Daniel and Kelly’s Extraordinary Universe - What broke the grand unified force in the early universe?

Episode Date: June 25, 2026

Daniel and Kelly walk through the early history of the Universe to explain how forces can be unified, or broken.See omnystudio.com/listener for privacy information.

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Physicists talk a lot about what happened in the first moments of the United States. universe. Non-physicists sometimes ask, why do you all care so much? Why should anyone care? This question flabbergasts me. Of course we should care. Who wouldn't want to know? It's the literal cosmological context of our lives. Knowing what happened in the early universe would be a huge clue in all sorts of late-night dorm room conversations about why are we here and it could inform, you know, how we should live our lives. How is it possible, I wonder, for people to walk around not caring about the early universe, not wanting to know about the origin of the cosmos. If you were expecting me at this point to drop some personal growth story where I came to realize

that not everyone cares about these things, I'm going to disappoint you. I'm still totally flabbergasted if people aren't interested in the early universe because we think there are so many answers to the big questions waiting for us there. When the universe was hot and dense, it behaved differently. The forces were unified and things were harmonious, described by a grand unified theory, which we hope has mathematical beauty enough to transcend its unfortunate acronym, gut. As the universe cooled, these symmetries cracked and broke into the laws that we have today. On today's episode, we're going to be digging into those first few moments. It's explaining how forces can be unified and what it means for them to break apart.

We'll take a journey through time, space, energy, and ugly acronyms. Welcome to Daniel and Kelly's extraordinary acronymical universe. I study parasites and space and gut is an acronym in animal behavior as well, but for us it means giving up time. Hi, I'm Daniel. I'm a particle physicist who all. also likes thinking about aliens, and I'm in a hot competition to be vice provost for gut research at the Whiteson Institute. Oh, but that means something totally different. That's not an acronym at all, though, right? That's just straight up guts.

No one's giving up over there. There's no depressing giving up. No, Katrina studies literal, actual guts. And I study hypothetical potential grand unified theories. And I feel like, you know, maybe that's more important. I don't know. We argue about it. No, you're wrong.

You're wrong. Giving up time has to do. With eating, though, it's about like how long do you forage in a certain patch before you decide that your payoff would be higher if you went to go forage in a different patch? Oh, I see. Because, you know, the cost of being in that patch and the amount of food available, etc. It's actually very mathy. A lot of math goes into figuring out giving up time.

But anyway. Well, maybe the grand unified theory all along was how we're going to unify all of our science into one acronym. Whoa. Yeah. I guess. The meta-acronym. Yeah.

Yeah, I feel like it should be something funny. Maybe gut is not going to be it. But here's my question for you. So today we're talking a lot about the Big Bang or the BB. The BB? Oh, no, that's terrible. Your outline says BB throughout. And so I assumed that was like a field thing.

Is that just a Daniel laziness thing? Nobody ever pronounces that acronym the BB. Nobody refers to the Big Bang as the BB. Although maybe now they will. And though I've written that many, many times, I've never even mentally thought of it out loud as like the BB. All right. Well, I am a real trendsetter. And so I suspect from now on when you go to conferences, it's going to be the BB.

Oh, no. Said quickly just like that. But so here's my question for you. So, you know, we talk regularly about how we know what it was like, like, at the moment of the Big Bang and then moving forward. But we can't know what it was like before the Big Bang because we can only see. what happened at the Big Bang and then afterwards. But what do you think it was like before the Big Bang? Because surely you have smoked some banana peels and thought about that. And so what, like if you had to guess, what would you guess? What is your like sleepy time dreaming? What is, what was it like before the BB?

Yeah. Well, great question. The pre-BB. That's a good question. You know, I really don't know, but I do have a mental image for placeholders. Like if I don't know, my, brain conjures up this black and gray fuzz where you can just make out the outlines of huge purple dragons. Really? Which in my mind represent like the crazy unknown thing we might discover, who knows? Like, you know, when we turn on the Large Hadron Collider, who knows what's going to come out? Maybe huge purple dragons. And obviously I don't expect literal purple dragons, but just sort of like a stand-in. And then my mind just sort of goes there visually also. So if I rewind past the big, Big Bang, I literally imagine a dark cosmos with purple dragons in it.

I love that. Does that have anything to do with, like, I think in maps when we didn't know what there was, we used to write here, there be dragons. Was that, no, that's not where that came from, though? If I was smarter and more literate, then I would claim that that was inspired by there be dragons, yes. That would make me sound a lot smarter. Well, I could be totally wrong about that.

Maybe that might be some, like, internet lore that I picked up somewhere, but I liked it, so it stuck. And okay, that's very cool. All right, yeah. Well, there you go, a little insight into Daniel's banana peel brain. Yeah. I don't have a brain that makes pictures quite as much as yours does, I think. All right.

So then in Kelly's brain, what's the pre-BB look like? I don't know. I haven't really thought about what the pre-be-be. I mean, I don't have as much, like, information to... But I guess you would say that you also don't have information to inform what the pre-BB was like. So none of us do. And so, I don't know, maybe just...

Just pure darkness. Interesting. That's like the default state of the universe. Yeah. I don't know. I like the idea of dragons. Well, let's dig into the episode and maybe at the end of it, we can check in with Kelly to see if it's changed her mental picture of the pre-BB. Okay.

But today's episode is actually inspired by a listener who watched a YouTube video about the early universe and the grand unified force and how it broke into four other forces. Didn't really understand it and wanted more details. Here's a question I got from DeLair Mustafa from New South Wales. Hi, Daniel and Kelly. My understanding is that after the Big Bang, there was one grand unified force. And then this force started breaking down to the four forces which we know now. My question is, what was there when this force was around in terms of energy and particles? Like what was the form of energy?

Was it in form of photon or whatever? type of energy and also what particles, boson or fermion and so on. And what caused this force to break down? And if you can explain, what were the steps of this breakdown, what happened first and so on, and which particles or energies resulted from these transition steps, and also what forces and particles and energy we have today. Thank you very much, Dan Ellen Kelly. There's a lot of questions in that question. There are, and this happens a lot that people watch a video or listen to some other podcast, and they don't quite get it, and they write to us, and they're like, what does this mean?

And I'm not mad about that at all. I'm very glad that people are watching videos and listening to Science Podcasts. I'm also glad that they reach out when they don't get it, that they come to us because it means that they think we'll answer because we do. Yes, and that's an honor, I think, to be the person that people go to. to when they're like, oh, I was still confused. Can you break this down even more? So thank you for trusting us with your questions. And this is a super fun topic and what I really did want to dig into because it's an opportunity to clear up, I think, a lot of misconceptions about what it means to unify forces and how things break in the universe and how the universe has changed over time and what

that really means and dig into the sort of philosophical underpinnings of effective theories, which I always want to talk about. But before we get there and before Kelly has to rein me in, I checked in with the Externaries to hear what they thought about this topic. I asked our cadre of volunteers, what broke the grand unified force in the early universe?

If you enjoy hearing this segment and want to add your voice to it, please don't be shy. We would welcome you in this chorus. Write to us to Questions at Daniel and Kelly.org. Here's what listeners had to say about what broke the grand unified force in the early universe.

Which force are we talking about again? Varyk energy. I think the overall energy dropped below a certain threshold, and that's what broke it. I would say that the Grand Unified Force broke itself in the same way than an egg breaks when the chicken cracks out of it. Something to do with the balance between matter and antimatter. As this is Daniel Kelly's extraordinarily fecal at times universe, it might be poo. Did a cat knock it off the shelf?

I think the emergence of light. Chuck Norris That expanded. unified force. Does it have anything to do with Planck's constant? We don't have a grand unified theory yet. Some kind of symmetry violation that only manifests below a certain energy threshold. It wasn't me, honest. Oh man, Chuck Norris, rest in peace. I got to be honest. I don't know anything about Chuck Norris, the man, but I knew a lot about Chuck Norris, the myth. I really loved those Chuck Norris jokes at like some point in my life. And Chuck Norris is no more.

He lives on forever in you, Kelly. Uh, okay. That makes you uncomfortable. A little. A little. If that's the case, I probably should learn a little bit more about the man, but I enjoyed the jokes. Or maybe some martial arts. Yeah. But we're not talking about Chuck Norris mostly today. We're talking about the universe and we're talking about how the universe changes over time. And before we dig into what are the fundamental forces and how do they unify high energy and then walking through the timeline of the very early universe to explain all of that,

I want to spend a minute on the big picture of what it means to have the laws of physics change because that's what we're talking about. The grand unified force broke into other forces. And what does it mean to have one set of forces and then later another set of forces? How does the universe change its forces? Is that going to happen in the future? What's going on here? And so I want to remind everybody that there might be some fundamental theory of the universe that describes everything that's happening and controls everything that's happening. We don't know what that is. We don't have it, right? It's not something we possess now.

We can't write it down. And even if we did, even if, like, advanced aliens showed up and they told us, here's the equation of the universe, it might not be very useful, right? The way, like, even particle physics today, which describes a lot of the quantum mechanical, details of our universe is not all that useful. You don't use it to predict how a ball flies across your backyard or whether Tesla stock is going to go up or down or whether this hurricane is going to be your left or right. It's not very useful because it's very disconnected from those level of details. You need to simulate like 10 to the 37 electrons to predict what's going to happen in that hurricane. It's not a useful theory. So number one, we don't have it. And number two, even if we had it,

It wouldn't be the kind of thing we needed to describe the universe we experience. Does that make sense? It does. So this sort of sounded familiar. And so is this what we were talking about with Ethan Siegel when we were talking about single unified theories in physics when we had him on the show on October 15th, 2025? Oh, my gosh. What a photographic memory you have.

No, no, no, no. Looked it up. Looked it up. Looked it up in advance, even. Look at that doing your homework. Yes, exactly. We were talking about whether there is an underlying. theory or not. Because we don't have that theory. What we do have are what we call effective theories

that are useful in various situations. And we don't think and don't pretend our fundamental theories of the universe. There are approximate theories that help us out in certain situations. The way, for example, Newton's laws can predict what happens when you throw a ball across your backyard. Nobody thinks Newton's laws are written in the cosmos, right? That they're fundamental in some way. But they work. they're effective. They're an approximate description of physics at this scale, and they work, but they work within boundaries. If your ball goes near the speed of light, it doesn't really work anymore. If your ball goes near a black hole, it doesn't really work anymore. And so rather than having a deep fundamental theory that isn't useful for anything, we have a patchwork of effective

theories that are each useful for something. And I think maybe an illustrated way to think about this is to think about the phases of water. Like, we have a theory that describes how water behavior when it's a vapor, the ideal gas law. We have a different theory that describes how water behaves when it's a fluid, right? Fluid dynamics. Very different set of equations, right? They both describe water. It's the same stuff, right?

Probably in the core of the universe, there are fundamental laws that are determining what's happening to water molecules in both scenarios. But when you zoom out, you end up with very different effective laws. And if you freeze it, you get an even different set of effective laws, right? So what's happening there? The universe is not changing the rules. We are changing how we describe the universe because we want a useful theory. And the useful theory is different in each situation, frozen water, liquid water, gaseous water.

Does this reflect that we are missing something? And if we were like smarter, we could have an equation that would describe what's happening in all of those situations. Or is this just the nature of the universe? There are like discontinuities and you need different equations to describe things at different times? Or, yeah, what do you think is going on there? Yeah, we don't know, right? We don't know if there really is a deep truth. It could be that all there is are these effective theories, these approximate patchworks that maybe come together or maybe there are gaps between them.

We don't know. Even Siegel believes strongly that there is a fundamental theory and that all of these effective theories are just approximate versions of it. And he might be right, but we don't know. And even if we did, we probably wouldn't use it, right? Because it wouldn't be very useful. Like, if I could tell you that the universe is made out of squiglions right now, doesn't mean I could calculate the flow of sewage through pipes using squigglyon physics, right? Because it would take like enormous number of calculations and be dependent on all these details. I would probably still use Bernoulli's equation or whatever fluid dynamics people use, right? Because that's effective. That's very useful. So these effective theories are, are very powerful. And as the situation changes, as the phase changes in water, you change your

effective theory. And this, I think, it really helps you understand what's going on in physics a lot of the time that we summarize a lot of the microscopic details we don't know, often in terms of one like unexplained number. We give you another example. We often talk about light flowing through material and moving it slower than the speed of light. People say light moves through a vacuum at the speed of light, but through glass it's a little bit slower. And through other materials, it's at a different speed. And there's a number, the index of refraction that tells you how light bends and also the speed of light through that material. And that's confusing to people because they're like, how do you slow down a photon? What's going on? And that's an example of an effective theory.

It's not that photons are slowing down and moving it slower than the speed of light. Instead, it's as if light was moving slower. It's not that that's what's happening, but if you describe it that way, all the predictions come out. So if you really want to zoom in on the microscopic details, you see light never moves as slower than the speed of light. It like zaps between atom and atom and there's an excited wave of electrons that move through the material. It's very, very complicated. The microphysics of light moving through material, super duper complicated. If you don't know how that works or you don't care, you can just zoom out and say, it's kind of like light moves a little bit slower than the speed of light, and all the numbers work out. All those microphysics get wrapped up

in this one number you can measure, the index of refraction. It's the same thing for like friction. Friction is super-duper complicated. You have two surfaces. They're sliding over each other. How they interact depends on lots of details. You can sum it all up in one number. It's called the coefficient of friction. 0.7 or 0.42 or whatever tells you how two things slide across each other if you don't know or care about the microphysics. So often we have these effective theories where a lot of the high-end details are bundled up into a single number we just measure that encapsulates all those details we don't know or don't care about. So as we move to the timeline of the universe and we talk about we're using this theory. Now we have two forces. Now we have four forces. What was changing was not

how the universe works. There's still that fundamental law perhaps that Ethan Siegel believes in. what is changing is how we effectively describe the universe because the phase of the universe changes. The universe goes from very hot to very cold. And as it goes through those temperature changes, the effective laws change. What's useful, what's practical for describing the universe changes. I'm going to go ahead and admit that I was moderately quiet during the conversation that you and Ethan had because I was, you know, not. I didn't have much to add, let's say. But I thought that I remembered Ethan arguing that there was like a unified thing that was happening at the early universe, but then something about like the energy state went down over time.

And then the unified thing broke down as the energy changed phase or there was less of it. And so what was he saying and how does it differ from what you believe? Because I remembered you two didn't quite agree on something. Yeah, good memory. There's something else really cool that happens. As you increase the temperature of the universe, you go to hotter and hotter phases of the universe, then the differences between the various effective theories tend to sort of wash out. They become irrelevant, sort of like, you know, the way the shape of a mountain matters a lot if you're climbing over it, but if you're flying over it, it doesn't really matter so much. And so we'll go into detail about exactly that.

But I think Ethan and I had a different concept of what it meant to unify physics. He was thinking about the very early universe when things are very hot. Can you ignore the details between all the forces in your effective theory? And I was thinking about whether there's an underlying theory even now that does unify everything, but maybe isn't a useful effective theory. Got it. All right. Let's take a break. And when we get back, let's remind ourselves what the five forces are today.

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That's the kind of conversation we're having every episode. Listen to the Radio 831 podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. All right, we're back. And Daniel, it's Physics 101. What are the five forces that we recognize today? All right, this is a lot of fun. And already people will disagree because some people say there aren't five forces, there are four forces, there are three forces, whatever. Let's start with the broadest categorization and say electricity, right?

That's the thing that powers your laptop. and then magnetism, which will count separately and then unify later. And magnetism, of course, is, you know, refrigerator magnets and loadstones and compasses and stuff like that. Then there's the weak nuclear force, right, which is responsible for, like, radioactive decay. It doesn't play a big role in our lives, but is there and important. Then there's this strong nuclear force that's very important for fusion and holding the nucleus together.

So those are the four quantum fundamental. forces, and then there's gravity, which seems like a force to us, and I think deserves to be included on this list, because we hope to unify it with these four eventually. But that's the starting point. Sort of historically, humans have been poking around and noticing stuff happening, and we built up this sort of phenomenological history. Like, we see this kind of stuff, let's call it one thing. We see that kind of stuff, let's call it one thing. We started out with basically five fundamental forces. And so first, we can talk. about how we unify them as we go backwards in time.

And then in the next segment, let's walk through the history of the early universe and see them breaking as time goes forwards. Okay. Real quick, before we do that, do we think that we've got all of the forces that matter or because we don't understand dark matter? Or is it dark energy? Which one do we not understand both? Neither. We understand neither of them. Okay. Is it possible if there's more forces we're going to have to unify once we understand things better? and these are just the five we're working with right now? Oh, yes, and I hope so. Okay.

There could definitely be more forces involved in the dark sector. If there are lots of dark particles out there, they could interact with each other using new dark forces. There could be dark photons and dark Ws and dark Zs and all sorts of stuff. There could be a dark strong force. And dark energy could be the product of another force we don't know about it either. It's really a big question mark. It's basically flying purple dragons where physics is concerned.

So, yes, these are the ones we know today. point. And I love the idea of a dark sector. I feel like that needs to be in someone's book somewhere. But okay, so how do we unify them? Yeah. So what we do is we notice that forces have things in common, right? But that doesn't mean they have to be the same. When we unify forces, we don't say electricity and magnetism are the same thing. What we do is we notice that they're two sides of a coherent whole, that they make more sense to click together than to keep track of themselves. separately. Like if you have literally two sides of a coin and you never noticed that heads and tails are related to each other and you kept track of like heads and tails separately. And somebody told

you, oh, did you realize that heads and tails are literally two sides of the same coin? That's where the expression comes from, right? It's much simpler because then you notice patterns like you can only see heads or tails. You can't see them at the same time, right? There's a relationship between them. If you add up the number of heads and tails, you get the number of flips you made. for example, right? So that doesn't require heads and tails to be the same thing. It requires them to be two parts of a larger hole. It's the same with like elephants. If you study elephant trunks and you study elephant tails and you don't think about the relationship between them, you're missing out in the bigger picture. And your theory of elephant tails and trunks is not nearly as good as a unified theory

of elephants that includes tails and trunks, right? So again, it's not saying tails and trunks are the same. It's saying that they're part of a larger hole. Got it. Yeah, you don't want to miss the whole elephant because it could squish you. That's right. And we made a lot of progress early on in finding connections between the electricity and magnetism with Maxwell. There were these different observations of electricity and magnetism and people had the various laws about it, Gauss's law, Amper's law, et cetera. And Maxwell put together the four laws that related these things, and he saw a bunch of symmetries here. And he noted that the equations were very similar

from the magnetic and electrical points of view. Basically, you could swap the electric fields and the magnetic fields in the equations and they wouldn't change. There was a symmetry there, right? That they were playing like the same role. Asterisk, right? Almost.

He actually noticed that there was a gap that that symmetry almost worked and he wanted it to work. He was like, ooh, this would be so much prettier if it did work. So he penciled in the missing term that would be needed to provide this symmetry. There was no evidence for this term in the universe yet. He just said, hmm, boy, this would be more symmetric if it was there. And then he went out and looked for it and found it. That's displacement current. And so that shows you like, wow, there really is some symmetry in the universe. You know,

following mathematical symmetries leads you to physical discoveries. I love that. And so we know now that electricity and magnetism have a very close relationship, you know, if you have electric charges and they move, that gives you magnetic fields. Or if you have magnetic fields that change, that can generate electric fields. So the two are very closely coupled. One can make the other one. Okay. But sometimes the universe has mathematical symmetry and other times it doesn't. Yes. So it's nice that it's here. But then we find that, like, Like water needs different equations for different phases, and that doesn't feel very satisfactory. And so I guess I'd like a whole episode on when you should expect the world to be beautiful and symmetrical and when you shouldn't.

And I'm guessing the answer is we don't know. Yeah, well, even within electricity and magnetism, the universe seems to reject perfect symmetries. Because you can't actually just swap electric fields and magnetic fields in Maxwell's equations because they have a really big difference. And that is that we have sources of electric fields like particles. with charges are sources of electric fields, right? An electron just sitting there makes an electric field. But there is no such source of a magnetic field. Magnetic fields only come from charges in motion.

There's no equivalent of the electron, no magnetron, or we call in particle physics a magnetic monopole. So one of Maxwell's equations has a zero in it to reflect the fact that there are no sources of magnetic fields, and another one has a non-zero to count the number of electric charges. So there is a fundamental difference there between electricity and magnetism. So they don't have to be identical. They just have to play nicely together. Okay, so they're linked, but they're not like a gradient from one to the other. That's right. Exactly. Though there is some evidence that the dotted line we draw between them is a little bit arbitrary because, for example, in a given situation,

whether or not there is a magnetic field depends on who's looking at it. Like if you have an electron and you're holding it in your hand, it makes an electric field but no magnetic field. If I'm driving by in the highway at 100 miles an hour, I look at that electron, I see that as a charge in motion because it has a velocity relative to me. Charges in motion make magnetic fields. So my compass would measure a magnetic field from your electron.

Your compass would measure no magnetic field. Is the magnetic field? there or not. It's frame dependent. It's observer dependent. So where you draw that dotted line depends on the observer, which is a strong argument that it doesn't make sense to account for these things separately, right? These things really are deeply linked. And that is why you say that they're linked, but you said there's five charges and you gave them separate counts because they're linked, but they're different. They are different. And historically, they were separate. And then we brought them together. And there's a really powerful lesson in how we brought them together.

Because another factor in electricity and magnetism is the strength of the force. Like, if you're just sitting around at low speeds, electricity is much more powerful than magnetism. There's a law, the Lawrence Force law, that tells you the force on a particle based on the electric field and the magnetic field. And the magnetic field has another factor in it. It's velocity. If there's no velocity, there's no force from a magnetic field. field. Okay. Yep. That makes sense because you said if you've got to be moving in that prior example. But what happens if you're moving at the speed of light? If you're moving at the speed of light, that velocity term, we write the speed of light as speed equals one. And so now

the two things are totally equal. So magnetic fields are much weaker at low speeds, but at high speeds, they become just as powerful as electric fields. So at very high speeds, a very high temperatures, if you have a gas of particles whose particles are moving near the speed of light, the magnetic fields are equally as powerful as the electric fields. This is the kind of thing we mean by unification at high energies. At high energies, the differences between electric and magnetic fields start to wash out, it become less important. Are we going to find that at all of our forces at high energy, they become the same? That's the trend. It's very tantalizing. It's the kind of thing that make particle physicists feel like it's possible to unify all the forces. Definitely happens here

and it happens every time we unify a force, we find that it happens at higher energies. Okay, well, then let's jump to the weak and the strong force then. Exactly. So we have electromagnetism. It's unified. And the particle that mediates electromagnetism is the photon, right? There's a photon field, the electromagnetic field, right? And we did that by putting electricity and magnetism into a single framework. Well, it turns out you can do the same with electromagnetism and the weak force. You can put those into a larger mathematical framework that in the same way makes a lot more sense to keep track of and to think about like rotations and velocity dependence. It all works really, really well and explains a lot.

But again, it doesn't work perfectly. There are still differences between electromagnetism and the weak force. And the most important one being the weak force is really weak. It's weak because the particles that carry it, the W and the Z, are really, really massive. Like the W and the Z weigh 80 and 90 times the mass of a proton, whereas the photon is massless. So the photon can fly forever through the universe, whereas then W and the Z are very short range because of their mass. So it's a very weak force. So this is a symmetry between electromagnetism and the weak force, but it's a broken symmetry.

All right. So what do we do to the weak force to make the mass not mass? We crank up the energy because the mass of the W and the Z only matter if you don't have a lot of energy around. If everything has a huge amount of energy, the fact that one of them has some mass doesn't matter anymore. At the speed of light, the mass of these particles doesn't matter. So if you crank up the energy, then the weak force becomes stronger. It becomes just as strong as electricity and as magnetism. And those three strands all unifying to one beautiful theory called the ElectroWeak theory,

which unifies electricity and magnetism. He uses group theory, which French mathematical nerds invented 100 years earlier just because they were having fun with numbers. Yay, nerds! And it turned out to be foundational in building our modern understanding of the universe. Another reason why you should give nerds money and leave them alone. Amen. And this all works really well. but, you know, one obstacle is like, well, why is the weak force different than the other forces?

And that's the Higgs boson. Peter Higgs was like, how can you put these things together, but also break that symmetry? You've got to build the symmetry and then break it. So the Higgs boson is the thing that breaks the symmetry. It interacts with the W and the Z bosons and gives them mass, but doesn't interact with a photon, so leaves it massless. So not only can we unify electricity and magnetism, into one force that's beautiful and wonderful and high energies, they all act the same,

we could also explain how at low energies they are different. And the reason you have to be able to do both of these things is because we're pretty convinced at the Big Bang all of the forces were the same, and we know that now there are situations where they're clearly doing different things, and so we need to be able to explain how both of those things can happen. Is that right? A plus, Kelly. That's right. We live in a cold universe where things don't have the energy to fly near the speed of light.

And so these differences matter. The weak force is measurably weaker than electricity and magnetism today right now. But we do think in the early universe, it had more energy and it was able to ignore essentially the Higgs field. It didn't really matter because, yeah, you get a little bit of mass, but who cares? You know, it's like giving a speeding ticket to a billionaire, right? No, those billionaires. It's not going to slow them down. Shake my face.

They're always going too fast. All right. So things are going great, right? We were using group theory and mathematics to unify these things as we crank up the temperature. Things seem to unify beautifully. It's amazing. So let's get to the strong force. And here we're stuck.

We would love to unify the strong force into some grand unified theory of physics that brings together all of our quantum theories. But nobody's been able to make it work so far. This is a target of physicists for decades and decades, even putting aside gravity, right? We haven't even gotten to gravity yet. We don't know how to unify the strong force with the electro-week force. So we really only have two quantum forces at this point, Electra-Weak and the strong force. And we would love to stick them together. And I don't want to make light of the suffering of physicists, but I'm just going to note that Electra-Weak appears to be abbreviated as, ew.

and grand unified theory appears to be abbreviated as gut. And so your notes say, we cannot yet unify ew into gut. And so that's the kind of problem you guys are working on right now. Maybe somebody needs to come up with like a bleh theory, or like a vomit theory or something. And then the dragons can get cleared away. The theory of dragon vomit is going to bring us all together.

That's right. Good luck. And so there are obstacles here. And one obstacle is that what happens when you turn, up the energy, then the strength of these forces changes. So all these forces have a strength that depends on the energy. And with electricity and magnetism, as you crank up the energy, the two come together that have the same strength. And the weak force also merges together into one single strength. Really beautiful. As you crank up the energy, the strong force also changes, but it doesn't

seem to grow together with the electrow weak force. It's like a parallel line. It's not coming together. So that made people wonder like, hmm, is there something missing? And, you know, we've talked a lot of times on the show about the theory of super symmetry. It's a theory that suggests that for every particle, there's a new kind of mirror particle called the super particle. And it was created for other reasons to explain why the Higgs mass is so small, which is a mystery we don't understand. And it's also this, like, beautiful example of symmetry. And the math nerds love it. And they would love for it to be true in the universe.

We've never seen any evidence of it. But if you add supersymmetry to the theory, then as you crank up the temperature, these forces do grow together. Oh. So it's like maybe we have a missing piece. Oh, there's a piece over here that people invented for other reasons. If we stick it in, boom, then at high temperatures, everything comes together. That's amazing. Wouldn't that be beautiful?

That was a real motivating factor for people believing in supersymmetry. It felt like a huge clue. Unfortunately, we turned on the Large Hadron Collider, we didn't see supersymmetry. So, yeah, that would have been nice. But for now, we don't know how to unify the strong force with the Electro-Weak force. It might happen. It might be then in the early universe. Something happened to bring these together, but we don't know how.

And so we can theorize about a gut theory, a grand unified theory, but we don't know yet what that would look like. But looking ahead to when we do, what will the acronym be? Will it be ooze? Electro-Weak strong? Ouse? Ouse, I think, is going to be a strong contender. That's excellent. It's important to figure this out.

Or so, strong electro-week? I don't know. All right, anyway, we can move on, I guess, to more important things. And if I am remembering our list of five correctly, the last one is gravity that we haven't got into yet. Exactly. And this one is even harder because, you know, unifying the strong force with Electro Week is challenging because they look different and the details are different, but at least they're both

quantum forces. You know, there are fields involved, there's momentum propagating through those fields, all that stuff. Gravity is a real head scratcher because it's a classical theory. You know, Einstein tells us that mass bend space and that space tells mass how to move, but it requires like mass to have a location and to have a classical trajectory to always have a place and a time where it is and to go from here to there by going through space from here to there, And quantum objects don't behave like that. So we just don't know how to interface it with gravity. The search for quantum gravity is like, you know, the most frustrating and exciting and terrifying and challenging thing happening in physics in the last hundred years.

We talked about a lot of times on the show. If you could unify a gut theory with gravity, you'd have a theory of everything. So you'd have a toe. And then you could make the theory called the toe ooze, I guess. Nice. Nice. I've already lost. What does toe stand for again?

Theory of everything. Theory of everything. Got it. And how is that different than the Grand Unified Theory? Grand Unified Theory doesn't include gravity. So Grand Unified Theory is Electro-Week plus Strong. Okay. And if you, I want to add gravity, that's a theory of everything.

Got it, got it. Okay, so gravity is the queen of the Purple Dragons. Yes, exactly. So force unification is very exciting and promising. Electromagnetism unified beautifully. The weak force added beautifully. Lots of Nobel Prizes won. Beyond that is speculative.

So grain unified theory that includes the strong force, speculative. Quantum gravity that includes everything, a theory of everything, still very speculative. But we're hopeful. Maybe we'll figure it out. Okay. And we need to figure it out to really understand the Big Bang. Is that fair to say? Yeah, I think so.

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We're back, and Daniel's going to start with the Big Bang and take us forward and tell us about how these forces split up. Question marks. Exactly. So we've been telling the story sort of backwards. We've been talking about taking the current universe at very low temperature, everything is very cold, and heating it up to unify things. But time flows forwards, right?

And so let's instead tell the story forwards, also because that's the only chance. we have to potentially maybe answer De Laire's question, which was the whole point of this podcast. And so we're going to talk about the timeline of the early universe, and we're going to use some crazy numbers to talk about time. And so what do these times mean? Remember that we don't have a lot of data for these very early moments. You mentioned in the top of the podcast how we've seen the Big Bang. We've actually only seen back to like 380,000 years after the Big Bang. Oh, that's right. That's when photons were made. So a lot of this stuff is speculative. but essentially you take our current model of the universe expanding and you run it backwards to predict

what would have happened deep deep in early history so let's go as far back as we can if you run the clock back 13.8 billion years you get to a place where you would need quantum gravity to continue like we don't know how to predict what would happen because quantum mechanics is relevant and gravity is relevant and so we call this time before the plank time because you know plank is one of the grandfathers of quantum physics, or this is the plank epoch. So before 10 to the minus 43 seconds in the universe, you can't go all the way back to T equals zero because we don't know how to do that, right? Like general relativity predicts a singularity. Quantum mechanics says you can't have that. So like, how do you even get to T equals zero? So before 10 to the minus 43 seconds, we say that's the

plank epic. And the laws that are involved are some theory of everything, some quantum gravity that we don't know. It's all the forces all unified. So it's 0.43.1 seconds before. Okay, all right. So we're really close to zero, and all the forces need to be unified. So we don't know what's going on,

but that's where we need to get. That's right. And DeLayer also asked, what is the content of the universe at every moment here, which I think is a really fun question. And at this early point in the universe, it's not meaningful to talk about particles because we don't think of the universe as filled with

quantum fields. I remember here we're always talking effective theories, right? What is a useful way to describe the universe? And to talk about particles, you need energy in quantum fields to be a useful description of the universe. And we don't know what quantum gravity will look like. Like maybe it gets rid of quantum fields. Maybe it has something completely different. It's totally new and it tells a different story about the universe. So we can't even talk about like what particles were around because particles are excitations in a quantum field. And we don't know if those are going to be part of the theory. We don't have the theory at all. So we don't know what effective descriptions are going to emerge. We don't know what the fundamental theory is. We don't know what the like useful

calculational tools are. We can't even really tell you. Wow. Okay. But exciting. Big question mark. Big purple dragons. All right. So as the universe expands, it cools. And so in the next era, which we call the gut epic, which is between 10 of the minus 43 seconds and 10 of the minus 36 seconds, this is after the threshold of 10 to minus 43 seconds when the universe has cooled enough that gravity splits off from the other forces. So we have in our heads an order in which we expect the forces to stop unifying. And we expect gravity to be one of the first to stop unifying. Okay. Why? Because it's so weird. and has always been the troublemaker? This is speculative, right?

It could be that we're wrong about this. It's just that in our heads, it makes more sense to combine them in the reverse order, like to combine the strong force with electric weak and then unify all the quantum theories with gravity, but we could be wrong. Okay. And it could be that like, you know, we got to do this in another order. So everything here is still very, very speculative, right? Depends on the details of quantum gravity, which we don't have.

This next segment depends on the details of the gut theory, which we don't have. So this is one way it could have gone, not necessarily definitive at all. Okay. Right. But in this version, then, you know, the universe cools to a point where we can start to ignore gravity. And we can only worry about the quantum forces when we're making predictions, right? And gravity is the weakest of all these things.

And as the universe gets less and less dense, gravity gets less and less relevant. And so it breaks off faster, right? Okay, that makes sense. And in this point here, we have gravity, which we know, plus all the quantum force is unified into one force, which we don't have a theory for, we just have like a placeholder for it. But we can't talk about particles now, because we have the universe mostly dominated by quantum fields, but we don't know what those fields are, right? They're the fields that are part of some theory. We don't have the gut field, right? And we talk about particles in those

fields. We call them the X boson, the Y boson. There might be some unified gut force, and these are the mediators of it. I'm sounding like I'm waving my hands a lot because we don't really know. Because again, we don't know how to unify the strong force. So this is just like a placeholder for what might have happened. But like the creation of the universe is so beautiful and you all came up with the gut force. Like I just, I, you know, every time you all have a chance to impress me, but it's your gut. To then just stick with grand unified force and never allow yourself to use the acronym. But all right, anyway. I'll call it the grand ut-force. with that. No, forget it. It's too late. It's too late. You already did it. Move on.

Yeah, I know. All right. So at this stage, we think we have a gut force, a single unified force carried by X and Y bosons. But we don't really know, right? It's speculative. The universe continues to expand, continues to cool 10 to minus 36 seconds. Now the strong force breaks off from the Electra Week, and now we have two forces, right? And this is the era that we can describe very accurately with our physics because we know about the electroweak force and we know about the strong force and the universe is cooled and as it cools these two forces start to behave differently. So again, the fundamental, true underlying reality is not changing the laws of physics somehow, but how we describe it effectively changes. Like it makes sense now to talk about the strong force

is different from the electrow weak force because the universe is cooler. We're in a different phase where the strong force acts very different. And so it makes sense to treat those calculations separately, right, instead of unifying them. The elephant's tail has run off by itself. Okay, but so now we would still have elementary particles and anti-particles, and do we know what kind of fields we're dealing with here? Is stuff pretty normal? Yeah, so now the content of the universe are particles we're familiar with.

They are the electro-week particles, the W, the Z, the photon. there are matter particles, quarks, electrons, all these kind of familiar particles. Okay. Right. So the universe is cooled and broken into these familiar fields, and we have ripples in those fields. That's the content of the universe. Then something crazy happens. The universe starts to inflate, meaning very, very rapid expansion.

So we were expanding before. Now we have kicked it into high gear. And this part also very speculative. But we think the universe expanded by a factor of like 10 to the 30. in a very, very short window of time. That's the equivalent of taking like a cantaloupe and expanding it to the size of the observable universe. Wow. So like really big deal.

Yum. And remember, we don't know the size of the universe, so the universe could be infinite, which means it went from infinite to more infinite, right? Or it could have been finite so that it went from small or large and finite to bigger. We just don't know if the universe is infinite or finite. And exactly when this happened depends on the details of your grand unified theory, et cetera, et cetera. But the crucial thing is that the universe became very dilute.

Like you had those particles already and the universe spread out massively. So now there's just not a lot of particles out there. And so the universe is suddenly very, very cold, right? Very, very dilute. Then inflation ends. So all this energy does powering the expansion of the universe, the inflation, that gets dumped into the other quantum fields. That energy gets dumped into quark fields and photon fields and all sorts of stuff.

And so now the universe becomes very, very dense and hot again. This is called reheating. And this is really the beginning of what we call the hot Big Bang. Before that, very, very uncertain. After that, things are described very, very well. Everything we just said up to now is speculative and could be totally wrong. everything we're going to say from now on is very, very well established. And what we mean when we say the Big Bang from this moment, somehow maybe via inflation,

dumping all the energy and reheating all the fields, the universe is very dense and very filled with familiar particles. And are we still dealing with the first second of existence? We are up to 10 to the minus 12 seconds. So still the first second. We're still dealing with the first trillionth of a second. Yeah. So that's pretty amazing that. Yeah. A lot of stuff has happened so far and just a few ticks on the clock, absolutely. And then something really exciting happens, which is, look,

you've created all these particles, you have matter particles, you have anti-matter particles, most of those annihilate. So the universe filled with matter, and then those particles find each other annihilate, and now the universe mostly filled with radiation with a little bit of matter left over because you had a little bit more matter than you had antimatter. And that little bit of extra is what turned into me and you and most of the stuff in the universe. This tiny little extra bit of matter. We don't know why there was that asymmetry. We can't understand it. We have some explanations for it, but not nearly enough to explain what we see. Wait, so that that tiny little bit of matter is all of the matter that still exists today? That seems like a lot of matter. It's a lot of matter,

but it's a tiny fraction of the original amount of matter. All right. I guess it's all about perspective. Yeah, so the universe was dominated by radiation. Like most of the energy in the universe at that time was in photons from annihilation. Okay. And so then things keep cooling. When we get to 10 to the minus 12 seconds, this is another phase transition of the universe,

where we're now cool enough that it matters that the weak force has heavy massive bosons. And so now we say the weak force breaks off from electromagnetism, by which we mean, you know, none of the fundamental rules have changed. The only thing that's changed is the temperature of the universe, but now it doesn't make sense to treat these two things together because it matters that one of them has really massive bosons and the other one has a massless boson. They start to act very differently, right? We're now hiking over those mountains instead of flying over them, and it matters if you're carrying a really heavy pack or not. And so we break those up and we treat them differently. Does that make sense? How did this all happen in a second? My brain is kind of trapped

on something else. How did this all happen in a second? And how did we figure out the timing down to the like point oh, oh, oh, oh, oh, but like, wow. I know. It takes me like an hour to get up and have coffee, even think about getting anything done. And by that time, the universe is already done all this stuff. Right. Yeah, it's crazy. And it's a fair question. How do we know this stuff? We know this by extrapolating our model backwards in time. And these models we've built from lots of lots of observations, the cosmic microwave background radiation, large scale structure, barionic acoustic oscillation. We've talked about all those things in the podcast, dig into those. We have really, really well-constrained models for a lot of this stuff. Those models don't work perfectly.

They're still like mysteries right now about that expansion. It's called the Hubble Tension. So there's definitely things to learn, but it's a pretty well-established timeline after the reheating. That's incredible. That was one hot set. Yeah. And it's still going. Between 10 and the minus 12 seconds and 10 of the minus six, you still have enough stuff in the universe to have a cork gluon plasma. Things are still pretty dense. So quarks and gluons are floating around and it's still hot enough that they're not bound into objects, right? They can just smush around like at the heart of a neutron star. And then around 10 of the minus six seconds, things cool down and you form protons and neutrons. And then for like literally,

two minutes, there's enough heat and temperature in the universe for fusion to happen. Like the kind of fusion at the heart of stars that makes heavy stuff, that happened in the universe for a hot minute. And it made a little bit of helium and a little bit of lithium. And that's called Big Bang nucleosynthesis. And that's very, very sensitive to lots of the details. How hot was it? How many quarks were there? We know a lot about the early universe just from measuring how much helium and lithium was made in that. And that first minute, super-duper-powerful test of the early universe. And then 400,000 years later, of course, things cool down and electrons find their protons, and we get neutral atoms, and the universe becomes transparent, and we see the cosmic microwave background radiation. So that's the sort of timeline in the universe from the beginning when it was really hot, cooling down, and we enter different phases of effective descriptions of the universe. Wow, it feels like the universe, like burned out. Like it had its first second was really, really active and then it was like,

you know what, I did too much. I did too much. And then it just really slows down and, yeah. I don't think it has any regrets. You know, I think what happened in the universe is beautiful. It is, it is. So back to DeLaire's question, he asked what was the form of energy?

Was it in photons? And so, you know, when the grain unified theory was ruling as the effective theory of the universe, we don't know because we don't know what that theory looked like. We call it the X and the Y particles, just this sort of place. holders because some candidates have those two bosons in them, but we don't know. He asks, what caused the force to break down? It's just that the universe cools. And so then the rules you would use to

describe things in a useful way change. Not that the rules of the underlying universe have changed, just our effective theories, our descriptions of the universe change. He asks, what were the steps of the breakdown? So we think that gravity broke off first, and then the strong force, and then the weak force, and now electromagnetism. He asks also, is there any relationship between the breakdown of this force and the inflation? Did one cause the other? So we don't know for sure because a lot of this is before the time that we can study really,

really well. Inflation is still speculative and how grand unified theories treat inflation depends on the particular grand unified theory. So there's a complicated interplay there that we just don't know about really concretely. So what I got from your conversation with Ethan was that there's, maybe a lot of disagreement about a lot of stuff. And so if I talked to, you know, 10 different people who had the same job as you, and they were given the same set of, I know, none of them as good as you, of course, of course.

And they were given- You're going to talk to other particle physicists, Kelly? No, never, never, I promise. You're being very controlling, Daniel. I suddenly have these feelings of jealousy, oh my gosh. Oh, get over it. All right. So if those sets of questions got sent to 10 other particle physicists, would they give a roughly

similar description or talk as you gave? Or would they have a very different sense of what we know and where the uncertainties are? I think that we would all agree on the later stages, because those are things we've measured. We are pretty confident about the very early universe stuff, inflation. And before that, very speculative, you might get different opinions about what's most likely to work out eventually as a model. And I think probably I have an unusual philosophical take for a particle physicist. It's thinking about emergence and effective theories differently than many particle physicists. But I'm not sure.

Okay. Maybe I'll ask them over lunch and try not to disturb their guts with too many ease. Well, I hope that goes well. Let me know. This was a ton of fun, and we learned a bunch. let's hear what our listener has to say. Thank you, Daniel and Kelly, for answering my question. That was really great.

I have a better understanding now of the early stages of the universe. And I think what helped in that clarification is that two approaches you use to address the issues or the questions. One is the energy level. What happens when we crank up the energy? And the second approach was chronologically how the universe evolved and the force, got broken down. And also what helped understanding it also the example of the different faces of the water. So looking at the universe and the forces in that way also makes it easier to understand. And also I like very much the clarifying question by Kelly. It seemed like she

was channeling my questions. So that was really very, very great help in getting Daniel to explain the answers in a very clear way and very logically. So once again, thank you very much. It was really fun listening to it and I enjoyed it. Thank you. Bye. All right. Thank you very much, DeLaire, for sending in your question and for writing with us on this journey back in time to unifying all the forces and then forward again to breaking them up

to explain the universe we know and love today. Thanks for joining us, everyone. Until next time. Thanks everybody for listening. Please go and do us a favor and rate the show on whatever podcast app you're using. It really helps people find us. Daniel and Kelly's Extraordinary Universe is edited by the amazing Matt Kesselman. He really is a wizard.

You can also find us online on Blue Sky, Instagram, and X, D&K Universe. Come engage with us. You can email us at Questions at Daniel and Kelly.org. We really do want to hear from you. And you can find our website. www. www. Daniel and Kelly.

Where you'll also find an invitation to join our Discord where everybody comes and talks about the amazing universe. And we also have the most amazing

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