Daniel and Jorge Explain the UniverseDaniel and Jorge explain how we might be able to listen to a new kind of gravitational hiss and what secrets it could reveal.
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Hey or Hey, I have a question for you about drawing cartoons.
Hey, finally something that I'm an expert in. What's the question?
Well, it's more about the backgrounds. Actually. How do you decide whether to leave the cartoon background blank or fill it in with a collar, or like draw a full scene in the background.
Hmm, well, I guess it can be whatever you want the background. I mean, you just don't want to take away attention from what's happening in the panel.
So the background is just like filler to be ignored.
Ah, you know, it's all odd.
You know.
Sometimes that's where the treasures are hidden in the background.
Oh, sometimes that's true in physics as well.
Really, you find Waldo sometimes in the background.
The cosmic Waldo background.
Hi am Hoorham, a cartoonist and the creator of PhD comics.
Hi.
I'm Daniel. I'm a particle physicist, and I have never found Waldo in one of those books.
Never. I think maybe you just haven't tried.
Daniel.
I think that's it. I get bored after like two seconds. I'm like, why do I care about finding this guy and.
Yet finding the Higgs boson and a bunch of noisy data that's somehow more interesting.
Yeah, I can write a computer program that scans it for me automatically. If I could write a computer program that looked for Waldo, that would be more fun.
Do it? Yeah, I mean, what else are you doing? What else could you be doing? It's more productive than that. Welcome to our podcast Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which we don't look for Waldo, but we do look for answers to the deepest questions humans have ever asked, Questions like why is the universe the way that it is? Could it have been another way? How big is it? Anyway? How did it start? How will it end? And what is it made out? If we don't shy from any of these incredible questions which inform the entire context of the human existence, and we admit ignorance. We don't pretend to know all the answers, but we try to explain to you what science does and does not know.
Yeah, we'd like to talk about all of those big questions that are at the forefront of science and the exploration being done by physicists and other sciences. We also like to talk about what's sort of in the background of science as well.
That's right, there are wonderful surprises out there in the universe. Sometimes you know what you're looking for. Sometimes you stumble across something accidentally when you were looking for something else. Sometimes you see an exciting discovery that jumps right out you. Sometimes it's treasures hidden in the background.
Yeah. And the crazy thing is that it's all there right like literally, when you look at the background of any picture that you take, and you look at the sky behind you or behind the person that you're taking the picture of, there's information there about the universe. There might be hidden secrets and interesting things to discover right there where you least expect it.
That's true. Even the darkest spot in the night sky have some photons coming to us from very very distant locations. If you pointed the hubble instead of your iPhone at some point in the sky, you would see millions and billions of galaxies so faint and so distant that their photons arrive like every few seconds or every few minutes, rather than all the time like the sun. But there is information in every little tiny patch of the sky about the nature of the universe and what's going on out there.
Yeah. Can you detect waldogs maybe in those signals standards.
No, their bosons, of their waldinos.
They're hard to find, I guess.
Yeah.
If I discover a particle, can I call it the waldo? Like if it's in the background of the you know, the universe.
I think that would disqualify you from participating in any experiment because they would not want you to give it that name.
I see a pre screen for name giving.
We don't, but we should because there have been some bad choices in the past.
But Yeah, we like to ask all of the big questions that are out there that scientists are wondering about and that scientists are speculating about. There are interesting things that scientists think might exist and things that we have not yet detected.
That's right. Every time we open up a new kind of eyeball and look out there in the universe in a new way, we find all sorts of crazy, exciting stuff, and sometimes we know what we're looking for. We think that there might be a hint a message out there in the universe we haven't detected yet that could give us fantastic clues as to the nature of the Big Bang, what happened in the first moment of the universe, or what's going on with black holes dying all across the galaxy.
Yeah, because you never know where the next big answer is going to come from, or where the next big question might come from. For example, we have this great question from JJ who's five years old? Who sent us a listener question through the internets?
That's right, here's a question that JJ's father sent to us because he couldn't answer it himself.
Here's JJ as to some supernoa by the black come in the middle of our galaxy.
Oh, he's so cute.
Wait was it a question or a statement? Is he saying he's going to make a star go supernova?
This is not the youngest supervillain listener that we have. He's asking what would happen if our star wins supernova next to the super massive black hole in the center of our galaxy.
Wow, it's amazing that a five year old knows the word supernova and kind of what it means.
Yeah, So thank you Lowell for encouraging the scientific curiosity in your children. In the next generation. You are fueling human progress.
So that's a complex question. Here, JJ is asking what if our son was somehow next to the black hole in the middle of our galaxy and then what if it suddenly went supernova?
Right, that's right, and JJ doesn't have to worry. Our sun is not going to go supernova. Supernova is one of the end stages of a star when it gets so massive that it collapses gravitationally. But in order for that to happen, you have to have a lot more mass than our Sun does. You need like eight times the mass of our Sun is the minimum threshold for a star to end up as a supernova.
Right, But so it can go supernova on its own, but it might eat something else and then get bigger, right, and then go supernova.
That's right. There is one possibility our Sun will probably end its life as a white dwarf. So there will be a phase where it becomes a red giant and blows out a lot of stuff, and then it collapses again, and then it blows out more stuff, and at the core will be left a very very dense blob, a white dwarf, which is essentially just a hot rock. And it's white not because it's fusion going on inside it, but because it's glowing white hot. And these white dwarfs usually just sit around for trillions of years and cool off. But sometimes if there's another star that comes nearby, they're so dense they can eat those stars, and if they eat enough of them, that can trigger a supernova.
Wow. Yeah, I sometimes feel like that. In Thanksgiving don't bring around another pie. I'm gonna blow no more turkey, please, I'm gonna supernova.
Do you gravitationally attract dessert if you eat enough turkey?
No, I'm just a easy magnetic course for that.
Nice.
Nice.
But I guess the question is what happens if its a star or I guess any star blows up next to a black hole? Like, does it blow away the black hole? Does it like evaporate it? Does it do nothing? If you explode it next to a black hole? What happens?
Well, first of all, you cannot explode a black hole. Like, There's nothing you can do that will destroy a black hole, because remember, a black hole is a dense blob of matter that will eat everything else, and that includes other forms of energy. So you shoot a particle beam, you shoot antimatter, you shoot dark matter into a black hole. It eats everything. It's like fororehead, Thanksgiving dinner. Right, It's happy to take anything. And so anything you blow in there, including the remnants of a supernova, will just make it bigger and stronger.
Oh, I see, So it's not possible to like blow it away or move it, or at least maybe like push it a little bit. Is it possible to push a black hole.
It is possible to push a black hole to accelerate it. It is a mass, and so you can tug on it. For example, if you bring another black hole near it, those two black holes will accelerate towards each other, swirling around and creating all sorts of like gravitational radiation. And it is interesting because a supern near a black hole like that super nova will eject the material really really violently. It's a huge explosion. The stuff comes out like a significant fraction of the speed of light. But it won't necessarily just like it all immediately eaten by the black hole because it's moving at such high speed, unless it pointed exactly at the black hole. They'll probably like miss a little bit and then swirl around the black hole and become part of the accretion disc. This like stuff around the black hole that's hot and glowing.
Interesting, But the explosion itself, it won't impart any like momentum on the black hole to push it a little bit.
Even No, it will impart some momentum on the black hole. But you know, a super massive black hole like we're talking about has a mass that's millions of times the mass of the Sun, and the kind of supernover we're talking about comes from a white dwarf, which is like less than the mass of our Sun. So the energy from it is just going to be like a pinprick. It's like if a mosquito lands on you, does it knock you over?
It depends on the mosquito, you know, it depends on what it's carrying.
I mean, I've never been to Panama. I don't know how big those mosquitoes are.
They get big, Yeah, as big as the Australian ones. Everything is bigger in Australia.
Well up here in the States. I've never been knocked over by a mosquito, but you.
Know, not yet.
Yeah, I'm getting older and they're getting bigger.
Well that's the answer for JJ. Thanks for sending in that question. And so today we'll be talking about sort of what's happening in the background of the universe. We know that there is the cosmic microwave background radiation that's light, that's electromagnetic energy that's out there hanging out from the Big Bang. But there might be other things in the background that may or may not be there that we might have information also about the universe.
Right, that's right. The cosmic microwave background is a really really rich source of information about what happened in the very early universe and what's going on now. But it's just another form of light.
Right.
Microwaves are electromagnetic radiation. But recently we figured out other ways to look at the universe, like we were just talking about black holes generate gravitational radiation, which is a completely different form of radiation and another way to look at the universe. And so now we're wondering about whether we can see a background to the gravitational radiation in the universe.
So today on the program, we'll be asking the question what is the cosmic gravitational background.
And who will win the Nobel Prize for finding it?
Interesting. So, it's something that we haven't found yet.
It is not something that we have seen exactly. There are some hints it. We'll talk a little bit about some of the experiments that claim to maybe see it and have claim to see it and been debunked. But so far as it's not an established thing, it's like potentially a discovery in our future. It's like a treasure chest we haven't dug up yet.
And it has a very similar name to the cosmic microwave background. This was the cosmic gravitational background. Now, couldn't you use a different name, like the universal gravitational background, just to like set it apart a little bit.
No, we want to use the same name to show the similarities. Like, it really is very similar in concept to the CMB, So we wanted to use a name that you know, conveyed the parallelism there. It's a little bit more awkward to CGB is harder to say than the CMB.
The CGB that sounds like a rap group maybe.
Or sounds like a type of monitor if you have an LED or a CGB, or.
Maybe a Supreme Court justice maybe the RGB.
Yeah, I'm done with cgbgg right.
Yeah, anyways, it sounds cosmic, and we were wondering how many people out there and knew what it was or had even heard of the cosmic gravitational background. So, as usual, Daniel went out there into the wild of the internet to ask people what is the cosmic gravitational background?
So thank you to all of our listeners who do live in the wilds of the Internet and allow me to visit if you'd like to participate and send your speculation to Crazy Physics Topics in the future. Please don't be shy. Send me a message to questions at Daniel and Jorge dot com and you can show off to all your friends hearing your voice on the podcast.
So think about it for a second. What do you think it might be. Here's what people had to say.
Heard of the cosmic gravitational background. I'm going to guess that it has.
To do with the overall mass of the universe or density I guess of mass in the universe.
It makes me think of cotton candy and when you're making it, you know, you get the big glob of cotton candy on the paper cone. But after that, there's still fragments and strings of it blowing around in the machine. And that's how I picture cosmic gravitational background, just a little snippets that are left after the planets and the stars congealed.
I'm eighty five percent sure that the cosmic gravitational background is an option on Zoom.
Well, this is something recent. It might work like a cosmic microwave background. I think so.
Never heard of the cosmic gravitational background. I guess it might be some low level gravitational wave background or some general gravitational pool of the universe towards a specific point or from the Big Bang.
I have not heard of the cosmic gravitational background, but since it has a pretty similar name to the cosmic background radiation, I'll guess it's some sort of map of the very early universe and how gravity like began to clump mass after the initial quantum fluctuations that caused the clumping. Just how gravity was distributed in the very early universe right after the Big Bang and all that stuff.
Well, I have never even heard this term before. This is a wild guess. I'm assuming it has something to do with the gravitational field of the entire cosmos. I don't know how you would measure it, but that would be my guest.
All right. I like the person who said it's an option on Zoom. You said your background to the cosmic gravitational background. Technically that is true, maybe right everything. I guess anything in the background does have gravity, so and it's part of the cosmos. So really all Zoom backgrounds are cosmic gravitational backgrounds.
Yeah. I wonder if astronomers even thought to look on Zoom to find the CGB. I mean, maybe it's just that easy, right, just an option on Zoom, who win.
Nobel Prize at least in some other noble category.
Maybe, well, there's so many options on Zoom. Who knows what's buried in some of those sub sub menus.
Right, maybe there's a Waldo option? No, please no, because we know and sometimes meanings are not that exciting. It'd be nice to have a lit of a bit of a puzzle.
There, find Waldo in the background of your Zoom speaker.
Yeah, there you go.
That's actually a pretty good idea. Build games and a Zomed to keep people entertained.
So this is pretty interesting, Daniel, I didn't even know that there was a cosmic gravitational background, like most of our listeners and or people who responded so step us through. I'm guessing it has something to do with gravitational waves and maybe the Lego experiment that we have to detect.
Them exactly it does. To understand what the cosmic gravitational background is, we first have to understand what gravitational messages are and what the gravitational foreground is. So let's start off with what is a gravitational wave? Is for those of you who don't recall, space itself can ripple because mass changes the shape of space, like if you have a black hole or a heavy object or something that bends space. We know now that gravity is not just a force between two objects. It's actually space curving around masses and changing the way things move. So gravity is this like apparent force. That's just because we can't see the shape of space, the bending of space, so that mass can bend space. Well, what happens when mass moves or mass changes, Well, that causes ripples because if a mass moves, then its gravitational field is going to move. So that's what causes ripples in space time, and that's what we call a gravitational wave.
Right Like, gravitational signals, like the effects of gravity take time and space to move. Right Like if our sun suddenly disappeared or started moving or jiggling, it would take like eight minutes for us to feel those changes in its gravity.
Exactly because information takes time to propagate, the gravitational fields don't change immediately. There's a ripple. There's like a wave that passes through the gravitational field sort of updating everything, and that wave moves at the speed of light. And this is not unique to gravity, right, it happens to lots of things anything in fact, where information takes time to propagate. Like think about a simple string. Maybe you're playing jump rope with your friend. You pull the string up. The whole string doesn't move at once, right. You see, first the first bit of the string moves up, and then that pulls the next bit of the string, which pulls the next bit of the string, and what you get is a wave moving through your jump rope. Right. So, anytime you have a material where information takes time to propagate, where it's not just like instantaneous transfer, that's because everything is local, then you get a wave. And so that's true for a jump rope, and it's also true for electromagnetism.
Right.
We can think about photons as electromagnetic waves because if you take an electron, for example, it has an electric field. If you shake that electron, then the electric field moves with it, but again not instantaneously. It goes back and forth with the electron. And that's what a photon is. It's just an update in the electromagnetic field.
Yeah, it's like a wave. It's like electromagnetic ripple, right, is what a photon is.
Yeah, And that's how you generate photons. You have an antenna and you wiggle the electrons up and down inside the antenna, changing the electromagnetic fields around the antenna in a regular pattern, and that causes waves in those fields, which are photons. That's what the light is. What light is, yes, exactly, And a gravitational wave is the same concept, except instead of wiggling a string or wiggling the electromagnetic fields, you're wiggling space itself, which is bonkers and super fun to say.
Yeah, and it's a true thing, right, Like, we have measurements of how space wiggles out there in space, out there in the cosmos, right, Like we've detected gravitational ways from black holes swirling around each other and things kind of exploding.
Right, Yes, we have seen it, which is amazing. This is an idea that's one hundred years old. When Einstein developed general relativity, one of the big steps forward, there was this concept that information doesn't propagate instantaneously. Right in Newton's gravity, information was instantaneous. If the Sun disappeared, then gravity on Earth changed instantaneously. So when Einstein introduced this concept of the limited speed of information, which of course came from his special relativity. This was one of the big predictions that if general relativity was true, we should see gravitational waves. But everybody thought these tiny they're going to be really, really hard to see because gravity is so weak. And then in the seventies people saw some clues about gravitational waves because they saw pulsars and those pulsars were orbiting each other and slowly falling into each other, and the only way that happens is if they're losing energy somehow, radiating gravitational waves. We didn't see the waves themselves yet, but we saw the things were radiating some kind of energy, so it must have been gravitational That won a Nobel prize. And then decades later people built machines that actually were able to measure the gravitational waves directly, like to see the waves themselves, operate on machines here on Earth, to see the physical effects of these ripples of space and time.
Yeah, those the big Lego experiment a few years ago that made a big splash in the science community. And now we have other observatories to listen for these gravitational waves.
Right, yeah, this is one of the Nobel Prizes I didn't win.
Do you keep track of all the Nobel Prizes you don't win.
I mean there's a lot more new prices I haven't won than I have one, so it's more work to keep track of. But I interviewed at Caltech and was thinking about going to grad school there to work on this project. But I remember thinking to myself, they're never going to see these things. Oh my gosh, this seems too hard, and I decided to go to particle physics instead, And you know, they won the Nobel Prize. Congrats to them. They prove me wrong. So one more mistake I've made in my career.
Well, you were involved in the discovering the Higgs boson that won a Nobel Prize, right.
Yeah, I didn't win a Nobel Prize for that, but that went to the theorist, But yeah, I was involved there. Anyway. We have the Lego and the Virgo experiments. These are observatories around the world that use interferometers to see these wiggles, and these wiggles come from really big events, dramatic events out there in the universe that generate gravitational waves. Like anytime any object with mass accelerates, it generates a little gravitational wave. But you know, gravity is so weak. In order to see these things, you need a big gravitational wave. Things like two black holes swirling around each other and merging and eating each other generates a lot of gravitational wave energy. That's the kind of thing that we can see here on Earth. But even if it's that big, even if it's a huge, dramatic cataclysmic event, we can just barely see it here on Earth because the ripples in space and time are very very small. For example, if you took a rod, then when a gravitational wave passed, it would get shorter by one part in ten to the twenty. And then we get longer as the wave passed by one part in ten to the twenty. So to see these things you need really really accurate measurements of distance.
Yeah, it's a crazy amount of engineering to be able to measure things so small, and it works. And they've seen a lot of these crazy events like black holes colliding with each other or black holes with neutron stars, and we've seen like maybe a dozen of them, and so they're common and they sort of form, as you say, the foreground of the gravitational universe, right, the gravitational field in the universe, And so there's the question of whether or not there's also kind of a background in gravitational waves, and whether or not that has some interesting signals that might tell us about the origins of the universe. So let's get into that. But first let's take a quick break.
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All right, we're talking about the cosmic gravitational background now, Daniel, we talked about gravitational waves. That's when like big things like black holes and neutral stars swirl around each other and collide. They generate these ripples in space that we can detect here. That's what you call the foreground of the gravitational universe, right.
That's right. Those are like big shouts in the gravitational spectrum, huge events that we're listening for. Specifically in their short they're localized that come from one direction.
Right, then they're big events where like things happening, but there might be things happening in the background as.
Well, that's right. Just like when you look up at the sky you see photons from stars, those are of like little localized events. You can see them. But then in between all the stars is the background, right, It is the thing that you're not looking for usually, And what we're interested to hear is like, is there a background to gravitational radiation? Are there things between these big events, these big gravitational shouts that we can listen to and we can hear and maybe learn something. The concept of a background is something everybody should be familiar with. Like if you're at a party and you're trying to talk to your friend, there's sometimes a lot of background noise, or at a restaurant or there can be a lot of background noise, which you know isn't the information you're looking for. You're trying to listen your friend's story, trying to see them interested in it, but there's always other noise around you, which sometimes makes it hard to listen to it. Right, that's the background.
Yeah, there might be stuff that you don't maybe want to pay attention to, but maybe you do. Maybe there's some interesting conversation sort of behind your friends.
Yeah, that's right. I like eavesdropping at bars, listen to other people's conversations. In this case, it's interesting because we only recently became able to observe the cosmic gravitational foreground. Right, We were listening for gravitational radiation and didn't hear anything for years and years and years because we were making our instruments more and more sensitive. It's only recently we've been able to hear the loud shouts. Right. Only recently we've been able to hear your friend at the bar talking in gravitational radiation. Now we're getting ambitious. We're like, ooh, can we hear what's going on between the shouts? Is there anything there?
Also? And it's interesting that you call it gravitational radiation? Can I use it? Is it a technical term? Gravitational radiation.
Yeah, absolutely, it's radiation. It's trend submission of energy. And when two black holes collide, the new black hole that's formed has less mass than the masses of the two black holes that formed it combined. And the reason is that a lot of the energy is lost. And it's not lost to light, it's not lost to particles. It's lost through gravitational waves. And sometimes it's like, you know, five times the mass of a Sun is lost in gravitational radiation. So yes, it's absolutely the radiation of huge amounts of energy.
Interesting, Yeah, like it's quantifiable, like it carries energy. It's not just information. It has some sort of substance to it.
Yeah, it has some energy to it. For example, you could take a seventy solar mass black hole, combine it with a thirty solar mass black hole, and end up with a ninety solar mass black hole, which means that ten masses of the Sun's worth of energy was dumped out into the universe in gravitational radiation. Now, don't be confused, we're talking about gravitational waves, not gravitons.
Right.
Gravitons are not a thing we know exists. It's the theere quantization of the gravitational field. So we know that gravitational waves exist. Those are waves in the gravitational field, but we don't know if that field is quantized into like the smallest piece, which would be gravitons. So gravitational waves exist, we don't know yet if they're made of gravitons.
Interesting, So then how would you define as the cosmic gravitational background. Is it like fainter signals or is it like a hum or? Is it like actual noise in the gravity of the universe.
Yeah, it's all of those things. It wouldn't be localized in any particular direction. It's not something that comes from one particular event, like one of these cataclysmic mergers, and it should be an overall hum And I think it's useful to go back to the cosmic microwave background radiation as a sort of comparison because it's very similar in concept. Like when you look at in the night sky you see stars and galaxies, but also between those stars and galaxies, we are getting light from the very early universe. That's the cosmic microwave background radiation. And it's in every direction, no matter of where you look, and It first appeared as a hiss in a radio telescope. We had a whole fun episode about how that was discovered. And it's left over light from the very early universe from four hundred thousand years after the Big Bang. The reason we call that the background radiation is again because it's sort of everywhere the universe was filled with this plasma and emitted this light which was going in all direction.
It's like it's not coming from a particular thing like a sun or a pulsar or something like that. It's like it's coming from everywhere, Like you hear this hiss in the signal of the universe everywhere you look.
Yeah, and it's sort of a funny naming thing because it's called the background radiation for that reason that it doesn't come from any particular direction. But you know, now we have experiments dedicated to just looking for this. So it's like you're looking for the background. Is it really still the background? It's sort of like your target, it's your signal, right, no longer the background.
Right, the background becomes a foreground than what's in the background.
The background background. We have multiple Nobel Prizes awarded, specifically four experiments measuring an analyzing ripples and wiggles in the cosmic microwave background. So it's definitely you know, worth calling it a foreground something.
But anyway, you have to call it a background Nobel prize, like a noisy you know, static y Nobel price.
Maybe it's like an off the record Nobel Prize, you know, like this is on background only.
Oh yeah, there you go. Unofficial.
The universe doesn't want to speak up about his secrets. It just wants to like slip it to us on background.
I think that would conveniently reduce a number of Nobel prices. You haven't want, Daniel.
That would be difficult. I could be like, look, I have this secret about the universe, but I can't tell you how I know it because I got it on background from a source. I'd be pretty disappointing.
And of course you got to protect your sources.
Right, absolutely, even if it's the universe.
Yeah. So then the cosmic microwave background is a sort of leftover light from the early universe. What would be the cosmic gravitational background.
So we don't know exactly because we haven't seen it. We only have theoretical ideas for what could be creating it what it might look like, but we're excited to see if it's there. The limitation on seeing this thing is making our experiments sensitive enough to listen to these very very quiet signals. And the challenge there is making your experiment insensitive to other things that look like gravitational waves, right, that look like this hum or, this hiss, because the way these experiments work. For examples, you have like two mirrors hanging miles apart in a tunnel underground, shooting lasers back and forth to measure the distance between them, and it's very easy to get wiggles in that distance if like the mirrors shake a little bit, or a breeze flows through and moves the mirror. You know, the size of a gravitational signal is a tiny, tiny fraction, much smaller than the width of human hair, and so it's very hard to isolate these things and just sort of like experimentally to make these systems work so you can see these signals, and so the challenge is making them work even better, making them even more sensitive. So now we could pick up like a low level omnidirectional hum if it exists, right, And.
I guess the tricky thing is that you know, you have your instrument, it's listening to the universe, and it's picking up noise from its environment, from the Earth, from the circuits in your instruments, but it's also maybe picking up noise from the universe itself in gravity, right in the gravitational spectrum of the universe. And so you want to be able to say, like, Okay, this crazy random noise here, that's from my experiment, and this crazy random noise here is actually from the universe's gravity exactly.
It will be a much more difficult thing to demonstrate we've seen than the gravitational waves we've seen so far, because it's easier to see things that are like individual catexclismic events. First of all, they're isolated, so you can say it happened and then it's stopped, whereas the gravitational background is going to be like all the time, so you can't say like, oh, we turn it on and off. And here's the difference. And also, the gravitational waves that we have seen have a very particular signature, like when black holes swish around each other and create these gravitational waves. It's not just like a big screen and we know exactly what it should look like. It should start small, it should get bigger and bigger and faster and faster as they swear closer and closer. So the gravitational waves we're looking for in the foreground have a very particular pattern when we first saw them. That's what convinced us that we had actually seen them right, that they look just like what we expected. We do these numerical relativity calculations that predict the signature, the fingerprint of these gravitational waves. That makes it much easier to see they come from a particular direction, they're very short lived, and they look different from everything else. But as you say, we're just looking for a hiss, it's much harder to know if that hiss is coming from gravitational waves or from wiggles in your detector. So experimentally it's much much more challenging.
And I think the idea is that this noise in the gravity of the universe and the gravitational spectrum of the universe, again, it's not coming from anywhere in particular, it's coming from everywhere, and it's maybe made up of you know, what would it be made out of? Is it made out of like big explosions somewhere else that have sort of propagated and diffuse throughout the universe. Is there any idea?
We don't know exactly what it might look like, and we can dig in a minute about the possibilities what could be generating it and what it would look like. But you know, one possibility is that it looks sort of like a hiss. Another possibility is that it's like very slowly changing, you know, like instead of having a wave that changes over seconds, the way gravitational waves from black holes do, it might be like a very gradually changing tide, Like we're seeing a gravitational wave that oscillates over like years or decades or centuries, And that would again, would be much harder to see because we like to look for the pattern that tells us that there was actually something there.
But again I think it is that you we'd be looking for like a hum in the distortion of gravity in the universe, right, we have to make sure that you know you're measuring the wiggles in gravity really well, and then you'd be looking for some sort of activity there that wasn't like coherent. There wasn't like a spike or a particular shape.
You need to establish that you're seeing a signal that is louder than the noise in your device, that your device has less noise than the signal you are seeing, which is tricky. And then what it would look like is, as you say, it would be like sort of random ripples in space and time, the way the cosmic microwave background radiation sort of has like little ripples in it. You'd get like a little distortion this direction, a little distortion that direction, a little distortion in the other direction, and all these distortions would be much much smaller than the signals we've already seen. They'd be even fainter.
Right, And what's cool is that with our instruments now like Lego and Virgo, we can tell which way gravitational waves are coming from. Right, so this would also tell you that they're coming from everywhere, not just in a particular direction.
That's right. We can tell the direction of gravitational waves mostly because they take time to propagate, and so we can see them in different observatories around the world. There's one in Louisiana, one in Washington State, and one in Italy. That's Virgo, and as the gravitational wave sort of washes over the earth. It arives one place before another place, and that timing information allows us to figure out sort of where in the sky it might have been coming from. But as you say, it is cosmic gravitational background would be coming from all directions.
Cool. Well, let's get into now whether or not this gravitational background really does exist and what could be making it and what he can tell us about the universe. But first, let's take another quick break.
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All right, we're talking about finding waldos in the cosmic gravitational background of the universe. Daniel like, is there something interesting in the background noise of gravity in the universe.
Yeah, we don't know yet whether it's out there, right. This is like an idea people have had. There are theories that suggest it might be out there and could contain like treasures of the universe, and people have looked for it, but we don't really have a conclusive evidence for it so far. Like Ligo and Virgo, our best gravitational wave detectors have looked for us very specifically. But what they see sort of in between the loud shouts from black holes, looks to them just like noise in their detector. Like they can estimate how much noise they expect to have in their detector, and that's what they see, and their estimate for their noise is above anything that anybody predicts for cosmic gravitational background. So it's sort of like you go into a library and you're listening for whispers, but you know you can't hear the whispers yet. All you can hear is like electronic noise in your device, and that's what you hear, so you don't see anything. We also haven't really learned anything yet. You just learn that you need a better listening device.
Oh, I see. It's like we know we have kind of a crappy microphone, so we know that with our current set up we can't listen to this background radiation, which means that we maybe don't know if it exists or not, Like this is a theoretical concept, then this cosmic gravitational background or does the theory pretty much predict that it exists, we just can't measure it.
Yeah, but first I got to stand up for Lego and Virgo because you just called them crappy microphones. These are like the world's most amazing, super sensitive microphones, just not quite super sensitive enough to listen to this incredibly faint signal. But you're absolutely right. So this is something people have theorized, and you know, there were moments when people thought they might have seen it. Ligo hasn't seen it yet, but there was a moment people might remember this experiment Bicep to something on the South Pole, which thought they saw the effect of this radiation on the cosmic microwave background radiation, which they can measure, and they had this whole claim that they had discovered it and it was evidence that there was cosmic gravitational background radiation which was tweaking the cosmic microwave background radiation. But it turns out it was wrong. It was just that they didn't understand how much dust there was in the universe. It caused a fake signal. So there was a real excitement there for a while, and then that faded, and that recently there's another experiment which is super cool, which it uses a galaxy size measuring device, and they think they've seen evidence for this cosmic gravitational background.
Why you mean we have something that big that we can use to measure the gravitational waves of the universe. We have something the size of a galaxy.
Yeah, well, we have our galaxy and people have figured out how to use the galaxy as a gravitational wave observatory. We're going to do a whole episode on this because it's super awesome. But very briefly, if you look at pulsars out there in the universe. These are fast rotating neutron stars that send us super super regular pulses of light. You can tell if those things get pushed away from us or pulled towards us when a gravitational wave passes, because it changes when those light pulses arrive. And there's an experiment that's looked at some of these very regular pulses. It's called NANOGrav and they see a signal which looks like cosmic gravitational background radiation. But you know it's early days, and there have been false claims before, so nobody's really accepted this yet. It's just sort of like and exciting possibility, all.
Right, So we're actively listening for this background or trying to listen to it, and it's sort of a funny thing, right, It's like trying to see if you can listen to the background noise and in a noisy bar, right or in a quiet library. It's sort of like you're trying to get everyone to quiet down so you can see if there is sort of a hiss there or not.
Yeah, exactly, if you're curious about what's going on outside your house, for example, you turn off the TV and Telebrady shut up and listen to see if it's just crickets or something else going on outside.
Did just turn this into a scary movie? Like what's out there? What's out there that we can't hear?
Yeah, And there's a whole range of reasons why we think the cosmic gravitational background might exist, and there's some like kind of boring possibilities to explain it, and then there's some crazy, exciting, bonkers possibilities.
I guess, you know, maybe as many of our listeners might be wondering, like, does it make sense that there wouldn't be a cosmic gravitational background, Like things are everything causes gravitational ripple, so why wouldn't there be a background exactly?
And that's the sort of most boring explanation is everything causes gravitational waves, Like every time you accelerate in your car, you cause a little gravitational wave. As the Earth moves around the Sun, it's generating gravitational waves, and there's a lot of that stuff going on in the universe. Most of that is really really small, but the idea is that it sort of adds up to this like overall stochastic background, and in particular, black holes that are too far away for us to like make out individually should add up to like an overall sort of noise level in the background. There should be black holes merging and neutron stars colliding with black holes all over the galaxy. Some of them are too far away for us to like pick out individually, but they should turn into like a hum the way like a whole crowd in a baseball stadium. You can't make out their conversations, but you can tell the people are talking. You hear this like overall hum So people expect that that can tell us something about like the overall rate at which gravitational waves are generated by distant objects.
Right, But I guess is it possible then? Maybe? You know, I'm just thinking like maybe the universe could be quantized enough so that at some point there is no noise, do you know what I mean? Like, maybe at some point there's a minimum size to these irritational waves. At which point and since everything's so faint, everything just flattens out. Is that possible?
It's possible, But you know, tak an analogy to photons coming from distant galaxies. You look up in the night sky, you can't tell that there are galaxies there because they're so distant, because the photons aren't arriving very frequently because they're quantized. Right, It's not like you can get zero point zhoho one photon. Every second you get one and then a minute later you get another one. But they are still coming, and so there is a non zero rate there. It's not like it goes exactly to zero. They just get less and less frequent. And so if gravitational waves are made of gravitons and they're super duper distant and faint, we should still be getting gravitons every once in a while, even from the most distant sources of gravitational radiation. But yeah, that would be really challenging to pick up.
Right, But I guess you know, photons are sort of ties in direction, but a gravitational waves kind of goes out in all directions, right, like a ripple in a pond.
Yeah, but you know, a star sends out photons in all directions. It's really equivalent when a black hole merger event happens. It sends out gravitational waves in every direction. We just only pick them up from one direction. And so we would be picking up the gravitons from those events. And if these things are happening everywhere, we should be getting gravitons and gravitational waves in every direction from all these distant events. And so this is, you know, what people expect at a very bare minimum. Because we know that black holes are generating gravitational waves, we should be able to pick out at least the gravitational background from these distant events.
Interesting, so we think there's definitely a cosmic gravitational background from just because we know there's things happening in the universe that are making them. So it makes sense that we would be sort of inundated with the faint ripples from all of these events.
Even things other than you know, black hole mergers. Even for example, when a star collapses into a black hole, you get gravitational waves. When a supernova goes off, you get gravitational waves. If a supernova goes off near a black hole, like JJ was suggesting, you get gravitational waves. So we should be able to see all these things sort of like added up all together.
Right, But it might be so faint that we can't see them or hear them.
They are definitely too faint for us to see them now. But you know, with future observatories and improvements in our technology, we should in principle be able to see it if it's there.
Right, all right, So that's one possible source of a cosmic gravitational background. What are some of the more bonkers sources that might be out there?
One really exciting one is a mission of gravitons. You know, in some theories, gravitons are the quantization of gravity. Remember that our theory of gravity general relativity, is a classical theory. It says that you can have any arbitrarily small distance, or any arbitrarily small amount of energy, or any arbitrarily small slice of time. Quantum mechanics says that's probably not true, and a theory of quantum gravity would have gravitons in it, as we've said before, like the basic element of gravity, and these things should be am basically anytime anything happens, you know, when a particle gets accelerated, it should emit a graviton. When an electron jumps down an energy level, it should emit a graviton. And so in principle, the universe is filled with these gravitons, and they would be very very high frequency. Like the gravitons we've seen so far have an oscillation time of like, you know, a few seconds. These would be much much higher frequency, like you know, mega hurts sort of gravitons. And so that's exciting because it would be like a signal of quantum gravity.
Right, But do they need to be gravitons? Like when an atom relaxes from an excited state, doesn't it also generate a gravitational wave like a regular wave.
It does, But now we're talking about a quantum particle, and a quantum particle should emit quantized bits of radiation, right, it can't emit classical radiation, And so this is something we don't understand. We don't know how to do gravitational calculations for a particle, for a quantum object. That requires a theory of quantum gravity. We just don't have so general relativity, like can't tell you what happens to the gravity of a particle. And we can't really do those experiments very easily because particles have almost no detectable gravity because there's so light in mass and gravity is so weak. So we just don't know. But you know, maybe all those particles out there are sort of like humming in gravity and we can pick up like the some of them. Like maybe all the particles in the universe sending us gravitons simultaneously would be something we might be able to detect.
Interesting. I think you're saying that maybe, like the gravitational background of the universe is not coming from these giant events and supernovas and black holes gliding. It might be being generated by everything like you and I just sitting here. It could be generating gravitational noise.
Yes, we should be in theory, and we should be able to measure it. And if we see it and we measure its frequency, we can use that to understand how they're being generated. The same way we look at the frequency of everything else, and we see emission from hydrogen, for example, at a particular frequency. We see transitions in lithium gas at a particular frequency. So this would be like a fingerprint for what's general. Gravitational waves in the universe would tell us something about gravity and the composition of the universe. It would be amazing to see gravitons emitted as the cosmic gravitational background.
And what about some of the other bonkers possibilities that are making this cosmic gravitational background.
One that gets me really excited are gravitational waves emitted from the very very early universe. Our theory of what happened in the very beginning of the universe is called inflation. It suggests that space was stretched by a ridiculous amount, by like a factor of ten to the thirty and a short amount of time like ten to the minus thirty. And when you do that, you got to generate gravitational waves. Because that's like the biggest ripple space has ever seen, right, So there should be huge gravitational waves left over from that, because, like the CMB, it happened everywhere all at once, so it should be like a cacophony of gravitational waves. But over the fourteen billion years since, they've probably gotten stretched out and smoothed over. And so this would be exciting to detect because it would probe the very very very very early universe. Like people talk about how the CMB tells us about the Big Bang, it doesn't really. It tells us about a plasma that formed four hundred thousand years later. It's like reading about the birth of Jesus from somebody who wasn't there, who wrote like hundreds of years later. Gravitational waves from inflation would come from like ten to the minus thirty two seconds after the beginning of the universe. So it's like really a first hand account of the beginning of the universe. If we could see it, it would tell us a huge amount about how the universe actually began.
All right, Yeah, interesting, I guess you know, anything with mass and energy affects gravity, right or it creates gravity, or so anytime you have a change in the universe, or anything really happening in the universe. It's going to generate a gravitational ribble, right, yeah, yeah, even like the beginning of the universe that must have generated a bunch of ribbles.
Yeah, huge ribbles exactly, tsunamis of gravitational waves. You could probably serf.
Them, yeah, with your friend Waldo on a surfboard.
Look, I know you keep trying to include him. I just don't like the guy. Okay, what do you have against Walden? I don't know. He just won't let me find him.
You know, he's just elusive. He's a sneaky guy, but a sneaky particles are totally.
Cool with yees. Sneaky particles.
All right, So then what's the last possibility here that might be causing a background in the universe.
The last possibility is the most bonkers, and these are cosmic strings. We talked about this one in the podcast Now a couple of years ago. They're like cracks in space and time. We are used to space time being sort of smooth. If you go from here to there, you can sort of just like slide through space. But we don't really understand how space was formed, and as the universe cooled, it might have like formed differently in different regions, leading to like weird disccontinuities, like parts of space are a little different here and a little different there, and at the boundaries there can be these cracks where like space changes from one state to another, and so these would be like long filaments, maybe even you know, like thousands of light years long, that have like cracks in space and time, and as they wiggle, they would generate gravitational waves. And there's some theories that like the ends of the might like wiggle really fast like a whip, causing these crazy gravitational waves. So if that's happening all over the universe, we might be able to pick up some of those signals and detect the existence of these cosmic strings.
Whoa, it's like the universe was a giant guitar and like there's giant light years long strings that are you know, basically making no it's music, right.
Yeah, we don't know if they exist. It's a super cool theory. If we do find them, we would tell us that we understand something deep about how space and time it's sort of formed in the early universe. You can think about them sort of like the way ice isn't always clear. If you take water and you cool it down, you get these cracks sometimes because it doesn't all crystallize in exactly the same way. And so this would really tell us something about how space itself formed in the very early universe. Would be super awesome.
That's my new theory of the origin of the universe, Daniel. When you religion, it's that the universe is just a giant guitar played by a giant rockstar named Waldoo. The go waldoism. We just found it any religion, all right, So I guess one final question is how would we know which of these origins of the background might be, Like do we have any hope at all of ever knowing like, oh, this noise is coming from inflation or gravitons or cosmic strings.
They would look different, right. Gravitational waves have frequencies, just like light does. Different frequencies of light look different. You got radio, you got infrared, you got X rays, you got gamma rays. In the same way, gravitational waves have frequencies. The ones we've seen have frequencies of about a second, and we have sensitivities due to our detectors to arrange your frequencies. So if they come at very very high frequencies, we think they might be gravitons. If they come at lower frequencies, they might be from inflation. If they're sort of periodic and spastic, then they might be from cosmic strings. So each one of these has like a different fingerprint. If there's just nothing there except for like the low level hum, then that should be pretty flat and lots of frequencies represented. So the sort of spectrum of the frequencies there is a fingerprint that tells you what generated it.
We just got to, I guess, tune up those microphones and turn up the amplifiers and reduce the noise just to be able to maybe listen to these signals and be able to tell which one is which.
Yeah, and we have some exciting plans. The Ligo Observatory is based here on Earth, and what they'd like to do is build a much bigger version out in space. They want to put three satellites out there that manage to somehow keep a very precise distance from each other and then shoot lasers back and forth to detect the passing of gravitational waves or the cosmic gravitational background. It's not science fiction. It's a real experiment. Might really happen sometime in the next ten or fifteen years, and it can really teach us something deep about the universe.
It's ligo in space.
It's called Lisa.
All right, Well, that's what the cosmic gravitational background is. It might be there, it might not be there. We think it's there, but we don't know quite what's causing it, right.
We don't know if it's there, we don't know what's causing it. But we're desperate to listen to these signals that might contain new treasures about the early universe.
So stay tuned once again, and maybe think about that when you look up at the night sky once again. Then how much information is bathing over us as we speak, and that might tell us about the very origin of the universe.
And think about all the gravitational radiation that you are generating. Every time you go out there and get exercise or hit the accelerator in your car. You are contributing to the cosmic gravitational background.
Some of us more than others. Like if you're a couch potato, technically you're just creating less noise for the universe.
Right, that's true, but you're getting more and more massive, so if you ever do get off the couch, then you can be pretty noise. Right.
There's a trade off there, all right. Well, thanks for joining us. We hope you enjoyed that. See you next time.
Thanks for listening, and remember that Daniel and Jorge explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.
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