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Part 2! What was going in the very early universe? How does inflation provide the seeds of larger structures? How can we possibly test this? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
Welcome back. Our bedtime story today starts when the universe was a mere 10 to the minus thirty six seconds old. Notice that we don't even bother with cool sounding Greek prefixes here. There's no nanos or picos or femtos. It's just 10 to the minus 36.
Zero point zero zero zero many more one seconds. You may want to listen to part one on this two part series on inflation. But if you're chill and just want the words to wash over you with no meaning or context or significance, that's fine by me. Five stars on iTunes, please. I tried to find a cool metaphor to express how quick this moment is.
The moment leading up to inflation and then the event of inflation itself. I couldn't find one. Here is something that falls far short. You have a billion heartbeats in your life roughly. Start counting and it kind of stresses you out to count, but you have a billion heartbeats in your life.
Compare the length of your heartbeat like boom boom to the length of your life. Your life is much much longer than your heartbeat. You can get a vague sense of the extent of your life if you compare it to the length of a heartbeat. If your heartbeat lasted ten to the minus thirty six seconds, you would need a billion, billion, billion, billion lifetimes to reach one second. So, yeah, it's it's not long.
I guess I could just say that. It's not long at all. The universe at this epoch is almost, and that's the key word here, almost incomprehensibly different than it is today. It's a little bit comprehensible and we're gonna use that to our advantage. And it really is an epoch.
It really is an age in the universe, even though it's a blink of an eye, less than a blink of an eye. The physics here that are operating are game changing. And I wanna make a quick aside here about the cosmic calendar that you sometimes see in shows like Cosmos. You know, these things that say, oh, when the if the big bang started on January 1, then, you know, stars appear in March or whatever. I guess it'd be more like January.
You know, the solar system shows up in October, and then humans don't even arise until the last minute on December 31, blah blah blah blah. It's designed to show you how insignificant human lives and events are on a cosmic scale, but that's biased. It's a biased perspective because here we are now in this episode, we're talking about events that take 10 to the minus thirty six seconds to start and then ten to the minus thirty two seconds to complete. And so much interesting physics happens that completely alters the future evolution of the entire universe. Compared to that time scale, we, human beings, even your heartbeat, are impossibly long lived and ancient.
So just dividing up time that way doesn't always make the most sense. Feel free to ask, and I'll give you guys an episode on a cosmic calendar that has, you know, proper scaling. So anyway, inflation. Yeah, a long time ago it seems that the universe got really really big really really fast. Okay, but how?
In 1980 Alan Guth proposed an original model and he coined the term inflation. And in his model, the universe got stuck in between phase transitions. In this moment, the universe was transitioning from having a grand unified theory with three forces in nature, strong nuclear, weak nuclear, and electromagnetism all bound together. It cooled off just enough for the strong nuclear force to peel away. That was a phase transition.
The the universe changed character in that moment. But in Guth's model, and we saw in last episode that during this phase transition, a bunch of monopoles were produced, and monopoles are bad news, we wanna get rid of them, and inflation is a way to get rid of them. And there is a way during this phase transition for the universe to undergo really, really, really fast expansion, AKA inflation. And it does this in Gauss model by getting stuck in the phase transition. An example of this is superheated water.
Right? You can you can heat up water in a microwave past its boiling point, and it's much, much, much hotter than the boiling point, but it's not boiling yet because it no bubbles have nucleated yet to actually start the boiling process. And so it's it's super it's in this super heated state, and then if you if you tap it, if you touch it, then, blah, the whole thing blows up. Another example is you can take, again, water, and you can freeze it very, very carefully to below its freezing point where it's still a liquid, but no nucleation has happened to actually start turning it into ice. So it stays as a liquid, and then you just touch it.
You just you just breathe on it, and then whoosh, the whole thing turns into ice. Systems can get stuck in the middle of a phase transition, and they have some weird properties when they get stuck in the middle of a phase transition. It's possible that in the early universe, when the strong force went to go split off, when it was time, it's like, okay, guys. It's been great. This ten to the minus thirty six seconds have felt like a lifetime, but it's time for us to go its separate ways, but then it gets stuck.
It gets stuck in what's called a metastable state, where as long as nobody breathes on the universe, it's in this awkward state, then as soon as there's some bump or wiggle, whoosh, it switches into the lowest energy state possible. An example of this, another metaphor visual, is imagine a ball rolling down a hill, it's heading down towards its lowest energy state, the bottom of the hill where it's gonna be nice and stable. But if it gets stuck in a little crevice, it will hold there. It's not in its lowest possible state, but it's not moving anywhere. But if you were to kick the ball, then it would get out of the crevice, out of the nook, out of the cranny, and down into the bottom of the valley.
This is called a metastable spit state. And in this metastable state of the universe, it had an unnaturally large amount of vacuum energy. There was extra vacuum energy hanging around that should have been in its lowest energy state, but it wasn't. It got stuck. And like we learned a couple episodes ago about the nature of dark energy, if you have a lot of vacuum energy, you get accelerated expansion.
So this is just like dark energy. So go ahead and pause and listen to the dark energy episode again. It's all the same mechanics. It's all the same physics. It's all the same justification, only more so.
Dark energy, there's a little bit of vacuum energy left over that's giving us this very subtle accelerated expansion. Back in the inflation days, there was a lot of vacuum energy driving a ridiculous amount of accelerated expansion. But then inflation has to end, and this inflation event ends when the universe, unstucks itself. It gets out of the crevice. It gets out of the nook.
Someone finally breathes on it or taps it. And in Goose Model, this happened by a process called Quantum Tunneling. I haven't done an episode on Quantum Tunneling. Feel free to ask. I'd love to dig into it.
So for now, we're just going to go with the universe got itself unstuck through magic or a plot hole or Batman. It it just it just stopped being stuck, and that's what matters for our discussion. By the way, some people wonder if the universe is still in a metastable state that will eventually get unstuck from in the future, that's another show. But the problem with this model, and and this this all this this model fits within the framework of quantum field theory. It fits within the framework of the gut phase transition of the strong nuclear force splitting out.
It's it's not really anything special. It's just saying, okay, in this this process wasn't perfectly smooth. And because this process wasn't perfectly smooth, we experienced this inflation event. But the problem with Guth's model, the original model of inflation, was that it ended too soon. The universe inflates, goes whoosh, feel free to insert your own lame sound effect here, and then it's done.
And we end up with a flat universe that's causally disconnected, it's free from monopoles, and it's 100% lifeless. So how do we fix this? Have you ever heard the phrase, you gotta slow your roll? Well, that's the solution we're gonna apply here when the universe was only 10 to the minus thirty six seconds old. Inflation is great, and Guth's inflation was great, but it has to slow its roll.
And I'm not even making this up on the spot. This model is literally called slow roll inflation. Instead of the universe getting stuck mid phase transition then magically Batmanning itself out of the situation, we're gonna introduce a new character to the universe, a new thing, a new component. We're gonna call it the Inflaton. No.
No. No. Wait. I I think it needs more grander. It needs more oomph.
Let me try again. In its darkest moment, when the universe needs it the most, a new hero appears. The Inflaton. That sounds better. The Inflaton which I meant is really, really silly to say out loud but hey at least you're expanding your vocabulary, is the thing that's going to make inflation happen.
It's a quantum field. Quantum fields are the stuff that permeate all of space time, and quantum fields are what generates matter and radiation in our universe. For everything like an electron or a quark or a photon, there's an associated quantum field that soaks all of space time. We're gonna add a new character. We're gonna add a new field.
There's electrons, there's photons, there's quarks, and now there's the INFLOTON. The INFLOTON starts off, for lack of a better term, like a ball sitting at the top of a hill. There's a lot of potential energy there. And then gently rolls down the hill releasing that potential energy, and this inflaton has a lot of vacuum energy. And it releases that potential energy, turns it into kinetic energy.
This is Physics one hundred and eleven. You just just imagine a ball rolling down a gentle slope. I want you to imagine that the Inflowton doesn't really roll down a grassy hill on a summer day, but what matters is that this Inflowton, I can't resist saying that that way every single time, starts off with a lot of potential energy and converts it into kinetic energy and the kinetic energy is the driving force of inflation. The inflaton, the act of it rolling down, of it losing potential energy drives the accelerated expansion of the universe through its intrinsic vacuum energy. Then it reaches a steeper part of the hill and goes faster that shuts off inflation, And then the inflaton eventually fades from the cosmic scene altogether and never becomes a player ever again.
Now I know what you're thinking. This is dumb. Physicists are just making things up with no rhyme or reason. Surely, there are more compelling ways to solve the flatness problem and the horizon problem, and the monopole problem may not even be a problem at all because we don't really understand gut error physics. So maybe the whole thing is off.
It seems like this story of inflation is just spinning a bedtime fairy tale story that sounds nice, but it's designed to fit the situation. Like, oh, maybe the universe does this. We'll cook up some scenario to make it accelerate its expansion. Oh, that doesn't quite work. I know.
So we'll introduce this potential energy and the Inflaton. And then the Inflaton does this, and that's what makes it. And it's it sounds fishy. And you're right. You're absolutely 100% right.
Inflation sounds fishy. The inflaton sounds dumb. It's a cute story, but is it physics? Does it matter? Does it matter if it's a story or physics?
No, what matters is that you contribute to Patreon. Patreon.com/pmsutter is how you keep this show going. Deeply appreciate all of your support. It really means a lot. Also go to pmsutter.com/book to pre order and or regular order my book, Your Place in the Universe, which is a fun, fact filled journey with lots of lame metaphors and puns that you know and love in book form.
And I guarantee you're gonna read the whole thing in my voice. That's pmsutter.com/book. So yeah, is inflation a story or physics? And there's a reason that inflation, this idea, has stuck around for so long, since 1980, as an interesting idea. And that has to do with what inflation does next and how we can actually test this.
So inflation right now, this whole story has been designed to explain some observations. But in order to do physics, we have to actually make predictions. And what the interesting thing is is what has to do with the end of the inflation event, especially the slow roll inflation event. By the way, slow is relative here. Inflation still takes something like ten to minus thirty seconds or ten to minus thirty two seconds to complete.
It's just, you know, slower than what we had thought but it's still ridiculously fast. We have to ask at the end of inflation, what happens to the Inflaton field? It goes away. It decays. It disappears.
The inflaton eventually releases its energy, get this, in the form of matter and radiation. As the inflaton dies, it decays and refills the now empty universe and reheats it. Keep in mind at this time, the universe is the size of, like, a golf ball, so it doesn't exactly have to work very hard. This kind of process is a normal thing in high energy physics with one kind of particle decaying into others. For example, go back and listen to the OMG particle episode.
That was one kind of particle decaying and transforming into a shower of different kinds. And since particles are just chunks of a field, we can totally legit have fields dissolving into other kinds of fields. That's not a weird thing in high energy physics. How this process takes place is a little bit sketchy. This is after all super duper high energy physics and all that.
And the inflaton field is totally hypothetical. We're kinda making it up on the spot to explain it. But in order for this inflation story to work, inflation has to decay, it has to end, and then it has to refill the now empty universe with matter and radiation. This process called reheating also occurred in Alan Guth's original model of inflation, but there wasn't enough time for a proper reheat. Like, if you're just trying to start that lawnmower and you don't quite yank the cord hard enough, it goes, boom, boom, boom, and then it just quits.
Alan Guth's original inflation model reheated the universe, but not quite enough. Slow roll inflation fixes that by giving the inflaton enough time to both drive the accelerated expansion of the universe and and then decay into all the matter and radiation that we know and love. But wait, there's more. That's not quite the juicy bit. Here's the juicy bit.
Reheating is nice, but it's still just a story. The prediction comes in with how exactly the reheating takes place and what it does to the universe. Our universe is cosmically boring, right? At at least at large scales. It's flat.
It's homogeneous, which means any one patch isn't really different from any other on large scales. It's isotropic, which means you look deep into the universe in any one direction. It doesn't really look any different from any other direction. That's at large scales right. Very, very big scales.
Much, much larger than galaxies. Like, somewhere around 400,000,000 light years, that's when you start to get into this homogeneous scale. But at small scales, there are all sorts of interesting things which has fueled basically this entire podcast. There's galaxies and planets and bad metaphors. There's all sorts of stuff.
How did small scale bumps and wiggles, like galaxies, and in the cosmological perspective, a galaxy is a small scale bump and wiggle, how did small scale bump and wiggles appear in an otherwise big, flat, boring universe? No points for guessing correctly. The answer is inflation. What I'm about to say is so ludicrous you could almost accuse me of being a madman, but no madman would have the creativity to actually say what I'm about to say. Remember the inflaton?
The field that drives inflation? It's a quantum field, just like any other. Which means that in the pure vacuum it's actually vibrating and humming and buzzing. The same thing we were talking about with dark energy, the same thing as vacuum energy, the same as any other quantum field. If you just look at the inflaton field, it has energy associated.
It's buzzing, it's vibrating, there's something there in the vacuum. And if you're wondering, by the way, why quantum fields appear so much in all these episodes, it's our fundamental theory for all of matter and radiation, so yeah, it's gonna crop up a lot. These vibrations in any quantum field, in the inflaton field are super duper tiny. They're below microscopic, they're below nanoscopic, they're below femtoscopic, they're just tiny. But when inflation happens, those bumps and wiggles get big.
Those vibrations, those fundamental vibrations in the quantum field itself get stretched and pulled. They get carried along with the inflating universe just like anything else. They go from almost impossibly small to just, well, normally small. And those bumps and wiggles imprint themselves onto space itself. There are now, after inflation, tiny, tiny, tiny density differences.
Little pockets, little corners of the universe have a little bit higher density or a little bit lower density because the quantum foam itself, these fundamental vibrations got scaled up and imprinted themselves on space time, which led to slight density differences made of slightly bumpy fabric of space time. Then when the inflaton decays, when it spews out matter and radiation, when it reheats the universe, it doesn't fill it up perfectly smoothly. There's tiny little pockets of slightly higher density over here and over there. Oh, and right over here. Those pockets of slightly higher density have slightly stronger gravitational attraction.
They attract matter, which leads them to have even higher density, which leads them to have even stronger gravity, which meet leads them to have even stronger gravitational attraction in this process continues. It's a chain reaction where these densities grow over time. How much time? How about thirteen point eight billion years? If the inflation fairy tale story that I'm telling you is correct, then all density differences in the universe were seeded here when it was less than a second old.
All the bumps and wiggles in the temperature of the cosmic microwave background, which we can observe, started here in inflation. The eventual formation of the first stars hundreds of millions of years later started here in inflation. Galaxies, the cosmic web, here. The great attractor got its start here. Molecular clouds, solar systems, planets, you started here in inflation.
Physics is about starting with initial conditions and following causes and effects to find the consequences. We have, with inflation, an unbroken chain of cause and effect that stretches back thirteen point eight billion years that tells us that the seeds of all structure in our universe formed in the microscopic quantum foam itself that was inflated to get this ball rolling. So let this moment sink in what I'm telling you. Just absorb this concept, that enormity, that the surprise, the, dare I say it, the beauty. The reason you are here.
A high density bump in an otherwise boring universe, is because 13,800,000,000 ago, a random quantum vibration was inflated and set the seeds for later structure formation. But still, that's just a bedtime story. How do we know? Because inflation predicts certain properties that those bumps and wiggles ought to have. There are two jargon words here that we need to that we need to introduce into this story.
Scale, invariance, and Gaussianity. I want you to imagine an orchestra. You know, all sorts of different instruments. Some make high pitched sounds, some make low pitched sounds. If all the instruments play at the exact same loudness, like the flutes are here and there's certain loudness, and there's, a tuba.
I don't know exactly what goes in a full orchestra, but there's something that really deep or really low pitch instrument. And everything's the exact same loudness. You have something called scale invariance. Low pitches and high pitches are equal. Inflation predicts, predicts that the seeds of structure should be scale invariance.
That's because if there's these quantum wiggles, they're just doing their thing, minding their own business, then boom inflation happens. Inflation is an exponential event. It exponentially increases the size of the universe. So it takes one wiggle happens and inflation gets started, takes that wiggle, starts stretching it out. Then another wiggle happens and inflation's gonna grab that and stretch it out right behind it.
Then another wiggle's gonna happen in the quantum foam. Inflation is gonna grab that stretch it out. So it keeps stretching out these wiggles one after another with the exact same amount of strength because that's what inflation does. Inflation is this exponential event that expands everything equally, inflates everything equally. So you end up with a universe filled with very long wavelengths, medium wavelengths, low wavelengths, all these different kinds of bumps and wiggles with the exact same loudness.
Inflation doesn't work perfectly smoothly. It has a beginning and an end, for example. So it's not 100% perfectly scale invariant, but it predicts it's nearly scale invariant. The other term, Gaussianity, if the instruments in the orchestra, you know how they're all, you know, kinda playing from the same score? Like like, each instrument has its own music, but there's this overarching theme so that everybody's connected and it makes sense when you listen to the whole ensemble.
Imagine if you went around passing out to all the different instruments a completely different set of sheet music. Like you're gonna play from this song, you're gonna play from that song, you're gonna play from this song over here, you're you're gonna play that one, and you just say, okay, go for it everybody. And the trumpets are playing one song, and the violins are playing from another song, and the tuba, which apparently is in this orchestra, the tuba is playing its own song. If they're totally disconnected from each other, all these different frequencies, all these different wavelengths, all these different pitches are totally disconnected from each other, that is something we call Gaussian. There's no overall score to harmonize.
The quantum wiggles that inflation causes to, inflate don't connect with each other. They're not related to each other. There's one bump and wiggle and then that could stretch out. And then another random bump and wiggle, that gets stretched out. And then another random bump and wiggle, and that could stretch out.
They're not they're not talking to each other, so they end up doing their own thing. They are Gaussian. So we have a prediction. If we look at the cosmic microwave background or the structure of the universe, we ought to find nearly scale invariant Gaussian bumps and wiggles. Technically, the word here is perturbations, but I'm not in the mood for saying that word a lot right now, so we're gonna go with bumps and wiggles.
What do we find in our universe? Nearly scale invariant Gaussian bumps and wiggles. Where do we find it? Cosmic microwave background. We can't see the event of inflation itself.
It's too dim, too blocked from our view, but we can see how the universe evolved since the moment of inflation. We can see the effects that inflation might have on the young universe, and one of our best ways to see that is with the cosmic microwave background. And we see nearly scale and variant Gaussian bumps and wiggles, exactly what inflation predicts. We've measured something called non Gaussian, like like how this cosmic microwave background, this light from the early universe appears to be Gaussian. Can we measure any sort of non Gaussianity at all that would tell us, like, oh, you know, there's some exotic model of inflation, maybe multiple kinds of inflatons running around, maybe there's some worse stuff.
Perhaps, and we've done this, the measure of non Gaussianity in the cosmic microwave background is perhaps the most precise measurement ever completed in all of astronomy. There are absolutely no signs of deviations from Gaussianity in the cosmic microwave background. The only theory, the only model that is able to explain that is the inflation model. So is inflation just a fairy tale bedtime story, a made up solution to a made up problem? Well, it solves the flatness and horizon problems.
It predicts how structure forms in our universe. It predicts how that structure ought to look, nearly scale invariant, Gaussian. These initial primordial perturbations have been evolved by gravity since then to grow up to become stars and galaxies and clusters of galaxies and the cosmic web and you and me, but we started out as nearly scale invariant Gaussian bumps and wiggles. But we don't understand its mechanisms. Has some problems of its own for sure.
Why does the Inflaton behave the way it does? Doesn't it seem still a little bit fine tuned? It brings up some troubling things like the concept of the multiverse, which is another episode. Feel free to ask. Raises some questions like is it connected to dark energy?
These are both explained by quantum fields, both periods of accelerated expansion, but totally different time scales, totally different energy scales. Are they connected? Probably not, but maybe. It seems that something like inflation happened. Our universe appears to have underwent a period of rapid acceleration, of rapid expansion in its early days that imprinted the quantum foam into the fingerprint of the universe itself.
We have no other satisfactory explanation for the data that we have. But our only explanation isn't entirely satisfactory. We don't understand gut error physics. We don't understand the mechanism of inflation. We don't understand the properties of this hypothetical inflaton field and so on.
It seems like a complete answer is just around the corner. There's a chance that complete answer that gives us the full picture of what happened in inflation might totally rewrite this entire story. So I'll just leave it there, and I'll tell you good night and sweet dreams. Thank you so much to all the amazing questions that led to this two part episode. We've got Massimiliano s on Facebook, Lorenzo b on email, at Zac Cody on Twitter, p e on email, Christian w on email, at Uproar on Twitter, Vicky k on email, Thomas on email, Banda c on Facebook, Steve s on email, Evan w on email, Andrew p on email, at Mark Gripa, Twitter.
So many people asking so many questions about inflation. At LouvreDade on Twitter, at Kazoukas on Twitter, Gordon m on Patreon, Jim w on email, Cosmic Wakes on YouTube, Florin h on YouTube, Gabby p on email, at Scared Jackal at Twitter, and Amanda z on email. Remember, you can go to patreon.com/pmsuder to help support the show. Big thanks to my top Patreon contributors this month, Robert r, Justin G, Kevin O, Justin R, Chrissy, and Helgeb. It's those folks and all the other fine folks that are keeping this show on the air.
If you don't want to contribute money, that's fine. Can you please go to iTunes and leave a review? That helps get word of the show out there. And of course, go buy a book. If you're in the mood for a book or even if you're not, go to pmsutter.com/book.
There'll be links to Amazon, Barnes and Noble, Books A Million, Indie Bound, all the places where you can buy books. You can buy Your Place in the Universe, which is the book I wrote. And it was lots of fun to write and I guarantee it's lots of fun to read. And I'll see you next time for more complete knowledge of time and space.