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How do cracks appear in spacetime? What does it have to do with symmetry? Could we ever hope to find a cosmic string? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
In so many myths, both past and present, there are stories of creatures, right, that are hidden, that are deep, that are asleep. And then through some bumbling of humanity, usually our greed or, our sense of adventure, we we find them. We awaken them like dragons and their hoards of gold or the Balrog. You know, if you dig too deep and too greedily, you're gonna awaken some ancient terror, some Lovecraftian nightmare. The point of these stories is probably more about human nature than nature nature, but nature has secrets.
Nature has some deep stuff. Nature has some scary things that, in some cases, we're probably better off not knowing that they exist. And the Balrog, the dragon, the monster that I'm talking about today is cosmic strings. Cosmic strings are a potential leftover, a potential remnant of the very early universe. And the very early universe was just a different place, man.
And I'm not talking like when the universe was a thousand years old or a hundred years old or a second old. I'm talking about when it was a billionth of a billionth of a billionth and probably a few more billionths of a second old. Some tiny fraction. Life in that universe would be completely unrecognizable to us today. The forces of nature that we identify and we recognize and we enjoy, the structures, the chemistry, just just everything is different.
Life in the early universe was symmetric. Life in the early universe, especially when it comes to the forces of nature, had a certain symmetry to it that is completely and totally broken today. The symmetries, it it's it's a little bit hard to describe the symmetries, and one of these days, I'll do a whole episode just on symmetries and forces of nature. Feel free to ask. But the symmetries are symmetries in the mathematics.
You write down the equations that describe these forces, and then there are certain reflections. There are certain things that stay the same if you flip things around or move them to a different point in space or change the charge or something. Some things stay the same, and those are called symmetries. And each force of nature represents a certain symmetry in nature. So there there's a symmetry in nature and in the mathematics, and then that symmetry is the electromagnetic force and weak nuclear, strong nuclear.
And you can combine these symmetries into almost like an overarching symmetry, a group of symmetries, some some higher order mathematical structure. And I know I'm being very, very vague here, and that's on purpose. You'll see why. You can combine symmetries and make higher order symmetries, but these symmetries only appear at high energies. So if we run a particle collider experiment, we start slamming atoms together, in that moment, in the heat of that moment of the collision, certain symmetries can appear that don't appear in normal everyday universe.
And this means that the forces combine. The weak nuclear and the electromagnetic forces, each one representing a certain symmetry, At high energies, they combine, they unify into a single force. We call it the electroweak force. And the electroweak force has its own symmetry that must be broken apart at low temperatures to give us electromagnetism and weak nuclear. These are the unification theories.
The unification theories are physicists trying to find higher order symmetry in the universe. And that's basically what modern physics is. It's a search for for deep symmetries. So life in the early universe was ruled by these higher order symmetries. And by the way, we don't know all of the higher order symmetries.
We know the combination of weak nuclear and electromagnetism. We've got that down. That's old news. We're working on combining strong nuclear. That's called a grand unified theory or GUT.
If we can fold that in and understand all the mathematical nuances, that'd be awesome. We're not quite there yet. And then if we know how to unify all four forces of nature under a single symmetry, that's a theory of everything, which we don't have at all. But the early universe at high energy was in this symmetric state. It was simply different.
Just the forces of nature that we identify now were gone. They didn't exist yet. They were replaced by this higher order thing, and I don't want you to think of electroweak as just electromagnetism and weak nuclear. No. It's a new thing.
It's a different thing. It's electroweak. There's different force carriers. They have different properties. It's a new thing.
When we run our particle collider experiments, in those moments, it's a different universe. And at one time, the whole entire universe was a different universe. And then it wasn't. And it wasn't because it cooled down. Our universe expanded and cooled and so it underwent a phase transition.
It changed state. It changed character. The exact same way, you can have a glass of water and you can stick it in the freezer. And after a while, it's going to change state. It's gonna go through a phase transition.
And that water is gonna go from a highly symmetric state, like you can hold the glass of water and it doesn't matter how you look at it. It still looks like water. And after it freezes, it's not asymmetric, is it? Because there's edges to the ice. There's boundaries to the ice.
An ice cube doesn't look the same from all angles the way a chunk of water does. The act of freezing, the act of cooling has reduced the symmetry of the system, in this case, the water. And the water had to go through a phase transition to get from that really symmetric state to the not so symmetric state. The universe had to go through a phase transition to get from a really symmetric state where the forces were unified to a not so symmetric state where the forces were not so unified, and phase transitions aren't perfect. Think of what happens inside of that water when the phase transition actually goes down.
It's not just like, boom, instant ice. No. It has to start somewhere. You've got these water molecules. They're all jiggling around randomly doing whatever they want, enjoying themselves being water.
But then as they lower and lower and lower in temperature, they're like, fine. Okay. We'll line up. We'll make a crystal lattice. We'll become an ice cube.
Fine. That has to start somewhere. There's a nucleation point. And from there, other water molecules start joining up. They start lining up, like, getting a bunch of kids out on the playground to line up to come back into class.
Like, you gotta start with one and then one steps behind them and then behind them, and they start lining up in all the same way to form that crystal lattice that we call ice. And so it starts from a point and spreads out throughout the water, and this takes time. It doesn't happen instantaneously. There's nothing stopping another piece of the water in some random place, maybe in opposite corner, from also doing the same thing. And deciding, okay.
You know what? I'm ready here right now to become ice. But I'm gonna line up in in a different way because there's no rule about how I where I have to line up. Some on one side, the water might decide to line up up down to form the crystal ice, and on the other side, the water might decide to line up left right to form a crystal ice. Either way, you get ice.
Either way, you get the exact same thing, a cold chunk of water, but there's a symmetry in the water that must be broken. The water molecules can't just jiggle around randomly. They have to pick a direction. It doesn't matter what direction they pick, but they have to pick a direction. So all the kids on the playground, you could start forming a line over here facing one direction, but on the opposite side of the playground, another line could be formed facing another direction.
Either way, the kids are lining up, the symmetry is broken, but the lines must be formed. The phase transition must take place. So if this happens in an ice cube and say you have two nucleation points, you have two different ways of arranging the ice, they're gonna spread out from their nucleation points, and they're gonna meet. There's gonna be a boundary. There's gonna be a a line.
There's gonna be a defect, and you can literally see the defects in the ice. You look inside of an ice cube. It's not perfectly crystal clear. It's not crystal clear because it has all these boundaries of where different nucleation points led to different ways of arranging the molecules to turn it into ice, different ways of breaking the symmetry. The end result is the same all across the ice cube.
It's ice from end to end, but you can just have different arrangements of that ice. And where they meet, there'll be a defect. What goes for ice cubes goes for the early universe. Our entire universe went through a phase transition. It lost some symmetry.
Some symmetry was broken, and it may not have been perfect. When the forces of nature split off from each other one by one, first it was gravity, then strong nuclear, then weak nuclear. When these phase transitions happened, they didn't have to happen simultaneously all across the universe in an instant. No. There was this there was a nucleation point.
There was a point where the phase transition started, and it spread from there. And it took time. And there could have been another corner of the universe with its own phase transition. So fine. Fine.
I'm done having unified forces. I'm just gonna split off whatever. I'm ready. I'm cold enough. And then another point somewhere else in the universe, you know, no.
I'm ready. I'm gonna start the party. In another corner, no. No. No.
No. I'm gonna and they're all starting at the same time or relatively the same time. The end result is the same. You get the same forces of nature at the end of it. But in the mathematics, the symmetry must be broken, and it can choose where to land in that symmetry, just where the water can choose how to line up to form ice, it's still gonna make ice no matter what.
So you get different regions of the universe. You potentially you get potentially different regions of the universe with the same forces of nature, but there'll be boundaries. There'll be wrinkles. There'll be cracks. There'll be creases.
We call them cosmic strings. These are not the strings of string theory. If you're curious, technically, those are called super strings because, of course, they are. These are called cosmic strings. It could be there is a way to make cosmic strings from string theory, but I feel like I first need to do an episode on string theory.
Welcome. I'm happy to do it if you ask. But for now, we're just gonna stick to cosmic strings. These are cracks in the universe, folks. These are defects in the fabric of space time itself, And they might be around today.
They were formed in the early universe. They were formed in that phase transition moment, and they persist. Just like the boundaries in the ice cube are formed when it becomes ice, but it persists. As long as there is ice, there are defects in the ice. So there just might be giant cosmic strings wibbling and wobbling around all over the place.
What's life like around one in these cosmic strings? The defining aspect. Because they're a fold in space time, they're a wrinkle in space time. Circles around them do not add up to 360 degrees, which sounds weird, but welcome to curve geometry. This is geometry of general relativity.
If you were to start in one position and then make a loop around the cosmic string, when you returned to your starting point, you will have traveled less than 360 degrees. Because it's pinched space time in a certain way, that circles don't add up to 360 degrees. And that's weird, and I'm sorry, but that's also life. The width of a cosmic string depends on the theory, like, it depends on when the phase transition happens and the energy scale of the phase transition, all that. And since we don't have a full understanding especially, of the grand unified theory and when the strong force breaks off, we're not exactly sure of how wide the cosmic strings might be.
They're typically around the width of a proton. You know, that's the scale of the strong nuclear force anyway, so that's that's why we get that number. The length of these cosmic strings are basically the entire universe. Because you can imagine in the early nugget universe, these regions forming and cooling down and and phase transitioning, and then they spread through and then there's a crack, that crack is pretty much gonna be the width of the universe, you know, plus or minus something, but pretty much the width of the universe. And because they're built into space time itself, as the universe expands, the cosmic string will grow.
It'll pull on that string because it's right there in space time. And if space is expanding, and a string and there's a defect in space, like, you're you're just gonna pull pull it right along. Because they're a defect, because they're this wrinkle, they have a lot of tension. You know, they're folding, they're pinching space time, that's attention, and in general relativity that means mass. So even though they don't they're not made of anything.
It's not like cosmic strings are made of some really exotic material. No. They're just a fold in space time, but a fold in space time is mass. Like, that's general relativity, causes an acceleration. And really only one word can describe them.
If you were to actually see one in action, only one word can describe a cosmic string, and that word is generous because they contribute to Patreon. That's patreon.com/pmsutter. It's how you and Cosmic Strings keep this show going. Thank you so much for your support and the support of Cosmic Strings throughout the universe. Another word to describe Cosmic Strings is ridiculous.
Think of it. A cosmic string is not made of anything. It's not a material. It's not a substance. It's a fold in space time, and one inch weighs the same as Mount Everest.
1 mile of cosmic string weighs as much as the Earth, and they're not made of anything. It's just a defect, an exotic defect left over from the early universe. It is a demon. It is a Balrog. It is a dragon from this exotic early deep age of the universe that might still persist to the present day.
Cosmic strings, you know, imagine a string, it's gonna be wiggling. It's gonna be alive. It's gonna be moving. They're always bending and curling and looping, and they're also vibrating because that's what strings tend to do. There are kinks and cusps that travel up and down the string at the speed of light.
Remember, these are they're not made of anything. They're just folds in space time itself. So these you can get a kink, and the kink will travel up and down at the speed of light. So you're making a really heavy thing move really quickly that generates gravitational waves. They can also occasionally, just through their wibbles and wobbles, end up folding in on themselves like making a little loop, and the loop will get pinched off.
So the string will reform, but there'll be a little piece of it detached. Detached, and the loop, there'll be a little closed loop that goes on to live its life. The loops are not stable because they emit gravitational waves, and so they will slowly lose mass and energy, and they will decay. But these decays are very slow, very slow. So every once in a while randomly, a cosmic string will overlap itself and make a loop, and then the loop will slowly decay.
Every once in a while randomly, a cosmic string may intersect another cosmic string, and they may cut themselves off that way and emit gravitational waves. Gravitational waves are really, really, like, slow inefficient ways to bleed off energy because gravity is the weakest force. Gravitational waves are a weaker thing on top of that weak thing, so it's, like, double weak. The point is that if cosmic strings are formed in the early universe, they they're gonna hang around. Even though they're losing energy, even though they're losing mass, even though they're emitting gravitational waves, they will stick around.
Thirteen point eight billion years is nothing to a cosmic string. And because there's probably more than one cosmic string out there, because probably in the early universe, this phase transition, these nucleation points happen many many many places, and so at many boundaries, many places where they intersected, many cosmic strings, there might be like a network of cosmic strings permeating the universe, and then a whole bunch of isolated smaller loops all over the place just from my from all the random overlaps. And they're just out there hanging out, vibrating, doing their stringy thing. That's cool. Is that true?
Well, maybe. We wondered for a while because there's, if there's this network of cosmic strings out there and they're gravitationally attractive, they're doing their stringy thing, that if there's a bunch of matter in the universe, dark matter, normal matter, all sorts of matter, they might get attracted to those cosmic strings. They might get attracted to the network, and the network of cosmic strings might form the backbone of the cosmic web. The large scale distribution of matter in the universe when you zoom out to the biggest scales, matter, galaxies, and dark matter take on this form that looks like a giant web. Maybe the cosmic strings form the spine of that cosmic web.
But no. This was ruled out by experiment about ten, fifteen years ago. We observed the cosmic microwave background, this background light from the very early universe, and we looked at the light from that period. The light from that period told us what the temperature and density was doing across the universe. We're looking at baby picture of the universe, and the short answer is if cosmic strings are responsible for the large scale structure of the arrangement of matter, they will leave a certain kind of imprint in the cosmic microwave background.
The baby picture will look different. It'll have a different face, and we didn't see that face. Instead, we saw a face driven by what we call inflation, which I did a couple episodes on way back when. So there aren't enough cosmic strings to explain the large scale structure of the universe. Okay.
It was worth a shot. That's science. We can also look at lensing because cosmic strings will split light. If if there's a bunch of light from background galaxy approaching a cosmic string, this is a wrinkle. This is a fold in space time, so some light will go left, some light will go right.
You'll get a split image. We looked for this, of course, in the cosmic microwave background. Haven't seen anything. And we've also looked for it in galaxies, like, if we just happen to get lucky and a cosmic string passes between us and a distant galaxy, and we might see a split image of that galaxy every few years, split image of that galaxy every few years that, you know, some astronomers claim or there's a group that says, hey, we think we found, like, a split image, then we dig into it deeper and it's just like a coincidence, or two galaxies that just happen to look alike but weren't exactly alike. And so there there's been nothing firm.
Our best hope now for detecting cosmic strings is through their gravitational waves. You've got all these strings, possibly, with all their kinks and cusps saying, you know, traveling up and down doing their wibbly wobbly thing. You've got all these loops hanging out, these pinched off off bits and pieces that are vibrating. Everybody's emitting gravitational waves. Our current gravitational wave detectors like LIGO won't be able to detect, like, the general background gravitational wash from all these vibrating strings.
They can only detect something called a whiplash, like, if there's a big cusp and it reaches the end of a string, it's like, like the end of a whip. That's right. You heard that right. But in gravitational waves, not sound. LIGO can hear something sharp like that and we haven't heard anything.
The next generation of gravitational wave detectors like LISA or the Big Bang Observatory, if those ever get off the ground, because they're gonna be in space, If those ever operate, they might be able to detect the general background of gravitational waves. And there's also a way to detect gravitational waves with pulsar timing. If you look at a whole bunch of pulsars at once, as gravitational waves ripple through the universe, you'll see very, very, very slight changes in the pulsar timing. And so we have arrays dedicated right now that are, like, are trying to do this, and this might be sensitive if the cosmic string network is adding to this low level background of gravitational waves in the universe. It might affect the timing of pulsars in a way we can measure.
Maybe, maybe not. We're working on it. We don't fully understand cosmic strings, theoretically, because they are creatures of unified theories, and we don't fully understand unified theories. So they might connect to other physics. You know, like, they don't just have to be folds in space time that emit gravitational waves.
They might, through somehow or other, connect to electromagnetism. They might connect to weak nuclear. They they might produce particles. They might produce gamma rays or or cosmic rays or radio emission. Who knows?
If we had a solid unified theory of strong nuclear, weak nuclear, and electromagnetism, we might be able to say something much more definitive about the cosmic strings. Because we don't have any evidence yet for these monsters from the distant past inhabiting our present day universe, they have to be very light. Like, the like, if you're gonna say, okay. If there is this network of cosmic strings out there, then the fact that we haven't observed any evidence for them, either in large scale structure or gravitational waves or lensing or anything, means they can't be very heavy because if they were out there and they were heavy, we would have seen them by now. So whatever these monsters are, they have to be really lightweight monsters.
Much, much lighter than predicted by Grand Unified Theories. Like, the theories that we do have, the hypothetical ideas that we do have produce cosmic strings of a certain weight, a certain tension, but we simply don't see that. But grand unified theories also produce cosmic strings in the early universe pretty generically. It's base it's hard to get phase transitions in the early universe without also making these defects, without making these balrogs, without making these wrinkles in space time. It's a pretty generic prediction of Grand Unified theories.
And they also predict them to be of a certain mass, like this Mount Everest in an inch and Earth in a mile, that kind of mass. That's what they predict, and yet we don't see it at all. So what's up? New physics, that's what's up. Either we don't understand grand or unified theories, which is likely, or we don't understand something about cosmic strings, which is also likely.
Either way, we don't understand something about the universe, which means we still have jobs. Thank you to Raul p on email for today's question. And, hey, I've got something special to talk about. So, you know, these AstroTurus things I'm doing, they're lots of fun. Side gig, I'm gonna go see the twenty twenty one eclipse that's gonna go through Antarctica.
I've been invited by Poseidon Expeditions to be the the guy who talks about science on the ship. Do you wanna come see an eclipse with me in November of twenty twenty one in the Antarctic area? It's it's a pretty cool cruise. You just email salesusa@Poseidonexpeditions.com or call (347) 801-2610. And say you wanna you wanna be with Paul Sutter.
They'll they'll get you all the right details. That's gonna be really fun. I can't wait. I have to wait a long time for that, but I'm willing to wait because it's gonna be awesome. Thank you to my top Patreon contributors this month, Matthew k, Helgeb, Justin c, Justin g, Kevin o, Duncan m, Corey d, Barbara k, Nuder Dude, Christy, Robert m, Nate h, Anderev, Chris l, John, Elizabeth w, Cameron L, and Nalia.
You can join their illustrious ranks by going to patreon.com/pmsudder to keep the show going. You can also shoot me questions at askaspaceman@gmail.com. Use the hashtag askaspaceman or go to the website askaspaceman.com. Leave a review on iTunes. Tell your friends.
Tell them all about phase transitions. Next time you look at the ice cube, tell them about bow wrongs and cosmic strings and nobody will be confused. And I'll see you next time for more complete knowledge of time and space.