bite of the universe. I'd have to like eat myself.
Wait, are you saying you're the most delicious thing in the universe?
If I eat everything else, then I'm the only thing left in the universe, and I'm both the most and least delicious thing in this very realistic example.
Right right, Yeah, yeah, I don't know if that. I think our whole analogy just just broke down.
Yeah, Daniel contains super massive black holes.
But anyways, we have found out that the pie of the universe is made out of only five percent of the stuff that we're familiar with, the rest of the stuff is a big mystery. And as you said, Daniel, these percentages have been changing exactly.
You might think that five percent is a minuscule fraction of the universe, but wait till you hear about the fraction that it's been in the past.
Well, it's usually what we were wondering how many people out there had thought about the pinhness of the univer and it's composition, And so Daniel went out there and asked folks, has normal matter always been five percent of the universe?
Thanks very much to everybody who answers these questions. I really appreciate it, and we love hearing your voices on the podcast. Gives us the sense that the podcast is not one directional, it's an interactive learning experience. So if you'd like to participate in the future, please write to me two questions at Danielandhorge dot com.
So think about it for a second. Do you think normal matter has always been five percent of the universe? Here's what people had to say.
Well, I guess you've got normal matter, and you've got dark matter, and you've got dark energy. I know that dark energy is getting bigger as the universe expands, so I would guess that when the universe was really small, just after the Big Bang, there wasn't any dark energy, so therefore normal matter would have been a much greater proportion.
I don't think that there is a reason for that percentage to be constant, but to assume that is also to assume that regular matter, dark matter, or dark energy to be created spontaneously.
No, norm matter has not always been five percent of the universe dark during the Big Bang. As far as I know, normal matter an inchie matter almost out of one on ratio, but no matter one out is there for part of our own universe. As for dark round and dark energy, I don't know about that part.
All right, awesome answers here. Most people just assume that it hasn't always been five percent.
Yeah, I think people are ready to be surprised.
I guess maybe people are used to having a universe be always changing, so like, why would it be the same.
Yeah, that's true. Although the universe is fourteen billion years old, so you might speculate that it's kind of grown up already and not going to change that much anymore.
M All right, well, let's dig into it. Daniel let's first of all talk about this pie and where it came from, and what we know about the pie of the universe, how much normal matter is there now, and is it really that normal?
Normal is really a very inappropriate label for our kind of matter, because we're not the biggest chunk of the pie at all. Really, we should call that like familiar matter or just our kind of matter.
Or something delicious matter. Apparently we are the most delicious things in the universe.
And so on the podcast, we often talk about the universe pie in three different categories normal matter, dark matter, and dark energy. Physicists usually break this down into four different categories, where they distinguish normal matter and radiation into two different categories. So let's break that down. Normal matter is the kind of stuff that you and I are made out of. Things built out of quarks and electrons, which usually go to making protons and neutrons, and then electrons make atoms, et cetera. So all the atomic matter, but it also includes things made out of exotic quarks, top quarks and down corks and bottom quarks, stuff that doesn't make up the proton and the neutron but can be created in the universe. All this kind of stuff we call baryonic matter or normal matter.
Would you categorize all that stuff stuff that has mass, like, those are all particles with mass.
Those are all particles with mass, But not all particles with mass are in that category. Dark matter, for example, we think has mass, but is not in that.
Category assuming it's a particle.
Though, assuming it's a particle or some kind of matter with mass. So yeah, all the normal matter has mass. That's one of the distinctions physicists draw between matter and radiation. Radiation is things that are effectively massless, things moving at or very near the speed of light. So photons, for example, are radiation. That's obvious. There have always been radiation, they always will be radiation. Neutrinos are very very low mass particles that move at almost to the speed of light, so we count them as radiation, even though technically they're made out of stuff. They have mass to them.
Wait, neutrinas we don't count as normal matter.
Why not we count them in the sort of normal slice, but we don't call them matter. We call them radiation if they're moving near the speed of light. And you'll see why this distinction is important when we talk about how the universe expands, because the expansion of the universeffects radiation and matter differently.
All right, then what are the other slices of the pie?
One more caveat on the matter radiation distinction is that the same particle can be in a different category depending on its speed. If you take an electron, for example, and it's just sitting there, it's not moving, it has mass, it's not moving near the speed of light, you call it matter. Speed up that electron to nearly the speed of light. Physicists now call that radiation because it's effectively massless. Its energy is so much bigger than its mass. It's going near the speed of light, Now you'd call that radiation. So the same kind of particles can move from one category to the other as the universe cools or heats. So, as usual, physicists have given things kind of confusing names.
Also kind of arbitrary, isn't it. It's like, when it goes kind of fast, it's totally categorized it as one thing, and when it goes kind of slow, it's totally something totally different. Doesn't seem very scientific, Daniel.
Yeah, Well, there are differences in the way the unit verse treats things moving near the speed of light or at the speed of light and things moving slower. It's not a bright line between them. There's a transition there. But that's true for basically all of science. You know, what's it different between water vapor and water liquid. There is a difference, right, It's not a bright line, there's a transition there between them. But we notice these trends and we draw a dotted line and we treat them as different stuff. So here physicists notice that as the universe expands, matter and radiation get treated differently, so they've drawn this line between them. Though you're right, it's really part of a continuum.
All right. So then what are some of the other pieces of the pie?
So the really big pieces of the pie are dark matter? Right. Dark matter is something that's out there in the universe. We know it's not made out of quarks and electrons or photons or neutrinos or any kind of particle that we know about. It's some kind of matter that's out there. We see its effect gravitationally, so we know that it has some kind of mass because it bends space in the universe. It causes things to move. It holds together, galaxies that spind, It changes the structure of the universe, all this kind of stuff. So we know that it's out there, but we don't know what it.
Is, right, And so far we only categorize it as something different because we don't know what it is. It could be like, if we know what it is, we would maybe categorize it as normal matter too, right, Or if a physicist looks at it in squints, maybe it'll say it's radiation too well.
There could be dark matter and dark radiation, right. If you have dark matter particles moving near the speed of light, there would be dark radiation. We are very confident, though nobody can ever be certain, that dark matter is not made out of normal matter. It's not like a novel rearrangement of quarks that hides in dark blobs, or some weird combination of neutrinos, or just zillions and zillions of neutrinos. And if you're curious about more details about that, check out all of our episodes on dark matter. Briefly, we are pretty sure that dark matter is not made of quarks because we know a lot about the number of quarks in their universe that controls the amount of helium and hygien made very early in the universe, so we're pretty sure we can account for the number of quarks, and there's not enough to explain the dark matter. We know that dark matter is not neutrinos because neutrinos move at nearly the speed of light, and for dark matter to have affected the shape and the structure of the universe that we see today, the galaxies and their distributions, it can't be moving very fast. It's low speed, and so we're pretty sure dark matter is not made of neutrinos. All of which to say, we're pretty sure that dark matter is a different kind of stuff. It's not made out of normal matter. It really is a different slice of the pie.
All right, Well, let's get into maybe the deeper parts of our mystery pie, which is dark energy. So let's dig into that. But first, let's take a quick break.
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All right, we're eating up the pie of the universe here today. We're serving folks a big slice or a little slice.
We are serving up a big, fat slice of universe pie. And it's mostly a dark pie. I'm hoping it's dark chocolate.
You're hoping what else could something dark in a pie?
Casey, Yeah, mudpie is the most innocent.
Example, you boisonberry. Go. Well, the pie of the universe, as we talked about, is about five percent the normal stuff that you and I are made out of, that hamsters and stars and planets are made out of. About twenty seven percent is dark matter, at least matter we so far that we don't know whether where to categorize it or we don't know much about it. But there is the big piece of the pie. Basically, the filling of the pie is something totally different.
All the stuff we talked about today, dark matter, normal matter, radiation only adds up to be like thirty two percent of the stuff in the universe. Normal matter is five percent, dark matter is twenty seven percent. Radiation is almost zero. It's like ten to the minus four fraction. Dark energy is sixty eight percent of the energy of the universe, and we don't know what it is. All we know is that dark energy is the mysterious stuff that's making the universe expand faster and faster.
Yeah, I guess. One day we notice that the universe is expanding, it's accelerating in its expansion, and so we gave that acceleration or whatever it might be causing that acceleration a name. That's the name dark energy.
Yeah, exactly. And we measure that expansion and we think how much energy does it take to make that accelerating expansion happen. And that's what we call dark energy. And we have a couple different ways to measure dark energy. One is just by measuring the expansion in the universe, and the other is by looking at the overall curvature of the universe. All these pieces of the pie have energy, so they all contribute energy density to the universe. And Einstein tells us that energy density, not just mass, is what curves space and causes the effect we call gravity. So all of these things contribute to the overall energy density of the universe, which affects its curvature. As we recently talked about on the podcast. They all add up to make space nice and flat. So they all add up to what we call the critical density. We can measure the sum of all of these components by measuring the overall curvature of the universe.
It's a flat pie universe. It's more like a pizza pie.
You're saying, it's a thin and crispy pie, not a deep dish.
Well, I guess I have a question, which is, like what form is dark energy present in the universe? Like, is it actually there or is it kind of like hidden behind the scenes that it only comes out when to expand the universe.
We don't know what dark energy is like. We don't have a microscopic picture of what it looks like, or what it's doing or how it works. We know really very little about it. It fits into our model of the expansion of the universe and the accelerating expansion of the universe if it does a couple of things, If it's some kind of field the way that light is a wiggle in the electromagnetic field. We imagine some other new kind of field, some kind of field that has high potential energy the way, for example, the Higgs field has high potential energy. It's stuck in its little local minimum, which is why it has so much energy stored in it. So if you have some field a bunch of potential energy stored in it, it has this accelerating expansion effect according to general relativity. So that's basically all we know about it. We don't know what field it is. We don't know why it exists. It's a big mystery. If you try to ask, like, well, do the fields we know about can they provide that potential energy? Do the calculation? That turns out the answers know and the fields we know about are off by about a factor of ten to the one hundred. So we really very clueless about what it is that's creating this accelerating expansion. When you hear dark energy, you should really just think about it as a description of our observation of the expansion, not any sort of understanding of what's causing it.
I wonder if it's like if you take two bowling balls and you connect them with an invisible spring, and then you bring the bowling balls together. You're creating like some stored potential energy between the two bowling balls. So you can't see it, but it's there. There's that potential energy hitting there is dark energy sort of like that, Like there's something about space that has this potential energy stored in it that's making everything accelerate and get bigger.
Yeah, that's a great description. Potential energy can be sort of invisible, right, It's just the configuration of that field. That's what potential energy is. Like you put that bowling ball up on a shelf, it's got energy stored in its configuration, the fact that it's up on the shelf and not on the floor. Now you take a field and you displace it from zero, you say, okay, the field has some value. And fields can have all sorts of different potential energies depending on their values. And so this particular field, whatever it is, seems to have a lot of potential energy stored in it in whatever configuration it happens to be in. It's like an infinite number of invisible bowling balls stored on the shelves everywhere in space.
Or like in an infinite number of invisible springs tied between different everything together. Right, Yeah, that are precompressed exactly.
And there's a seeming contradiction here in dark energy. Like on one hand we say dark energy is sixty eight percent of the energy in the universe, you might think, wow, it's overwhelming, it's dominant, it controls everything. But in our lives and in our experience and in our Solar system, dark energy is basically negligible, Like we didn't discover it until we looked at the expansion of the universe on large scales. You can't see it in the Solar System. You can't feel it with gravitational experiences here on Earth. Because dark energy gets stronger over greater distances. It's the opposite of gravity. Gravity gets weaker over greater distances. Like we have two bowling balls next to each other, they feel each other's gravity. You put one on Jupiter, you can basically ignore the gravity the other bowling ball. But dark energy, because it's a feature of space itself. As you get more space between stuff, it gets stronger and stronger. So you don't see dark energy in less you're looking over really really large distance scales and so it has no effect on my life, or door life, or our life in the Solar System or super high precision measurements of the orbit of Jupiter can't detect any dark energy, but it is the dominant fraction of energy in the universe.
I guess it's the reminder of how big the universe is, right Like places like Earth where you have a lot of normal matter clumped together, or even our Solar System, they're pretty rare in the universe. Right Like, outside the Earth, it's mostly empty space. Outside of the Solar System, it's even more empty space.
In the same way that dark matter overwhelms the kind of matter in the universe that's also not in our experience, right like you can't detect dark matter in our Solar System by looking at like how the voyager probe moves in its gravity, and that's because it's spread out through all that empty space. Dark matter is just not nearly as clumped as normal matter. So all the space between us and other stars where there's almost no normal matter is smoothly filled with dark matter. So, as you say, the bigness of the universe means that that adds up to a big chunk of the.
Pie all right, Well, let's summarize the pie. Then we know that the pie of the universe is about five percent normal matter, twenty seven percent dark matter, almost zero radiation, and about sixty eight percent dark energy. That's the pie of the universe as we know it today. And so the big question of the episode today is has that always been the case? Has it always been those percentages? And will it change in the future? So what do we know of that, Neil?
So we know that these fractions can change and have changed, and we're radically different earlier in the history of the universe and very likely will be radically different in the future of the universe. And it turns out we live in a very peculiar time in the history of the universe when these fractions are at all similar to each other. Most times in the universe, one of these fractions totally dominated, and in the deep future of the universe, we expect one of these fractions to totally dominate. Sort of weird that things are kind of at all in balance right now.
Well, I guess there are different ways that the universe can change its comp decision, right, Like maybe a normal matter turns into dark matter or dark matter turns into dark energy, or something like pies is there, but the ingredients change from one to the other. Or you could have like the pies growing and just like every day there's more dark energy, there's more dark matter, and so the percentage is changed in that way. Right, So in which way is the universe changing?
So in all of those ways? Number One, A certain kind of stuff can turn into other kinds of stuff. For example, matter can emit radiation. An electron hanging out in the universe can give off a photon that increases the radiation fraction of the universe. Photons can turn into matter. A photon can convert into an electron, and a positron that's converting radiation into matter. Very straightforward, and these kind of processes happen. Also, we think between normal matter and dark matter that we can dig into that in a minute. Also, the universe is expanding, and as it expands, the various fractions get treated differently. They dilute differently as the universe gets bigger. Finally, the universe is cooling. It's lowering in temperature, and as the temperature gets colder, some of these processes that convert one thing to another turn on or off at various times. So all these things change through the history of the universe because of all of those reasons.
Interesting, So there are many ways in which the universe is changing. Although I feel like you forgot the third way in which the universe can change.
What's that puberty?
No, when a physicist you know, looks at it differently or wakes up differently one morning, it says, I think that's radiation. No, you know today that looks like normal matter.
There's always an arbitrariness to how we call these things. And you know, I look forward to arguing with alien physicists about the meaning of radiation.
But as you said, the universe has gone through a lot of changes in its body. I guess it's gone through puberty, or is going through puberty. And so Daniel maybe steps through what some of these changes have been, and what are they like, what happened in them?
So let's start at the very beginning. Zoom back to the earliest point we know, which is when the universe was very and very hot, right, filled with some kind of plasma. We don't know what happened before this. We don't know what came before this to create this hot, dense plasma. Their theories about inflation and infulton particles and all that is very very speculative. What we are very certain about is that about thirteen point eight billion years ago, the universe was very hot and very dense and everything was sloshing around. And from that point on we can model very precisely how that expands and how it cools and everything changes. And that's about as far back as we can go and be confident. We can speculate deeper and talk about like crazy theories about before that. But what we know very well precision cosmology takes us back to that moment when the universe was very hot and very dense, and at that moment, we think the universe was ninety nine point ninety nine a bunch more nine percent radiation. That was essentially all radiation, and everything else was a tiny fraction.
Like it was all basically light in the Trino's right, because that's the only thing you count as radiation is light in the trinas.
Oh, I'm going to disappoint you. Actually, if you had electrons back then, they counted as radiation because the universe was so hot, electrons were moving at the speed of light, and basically everything was moving it nearly the speed of light because the universe was so hot. Everything was so fast that it didn't really matter how much mass it had, even like a top cork if it existed back then. The top quark is the most massive fundamental particle we know about. But back then, in the very early universe, it was moving it basically the speed of light because the universe was that hot, and so everything gets counted as radiation.
I feel like you're basically just making stuf about that. I tried to warn you it's going to dig into it for a second. You're like, why is it important that if it's moving it closest to the speed of light you call electron radiation. Isn't it still an electron.
It's still an electron, absolutely, But it's going to be important as soon as the universe starts expanding and cooling. I just want to add that there are other reasons to think that the very early universe was filled with radiation, not just because we call it radiation, but also because frankly, it was very hot and filled with charge particles. And what do hot charge particles do? They give off photons. So there really were a lot of photons in the early universe, many more than we have today. It's not just a slippery naming scheme.
But it's partly a slippery naming scheme.
Yes, absolutely, it's partly a slippery naming scheme, but not entirely. Also, in the early universe there was matter and antimatter, and that annihilates and forms photons. So there really was a lot of photons in the early universe. So it's partially a slippery naming scheme calling stuff radiation that today we wouldn't But also there really was a lot of radiation stuff that we would call radiation today.
Well, then let's maybe be helpful to people and break it down out of this is what do you call radiation? How much of it was actually light and how much of it was electrons or quarts or you know things with that today we would call normal matter.
Thanks to the CMB and and bearing on acoustic oscillation, we can actually make measurements of like the photon to cork ratios. So we do have those numbers. They change sort of rapidly as the universe is cooling.
Okay, so then so then what was causing these changes between like electrons and photons?
So what happens very early on, is the universe starts to expand. Right. What's happening is more space is being created, but you're not creating more stuff, right, And so what happens if you have like ten particles in a tiny box and then you make the box bigger, Well, now the density decreases, right, and it gets more dilute, And that makes sense. So the energy density is dropping, dropping, dropping, So that's how it works for matter. Right, you increase the box, you get a drop in energy density. For radiation, the rules are a little bit different. For radiation. The energy density actually drops faster as you increase the size of the box because the particles get red shifted. The expansion of space stretches the wavelengths of these particles.
The photons, But doesn't it stretch also the electron if.
They're moving very fast. If they are treated as radiation, And that's the distinction if you're considered radiation, it's because effectively you have no mass, which makes your wavelength expand as space gets expanded. That means that your energy density is dropping more quickly because not only are you getting more dilute, you're also getting red shifted. So as time goes on, the energy density of radiation drops faster than the energy density of matter, which does two things. One, now some of that radiation slides over into the matter category, and also the radiation slice of the pie starts to decrease relative to the matter slice of the pie.
Okay, I think what you're maybe saying is that as the universe expands, the amount of energy that's stored in momentum is changing. That's really kind of what's happening, right, And so at some point, for example, the electrons were mostly momentum because we're going so fast, but at some point they slowed down so much that they were mostly just the mass of the electron.
Yeah, that's exactly right. But remember that there's something else also happening for photons, right. Photons do get red shifted, and people write in and ask about this all the time, like what happens to the energy of a red shifted photon? Where did it go? Because we've had the concept of conservation of energy banged into our head for so many years, Well, expanding space does not respect the conservation of energy. If you have a photon in a chunk of space and then you expand that space, the wavelength of that photon also gets expanded because space itself is expanding, and so now it has less energy, and that energy didn't go anywhere. There's just is less of it because the universe doesn't respect conservation of energy unless space is fixed.
But I guess the question is, like, it sounds like our classification of what we call radiation and matter was changing very rapidly during those times. But were the actual like number of electrons changing, Were there more electrons being created or destroyed? Was there more light being created or destroyed? Or did that mostly stay the same.
In the very beginning that mostly stayed the same. You had a bunch of processes. These things are slashing back and forth. Photons can turn into electron positron pairs. Electron positron pairs can to annhilate back into photons. The very early universe, we think was in thermal equilibrium. You have all these things going in both directions and they basically equalized. That's what happens at a hot plasma. If you give it enough time, then the universe is expanding and some stuff gets slowed down, so it falls out of the radiation category into matter and stuff that got left in the radiation category is losing energy density compared to the matter category. So now the matter category is growing and the radiation category is dropping, and so eventually we get to a matter dominated universe. The universe started out dominated by radiation, but as it expands and cools, it becomes a matter dominated universe.
And so in that way, you would say that the percentages of matter and radiation in the universe were changing.
That's exactly right, and that happened about fifty thousand years after the Big Bang. At that point, the matter and radiation sort of hit a crossover point. Radiation is dropping more quickly, but it starts out higher, and that's the point where they cross each other. Meanwhile, dark energy is humming along like a tiny fraction of the universe like zerz or zerzer one, this whole time playing the long game, waiting for its turn to dominate. But at around fifty thousand years the universe became matter dominated. It was cooled enough that a lot of stuff slowed down and became essentially called matter, and the photon energies got decreased because of this radiation stretching.
Interesting, let's get into then the long game of dark matter and the newcomer dark energy, and how it took over the entire pie of the universe. Let's dig into that, but first let's take another quick break.
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All right.
We're talking about the universe pie, the chocolate pie, the white chocolate pies, all the pies in the universe and what they're made out of, and how that's been changing since the birth of the universe and throughout all of its history and maybe into the future. And so if we've learned that at the beginning of the universe, physicists would qualify most of the stuff in the universe as radiation because it was going so fast. But as it cooled down and expanded, things sort of slowed down enough that they started to be called more a regular matter, which is the kind of matter that we're made out of. And so you were saying that dark matter was sort of waiting in the wings to make an appearance here in the history of the universe.
Well, it was more thinking about dark energy as waiting in the wings. Playing the very, very long, the billion year game. The first few tens of thousands of years were a battle between radiation and matter, including normal matter and dark matter. Both of those contributions combined were still smaller than radiation in the very early times. Than after about fifty thousand years, matter together overwhelmed the radiation category, and that includes dark matter and normal matter. But there's an interesting mix there between the matter and the dark matter fractions, like there were a bunch of electrons and there were a bunch of protons. But we also think there was a lot of dark matter in the early universe, and we suspect very strongly that in the early universe, matter and dark matter were turning into each other, that there was some kind of force that allowed one to turn into the other or back.
Wait, And so in the point zero zero zero zero zero zero one percent of stuff in the early universe that you call matter, you're also including dark matter in there.
Also including dark matter in there, exactly. And we think that there was a higher percentage of that that was dark matter than there is today, that at the matter section of the pie, dark matter plus normal matter, that there was more dark matter matter as a fraction than there is today.
And you're saying that dark matter can turn and us turning into regular matter and vice versa because it's so hot.
Yes, and we don't understand how this works, because we don't know what dark matter is and whether it's a particle and what forces it feels. But in order to tell the story and be consistent with everything we understand, there needs to be some mechanism for dark matter and normal matter to slosh into each other to explain how we got from having more dark matter in the early universe to having less of it today. The story goes something like this dark matter we think is some massive particle, some very heavy particle, and as the universe is cooling, matter and dark matter are turning back and forth into each other. But as things get colder and colder, matter can no longer turn into dark matter because dark matter is too heavy. No longer is there enough energy around to smash together normal matter particles and turn it into dark matter. So dark matter creation stops, but dark matter annihilation doesn't. Dark matter is still turning into normal matter right in that one direction, but the reverse process is no longer happening. So instead of being in balance, now dark matter is turning into normal matter, but it's not happening the reverse and so the dark matter fraction shrinks.
Whoa, So like the regular matters was growing.
In the early universe, some dark matter got turned into normal matter and then it got frozen out. It didn't get turned back into dark matter like it did when things were hot and slashing around freely.
Then the pie bake exactly.
Somebody baked the pie. And then as the universe expands, it cools further and there's less and less dark matter. Then there's not enough dark matter around for there to be much annihilation. So dark matter stops turning into normal matter and it gets freezes in or baked in. I guess you could call it in our analogy, but in physics terms they call it dark matter freeze out.
So then when did this freeze out or baking happen?
It happened about ten to the miners eight seconds after the Big Bang?
WHOA what? And since then it's been the same proportion of regular matter and dark matter.
Yeah, the dark matter normal matter fraction got frozen in very very early on as the universe expanded and cooled, and then the radiation normal matter fraction changes as things expand further. And that turnover point was like ten thousand years. So there's lots of really fascinating time scales here.
Okay, So then since ten to the minus second, since the Big Bang, and we've had the same amount of regular matter and dark matter. It sounds like what you're saying, And so those proportions stayed the same, right, it would be about five to one, right, m M. But then waiting in the wings, you're saying there was dark energy. So back then there was no dark energy.
There was not no dark energy, but there was less universe than there is today, and dark energy is built into space. Every chunk of space comes with dark energy, so you have less universe you have less dark energy, but it's constant in density, right, more universe, less universe. You don't change the density of dark energy. But as you expand the universe, you do change the density of matter and radiation. As we talked about, you expand the box, you have a smaller density of matter, and radiation drops even faster. So as the universe is expanding, both and radiation are having their energy density drop very very quickly. But dark energy isn't it's a constant density. You make more space, you get more dark energy. That doesn't happen for electrons. So as the universe expands, dark energy starts to creep up in its fraction.
Although I wonder if you can make the case that dark energy was always there, like if we don't know what it is, and it's just like a hidden, invisible potential energy that could may move things. Wouldn't you say it was already built into the universe from the beginning.
Yeah, that's exactly the model, right. We think it's an inherent part of space. As long as you had space, you had dark energy, and it was there during this first ten to the minus eight seconds. But the other stuff had such a high energy density that it swamped the whole pie. The dark energy was always there with its same energy density, It was just small compared to the energy density of the other components, which then faded as the universe expanded.
I guess what I mean is like, if you're closing the universe off at a certain point and you're saying that the energy of the universe is not conserved. There's more energy being pumped into it. But if you count the pump and where this energy is coming from, then maybe I wonder if you could say that dark energy was always there at the same percentage.
Yeah, perhaps, And there's lots of the theories of dark energy and what it might be is it's some kind of weird field. Is it some kind of other stuff? And so those various theories would upset these fractions if you included them.
Yeah, And so that's how the universe pie has been changing. But it seems like it hasn't really changed much since ten to the minus eight seconds, since the Big Bang, except just that dark energy has been growing.
Yeah, dark energy has been growing, which makes for a fascinating tug of war because dark energy is growing. But in the early universe, like a billion years in We're still matter dominated, right, Radiation is faded away. We're in a matter dominated universe. The universe is expanding, but that expansion is now decelerating. It's slowing down because the universe is dominated by matter, and what does matter tend to do? Pulls things together, right, It slows down the expansion of the universe. But because it was expanding still even though decelerating, dark energy is creeping up and up and up, and dark energy makes the universe expand faster, and eventually the universe expanded enough so that dark energy just took over. And around eight billion years after the beginning of the universe, dark energy was the dominant fraction. And you'll see that the universe start to accelerate its expansion because dark energy takes over. And that's basically the future. Dark energy is a runaway process. If nothing else changes, dark energy will continue to grow as a fraction of the energy density of the universe, making the universe accelerate faster and faster, increasing the dark energy fraction faster and faster.
All right, So then in like a billion years from now, what's going to be the percentage or a pie breakdown of the universe. If you had to.
Get Well, it's like sixty eight percent dark energy. Now it's just going to increase. It's going to go to ninety percent, ninety five percent, ninety nine percent. But this is over billions and billions and trillions of years into the future. But it's only going to crank up. But you know, we don't know that, right. Remember, we don't know what dark energy is. We don't know for sure what its behavior is going to be in the future. This model describes very very well the history of these energy fractions and how the universe pie has changed, with a couple of caveats, like the measurements of the dark energy in the early universe don't one hundred percent agree with our measurements in the late universe. There is this hubble tension. But mostly this picture of the sloshing pieces of the pie holds together very well and matches all of our data.
All right. So then in the future, normal matter, dark matter, and radiation, they're all going to stay the same amount, but the percentage is going to go down because dark energy is growing, and so in the far future it's going to be like ninety nine nine nine nine percent dark energy.
Then the universe, Yeah, exactly. And the universe started out dominated by one fraction of the pie, it's going to end up dominated by another fraction of the pie. And there's this little window in the middle where multiple fractions are not zero. It's a very unusual time to be in the universe when you have a bunch of different kinds of stuff around. You got dark matter, you've got normal matter, you've got dark energy, all at the same time.
But according to you, we're still in the undelicious part of it universe. Apparently, apparently, thanks things can't get worse, but they can only get better.
I wouldn't say undelicious, I'd say less delicious than the future. That's the optimistic way to think about it.
Oh, there you go. And let's hope that dark energy is the most delicious thing in the universe, because that seems to be the only thing that's going to be around in the future percentage wise.
And caveats for those of you who they like to think a lot about dark matter. This assumes a fairly simple model of dark matter that it was in thermal equilibrium with everything else early in the universe. That doesn't have to be the case. You can have other theories of dark matter, axions, et cetera that are so weakly coupled that hardly interacts, so they're not thermally mixed with the stuff in the universe. There's lots of other ways you could change this picture. This is like the simplest model we can make that describes everything we see, and it works really pretty well.
All right, Well, it's another big reminder that the universe is a big, mysterious piece of pastry out there. There's still a lot to learn, a lot to explore, a lot to taste, and a lot to bake as well.
And as you take a big bite of the universe, remember that not only is our kind of stuff unusual in the universe, it's getting more and more unusual. And we live in an unusual time in the universe when our kind of stuff and dark matter is a really significant fraction of the stuff out there in the universe, alien civilizations in the year three trillion, we'll have a very different kind of physics to deal with.
All right, Well, we hope you go out there and grab your piece of the pie of the curiosity and mystery of the universe. You hope you enjoyed that. Thanks for joining us. See you next time.
Thanks for listening, and remember that Daniel and Jorge explain the Universe is a production of iHeart Radio. Or more podcasts from iHeart Radio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases. Many farms use anaerobic digesters to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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