What are the weaknesses of inflation theory? How did the ekpyrotic theory attempt to improve it? How did it open up its own can of worms? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPT (AUTO-GENERATED)

Wouldn't it be nice if the universe was just always here? I mean, I love the Big Bang and all. Lots of drama, good action, the dying inflation field giving up its energy to flood the cosmos with light and particles, the grand mystery, the singularity. It's all there. And all annoying. Just how did it all go down? What exactly is inflation? Why did it go away when it did? And the singularity, gee, anchoring your entire cosmological model on a question mark? isn't exactly a compelling play, is it? On the other hand, there's a certain sort of comfort, I suppose, in thinking that the universe has always been here, even if it repeats itself, even if there are cycles. Like, okay, maybe this cosmos isn't doing so great. Well, that's fine. We'll get another shot. Sure, it may not be for another 14 trillion years or whatever, but we'll try again next time. And as for the past... To think that all this has been here before, that there have been generations of stars repeating in some grand cycle, that's comfort.

In contrast, the Big Bang Theory can sit a little too linear. This is it. This is our chance. The universe was born, it's already past its heyday, and it's nothing but expansion into the coldness vacuum from here on out. This is part of what makes the Big Bang Theory so aggravating when it was first proposed. Many modern and historical religions believe in multiple cycles of cosmic history. Even Christian faith has a certain sort of cycle built in, what with the flood of Noah and all of humanity getting a second chance. Okay, maybe most people died, but it's the thought that counts. And yeah, your life may be really hard now, but if you do good enough in this life, then when Jesus comes back, it's all gravy from there. But the Big Bang doesn't have any of that. So it's no surprise that in this century plus, since its first formulation, people have tried to make our cosmology cyclical. The first person to really attempt it was Richard Tolman way back in the 1930s. And this is when the standard Big Bang picture was even first emerging.

This is before Big Bang even had the name Big Bang. He envisioned a cyclic universe of Big Bangs followed by Big Crunches. That name wasn't around back then either, but it's the same idea. And... We get to it because when we're allowed to have this idea, because in Einstein's relativity, the universe can be expanding or it can be contracting. Whatever it is doing, it can't be sitting perfectly still. That is an unnatural equilibrium state that general relativity says it's just good luck getting the universe to sit still. It's like an excited dog. It's just vibrating. It's always going to be moving. And You have choices, though, expanding or contracting. We happen to find ourselves in an expanding universe. But there's nothing stopping the universe from contracting, at least in principle. And so maybe there are cycles where the universe expands for a while, slows down, stops, and then reverses back down to the singularity and then repeats again and again. But this idea of this basic idea of a cyclic universe runs into a big, giant problem.

And that problem is entropy. As time goes on, disorder grows. And this is not coupled or connected to the evolution of the universe. Just that is our arrow of time. Past to future is order to disorder. At least those two concepts are linked. We don't know if one causes the other or not. That's a whole separate discussion. But we know these two are linked. That past to future, order to disorder. And this works even in a collapsing universe. The collapsing universe, the big crunch, is still advancing towards its future, which means entropy is still growing, which means disorder is still getting greater. So every time you cycle the universe, you don't get to reset the clock. Every time you cycle the universe, your entropy just goes higher and higher. Your starting point for a new universe is whatever the entropy was left over from the last universe. And that becomes a major problem because after a few cycles, your entropy is way too high to have something like life. And then if you run the clock backwards, you're like, okay, okay, so to have life in our universe right now, we need a certain entropy floor.

Our cycle of the universe needs to start at a low enough level of disorder. which means the previous universe had to be even lower, which means the previous universe had to be even lower. And you work the clock backwards. And then you find out that no matter what, there is some sort of starting point, some sort of minimum entropy ground floor that the cycles of universes have to get started from to allow us to have life here. And it could be five cycles away. It could be a hundred cycles away, but it is a finite number of cycles. And then You're just back to where you started. You're like, okay, I don't explain the Big Bang. I got rid of the singularity. Now I have these cyclic universes. But it still has a beginning that I can't explain. And so Tolman's basic idea of the Big Crunch doesn't make a lot of progress. So for decades, it felt like the Big Bang with us having just this one universe, that's the only shot you get, would have the last lap. And then inflation happened. Yes, our good friend inflation.

I've done an entire series on inflation. It seems like every few episodes inflation comes up. And I will be the first to admit that inflation is kind of an awkward theory. It's so awkward that some people, up to and including some of the original developers of inflation theory, argue that it's not even theory. I'm not willing to go that far, but that's for a different episode. Feel free to ask. Inflation says the following... about the extremely early universe. These are the statements that inflation theory makes about the conditions of the universe when it was less than a second old. Says the universe was born, don't ask how, singularity, that's as far as we can get. That the forces were merged and then became unmerged once the cosmos expanded and cooled. This unmerging or splitting of the forces triggered a mysterious quantum field to drive exponential, as in really, really fast, expansion. The universe became super big, like, We're talking the size of a solar system. And then the quantum field that drove the exponential expansion gave way, lost energy, and transformed its last bits of energy into particles of radiation filling up the universe.

And then the hot Big Bang proceeds from there. Now, if you were just handed this idea on a sheet of paper, you might question my sanity. It's valid. But inflation is designed to answer some really major problems with the standard Big Bang picture. problems of flatness. Our universe is extraordinarily geometrically flat. We are able to measure that. That is unambiguous. The universe is flat, period, up to a very, very small margin of error, which means the incredibly early universe had to be even flatter than it is today for us to be able to measure this certain amount of flatness today. It had to be super flat way in the past. There's nothing in the standard Big Bang picture that gives us that finely tuned level of flatness. There's the horizon problem. When we look at opposite directions on the sky, they have roughly the same average temperature, the same structures, the same pretty much everything on a statistical level. It's not identical one side to the other, but statistically, if you get a room full of people and another room full of people, you don't think all the tiny differences average out.

No, it's different people in different rooms, but you take the average and you end up with the average height. You end up with the average weight, and then the two rooms are pretty much the same. statistically. But in the standard cosmological picture, there isn't enough time for these distant regions of the universe to figure out how to get along and agree on what the baseline is. So they all had to be crammed together for a lot longer than we thought, or were separated far quicker than we thought. And then there's the monopole problem. Generically, In our understanding of the early universe, we're producing, in the splitting of the forces, we're producing exotic defects like crazy, like the monopoles. But we don't see any monopoles. They have to go somewhere. Inflation solves all these. It answers a set of very, very difficult questions very elegantly with one idea. Like, oh, yeah, the universe underwent a period of extremely rapid expansion. And, and, this is the part a lot of people leave out that drives me nuts.

It gives some nice unexpected bonuses, like a bona fide prediction. That's right. Inflation theory gives us predictions about the universe. It explains some things and predicts other things. That is how you have a good scientific theory. Inflation theory predicts the seeds of structure. It predicts that quantum fluctuations get all grown up during the inflationary epoch, lay down the seeds of structure that become things like galaxies and clusters and groups and all that. So it gives us a way to fill up the universe with galaxies, which is pretty cool. And, and, and, that is a triple and. And it's able to make a very specific prediction for what those fluctuations should look like. How many big wiggles are there? How many medium ones? How many small ones? Inflation theory predicts the big, the medium, the small wiggles, how frequently we should see those. And it gets it right. It matches predictions. Our observations of the cosmic microwave background of large-scale structure of the universe match to very, very high accuracy what inflation theory predicts about the early universe, which is why inflation, despite being somewhat goofy, has stuck around for nearly half a century because nobody has a better idea.

But because it has its own problems, like what the heck, powered inflation, the problem of the multiverse, like you start generating one inflation, it seems very natural, at least in our rough sketches of these theories, to produce multiple universes with different kinds of properties. And then your ability to predict what our universe should look like tends to break down. You still need a beginning. You don't solve anything about the singularity. Everything in inflation says what happens after the singularity. So we have this tension. Inflation theory is really successful, but it also has some very, very obvious weaknesses. So it opens the door for everyone who thinks they might have a better idea to propose a better idea. Hey, if you can do it, if you can do it, if you can explain all the same things as inflation, make some predictions of your own and get rid of some of these nasty headaches about inflation that we don't understand. Boom. All right. Let's see what you got. Put your cards on the table.

Enter. Ekpyrotic theory, an attempt to beat inflation at its own game. Now, ekpyrotic theory is one of those cases where the math is beyond gnarly and also largely probably not trustworthy. If you thought inflation theory was a little sketchy, you should see ekpyrotic theory. It's a little gnarly and not entirely trustworthy because it itself is set in a framework of some of the more esoteric branches of string theory. And string theory is already itself an esoteric branch of physics where it's more math playground than grounded physical theory, which is OK as long as it doesn't advertise itself as being anything past that. So what I'm trying to say is you take string theory, which is already highly speculative. And we do not have pinned down. We don't even have a string theory. We have some ideas of what string theory ought to look like if we were to complete it. And you extend that a little bit and you get to ekpyrotic theory, the ideas behind ekpyrotic theory. So we're just way out here in math speculation land, which again is not a bad thing if it yields useful information.

But for this story, we're not going to follow the math because it's nasty. Instead, we're going to follow the pretty picture of what the math says should be happening in the universe. And what a pretty picture it is. Let's start off with the fact that ekpyrotic theory says that our universe is a brain. No, not a brain like you're thinking with right now. A brain. B-R-A-N-E. It's short for membrane. I don't know why. Listen. These string theorists, so they had strings, all right? And you can envision a string. It's like a little one-dimensional thing. Then they realized that the strings aren't enough, that there are more general versions of strings that can exist in like two dimensions or three dimensions or four dimensions where they obey the same mathematical rules that strings do. They just operate in higher dimensions. And the natural extension of a one-dimensional string is a sheet, but sheet was taken by something else. So they called them membranes, but they couldn't stop there because they're like, well, you know, membranes really, really evokes two-dimensional the way string evokes one-dimensional.

But there can be three-dimensional membranes or four-dimensional membranes. So instead of creating a new name for string-like object by operating in multiple dimensions because it has its own special set of mathematical rules that it has to obey, they just shortened membranes to brains and walked away. Listen. It was the 90s. It was a different time. Priorities were different then, okay? So we're stuck with it. We're stuck with the brain. We're going to say that our universe is a brain. What does that mean? It means our universe is a four-dimensional sheet or a three-dimensional sheet of A three-dimensional object that obeys a certain set of mathematical rules. And I said I'm not going to dig down deep into the math, so we're just going to live with it. And as we all know from our good friends down the hall working on string theory, where there's a brain, there's a bulk. I know, I've encountered this before. We've done this before. We've done brains and bulks before. Amongst my least favorite jargon terms, but we're stuck with it.

The brain is where our universe is. It's our stuff. It's the stars. It's the planets. It's the galaxies. It's the coffee shop down the street. Everything in the universe exists on the brain. But then there's an extra dimension that our brain is floating in. And we call that extra dimension, that extra volume, the bulk. If you think of the universe as like a sheet hung out to dry in the wind, the sheet is our universe. And then it's blown in the wind. And then everything surrounding our universe is the bulk. Except our brain is four dimensions. Don't try to think about it too much. And it can also fold in over on itself. There's that. That becomes important for equipyrotic theory. Now, you take this sheet. Take our brain. The thing that is our universe. And it exists in this extra dimension. It can wiggle. It can billow. And it can fold over on itself. Most of the time. These two pieces of parallel fabric, the brains, don't touch each other. But every once in a while, they approach, and when they meet, there's a sudden release of energy that ripples out through the sheets, and then they bounce off of each other.

That's it. That's our Big Bang. Not a singular point of infinite density, but our universe bouncing off itself in a higher dimensional space. We only interpret it as a Big Bang. Because we're stuck on the sheetwork. We live inside the brain. We can't look outside the brain. So we can't extract ourselves into the higher dimensional bulk to see what's really going on. We live here in the brain. We see a big explosion of energy and ripples in space and time. We're like, wow, that's crazy. We can't see through the bulk to the other piece of our universe that we just bounced off of to release this tremendous flood of energy. So to us, it looks like a singularity. To us, it looks like a Big Bang. To us, it looks like inflation. To us, it looks like the early universe. But really, it's our brain bouncing off of another brain inside the bulk. Now, there are a lot of versions of this story, and I'm going to weave a sort of middle ground so that you get the general picture of what the ekpyrotic story is, which is motivated by string theory.

and what it says about the universe. There are a lot of details. There are a lot of variations that I'm not going to get into. And what it says about the universe, what ekpyrotic theory says about the universe is this rosy, everything is solved and we can all go home answer, which is that there's no singularity. There's no Big Bang in the traditional sense. There's no point of infinite density from which everything emerges. There's just Contributions to Patreon. It's patreon.com slash pmsutter. It's how the early universe bounced off itself and kept this show going. And you can participate in that too. Why don't you bounce off of this brain? Patreon.com slash pmsutter. I really do appreciate your contributions. No, there's no Big Bang. No singularity. No point of infinite density. It's just two sheets blowing in the bulky wind. Attracting each other, bouncing, releasing energy is separating. And if you play your cards right, the sheets can do it again, and again, and again. In fact, they might have always been doing this, and will keep doing this forever.

Our universe always existing, repeating, cyclically bouncing off itself to trigger new rounds of big bangs. This is where the name Ekpyrotic comes from. It's taken from a Greek Stoic concept, and it means out of fire. It was a belief in ancient Greece amongst, you know, like four people. A belief that every once in a while, a great cleansing fire would consume the whole world slash universe, reducing everything to ash, giving all of creation another chance. And here we have the universe wiggling around in the bulk, doing exactly that. Let the universe live long enough. Let it expand. Let it have dark energy. Let it do all its normal stuff. then every once in a while will fold over on itself in a dimension that we can't see or perceive, and these two regions will approach each other, bounce off, flash of a new bang, everything resets, everything's flooded with energy, everything inside of it thinks a big bang just happened, and the cycle starts again. Oh, not only is there no singularity in ekpyrotic theory, there's no inflation.

The bouncing brain idea can be made... to include ripples in spacetime that behave exactly like what inflation did, laying down the seeds of structure. So you can get atoms and galaxies and all the good stuff that you expect out of a working cosmological model. Like I said, if you can replace inflation and make the exact same set of predictions, match the exact same observations, but then make some predictions of your own while saying something new about the universe, that's a viable idea. How does it match inflation? Well, look at the problems. A flatness problem handled. It does it through a different mechanism than inflation. Inflation just blows up the universe, so any curvature just goes away. In the ekpyrotic story, flatness is generated by the intense energy density of the universe as the brains are coming together. It just totally swamps out anything else, like as the brains approach each other. the energy between them becomes so immense that it drives the universe to flatness.

That's cool. The horizon problem? Hey, inflation handled it one way. It said, you know, a long time ago, everyone was partying and then inflation happened and flung everyone out to the far corners of the cosmos. Ekpyrotic handles that in a different way. It says the brains take long enough prior to the actual, you know, meat flare bounce situation to They take long enough to approach each other that everyone has time to get their act together, get their story straight, synchronize their watches before the expansion phase kicks in. Oh, and dark energy? Well, ekpirotic doesn't explain dark energy, and neither does inflation. Inflation and dark energy don't care about each other. They don't connect or talk to each other, but ekpirotic theory actually requires dark energy. In ekpyrotic theory, the brains spend a long time doing their own thing, expanding and growing. That's what we see as dark energy. But eventually, far into the future, dark energy shuts off, and that's what allows the two brains to slowly contract towards each other in this extra dimension.

So all you have to do to get a new cycle is just wait. With enough time, say 100 trillion years or whatever, long after the last star is burned out, the two brains will approach each other and bounce, triggering a new Big Bang phase like it always has. Oh, what about that entropy problem with Tolman's cyclic universe saying, Ekpyrotic's got your back? Don't sweat it. The problem with the Big Bang Big Crunch idea of Tolman was that entropy stayed confined to the universe. You build up the entropy, then you squish everything down. There's nowhere for the entropy to go. But in the Ekpyrotic theory, the universe never stops expanding. Even when the two brains meet and flash and ripple, that's just an event set against the overall expansion of the brains themselves. The brains, these sheets are getting stretched out all the time. Just two pieces of it bounce off of themselves, triggering a new big bang, but they don't stop expanding. This means you get to dilute the entropy throughout the enormous volume of the universe before you trigger a new big bang.

gives you a cheat code to essentially reset your entropy with every round. One universe, always existing, bouncing off itself in a higher dimensional space, powered by dark energy, whatever the heck that is, resetting entropy every time, getting the seeds of structure through the ripples in the fabric after the collision, no inflation, no singularity. As ideas go, it's not bad. In fact, it's good. It's a good idea. Yeah. It's just a shame it doesn't work. I mean, at least work as well as advertised. And why, after a couple decades after it was first proposed, it hasn't managed to unseat the traditional inflation story. First, we need to talk about the singularity. Ekpyrotic theory makes a bold claim that there's no singularity here. That it's just two brains merging, meeting, releasing energy. But... In the original formulations of the theory, that was more of a claim than a demonstration. It was more of an assertion like, oh, yeah, yeah, obviously, when these two sheets meet, there's no singularity.

But again, you know, ekpyrotic theory is based on string theory, and we don't have a string theory. We have ideas and sketches of what a string theory might look like and how it might behave. We do not have a working string theory. String theory actually can't make predictions. The math isn't there. I did a whole series on string theory. Feel free to listen to it. If you have more questions, let me know. So this was more of a cross our fingers, we're sure the string theory math will work it all out kind of situation. And it didn't actually work out. Other people dug into the math and discovered that no, there's no guarantee that That even with our barest sketch of what string theory should look like, that you can guarantee to get rid of the singularity, singularities may still appear in this model, which means there are points where our physics breaks down and we can't explain them anymore. Then there's this problem with dark energy. Ekpyrotic theory demands that dark energy shuts off far into the future.

While dark energy is on, keeps the brain separated, it's only once dark energy shuts off that two brains can start to approach each other well we don't know if dark energy is going to slow down or stop or reverse we don't even know what dark energy is so like pinning the entire structure of your cosmological model on another part of cosmology that we don't really understand okay you know it's it's a fair statement but also it's a weak one you got a lot riding on dark energy doing what it's told and if dark energy is taught as anything it doesn't like to do what it's told And then there are the brains. Just like zombies, it's easy for string theory to want more brains. And the generic picture that I painted for this episode is just one universe, just one mega brain, folding over on itself to trigger these new rounds of big bangs. But... The theory itself, the math is more generic than that. There's nothing stopping other brains from existing and other brains of other universes colliding into ours and we mutually create big bangs.

And so one of the objections to inflation theory is that it's too easy to make a multiverse. And once you have a multiverse, once you have every possible universe created... through your physics, then it becomes impossible to really say anything conclusive or predictive about our universe. And this is the game of science where you have to make predictions in order to win. We limit ourselves to ideas that are testable and multiverse is very, very difficult to test. Multi-brain is just as difficult to test because you're like, well, what does, what does ekpyrotic theory say about our universe? And if your answer is, well, it makes every possible universe and we're one of the possible universes. Well, then how am I supposed to test it? What am I supposed to say? What do I say to the astronomers running the telescopes? Look for anything and whatever you find that will validate the theory? That doesn't sound right. There are workarounds for all of these theoretical objections, or at least hand waves around all of it, to keep ekpyrotic theory alive, keep it stuck to a single brain.

maybe work out different mechanisms than dark energy, you know, et cetera, et cetera. But the real stickler with ekpirotic theory is in what we can observe. Ekpirotic theory actually makes predictions about the universe just like inflation does. Inflation makes a real deal prediction, not just about the seeds of structure of what would become galaxies and groups and clusters, but in what kinds of seeds there are, the sizes, the distribution, Inflation predicts the statistical properties of what kind of universe you're supposed to get. And while we can't observe inflation directly, we can observe the cosmic microwave background, which is the earliest picture of the universe we've got. And inflation agrees with those observations and made predictions of what we should see with our cosmic microwave background probes. And then that's exactly what we saw. We got the right statistics. Ekpyrotic also makes predictions for the statistical properties of the seeds of structure. I mean, you can't have a decent theory of cosmology without one.

And it too says something about the cosmic microwave background that we observe. And actually, the original version of ekpyrotic theory was way off. Like, stop talking to me, how did you find this office? Way off. You got it wrong. Inflation says there should be so many big... Wiggles, so many medium ones, and so many small ones. Ekpyrotic said, oh, no, no, no, no, it's this many big ones, this many medium ones, and this many small ones. You see, ekpyrotic theory was originally developed in the early 2000s, when we had only somewhat rough measurements of the cosmic microwave background available. And the theory was designed and tuned and refined to match the known observations. Then we went and built Planck. a satellite that measured the cosmic microwave background to the greatest level of detail ever, explicitly designed to test various ideas of the early universe. Inflation, others, ekpyrotic, there's others based on cosmic strings that form the seeds of structure, the whole deal. Planck gave us a much more refined, much more detailed measurements, and those measurements agreed with what inflation said about the early universe and not with what ekpyrotic theory said.

And there aren't many ways around that. The theory isn't dead. There are some hacks. There are some tricks people are trying to pull, continue to try to pull to try to get the right statistics out of it, the right properties of the universe out of it. So it's not dead, but it's marginalized. Not many people focus on it. It's got too many roadblocks. It's difficult to wrangle the math. It doesn't deliver fully or cleanly on all of its promises, like eliminating singularities and struggling with essential observational tests. like predictions of the cosmic microwave background and the rest of structure. It's a good idea, but good ideas aren't guaranteed to be right ideas. That's up for nature to decide, not us. So it still putters along with some people poking at it here and there, maybe because it's still a fun idea. And also, wrong ideas might yield interesting fruit down the line if you poke at them enough. It may not be act by erotic theory, but some nugget buried in there might be interesting and worthwhile.

But for now, Big Bang and inflation agree with all known observations. And all evidence suggests that this one universe, this one shot, is all we get. So make the best of it. Thanks to Henrik L. for the question that led to today's episode. Please keep those questions coming. It's askaspaceman at gmail.com or the website askaspaceman.com. If you can, leave a review comment on this podcast. It helps to show visibility on your favorite podcasting platform. And of course, if you can, I would appreciate any contributions to Patreon. That's patreon.com slash pmsutter to keep this show going. I'd like to thank my top Patreon contributors this month. They are Justin G, Chris L, Alberto M, Duncan M, Corey D, Michael P, Nyla, Sam R, Joshua, Scott M, Rob H, Scott M, Lewis M, John W, Alexis, Gilbert M, Rob W, Jessica M, Jules R, Jim L, David S, Scott R, Heather, Mike S, Pete H, Steve S, Lisa R, Kevin B, Eileen G, Debbie A, Michael J, Phillip L, and Stephen B. That's patreon.com slash pmsutter. And I'll see you next time for more complete knowledge of time and space.

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