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Part 1! What are some weaknesses with the standard Big Bang model? What are the horizon and flatness problems? How does accelerated expansion solve these? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
Imagine traveling to a far away country, like on the opposite side of the globe. You're you've been pumped, you've been prepping for this trip, you've got your passport, you've got your extra underwear, you have your snacks for the flight, the whole deal. What do you expect when the plane lands and you finally arrive after a million hour flight? Least you have a decent podcast to listen to to pass the time. But you finally land, the doors open, you walk off, and what do you expect?
You expect it to be exotic, right, and foreign. Maybe it's warmer or maybe it's colder. There's the people should be different, the language should be different, the the money should be different, even even the cheese should be different. Imagine traveling all day, landing, getting off the plane, and it's the same. The people look the same.
The air smells the same. The buildings look the same. Everything's in the exact same language where you left. It's like you never left at all. How could two cultures separated by thousands of miles and oceans and mountains and unnavigable rivers end up being exactly the same.
If this were to happen to you, you would face what cosmologists call a horizon problem. How could a culture beyond your horizon end up looking the same as your own? In order for that to happen, there needs to be contact, there needs to be trade, there needs to be a lot of time for the two cultures to interact and meld together into a single unified culture that despite their differences, their distances, they are essentially the same. That takes time. When we look out at the universe, we see vast stretches of the cosmos.
Alright, we we see a big chunk of the universe, and there are patches of the universe that we can see that look basically the same. They're not copies of each other. It's not like we see a group of galaxies over here, and then we look in the opposite direction on the sky and we see the exact same group of galaxies. It's not a copy, but it's the same culture. Just like if you were to do this imaginary plane ride, you wouldn't see the exact same people, you'd see the same kind of people.
It's not like you're gonna meet your own grandmother on the other side of the world, but you're gonna meet someone who looks a lot like your grandma. This isn't a big deal in cosmology if these two patches of the universe have had enough time to be in communication, to trade info, to share a culture, to allow lights to bounce back and forth so they can have roughly the same temperature and roughly the same arrangement and statistical properties and all that. That's not a big deal, but they're too far apart. They're too far apart for the amount of time that the universe has been around, thirteen point eight billion years, there are chunks of the universe that are too far apart for them to have the same kind of cultural identity and yet they do. The big bang model of cosmology suffers from a horizon problem.
To illustrate this with a little bit more details, imagine looking at the Cosmic Microwave Background, you know, this leftover light from when the universe was only 270,000 years old. The universe back then was only a millionth of the volume it is today. And the light that now reaches us from the Cosmic Microwave Background, it's it's spread out all over the sky. You are seeing light from other parts of the universe that are now totally disconnected from us. The expansion of the universe has carried that chunk of the universe far away from us, inaccessibly so even, but we can see the leftover light from when the universe was a lot younger.
And you can ask how big those chunks of universe are. If you run the clock backwards to when the universe generated the cosmic microwave background, it's one millionth of its present size and you can figure out, okay, what is a ball, one millionth the present size of the universe and project it on our sky? And you do some math, and it's it's not the hardest math in the world, but it's a you have to be a little bit careful about it. And that chunk is about two degrees across. So if you hold your fingers to the sky, your outstretched hand, and I'll let you choose which fingers you wanna hold up to the sky.
One finger is about one degree. So if you hold up two fingers next to each other, it's roughly two degrees. The width of those two fingers contains an entire observable patch of the universe, and we're seeing the baby picture of that chunk of universe, of an observable bubble of the universe the same size as our own that's now totally disconnected from us and really, really far away. There are about 10,000 such patches in the whole sky. So looking at our cosmic microwave background, we actually are seeing the origins of 10,000 patches of observable universe, of bubbles of the universe.
But the cosmic microwave background is smooth. It's smooth to one part in a million. It has the same average temperature everywhere in the sky to one part in a million. There are 10,000 bubbles of universes, observable patches of universes, chunks of universes that are all totally disconnected from each other now that have essentially the same temperature. So how?
How did all these independent chunks of the universe have enough time to get the same average temperature? This is the horizon problem. The universe hasn't been around long enough, especially in its early days. Especially in its early days, the universe didn't hang around long enough for everyone to agree and share notes about what the average temperature ought to be before expansion continued and separated these and put them on opposite sides of the sky. This is a weakness, like I said, in the Big Bang Model of cosmology.
That's right. Like any model of nature, there are going to be weaknesses. There are going to be things that the model can't explain. There are gonna be shortcomings. There are gonna be corners.
There are gonna be facets, facts observational facts that we can't fit into the framework. And in physics and in all of science, this happens all the time, you're always faced with a choice. If you have a model, say the Big Bang model developed in the twenties, thirties, forties, and fifties, and it comes face to face with an issue like the horizon problem, you're you're faced with a choice. Either we can find some way to extend the existing model and keep the framework, keep the house that the model lives in, but maybe add another bedroom or or redo the bathroom or or add a add a sunroom. Yeah.
That'd be great. Or maybe a pool outside, but it's still the same house. Or if the problem is big enough, if the issue is big enough, you you're not gonna fix it with a remodel or an extension. You just have to burn the whole sucker down and collect the insurance money and walk away and maybe build something new. This happens continuously.
This is the name of the game of science. This is the process of science that goes on over the course of years and decades and centuries, And it's never clear cut which way to go. You know, if there's a lot of problems, a lot of issues, then it's probably about time to replace the model. If there's a few corners but like the framework is still sound, if the foundation of your house is still is still good, termites haven't infested it, then maybe you should keep the house and just focus on remodeling. At this point in 2018, when I'm recording this episode, the Big Bang model of cosmology is virtually unassailable.
It's so darn good at explaining so many features of our universe. There are no alternatives left standing, but it doesn't explain everything without extensions. There are features of the universe that clearly fit within the Big Bang framework, but we don't fully understand, and we need to extend our Big Bang language in order to accommodate these observations. One of these is the horizon problem. Our best solution to it is something that we're calling inflation, which I'm going to explain in glorious and or gory detail, but I wanna be straight with you.
We don't fully understand inflation. This this process that I'm gonna lay out over this two part episode, we don't fully understand. We don't understand the processes, the methods, the physics. It's something of an ugly hack, And I wanna be upfront with you that there are some serious level headed, well respected physicists who say that inflation, the store I'm gonna tell you, is on the wrong track, including some of the people who were amongst the originators of inflation decades ago. But then there are others that insist that it's the only way forward, and we just don't have a full understanding of it yet.
And honestly, I don't know which way to go. I'm gonna give you the rundown of why we think inflation is necessary, what we think inflation looks like, and what some of the inflation the the weaknesses of inflation are. What got me started on this episode are a bunch of questions from Massimiliano s on Facebook, from Rolando b on email, from at Zach Cody on Twitter, from PE on email, Christian w on email, at Opera on Twitter, Vicki k email, Thomas email, banda c on Facebook, Steve s email, Evan w on email, Andrew p on email, at margreep on Twitter, at luft eight on Twitter, at kazookas on Twitter, Gordon m on Patreon, Jim w on email, Cosmic Wigs on YouTube, Florin h on YouTube, Gabby p on email, at scared jackal on Twitter, and Amanda z on email. Tons of questions. Very lots of people curious about inflation, and I think it's time.
I think it's time to dig in. And two these two episodes are gonna be a little strange because usually I address topics that we either know really, really, really, really well, where I can go into great detail about all the things we figured out over the past few centuries, or it's something that we're totally ignorant. Like, here's a some super mysterious part of the universe, and I'm gonna explore how we know about it and how we're trying to tackle it. Inflation sits somewhere in between. Where we do know something about this process and we have some solid evidence that this process occurred, but we don't know the details, and we don't understand the physics of the details.
My own perspective, which I wanna tell you since that bias will, of course, pervade these episodes, I think that something like inflation happened in the early universe. I think there is very strong and compelling evidence that it did. In factflation has been able to make successful predictions, which is the cornerstone of a good theory, but theory itself is probably too strong of a word, more like operating assumption and placeholder until we learn more. I'm glad I just did the dark energy episode for multiple reasons, and this is one. It's gonna it's like our placeholder for dark energy, but it's on some stronger grounds here.
But there's a fair chance this whole thing could come crashing down. I know I'm giving you lots of caveats because I wanna set the stage for this story of inflation. I wanna make sure you have the proper context before we dig into the meat. This is exciting times, right? Exciting times brought to you by Patreon.
Go to patreon.com/pmsouthern to learn how you can keep this show and all my education outreach activities going. Huge shout out, huge thanks to all my supporters. If you would like to preorder my book or if it's past 11/20/2018 when you listen to this, buy my book. Go to pmsutter.com/book. In that book, I tell the story of inflation using its separate language.
If you're wondering why I waited until, like, 50 people asked me about inflation, this is why. I wrote the book about a year ago, and I knew I was gonna write the book about a year and a half ago, and I wanted the book to have a fresh perspective than the story I'm gonna tell in the podcast. So I wanted to approach it in the book, then let it sit, let me forget about let me forget all my metaphors and and stupid jokes, and so I can bring in new metaphors and new stupid jokes into the podcast. So that's why I'm hitting the cosmology episodes pretty hard because my book is about cosmology, and now I can revisit those topics, and I'm not just reading the book out loud to you. So pmsutter.com/book for your place in the universe, which is the book I wrote.
So I I started this episode on the horizon problem. And if that was the only issue facing Big Bang cosmology, we we wouldn't really care about inflation because there's there are other ways to solve the horizon problem without invoking or saying the name of inflation. But Big Bang has some other issues at well that need to be addressed. For example, we know the universe is flat, right? We've measured it really, really, really well.
We've measured the flatness today. We've measured the flatness in the past. It is geometrically flat. It's neither open nor closed. It is so boring it is flat.
We've measured it within a few percent. First off, congrats to all the fine cosmologists over the decades who were able to measure something incredibly magnificent like the curvature of the universe itself with such incredible precision. Round of applause to my predecessors. But here's the thing with flatness. The universe could have had any old built in curvature it wanted, right?
It could be curved this way. It could be curved that way. Any value of curvature from, like, negative infinity to positive infinity. And in in of all the numbers, it picked no curvature at all. It could have been round like a sphere.
It could have been, not round like a saddle. It didn't pick either of those. Even though there's many more options, it picked flat. And not just a little bit of flat, but a lot of flat because curvature grows with time. If we measure flatness to within a few percent today, that means the early universe, as we go back in time, must have been incredibly flat, like one part in 10,000,000 flat in order to give us a universe that's flat to within a few percent today.
So, it must have been really, really, really stupidly ungodly, unnaturally, remarkably, significantly, imperviously flat in the early days of the cosmos. Why? Why that flat? Could have chosen anything in the world, universe, and it picked no curvature at all. Isn't that suspicious?
Doesn't that seem a little funny? This is what we call the flatness problem. What caused our universe to be flat? Because it just being flat by random chance seems a little bit weird. We think there has to be a cause for the flatness.
So the idea of inflation first appeared back in the funky seventies, and a cosmologist by the name of Alan Guth wasn't thinking about those two problems at all. Wasn't thinking about flatness, wasn't thinking about horizon problems at all. He was thinking of his GUT, and not in a cheese eating way, but in a high energy physics. You know, GUT stands for Grand Unified Theory. Grand Unified Theory is our model of how the forces of nature unify at high temperatures.
So, you know, we have the four forces, good old four, gravity, electromagnetic, weak nuclear, and strong nuclear. As you kick up the temperature, weird things happen to these forces of nature. At a certain temperature, a certain energy level, the weak nuclear force and the electromagnetic force merge together into a single force. So that means at high temperature, just let that sink in. At high temperature there's only three forces of nature.
There is gravity, strong nuclear, and something we call electroweak. If you kick up the temperatures even more, the strong nuclear force melds together with the other two, and you're left with two forces of nature, gravity, and what we call a gut or a grand unified theory. If you go all the way and merge all four, you have a, I'm not making this up, a toe or a theory of everything, but that's another show. Alan Guth was thinking about the early universe. The early universe was small, it was hot, and it was dense, and it was super energetic.
It was energetic enough in its earliest moments, we're talking when the universe was less than a second old, in its earliest moments, the universe had these forces merged together. There was a period of time when the universe had only three forces of nature. In a period of time before that where there were only two. Now we don't fully understand. We don't have a complete grand unified theory.
There are several options. There's a lot of things we don't understand, but we're pretty sure some sort of unification exists. That's another show. Happy to dig into that. Alan Guth was worried about this.
He was thinking about it in the early universe, and he was thinking about when the universe changes character, when these forces start to split off from each other as the universe is expanding and cooling, He was thinking about phase transitions. Right? We're all familiar with phase transitions, right? You know, water into ice. It's the same molecules as good old H two O, but it undergoes a complete change of character when you change the temperature, the pressure, the density.
You can change water into a solid, you can change liquid into a gas, you can change a gas into a plasma, you can change phases all the time. There are some really messed up phase transitions at really high energies like where you're changing the character of the forces of nature themselves. Needless to say, the earliest moments of the Big Bang, we're talking when the universe was less than the second old, less than a fraction of a second old, it was a bizarre time. It was a bizarre time where there were only two forces of nature, gravity and the gut, and at some point as the universe aged, when the universe was only 10 to the minus thirty five seconds old. Take a moment, a breather here to appreciate that we are even contemplating.
Like I can even begin to have an episode about the universe when it was 10 to the minus 35 old. That is zero point zero zero zero. There is no way I'm gonna say all those zeroes, 35 of them, one seconds old. That is when we think this key phase transition happened. When the strong nuclear force split off from the other forces and the universe underwent a phase transition from having two forces of nature, gravity in the gut, to having three forces of nature, gravity, strong, nuclear, and electroweak.
That happened when the clock ticked at ten to the minus thirty five seconds. Alan Guth was studying this in detail, and he found an issue with this phase transition. He found that when the gut went away I'm getting a chuckle every single time I say that. When the gut went away, the universe got flooded with magnetic monopoles. Yep, a magnetic monopole.
I'm pretty sure we're on our third or fourth detour by now, but hey, it's a wacky wild fun adventure journeying into the earliest and most exotic eras of the existence of our universe, so strap in folks because it's only gonna get worse. When this phase transition happened, when the strong nuclear force peeled off from the other forces, it created defects. It created flaws in space time itself because this phase transition didn't happen very smoothly. And we can see this all the time actually. If you turn water into ice, the water just doesn't turn into ice like with in a snap, in a flash in a snap where it's all uniformly ice all at once.
No. Little pockets will nucleate first, and they'll start forming ice, and the ice will spread out from that. And then in another opposite corner of the water, ice might nucleate over here and start forming ice over there. And eventually these two domains will meet. Where they meet, there will be flaws.
So if you pick up an ice cube, you might see little bubbles, little cracks, little walls. That is because the phase transition wasn't uniform across the entire block of ice. This is because it is an imperfect, flawed transition and leads to defects. So the next time you make ice and put it in your cocktail, you are providing a handy metaphor for when the universe was only 10 to the minus thirty five seconds old. When this phase transition happened, defects appeared that took a variety of exotic forms that I would love to dig into someday, such as cosmic strings or domain walls or magnetic monopoles.
Magnetic monopoles are a particular kind of defect in space time itself, a very odd creature indeed. Imagine a North Pole by itself. Hard to imagine because magnets as we know them always come in pairs. North and South together inseparable. You chop that magnet in half, what do you get?
You get two little mini magnets. You chop those two in half, you get four little mini mini magnets, and you keep going. You just get smaller and smaller magnets. You can never chop, you can never separate a North from a South Pole, but it might be possible in the early universe with this exotic crazy phase transition creating some exotic creatures. Basically you can think of a monopole as a physics balrog.
If you find one, it means you've dug too deep. And in Alan Guth's and others calculations, this phase transition that occurred when the universe was 10 to the minus thirty five seconds old should have flooded our cosmos with monopoles, which means we ought to see monopoles all over the place. Should there should be one, like, right next to you. There's a lot of monopoles, but we don't. We haven't seen a single monopole ever, and we've looked really hard.
So what happened to all the monopoles? Where did they go? Alan Guth was poking around with the physics of the gut transition, and I'm sorry I have to keep repeating a phrase like gut transition all the time like it's a totally normal everyday thing to say. And he found in the physics of this exotic early universe phase transition itself a possible escape hatch, a possible way to solve this monopole problem using the same physics that generated the monopole problem. He saw a clue in the phase transition itself.
You guessed it, inflation, he coined the term inflation. If the universe somehow, and we'll get to the some and we'll get to the how, got really, really big really, really fast, it would send all of those pesky monopoles far away from each other. It would dilute the monopoles in our universe. How big and how fast? Well, you can estimate because you can roughly calculate how many monopoles should have been produced, and you need them diluted to the level of, say, one per observable patch of the universe to make make them essentially unobservable.
And to do that, the universe needs to grow by a factor of 10 to the 26 in less than 10 to the minus thirty two seconds. That's going from the size of an electron to the size of a golf ball. To put that in perspective, it's if I took you, the the physical human being of you right now, and I snap my fingers and in ten to the minus thirty two seconds, faster than the fastest thing you could possibly imagine, and in that amount of time, I inflated you to be the size of our observable universe right now, 90,000,000,000 light years across. That is a dramatic phase transition to put it lightly. The universe has never seen an expansion like that ever before or ever since.
Inflation appears to be a singular event, but that is what's required to get all these monopoles away from each other so that each individual patch of the universe essentially sees none. So what this tells us in the picture of inflation is that we have our observable patch in the universe 90,000,000,000 light years across. The actual universe is far, far, far larger and contains many such little observational patches that never get to talk to each other, and they have each at most one monopole in that entire volume. This is the solution that inflation implies. Alan Guth and I'll talk about the mechanisms, don't worry, of how the universe can actually get so big so fast.
This structure, this idea of cosmic inflation was designed to solve the Monomole problem, but it also solves those pesky horizon and flatness problems. How? Well, it solves the horizon problem because it says, well, at one time, the universe was super duper tiny, hung out for a while minding its own business, and then got really big. This means that there was enough time before inflation for everybody in the universe to share notes, to share photons, to equalize to the same temperature, to make sure everyone's on board with the game plan, to synchronize their watches, to do everything like, yes, this is how we're gonna be as a universe, and then inflation sends these patches far apart from each other so that they're on distant edges of our sky, they're on distant edges of the universe, but they still maintain that memory. It solves the horizon problem because it says these two different countries a long time ago were connected before they got violently ripped apart.
And it solves the flatness problem because the universe is now so stinking big, it doesn't matter what kind of curvature it is, you'll only ever measure a flat universe in your observable patch. Just like the Earth is curved, but my backyard is flat because my backyard is so tiny compared to the entire surface of the Earth. It the Earth could be really curved. It could be curved a little bit. It could be curved like a saddle.
You know, like an inside out thing. You could do whatever it wants. It doesn't matter. My backyard stays flat. Well, if the universe is truly enormous, much, much larger than our observable 90,000,000,000 light year across patch.
It doesn't matter what curvature it has. It can have any curvature it wants, but the universe is so big that our little patch looks flat. So this inflation provides an explanation for flatness. It demands flatness. It says you're never gonna measure any curvature because the true curvature of the universe is on scales that you can't possibly measure.
It's bigger than your little observable patch. And I have to be careful here. I hope I've been careful throughout this episode about the definition of the word universe. Sometimes when I say universe, I mean our observable patch, the limit of what we can see. And sometimes I mean the whole big the whole enchilada, the big thing that inflation drove that is super duper gigantic, enormous.
I try to distinguish between these. When I say universe, I mean the whole thing, An observable universe is our little patch. Our observable universe, what we can measure, will always be flat. The whole enchilada can be shaped like an enchilada if it wants to be. We'll we'll just measure flatness.
So the exotic gut era physics in this picture of inflation that Guth developed causes monopoles, but also potentially generates the inflation mechanism itself, which gets rid of the monopoles and demands a flat universe and solves the horizon issue, and it all fits within the Big Bang framework. We haven't changed the Big Bang picture of our universe. We haven't burned down the Big Bang house, but we've added a a a nice patio. We remodeled the patio with this picture of inflation. Or not.
You see, Alan Guth's original model had a major flaw. It was too good. It drove away all the bad stuff like monopoles. It made the universe flat, solved the horizon problem, but it also diluted everything else. It spread apart all the particles from radiation and matter.
Everything that makes us us, that makes our universe full of light and stars and galaxies, that's bad. At the end of inflation, after inflation has done its thing, whatever its thing is, it's made the universe really, really, really big. It's diluted all the money. It's spread out all the monopoles. They're really far away, but we're left with a completely flat and empty universe, totally devoid of anything else.
So right now, inflation has solved a bunch of problems but introduced a problem of its own. How can we fix it? Can you smell that? Smells like a cliffhanger. I'll see you next time.
Thanks so much to all the amazing people who asked the inflation questions for this episode and the next. Remember, you can ask your own questions. Go to askaspaceman.com. You can email me at askaspaceman@gmail.com. Hashtag on social media, hashtag askaspaceman.
You can also follow me directly at paulmad sutter. Keep those questions coming. Go to iTunes, submit a review, tell other people how much you like this show. I really appreciate all the support, the support on patreon.com/pmsutter. Any way that you can help keep this show going, I owe it to you, And I promise next time, I will solve all your problems and provide, finally, complete knowledge of time and space.