Daniel and Jorge Explain the UniverseHow to use the whole galaxy to hear huge gravitational waves
Daniel and Jorge break down the recent discovery of huge gravitational waves by NANOGrav, and what it means about supermassive black holes.
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Good job. Thanks, Hey Daniel. What's the biggest science experiment ever built?
Ooh, well, the Large Hadron Collider is pretty big. It's thirty three kilometers around.
That is pretty big, I guess for human scale, but compared to the universe. I mean the universe is huge, right.
That's true, And I guess astrophysicists use the whole universe as an experiment when they like watch black holes collide.
Mmm, the whole universe is as an experiment. What are the results?
They're pretty universal?
Are they positive or negative?
We have only one data point so far, so I think it would be presumptuous to make any conclusions.
What's a hypothesis for the universe? It's awesome or not awesome, it's bonkers. I guess you use data from the whole universe. But usually you only detect things with a small telescope, right, or a small Parkle collider.
Yeah. The kind of eyeballs we build are usually small, even when we are observing really large things. But there are some really clever approaches that use like the whole galaxy as a detector.
Mmm.
What is that experiment called?
Because it's so gargantua and of course they use the word nano in the title of the experiment.
What.
I guess it's nana compared to the size of the universe.
I can't defend astronomers when it comes to aiming their experiments.
But you can defend particle physicists. He didn't defend anyone in science or how they named things.
I'm gonna plead no contest, the.
Experiment says no.
Hi.
I'm Jorhem Mac, cartoonist and the author of Oliver's Great Big Universe.
Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I think of a universe is pretty well named.
Apparently not because there's something called the multiverse. So I think if you have to put something in front of it later, it wasn't well named.
Oh, well, like Superman is not well named because he's a super version of a man.
Well, he's special, but if you have to sort of redefine what that is like, if you had to later name him ultra Superman, then maybe he didn't name him well the first time to Superman. Maybe there's a universe where Superman was well named. Or physicists name their experiments in easy to understand ways.
That would be like a superphysicist.
But anyways, welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which we use our definitely not super brains to try to understand the super mysteries of the entire universe. We hope that everything out there in the universe can be described by simple mathematical recipes that make sense to our mammalian brains, and we do our best to apply those rules to the whole universe to see where they work, where they break down, and where we can explain all of them to you.
Yeah, because it's amazing that our tiny little brains can understand universe, that we can look out into the cosmos to get data and figure out how things work. Fortunately, the universe likes to reveal itself sometimes.
Sometimes we have to be clever and build really interesting little apparatusus to like smash particles together or balance against each other in order to extract some information from the universe. We like concoct special setups that we hope will reveal deep truths about the universe. But not everybody gets to do that. Not everybody gets to build their own experiments. Some people have to go out there and find the experiments happening in nature.
Yeah, because science is a continuing story and there are new things being learned every day and new ways to look at the universe being discovered every day. And so this week there was a very special news about a new experiment that just revealed the data it's found.
That's right after analyzing fifteen years worth of observational data. The NANOGrav experiment has just made a dramatic announcement about their fantastic new discovery.
You might have seen it on the news it's sort of a big deal for this community of astrophysicists.
It's literally sending waves through the cosmological community.
But hopefully the results aren't the wavy or shaky.
It's a bit of a treacherous territory because, as we all know, claims of the discovery of gravitational waves have either been verified and led to Nobel prices or have been debunked and led to some quite red faces in the cosmological community.
And so this week there was a big announcement by an experience called nanograph. Now, Daniel, is that an acronym or do they just like the word NANO?
You know? I think the answer to that is yes on both counts. It is an acronym. It stands for North American Nanohertz Observatory for Gravitational Radiation. So they both like the word NANO because it's in their title and it's an acronym.
Wait what, it's a recursive title, like the word nano is in the acronym nano exactly.
And because NANOGrav is one of these really clever devices that uses the entire galaxy as a detector for gravitational waves, you might think that they would choose something which describes the scale the grandeur of that experiment. But instead they've chosen nano, which reflects the very short frequency of these gravitational waves.
Mmmm. Yeah, I saw. Did you said the word nano hurts? I guess that's the frequency of gravitational waves that they have detected.
That's right. The huge scale of the galaxy allows them to measure really long wavelength, low frequency gravitational waves that other gravitational wave detectors Ligo and Virgo and all of those could never see, giving us a whole new window into what's going on in the universe.
So today on the podcast, we'll be tackling the question how did the nanograph experiment use the galaxy as a detector?
Now?
Did they ask the galaxy's pervision before they did this?
We've been over this before. The universe has no right to privacy inherently, man, we can ask whatever question we want.
Is it in the fine print or are we going to get sued later?
I checked with our legal team. They're fine with it.
We have a legal team.
I'm the legal team.
Yeah. I think that means we don't have a legal team. That's I check.
You're not a lawyer, that is correct, do not take any legal advice for me.
So this is one of those large physics collaborations. And I guess nanosoundsmoll because we're used to associating the word nano with distances, right, like nanometer as being super tiny, or the scale of atoms and things like that, But here it sort of refers to frequency, which actually is sort of like the inverse kind of our intuition a lot of times, right, and so nano here actually means big.
Yeah, that's right. You have to think about waves wiggling, right, and waves that wiggle at really high speed. Things like gravitational waves, which move at the speed of light, have a connection between their frequency and their wavelength, just the way light does. So very high frequency light has very short wavelengths. Very low frequency light like radio waves, has longer wavelengths. And the nanograph experiment is looking for huge gravitational waves, gravitational waves which dwarf the size even of our solar system, and so they have to look for very very low frequency, very tiny frequencies nano hurts, which means like a frequency of one times ten of the minus nine.
Yeah, Because I guess when you hear the word like giga hurts or megaherts, it's a super high frequency that happens really fast and very short wavelengths. But if you hear nano herds, that means that it's like a super low frequency, like it takes years and years for a wave to go by.
Maybe that's right. One nano hurts means one wiggle every thirty years. Whoa.
And so they recently announced a big result after a long time that they've been at this using their technology to detect gravitation waves, and this week they made the announcement, right, which made the news.
That's right, And they've been hinting at this for several weeks that they have something very big to share, and people have known that they were going to have enough data to say something interesting for a little while. We covered them on the podcast a few years ago when they had very preliminary data. They didn't have yet conclusive results, but we were very excited to hear what they were going to have to say in a few years. And that day is today.
So let's dig into what their result was and how this experiment works and how they use the whole galaxy basically as a detector so what.
They're looking for are gravitational waves, which you have to remember are not a wave like a wave in water, that they have a lot of similar mathematical properties. Gravitational waves are waves in space itself. General relativity tells us that gravity is the curvature of space, which really just means you're changing their relative distances between points in space, and so gravitational waves are like ripples through space where things get closer together and further apart, closer together and further apart, like space itself is oscillating.
Yeah, because we know space isn't just like the emptiness of the universe. It's actually sort of like a thing, right, like you can bend it and squeeze it and curve it. Right.
Yeah, that's exactly right. And those features of space are what give rise to our sense of gravity. If space was totally flat and smooth, there had no curvature to it, then things were just moved through it in straight lines that look straight to us. But because space has this sort of invisible curvature to it, things in free fall tend to follow the curvature of space, following those curves and bends and wiggles, which to us look like something is pushing on it to make it change its direction.
That's kind of the Einstein view of the universe, right, that gravity is not a force that pulls on you, but it actually kind of bends space time around you to make you move in certain ways.
That's right. That's general relativity in fifteen.
Seconds done in nanotime, nanorelativity nanoscience podcast. But yeah, it's kind of this idea that space kind of bends right and can wiggle, especially when you move masses through it.
Exactly. And we know that Einstein improved on Newton's idea of gravity. Newton had the idea that gravity was a force, and Einstein replaces it with this concept of space being bent. But there's another important consequence of Einstein's update to Newton's idea which gives us these waves, which is that gravitational information is not instant According to Newton, if you had deleted the Sun from the universe, you would instantly feel across space and time the absence of the Sun's gravity. But Einstein tells us that if you delete the Sun, that information takes time to propagate. Space doesn't like snap back instantaneously. Everywhere in the universe. There's propagation of information, and you can think of gravitational waves sort of as the propagation of information, the same way that if you wiggle an electron, you make wiggles in the electromagnetic field, which we can think of as photons. If you wiggle the sun, or if you move any massive body, you make wiggles in space, which we can think of as gravitational waves.
Yeah, it's interesting you call it the propagation of information. It's sort of I guess like if you stand in the middle of the field and you scream, it's going to take a while for that scream to get to places.
Man, what a dark example. Why are you standing in the middle of fields and screaming. Is this like some new performance art.
Yeah, I'm screaming with joy. Yeah at the fact that nanograph has discovered that's something interesting this week.
Yeah, maybe what we've actually discovered is aliens all across the universe screaming in their fields. Yes, but you're exactly right. Information always takes time to propagate, and that includes the curvature of space time. So anything that wiggles, anything that's accelerating in the universe is making gravitational waves. Now, gravity is a really weak force, so when you accelerate your arm up and down, you are technically making gravitational waves, but they're super duper weak. So in order to see gravitational waves, we tend to need really really high masses undergoing huge accelerations, which is why the target for seeing gravitational waves are typically things like black holes swinging around each other super duper fast the moments before they merge.
It's interesting just going back you describe it as how information about it propagates. I guess it's sort of like if the sun suddenly moved a meter to the right, it would take some time for us and the gravity we feel from the sun to feel that shift in the sun right. And then if the sun went back to its original position, it would take a little bit of time for us to feel that it went back to its original position. And so that kind of going back and forth is kind of what you can call gravitational wave. Right, It's like the effect of the sun through gravity. Oh, the effect of the sun propagates.
Mm hmm. If you're holding a string and your friend a mile away is holding a string and they wiggle their end of the string, you're not going to feel your end wiggle instantly, It's going to take a while for that wiggle to travel down the string. If you wiggle the Sun, it changes the curvature of space, and it takes a while for that wiggle to get to Earth. Those are gravitational waves.
So I guess you could describe it two ways. It's like it's the Sun bending the space around it, or it's also you can think of it as the effect of the Sun through gravity being kind of changing over time.
Yeah, I think those are both accurate. I tend to think of it the first way. That's the Einsteinian way. It's changing the curve sure of space time around it. Gravity is an effect of that curvature.
So a gravitational waves sort of made the news maybe like five I think, I want to say five maybe years ago. That's kind of when they entered the popular culture, because that's when we started to be able to listen to them, right through an experiment called Ligo.
That's right. We had strong hints that gravitational waves were a real thing decades before that, when we saw neutron stars orbiting each other and their orbit decayed and exactly the way you'd expect if they were losing energy to gravitational radiation. But the first direct proof was observations by Ligo and Virgo in twenty sixteen of black hole mergers giving off these gravitational radiations using this incredible brava technique of these lasers underground between mirrors kilometers apart, looking for variations in the distance between these mirrors of one part in ten to the twenty. So that was really a very spectacular confirmation that gravitational waves are a real thing in our universe.
Yeah, we know they exist, and they're pretty awesome because they tell us a lot about these huge events going out there in the cosmos. But apparently these things only tell us about a very maybe narrow window of things that can happen with gravitational waves out there in the universe exactly.
Logo and Virgo are sensitive to gravitational waves of a certain frequency basically because of their size, Lego and Virgo can only really see gravitational waves that are about the size of their detector. The gravitational wave was much much bigger than it wouldn't have an observable effect on their detector. It would like wiggle too slowly. So Logo and Virgo are built to be able to see stellar mass black holes like black holes of ten or twenty or thirty or forty times the mass of our Sun emerging, and they've seen dozens and dozens of those. It's very, very exciting. But they can't see things like super massive black hole mergers, which are predicted to happen when galaxies collide. And they also can't see like echoes from the Big Bang itself, which would leave gravitational waves with huge wavelengths.
Interesting. All right, well, let's dig into these other kinds of gravitation to wave events that can happen out there in the universe, and how the nanograph experiment has apparently maybe seen some of those. So let's stick into that befirst. Let's take a quick break.
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All Right, we're talking about the recently announced results of the nanog Gra experiment that have to do with gravitational waves. Big gravitation waves.
That's right, and not to take anything away from Lego and Virgo, it's really impressive that they saw anything at all. I mean, I remember having the opportunity to join those experiments when I was picking graduate schools, and I thought, those guys are never going to make that work. And hey, that's not the first Nobel Prize that I've missed out on.
You've had several, haven't we all.
I mean, there's this thing in research where you have to sort of make a gamble, like is this going to be a fruitful thing to do? And part of it depends on your smarts and your hard work, and part of it just depends on the universe. Is there something there to discover? You build this new kind of telescope and use it to look out into the world. Is there something there for you to see? Lego and Virgo, We're definitely lucky because there are a lot more black holes out there than we thought.
Well, are you sort of assuming that if you had joined that collaboration, they still would have discovered gravitational waves? I mean, aren't assuming something there?
Yeah, you're right, I might have put in the entire process, or maybe it would have happened much faster. Who knows, We'll never know.
Or never who Right.
No, you never really get to open those other doors and know what your other lives might have been.
You need like a superhero movie to explore the moltiverse.
But we all do make these choices as sciences to say I'm going to devote my life to this one particular way to maybe learn something about the universe.
Cool. Well, we're talking about there recently announced results from the NANOGrav experiment, which is a big collaboration of many different places, right, not just in North America. I mean the name is North American nano Hertz Observatory, but actually they use I think places from all over the world.
Well, NANOGrav uses Aricibo and the Green Bank Telescope in West Virginia and the very large array in New Mexico. But you're right, there are definitely pulsar observatories all over the world in Australia and in Europe and in China, and there are other competing collaborations. So NANOGrav represents like the North American slice of the world.
And so we were talking about earlier how nanograph looks for gravitation waves in different wavelengths of gravitation waves than Ligo and virgo.
That's right, NANOGrav is looking for something much much bigger than Logo and virgo is looking for, and something much bigger than Lego and virgo can even see. Something important to understand is like when black holes merge, the ones that Ligo and virgo do see, they're emitting gravitational waves during the entire merger, but Ligo and virgo can only see to the very end because it speeds up and gets shorter wavelengths, and that's what Ligo and virgo can see. So they're only seeing like the last little bit of that merger when it's in their little window.
Now, is that because of the some sort of frequency limitation or is that because when those black holes merge, that's when the gravitation waves get really big, right when they're closing in on each other. And circling each other really fast. That's when there's a lot of acceleration by those masses, and that's when maybe we get waves that we can detect here on Earth.
Yeah, it's both factors. The amplitude is too small during the earlier part of the merger, and the frequency is wrong. Lifo and Virgo is sensitive across a certain frequency range, and it's limited by a couple of things. One is just the size of their detector. A few detector is only a few kilometers long. You can't detect changes over light years, right, there's like no variation across your detector.
Wouldn't be easier though, Like if you you know, like a slower wave, wouldn't your detector be able to catch those better if it had the same amplitude.
Well, think about it in terms of the distance. Like, it's much harder to measure the curvature of the Earth if the Earth is much much bigger than your ruler, right, Like to us, the Earth looks almost flat. You can't even really detect the curvature, and so detecting small changes in the curvature is really hard. But if you're like the little prints and you're on a planet, where like a planet is basically the size of your ruler, it's much easier to measure the curvature and the changes in the curvature. So here, now we're like sitting on a huge wave. If the wave is like the size of the galaxy, there's no way that your little two kilometer ruler is going to be able to measure any change in that wave.
Unless the change is really right, like the amplitude of those low frequency waves, we would be able to detect them, but maybe we don't get those here.
Yeah, if it's big enough, then you can detect it in any way. But there's another factor, which is noise suppression. These things have to be able to distinguish real gravitational waves from other kinds of wiggles. And because Logo and Virgo are on Earth, there's seismic noise, and that seismic noise tends to be lower frequency, and so it sort of creates a wall that LIGO and Virgo just can't see beyond.
Okay, So then the nanograph is a different experiment from LGO, and the use is a totally different technology and method. Right, they didn't even sort of have to build really a detector. They just sort of used existing radio antennas that we have, right, Yeah, they.
Both use existing sources of radio waves meeting pulsars scattered through the galaxy and existing telescopes that can see those radio waves. So it's really just like a clever combination of stuff that's already out there. It's really one of my favorite examples of like just ingenuity and physics.
Yeah, it's pretty cool. And so the idea is that they're using pulsars to detect kind of how the whole galaxy reacts to a gravitational wave.
Right, Yeah, that's exactly right. They want to see really big gravitational waves on the scale of the galaxy, and so they want to see the galaxy itself sort of change shape. Like what Lego and Virgo do is they have this big l they construct with mirrors, and they see a change shape. They see one side get shorter, another side get longer, the whole thing oscillates. So NANOGrav and the other pulsar timing arrays want to do the same thing for the whole galaxy, but they can't like go out there and build lasers and mirrors that are on the scale of the galaxy. So they're just watching the pulsars, and they're using very precise timing measurements from these pulsars to measure how the galaxy itself is squishing.
Well, I mean, come on, they could build space lasers if they wanted to.
Who doesn't want to, right, I mean, when I go out in the field and scream, I'm screaming, let's build space lasers.
Yeah, pew, let's do it.
But this is really awesome because it takes advantage of the universe as sort of a natural set of clocks. These pulsars are really cool, fascinating end point of stars. The stars out there that are burning bright and turning their fuel into light, and then eventually they collapse with like a type two supernova and they leave behind this core, this very very dense object, a neutron star, which is something that has like a ten kilometer radius but weighs as much as our sun.
Yeah. I think we've had one or a couple of episodes about pulsars and neutron stars. They're not burning like regular stars, but they are giving off a lot of light, right because they are still really hot.
They are definitely still hot because they're super dense, but there's no fusion going on inside of them, so they don't glow in the typical wavelengths. Sometimes you can see X rays from cracks on their surface. But what we're interested in this case is the beam that they emit along their magnetic north and south poles. They generate radiation from like motion of charge particles on the surface on the crust of the neutron star, and they have this very strong magnetic field which slurfs those charge particles in that radiation sort of up the north pole and down the south pole creates these massive beams of radiation along the north and south pole of the pulsar.
Now this happens with every neutron star, or only some neutron stars have this beam of radiation going out of its poles.
Not every neutron star is a pulsar. And we do not understand very well both the source of the radiation. You think maybe it comes from a combination of the rotation of this object and the charged particles on the surface, And we don't really understand very well the source of incredibly strong magnetic fields from neutron stars. But not every neutron star spins this way and has these magnetic fields and is a pulsar.
Or can we see it right? Because I think the idea is that you have this neutron star that's a magnetic field. It's shooting out basically like radiation beams out of its poles, and it's also spinning. The whole thing is spinning sort of like a lighthouse, right.
And the magnetic north pole is not aligned with the spin, so the magnetic north pole and the beam sweeps across the universe.
Yeah. I guess it's sort of like when you spin a top on the floor and it starts to slow down, it starts to kind of spin in a kind of wiggly fashion, right, And so if it has like a flashlight at the top of the top, then that flashlight is going to kind of sweep around, sort of like a lighthouse.
Yeah. Well, these guys aren't wiggling, like they're spinning very very regularly, but the flashlight is sort of offset from the axis of spin, so it's sort of like you're spinning around. If you held the flashlight straight up, it would always point in the same direction. But if you held the flashlight at an angle from the axis where you're spinning, then it's going to sweep around the room and light up different corners. So that's what's happening with a pulsar is the angle of radiation is different from the angle of the spin of the star.
But I guess what would make the magnetic axis be different than the spin axis.
Yeah, I wish I knew. It's the same on Earth though, right. The Earth's magnetic north pole is not the same as our spin access north pole.
Right, And that's because the stuff inside of Earth is spinning in a kind of weird way, right.
Yeah, And again not something that we understand very well. And the Earth's magnetic field even flips every once in a while, as does the suns. So that's a whole murky era of research we don't understand very well. But something that is amazing, and it makes the physics that nanograb is doing possible, is that the whole thing is super duper regular. It doesn't wiggle a lot. When this thing spins, it spins at a very specific rate. And when a pulse reaches Earth, it reaches Earth very very regularly. Like, the time between the pulses is extraordinarily predictable.
I guess what makes it so predictable. I guess just because it's an object in space spinning, and so therefore it's pretty regular.
It's very dense, it's very high energy. I guess it's just not interfered with a lot. You know, in principle everything is predictable, but some things are more complicated because they are chaotic. They're like multi object systems. But here you have an intense source of read and an object that's sort of isolated. It's the left over core of this star.
Okay, so that's a pulsar, which is a kind of neutron star that has its neetic axis, ask you from its spinning axes, and so therefore we can sort of get hit by its radiation beams sometimes or regularly like a clock. These are kind of spread all over the galaxy.
Right, Yeah, that's exactly right, and they vary in their frequency. Some of them spin super duper fast. These are called like millisecond pulsars, which means that it's spinning so fast that the time between pulses is in the order of milliseconds, which means the whole neutron star spins around thousands of times per second. It's wild, it's really incredible, and by looking at the pattern of the timing, we can extract a lot of information about what's going on near that neutron star.
So you're saying the frequency sort of depends on the kind of the physics of that star, what's going on inside of it, exactly.
And you can watch one of these things for a long time and learn what its frequency is, and then if that frequency changes, if you notice, like, oh wait, there was a longer time between these last two blips, that tells you something about what's going on between you and that neutron star. If, for example, a huge gravitational wave wiggle through the galaxy, it would make some of these neutron stars further away from us and other neutron stars closer, and so it would vary the timing pattern of those neutron star pulse.
Arrivals because it's making the star further and pushing it away from us and then towards us. Or because it's sort of affecting this like the path that the light has to travel as it gets here.
What's the difference between those two.
Like you could maybe have a gravitation wave happen in the middle and it might not move necessarily the pulsar, but it might affect the photons that are on the way.
Well, this is really similar to the way we think about the expansion of space. You know, how the universe itself is expanding. Gravitational waves have that same effect. They expand or contract space and in the same same way. You can think about in two different ways, like the literal distance between us and the object is increasing, or that it's expanding the photons as they're moving between here and there. Fundamentally, those are the same picture mathematically. To me, the most intuitive way to think about it is that it's literally increasing the distance between us and the pulsar, and so it takes longer for those photons to arrive.
That's a pretty cool idea. I guess you're sort of listening to the blips coming from this pulsar, like it's going beep beep, beep, beep, beep, beep beep, and if the frequency changes like it only gets really fast and then really slow, then you know that something happened maybe to the distance between here and that pulsar, and that change in the distance could be a rotation way.
Exactly, and for an individual pulsar, lots of things could do that, Like we actually have discovered planets orbiting pulsars by how that planet has tugged on the pulsar and changed that series of blips. But what we're looking for here are correlations among many, many pulsars. So they're looking through the sky for lots and lots of these examples, and they want to see an overall effect where a bunch of pulsars are squeezed towards us and a bunch are squeezed away from us. And there's very specific predictions made by two physicists, Hellings and Downs, that predict a very particular kind of shift in the pattern of pulsars that would come from gravitational waves, and that's what nanograb and the other arrays are looking for.
MM So, I think you're saying, like, if you look out to a whole bunch of pulsars out there in the night sky and you see sort of a ripple go through all of these different frequencies of light that you're getting from these pulsars, then you know that maybe a gravitational wave kind of spread out there in space exactly.
And these could be gravitational waves from like the mergers of supermassive black holes, things that could have been washing over us basically our entire existence, but we could not detect that even logo and virgo could not see. So this is like opening a new kind of eyeball to a new kind of frequency of gravitational wave nobody's seen before, from a different kind of source that nobody's heard from before.
That's pretty cool. And so I guess how many pulsars did Nanograph look at for this latest set of results.
So Nanograph has been looking at sixty eight pulsars and studying them for fifteen years. So that's a good amount of data. And as you say, they're using existing facilities, but they still have to like occupy time on those facilities. They're not general purpose telescopes that listen to the whole sky. Got to like point the Green Bank telescope at the right part of the sky to listen to a pulsar. So it has taken a significant amount of our sort of astronomical resources that we could have spent listening to other stuff.
Oh you mean, like we don't listen to all sixty eight pulsars at the same time. We have to kind of go through them one by one mm hmm, yeah, exactly. And so you're pointing all your telescopes to the first pulsar, measuring its frequency, going to the next one, going to the next one, sixty eight of those and then you, I guess, you cycle back around and you start with the first one again, and you do that for a long time.
Yeah, you need lots of hours, and they observe each one monthly, so they know roughly the pattern of these things, and they sort of cycle through them. And these pulse arts are very faint, so you can't always hear them very well, So you need lots of hours to sort of like light these up in NANOGrav. Even though it's trying to use the whole galaxy as an observatory's only still really sensitive to the ones within a few thousand light years of Earth because the limitations just of like hearing these things, they're very faint sources.
And I think you have to do it for a long time, like you said, because these waves are so slow, right, like you said, they have a period of maybe thirty years, fifteen.
Years, thirty years, yeah, exactly, and so these are very slow moving things, and so to see slow moving things, you need data over a large period of time. It's hard to measure slow effects over a very short time period. The longer your lever arm and time, the better you're able to see effects that are very slow and.
So specifically, things that would give gravitational ways that are so big and so slow thirty year period. Those are very special events in the universe, right and just happen all the time, or maybe they do. I guess that's part of the question.
They don't just happen all the time, but they are very slow events. So when you have like two galaxies merging and then they each have like a super massive black hole that's like millions to up to ten billion solar masses, they don't merge instantaneously. They dance around each other for like, you know, sometimes twenty five million years before they actually coalesce, which means these things are generating gravitational waves for twenty five million years. And so while galaxy mergers aren't happening all the time nearby, their radiation does last for a long time.
And so just like lagoa, I guess we're looking for the moment where these black holes are right before they smush together, right like, because that's when they spin around each other super super fast and cause big waves in the gravitational fabric of space and time.
Yeah, but NANOGrav is sentenced to them well before they actually hit each other, so we can listen to almost any part of that. And the current results from NANOGrav they do see these galaxy size gravitational waves, but they can't pinpoint individual collisions. It's sort of an incoherent superposition of mergers all over the nearby part of the universe.
I imagine it's hard because these pulsars are different distances from us, right, So, like some of them are maybe one hundred thousand or I don't know, fifty thousand light years from us, and so like the data you get from them, they come from kind of different times in the universe, don't they.
Yeah, that's right. All the pulsars that we're looking at are a few thousand light years from Earth. But they definitely take this into account. But again, the events we're looking at are very very slow moving.
Meaning like maybe there's our two super massive black holes merging somewhere in the universe, but they've been doing it for you know, thousands and thousands of years, and so if you detect all your pulsars kind of wiggling at the same frequency. Maybe they come from the same event exactly.
But again, nanograph can't pinpoint individual events, not yet at least, but they see so far is totally consistent with a lot of gravitational waves adding up from lots of supermassive black holes merging.
Cool. Well, Daniel, you got to interview one of the scientists that works on this nanograph collaboration, Professor Kiaramingarelli, and so you have an interview with her.
That's right. I had a lot of fun chatting with her.
Cool. So when we come back, Daniel will interview Professor Kara Mingarelli, who's part of the nanograph collaboration, and she'll talk about some of the results and what that means for her and for the scientists that worked on it, and what that means for our knowledge of the universe. But first, let's take another quick break.
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All right, we're talking about the nanograph results unveiled this week, which are big news because I guess it. Let's us see another part of the universe we couldn't see before.
Right, that's right it. Let's us listen to things going on out there that we couldn't hear before, specifically the mergers of supermassive black holes, which are already things we don't understand very well.
Yeah, I mean I think they're tied even to like the mystery of how galaxies form, right, because we sort of are not sure about that, right.
We definitely don't understand how galaxies form and how they get these super massive black holes and why there are so many of them. The black holes at the hearts of galaxies are bigger than anybody expects, and they're everywhere, and they formed earlier in the universe. Then we understand.
Cool. And so you talked to one of those scientists that worked on this. How excited was she?
She was very excited. She had met her entire career since she was a graduate student on this project, and now she's getting the payoff.
Awesome. Well, here is Daniel's interview with Professor Kiera Mingarelli from you and the nanograph experiment.
All right, so then it's my great pleasure to welcome to our program professor Kiera Mongarelli, professor at Yale who works on NANOGrav Kierra, thank you very much for joining us today.
Thank you so much. I'm so happy to be here.
So tell me first, what is the basic idea of nanograph? How does keeping track of pulsars help you spot gravitational waves.
So Nanograph uses the galactic population of millisecond pulsars to look for gravitational waves. We can do this because millisecond pulsars are nature's best clocks. So they spin around about one hundred times a second. Their masses are maybe one and a half times the mass of the Sun, and they would fit inside Manhattan. So if you can imagine taking the Sun, shrinking it to the size of Manhattan and putting it in a blunder, that is a millisecond pulsar. And there are ticks when they arrive at the Earth. So every time they spin around, they send a flash of radio waves to the Earth. And those flashes are so precise that we can time them to hundreds of nanoseconds over a decade, so they are very very stable clocks. That point is really important because gravitational waves change the distances between objects, and so as a gravitational wave transits the galaxy, it will change the distance between the Earth and the pulsar. And so now those super stable radio flashes arrive early, and then they arrive late as the gravitational wave transits through the galaxy. So a little bit early, and then a little bit late, and then a little bit early again. And so what we look for are those changes and the arrival times of the pulses from these ultra stable pulsars. That's how we can turn the whole galaxy into a gravitational wave detector.
Awesome, Well, it sounds great in principle, but I'm sure in practice there are a lot of things to get right, what kind of like technical and data analysis challenges that you have to overcome to make this crazy idea work.
So it's taken a lot of people a lot of time to overcome all those challenges. In fact, we have some of the best people in the world working on the data analysis and the noise mode, because you're right, there's a lot of noise involved here. We can't walk over to our pulsars and turn them on and off again because we think something funny happened. Right.
One of the big challenges that.
We have is the interstellar medium, so the gas and dust between us and the pulsars. There are some maps of that that they're all very approximate, and this affects our signals because the radio wation the pulses are dispersed by the interstellar medium, and that happens at different frequencies radio frequencies, and so we have to take this signal that's then been spread out by the interstellar medium and d disperse it to make it a very clean signal again, and so that can add some noise to what we're looking for. And unfortunately that noise looks very similar to the gravitational wave signal that we're looking for. So we have to be very very careful when we're making these noise models for the pulsars that we're not going one way or the other. That is to say that we're not taking a gravitational wave signal and thinking that it's just noise in our pulsar because they look so similar, and therefore taking the signal away from any possible detection. And at the same time, we need to make sure that our signal models are good enough that we don't mistake some noise for a possible signal. And so this right now is really really important. Getting those individual noise models correct for the pulsars is really really important.
So how do you distinguish between them? How do you know you're getting the noise right.
So we can distinguish between the signal and the noise in a few ways. Number One, the gravitational wave background has some characteristic amplitude, and that's a function of the astrophysics of the source that's creating the signal. So if, for example, it's super massive black holes, and we'll probably get to that in a second, but if, for example, it's that that's creating this signal, then the amplitude of the signal is based on the astrophysics that governs their mergers. So how massive are the black holes, what gravitationally frequency are they emitting at, how far away are they how many super massive black holes are there per unit redshift? So what's the density of black holes in the universe. And we know that that amplitude has a predicted and characteristic way of varying as a function of frequency. So we have lots of different frequency detector bins in our experiment, and we know exactly how much signal should be in each one of those bins from our simulations, and we can measure what's there and the way that the signal is distributed in those bins can be characteristic of, for example, super massive black hole binaries generating the background. So that's one part of the puzzle. The second part art is something called the Hellings and Downs curve. And so back when people were thinking in the eighties about how do we actually go about detecting a gravitational wave background, the zero thworter thing to do, and I think a lot of us that have done undergraduate degrees in STEM fields have done, is a cross correlation analysis, where you say, all right, there's one signal in my data. So I'm going to take all of these different pieces of data and like bash them together and see what signal is present in all of my different pieces of data. And so that's what we do with pulsar timing. We take all of these different timestamps from pulsars and cross correlate them, and we look for the gravitational wave background signal. What's really interesting is that not only do you get this amplitude of the gravitational wave background that's correlated, but there's an additional geometric piece predicted by general relativity, and that additional cross correlation term varies as a function of angle between the pulsars, and so if pulse are close together, they have a stronger correlation, a stronger response to the gravitational background. If they're a little bit further away, it gets weaker. But then, interestingly, because gravitational waves have this quadrupolar shape, which is like a cosine sort of shape, the signal increases again right as they get further apart, And so to create a signal that looks like that, that kind of cosine style shape in addition to an amplitude, which is predicted by all of our simulations, that's impossible to fake. And that's what we've seen now in the nanograph data. So previously we'd seen evidence for this amplitude and it was so loud we where were scratching our heads for a hot second, being like can this be real or is this just noise? Right? And we had to wait until we had this distinctive Hellings in Downs curve, until we had evidence for that as well, because nothing can fake the Hellings in Downs curve the noise. You know, there could be unknown unknowns that just happened to be generating some sort of correlated noise and all of the pulsar signals. We don't know, but we do know for sure that this distinctive quadrupolar shape that's the result of this Hellings and downce curve can't be faked. So having those two things at the same time makes us very confident that what we're seeing now is evidence for the gravitational wave background.
Do you feel like you have to be extra careful about these claims in the wake of the BICEP two debacle.
Extraordinary claims require extraordinary evidence, And I feel like as a collaboration, we've done a really great.
Job at being very conservative.
When we first found the hint that what we were seeing was a gravitational wave background back in twenty twenty, we were very careful to say that this could be the first part of the signal, but we're certainly not saying that we've seen the whole signal, and this is certainly not any kind of evidence for the gravitationalive background, but heck could sure interesting. And we're going to keep timing the data and we'll let you know when something cool happens. So another part of, you know, why we're so confident that this is evidence for the gravitational wave background is that not only have we seen this signal, but so have the Europeans. So in Europe they have their own pulsar timing array, and in Australia they have a pulsar timing array and in India they have a pulsar timing array, and so far we've all been seeing consistent signals the O. There's different levels of evidence for the signal depending on the data set, but these data sets are all different, they have different systematics, they're taken with different telescopes.
We use some similar.
Analysis tools that we're now even having independent analysis tools for the pulsars, and so this makes us all very confident that what we're seeing has to be real.
So why did you choose this as your particular slice of physics to pursue. Doesn't it seem quite a risky bed for a young researcher.
Oh yes, when I was even younger, I had a lot of professors tell me why I was wasting my time looking for low frequency gravitational waves. And even you know, I've been doing this work since twenty ten, that's when I started working on pulsar timing rays and graduate school, and you know, even then working on LIGO was risky. I in fact started my life as a ligone and did a lot of work in helping to develop something called LOO, and that was terrible for me and my professor at the University of Birmingham. My supervisor, Alberto Vecchio, was like, well, maybe you want to do something with pulsar timing arrays and I was like, oh, I don't know, let's think about it. So the idea of a pulsar timing array, of having nature create a gravitational wave detector for you, if you're just clever enough to use it, really.
Blew my mind.
It was such a beautiful idea that I thought, you know, this is really the experiment for me. And I also felt like at the time that it was risky to join a collaboration that had a lot of the theoretical predictions already in place, and at the time pulsar timing arrays didn't have, you know, some of the fundamental papers that have now been written that I've helped to write, and so I felt like it was maybe less of a risk. I don't know, it depends on how you look at it. It was risky for sure to go into a field when it was very exotic, even compared to ligo, which at the time was also exotic.
Right, So it was writing.
Down the equations, trusting that the math was right, and then just kind of looking at nature hoping that the merger rates would comply, because there's really nothing you can do, Like, even if you have the perfect instrument, if super massive black holes never merge, you're not going to find a signal.
Right, So you're right. It was risky, and I.
Had several people tell me that I should absolutely not be doing this, that it should definitely not be my phobos.
But I mean it was good advice at.
The time, right that if you're a stake in your career on something that isn't a proven technology yet, it's.
A big risk.
I love the idea that we build a new kind of eyeball and we use it to look onto the universe, but we don't know what we're going to see. And in the case of Ligo and Virgo, they were lucky because there's so many more gravitational waves than people initially expected. What's the situation here. Are we surprised at the sort of amplitude of these gravitational waves? Is it more or less than we expected?
I'm very surprised at the amplitude of the gravitational wave background. It is firmly twice as loud as I thought it would be. From my own models, So that's very surprising to me and delightful.
I couldn't be happier exactly.
Thank your universe for loving super massive black holes as much as you love stellar mass black holes. But that's assuming that all of this signal is coming from these merging super massive black holes. So I guess one thing that I haven't said yet is that the signal that we found evidence for is this gravitationally background, and that comes from the cosmic merger history of supermassive black holes. So it's not one signal that we're looking for. It's this aggregate signal. It's the incoherent superposition of all of the super massive black hole mergers that have ever happened. So it's the total opposite of ligo in so many ways. It's very low frequency, so frequencies of nanohurts, and if you're not used to thinking about nanohurts, one nanohertz is like an orbital period of thirty years, So these are very slowly orbiting super massive black holes, and by super massive, I mean a billion times the massive of the Sun. Our signals are also very long lived. So one of the reasons that there is this buildup of gravitational wave signals at very low frequencies is because the black holes evolve so slowly.
Their mergers are so slow. They take about twenty.
Five million years to merge while emitting gravitational waves, and so our signals are rare in terms of rates. One super massive black hole binary system takes a whole galaxy merger to happen, right, and so that's not happening all the time everywhere you look. But the merger is so slow, and the signals are so powerful. There are a million times stronger than what you can see in LEGO. But when they build up, they create this wopping loud signal that we can look for with pulse our timing arrays.
So you say there're one million times more powerful than LEGO, that's like at their source or to that factor in also their relative distance, because they're further away also than what LEGO can see.
Right, So I mean those are slightly different questions. So when we talk about gravitational waves, our currency, the measurement that we use is the strain and so that's how much are the gravitational waves changing space time that you can think about that as a distance over distance or time over time, because we have space time, so you can pick one. So for Ligo, they can look for signals that have a strain that has a value of ten to the minus twenty one.
But what does that mean.
That's something like that's the fraction of the size of a proton over the length of their detector, and I think that's the tagline that was very successful for them.
For us, this is more like one meter per light year. But what's a light year.
I don't have a physical intuition for how long a light year is, and Americans are not good with meters, so I think that change in time over time is more intuitive. And so for us, the strain that we're looking for is ten to the minus fifteen. So that's one part in a million billion, and that's about one one hundred nanoseconds over a decade. So that's how much the gravitational waves are changing the space time fabric around us. And while one part in a million billion is incredibly small, it's still a million times louder than what LIGO is able to detect.
I see, yeah, that's the relevant comparison. Very cool, So you say that we can see this overall background hum and that we attribute it to super massive black holes, how do we know it's not also coming from like primordial gravitational waves from before the CMB, etc.
That's a fantastic question. The answer is that we are not sure. In fact, there's a whole paper about, you know, exotic physics that we can now constrain with this amplitude of the gravitational wave background that we found. So one of the things that we'll do in the future is to try to characterize the gravitational wave background and to you know, see how does the amplitud to vary as a function of frequency.
Right now, all.
Of the sources that we know of very in similar ways, right and the error bars will include all of your standard models. Right So, right now we have as our key targets. Are the prime suspects for sourcing this gravitation oid background are super massive black holes first and foremost you know only the cosmic merger history of super massive black holes of cosmological or primordial gravitational wave background, and that is due to quantum fluctuations in the early universe that were then blown up to the size of the whole universe. And then cosmic strings, which are these defects in the fabric of space time. There's matter density spaghetti in the universe that are vanishingly small that can also create gravitational waves.
Now, those three sources.
Might all be contributing to this signal and might help to explain why it's so loud, but it's not necessary to include them. But at the same time, right now, all of the predictions for how the amplitude of the signal varies as a function of frequency, you know, they're all about minus one. They all have like this almost minus one slope, and that means that we won't be able to know for another five years or so as to exactly what's happening and what's sourcing this background, or at least we can say what is the primary source of this background. And that's not even to get into the fact that it's still possible that there might be some noise that's leaking through the pulsars that's masquerading as some background amplitude when it's really noise. We know for sure that there is a background because we can see the hellings and downs curve and nothing else can fake that, So we know that it's not all noise. But we now need to figure out what's sourcing the signal and create you know, better custom noise models for our pulsars, to make sure that we completely the best of our ability understand how the astrophysics of the signals propagating from the pulsar to the Earth will affect things.
So then what are the future prospects for NANOGrav you talked about the next five years. Is that gathering more pulsars or more analysis of the data, or combining with the other pulsar timing arrays on Earth.
It's all of those things. It's all of those things. So the signal to.
Noise that we measure with our experiment increases as the number of pulsars that we add and as the square root of the time. So it's really important to add more pulsars. One way of adding these new pulsars is to go out and search for more pulsars. That's one thing that you can do, and that's one thing we should absolutely be doing. The more immediate thing that we can do is to collaborate with our colleagues and Europe and Australia and India and China and South Africa and share our data and make these huge mega combined data sets that will give us immediately access to the southern hemisphere to all of those pulsars. And when we combine our data streams from all the individual pulsars, we'll get denser data sets, and that can make us much more sensitive to individual inspiraling super massive black hole binaries. So now that we have evidence for the background, which, to be honest, is a foreground. It's what we were looking for. It's not a nuisance that was the thing. Now it will be a background. Now it'll be a source of noise that we're trying to get rid of. And so what's underneath right, well, there'll be some anisotropy and the gravitational wave background similar to the cosmic microwave background. We'll be able to make maps of the gravitational wave universe. And what I'm really excited for is when we find some sort of gravitational wave hotspot on one of those maps and there's no galaxy, right Like, what happens if there's gravitational waves that are coming from a place where there's no known galaxies, That's when I think things just start to get really interesting. So stuff like that could be right around the corner and that's super exciting. We can also do tests of general relativity, so general relativity predicts two gravitational wave polarizations, so just like light, you know, we have plus and cross polarizations with gravitational waves, but extensions to general relativity predict even more polarizations, like for example, there could be a breathing mode. So instead of gravitational waves stretching and squashing the fabric of space time like a plus or across, it breathes, so all of the space time, you know, goes out like you're taking a big breath, and then collapses in on itself like a balloon.
Getting bigger and then getting smaller again.
Great, well, this is very exciting. Congratulations and thanks very much for taking some time to chatt with me today.
Thank you very much.
I just want to really emphasize that NANOGrav is an experiment that's been taking da for fifteen years and it's taken a team of one hundred astrophysicists to get this result. So it's a huge, huge experiment, taken a lot of time and a lot of money, and it's just part of this global effort to detect gravitational wave. So I really want to give a shout out to my colleagues all over the world. I started my career in Europe, right so I was part of the European Pulsar Timing Array for many years. And I've written papers with the Parks pulsar timing Array in Australia, and you know, of course many papers with NANOGrav here in the US and in Canada. So I just want to mention that this is a huge experiment that's taken decades to get to where it's at right now.
Absolutely, and I love when scientists from all over the world can come together to make a project bigger than anyone scientist. Absolutely. Well, congratulations again and thanks for joining us.
Thank you so much.
Awesome, pretty exciting, it must be I when I've been experiment your work done for so long it pays off and potentially changes how we see the universe.
Yeah. I love the bit where she says that people warned her not to work on this experiment because it was such a long shot.
You would do you wish you had maybe listened to that advice or somebody had given you that advice.
No, I'm pretty happy with where I ended up. But I'm just glad that somebody's out there swinging for the fences and trying to discover crazy things about the universe. One of my favorite things about this discovery is that it was a surprise. You know, that they're seeing super massive black hole gravitational waves at like twice the amplitude that they expected. The universe is louder in super massive black hole gravitational waves than we thought.
Yeah, sort of the same thing happened with Ligo, right, Like, there are more of these events, or at least we could listen to more of these events than we thought was possible or actually happening.
Yeah, we have an episode coming up about how surprising it was that Lego saw so many black hole mergers when they did, And in this case also we were sort of lucky the universe was to our new kind of ear. Then we even dared hope.
The universe is out in the field screaming with joy and gravity.
And finally we're able to listen.
All right, Well, congrats to the scientists at NANOGrav and congrats I guess to all of us. Right, it's humanity opening up a new eyeball into the universe, that's right, and hearing new kinds of physics going on out there.
Hopefully in the future, as NANOGrav and their international partners stitch together their data into a massive data set and collect more pulsar as we can learn even more things about gravitational waves from the early universe and maybe even figure out how this whole crazy universe came to be.
And why it's in the middle of the field screwing for joy or not, there might be a different experiment. All right, Well, we hope you enjoyed that. Thanks for joining us, See you next time.
Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a product of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases, many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as Dairy dot COM's Last Sustainability to learn more.
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