Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - 212 | Chiara Mingarelli on Searching for Black Holes with Pulsars

Episode Date: September 26, 2022

The detection of gravitational waves from inspiraling black holes by the LIGO and Virgo collaborations was rightly celebrated as a landmark achievement in physics and astronomy. But ultra-precise ground-based observatories aren't the only way to detect gravitational waves; we can also search for their imprints on the timing of signals from pulsars scattered throughout our galaxy. Chiara Mingarelli is a member of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration, which uses pulsar timing to study the universe using gravitational waves. Support Mindscape on Patreon. Chiara Mingarelli received her Ph.D. in physics from the University of Birmingham. She is currently an assistant professor of physics at the University of Connecticut and a research scientist at the Flatiron Institute Center for Computational Astrophysics. Her Ph.D. thesis was selected by Springer Nature as an Outstanding PhD thesis, and she was selected as a "Voice of the Future" by the Royal Astronomical Society. She regularly contributes to science communication, including Amy Poehler's Smart Girls and the Science Channel's "How the Universe Works." Web site Simons Foundation web page Google Scholar publications Wikipedia Twitter

Hello, everyone. Welcome to the Mindscape Podcast. I'm your host, Sean Carroll. So we've just been through an eventful launch week for the book version of the biggest ideas in the universe, volume one, on space, time, and motion. That was fun. Gave a bunch of talks. And we had the podcast last week, the solo podcast, which highlighted one of the ideas from the biggest ideas, which was Einstein's equation for general relativity, the equation relating space time and how it curves to matter and energy, things like. that. And the payoff of that equation is that you discover the existence of black holes. Basically, neither Einstein nor even Schwarzschild, who went off and solved Einstein's equation immediately after Einstein came up with it. Neither one of them knew that they were predicting black holes. They went to their graves, as it were, not knowing that black holes were predicted by general relativity. Of course, things changed. You know, Einstein died in the 1950s, and in the late 50s and 60s, scientists really began to understand. what black holes are. These days, as it turns out, we observe them. Not directly, of course,

they're black. We can't actually see them give off radiation. But we absolutely know they're there because we can see what effects they have on the universe around them. We can image the matter giving off radiation near the center of our galaxy and other galaxies. We can get gravitational waves from two different black holes in spiraling toward each other. And of course, there's been indirect evidence for a long time from x-rays and quasars and things like that. So basically, we're moving into an era of black hole astronomy, where we don't just think about black holes, but we observe them using many different techniques and use what we learn from those observations to better understand the whole evolution of the universe. With that in mind, we're very happy today

to have Kiara Mingarelli on the podcast. Kiarra is a astrophysicist, physicist slash astronomer, I guess, who thinks about black holes and how to detect them. Now, the great thing is that the way that she specializes in detecting black holes is not one of the usual ways. Kiar is an expert in what are called pulsar timing arrays. And this is just a fun idea. It's one you'd be rooting for to work out, even if you didn't know anything about how sensitive and important it's going to be. Pulsar timing arrays basically come from the fact that black holes and other things, by the way, emit gravitational waves.

So it's really what we're looking for is not the black holes directly, but gravitational waves emitted by black holes and maybe some sort of background other gravitational waves from other sources, but black holes are probably

black holes doing things, spiraling in, you know, swallowing up matter. Those are the biggest sources of gravitational waves out there. And what happens is these gravitational waves pass by pulsars,

which are very tiny neutron stars rapidly spinning, and these pulsars turn out. to be really, really good clocks. They emit their beams of light in very, very regular pulses. So if you have a big, long gravitational wave that passes by all the pulsars in our galaxy that we're monitoring, it will slightly distort the timing of those signals that we get from

the pulsars. And you can figure out what kind of gravitational wave it is. So basically, you're using a bunch of stars scattered through the galaxy as a gravitational wave detector, which is not only a surprisingly good way to detect gravitational waves, it's a completely different wavelength range than we can look at here on Earth. So it's a different kind of physics underlying what we will ultimately see. We don't know, as we'll learn in Kiara's podcast, we don't actually have a claimed detection that we know for sure that the pulsars have seen gravitational waves, but we seem to be

very, very close. It's very, very tantalizing. So we're going to learn something about it, hopefully, in the near future. And we get to talk about, you know, black holes more generally. Could they be the dark matter? What did LIGO find? What does it all mean? Once again, a reminder that, unlike myself, who is a theoretical astrophysicist and likes to write down equations,

most physicists out there are actually looking at data, collecting information. Kiarra, by the way, is also a theoretical physicist, but she works very, very closely in the team that is looking at the data from pulsar timing. And so it's a real sort of honest combination of theoretical work and good old observational work. That's how we learn about the universe. So that's why you're at the right place. Let's go. Yarramingerli, welcome to the Mindscape Podcast.

Thank you. It's great to be here. We've talked about black holes before in the podcast and even gravitational waves. But there's never enough talk about black holes. I agree. I always like to ask the black hole-oriented guests, you know, how do you define? Black hole. What do you think about? It'll be different for a quantum gravity person than an astronomer, I presume. Yeah. So that's a great question. When I think about black holes, I think about water coming out of a water fountain. And I think about the water going up and then falling back in itself. And that a black hole is going to be some sort of ultra compact object, although formally and mathematically it's a singularity, this point of infinite curvature of space time, whatever that means.

I, you know, in my mind's eye and in my heart, I feel like a black hole is actually a thing. It's probably a very small thing. Wait, so what is the analogy? Why is it a fountain? So it's like the light coming out of the black hole, whatever that is. Like if you look inside the event horizon of the black hole, if you could imagine being inside on the other side, you would probably see light coming out and being processed around the singularity, like water coming out of a water fountain going up and then falling back down. Because the water itself can't ever escape.

So it might make a spray. It might kind of get close and then start doing weird and wonderful wiggles. But it's never going to just take off and go away. It's going to be like water going up and then coming back down and on itself. So that's the view from inside the event horizon. We're not going to get because we're astronomers now today. For today's episode, we're looking at the outside of the black hole, right? Yes.

And, you know, we've come a long way. I don't know how has our thought about black holes empirically, like in the universe, change since you started thinking about these things professionally. Right. Let's see. So when I was starting to think about black holes when I was a kid, right? I would save my babysitting money to buy Scientific American magazines. And I would take all the glossy photos and put them on my wall next to Jonathan Taylor Thomas and Jonathan Brandis.

Classic. You didn't. Well, my friends didn't. Yeah, I had a few friends. And so the black holes were just kind of like this, they were considered very theoretical, right? There was sort of evidence from Cygnus X-1. There was, you know, x-rays that were coming out of this compact source, and it could have been a block hole accreting something. So gaining material from a star around it and then, like, that material getting hot and ionized gas coming off of it,

emitting x-rays. Like maybe that's what was going on. but it was still very kind of fringe to talk about black holes. And I feel like today it's very concrete, if you want to say. So now we have evidence of black holes merging. We have waveforms from ripples in the fabric of space time that they make, which is incredible. We have images of supermassive black holes of two of them that have been directly imaged, which is absolutely amazing.

So I feel like black holes have gone from something that's very, very almost science fiction-y to something that's very hard science. And just to be clear, because I think that a lot of us are a little too quick when we talk about imaging or observing black holes. We're never seeing light coming from the black hole, right? We're seeing light coming from things around it and we're interpreting it. That's right. Absolutely. So we see like normally coming from an accretion disk around the black hole, so a material that's, you know, kind of in an orbit around the black hole with it. And sometimes it feeds the black hole and sometimes it doesn't and sometimes that material can be part of jets

that the black hole can make, can launch the jets and some people say that those come from the black hole. But you're absolutely right, Sean, that a lot of people get confused by the terminology and think that the light is coming out of the black hole. It is not. It's close to the black hole, but it's not coming out. People have probably heard of the idea of hawking radiation, but nothing that you're doing has anything to do with hawking radiation. We're never going to see hawking radiation from the black holes that you care about. No, that's right. Yeah, hawking radiation is not something that we care about right now.

Maybe, you know, when Boltzman brains start waking up and looking around, there might be some evaporating super massive black holes. Did you see that episode of Star Trek? The most recent episode of Star Trek, Strange New Worlds, had a Boltzman brain. You must have been delighted. No, they got it completely wrong. I mean, it's a great show. I love the show. But this was like a godlike Boltzman brain.

That's really just not probably. It could be. I don't know. Anything is possible. Maybe if you wait long enough. I don't think that was the simplest conclusion for them to leap to that it was a Boltzman brain. But anyway, so the people were weirdly for a long time sort of in the 20th century skeptical that there were black holes. So I think that I, my childhood, cutting out pictures of black holes phase was in the 70s.

And we already knew about Stativeness X-1. Yeah. This was the famous one. but we didn't even know for sure that that was a black hole and people were really, I guess they were properly cautious, but it was weird because they weren't sure whether they could be made. And in fact, it's just not that hard. I mean, nature wants to make black holes. Is that safe to say? Yes, I think that nature makes black holes in lots of different ways, right?

And, you know, if we think about the history of black holes, how there are these, you know, I don't know what to call them in layman's terms, but if you just think of them as like, these, you know, weird singular points in Einstein's equation. So they're the points where the equations can blow up. And so no one thought it was actually real, that maybe it's just some sort of artifact, like maybe we didn't write things down carefully enough. Or maybe we made an assumption that we shouldn't have. And, you know, is it really possible to have something that's like a black hole? And so I understand what people were skeptical because, you know, as scientists, sometimes we

make a lot of approximations. And sometimes it's fine. And sometimes it's not fine. And so I understand people being careful. But then going back to these, you know, Einstein equations, what boggles my mind right now is that if one of the solutions allows black holes, that means that the other one would allow white holes. That's true. And what does that mean? How can we believe?

We can talk about that, but that would be a very different episode than what we have coming up. This is, yeah, so this boggles my mind right now. Like, how can I believe one? And then the other one doesn't make any sense. Do you see arrow of time? That's the answer. But you should interview me for your podcast. Did you know a little bit about that, Sean?

Did you write a book on that, Sean? But you've mentioned that there's different populations of black holes, right? So there are ways that nature makes them, but they're a different way. So like what we have, again, a lot of recent new data and discoveries, but before that, what was our expectation for what the populations of black holes would be like? Right. So that's a fully loaded question. So maybe I'll start with how nature makes black holes and how nature seems to

seems to want to make black holes. So if we start with the very small end, potentially primordial black holes, so there were potentially at the beginning of the universe, these small fluctuations. And some of them could have been dense enough to create baby black holes. And so those were never stars. Like there might not have ever even been any matter or barons that went into creating those black holes, that it's just a kind of blemish in the curvature of space time. Like that kind of black hole is all curvature, which is so strange to think about, but it's entirely

possible. Then there's the black hole. And also, by the way, zero evidence that that actually happens. Yes. But it's something we can think about. Exactly. And they could even maybe be the dark matter. Who knows?

Yeah. Well, exactly. It could possibly be because you could possibly get some of them that are as massive as the LIGO sources. Right. And so the first detection from LIGO was to roughly 30 solar mass black holes. And some people think that those could be. be primordial black holes. Those could have come from the early universe. And maybe those are also

dark matter. Like maybe that's the missing matter in the universe. Who knows? Right now it feels like that parameter space is being squeezed pretty hard, that it's very unlikely that that's the answer. But it's curious. Well, black holes exist and they're black. So that's good. But it is hard to make the right number of them to be the dark matter, is my impression. Exactly. And there's a lot of things that you would have to discount like the lack of lensing events. Oh, okay. Sorry. Yeah. Say more about that. Well, that, um, you know, if, if you have black holes, you can have light that's behind them that gets lensed when they're traveling on the way to Earth. And if you were to have

so many black holes that they were the dark matter, they would create a lot of these lensing events. And there's no evidence for this at all. So I feel like it's being squeezed in a lot of different ways that there's a lack of evidence and a lot of different fronts for, these black holes to be the dark matter. So there still could be primordial black holes, but maybe not enough to be the dark matter. Yeah, or maybe not enough that are in that mass range. Okay. Right.

It's possible that, I mean, there's a lot of theories about different masses and black holes that you can make depending on the conditions that you had in the early universe and what you believe. But right now, the LIGO mass black holes, anything from like 10 to 100 solar masses, that's really hard right now to get those to explain dark matter. Okay. So how do you make them? So they come from the collapse of stars. Very massive stars at the end of their lives will undergo a gravitational collapse. And the remnant will be either a neutron star or a black hole. So the neutron star is kind of a halfway phase. It's a halt that happens when you have the electrons and the protons that come together and make a neutron. But there's not enough pressure to make the neutrons to continue to collapse. There's a, neutron degeneracy pressure.

That's okay. You can use those words and we can assume people know what it means. But anyway, neutrons. Basically, the neutrons, you need a lot of pressure to get them to, you know, continue to collapsing into a black hole to make whatever material is at the center, whatever quantum description you have of that, whatever your equation of state of that fluid or material or whatever quark gluon plasma you think makes up the central object inside a black.

hole, it takes a lot of pressure to get the neutrons to make, to turn into that kind of material. So some of them just stop there. And they're about one and a half to two times the mass of the sun. But if you can keep going, then you create black holes. And then the cool thing is that from black holes that are, you know, one or 10 or 100 solar masses, they can merge. And the final mass is the sum of the two black holes minus 5% for gravitational waves.

Okay. So we're making, if the thing that is collapsing weighs roughly less than the sun, it'll be a white dwarf or a neutron star. If it's enough bigger, it'll make a black hole. Yeah. Okay. So we expect to have a bunch of black holes that are more than one solar mass. Yes. And then if they merge, they can get up there. Exactly. But the fun thing, I think, is that there hasn't been enough time in the history of the universe to merge all the stellar mass black holes that are roughly the size of the. the mass of the sun to make a supermassive black hole. Okay, what are those? So supermassive black holes are around 100,000 to a million and potentially up to 10 billion times the mass of the sun.

They are the biggest black holes in the universe. No one knows how those black holes were made. There's, of course, a bunch of different formation channels that you can imagine. One is that you had these huge gas clouds in the beginning of the universe. It just directly collapsed into a black hole. Okay. But that's hard because it means that none of that. gas was heated, none of it fragmented to form stars, that it just kind of went and then created

a supermassive black hole and there you go. So that's kind of mind boggling. There's an intermediate kind of theory where you have the gas cloud and then it collapse and it makes these huge stars that live fast and die young and they make kind of intermediate mass black hole. So maybe 10,000 solar masses, a thousand solar masses. And those all quickly merge to make a supermass. massive black hole. How would we know? Is this something we're trying to discover with telescopes? I'm so glad you asked that. Yes. So one of the ways that we can find out what the, what we call them seeds, supermassive black hole seeds are, is by looking at gravitational wave signatures from the early universe. Because if you do have all of these merging intermediate

mass black holes that are, you know, building up to create a supermassive black hole, each merger will emit a gravitational wave signature. And so the laser interferometer space antenna, or Lisa, between friends, is going to launch in 2034. And that is going to be a huge ligotype instrument in space. And that will be able to detect those kinds of gravitational waves. Okay. Okay. We will be able to, all right, that's, this gets in, it's interesting how many things come together at once, right?

You need to talk about the astronomy of making these things and the physics of detecting them and so forth. But maybe tell me just a little bit more about the nature of these supermassive black holes because they're not rare, right? There's a lot of them. So there's at least one supermassive black hole in the center of every massive galaxy. And my own research is studying supermassive blockhole mergers. So when galaxies merge and we have lots of snapshots of merging galaxies, in fact, the JWST image that came out earlier this week had Stefan's Quintet. Stephen's Quintet.

It was breathtaking, five galaxies, yeah. Five galaxies getting close and two that were actually merging. So we know that galaxies merge, and it's also how we just think the universe works. There's this hierarchy and, you know, galaxies get bigger by merging with other galaxies. And so if that's true, then their central supermassive black holes should also merge. and those create the strongest gravitational waves in the universe. In fact, they're about a million times stronger than the ones that have already been detected at high frequencies. So maybe to explain this a little bit because the black holes, let me ask it this way.

Yeah. We use the words supermassive black holes and they're the center of galaxies. And I bet that in a lot of people's minds, the black holes are holding the galaxies together. Right. But they're not. They are a significant fraction of the, well, significant maybe one. 1% around 1% of the mass of the galaxy. But, you know, it's interesting that you say that.

It's actually also an open question. How did the supermassive black holes get to the centers of galaxies? Was it that there was a galaxy that formed a supermassive black hole formed somewhere else? And they found each other in the early universe. Is that how they were seated, we say? Where they formed in situ? That seems really hard to do. So that's another open problem.

How big is a supermassive black hole? Like how many light years is the black hole at the center of our own galaxy? Do you know? So let's see. So we have a fun trick. The relativists' units is to use seconds for everything, which is light seconds. So one solar mass is 4.9 times 10 to the minus six seconds. So that's how long it would take light to traverse the sun.

And so in the center of our galaxy, we have something that's about four million solar masses. So it would take maybe 10 seconds at most for light to get across the center. 10 light seconds. Yeah, exactly. If you're a photon, it'll take you less than that. So four times, yeah. But why I'm asking is that's very tiny compared to the size of a galaxy, right?

I mean, even if it's 1% of the mass or less, it's much, much, much less than 1% of the size because black holes are very massive. So how do the two supermassive black holes find each other when two galaxies merge? Why do black holes merge at all? Right. So that's a great question. And it's also an active area of research. There's very little known about the lives of supermassive black holes, mostly because it takes so long for anything to happen on cosmological time scales. So I've done a few calculations which show that supermassive black holes will merge in something.

like two or three billion years. But that's a sizable fraction of the age of the universe, which is about, you know, 13 to 14 billion years old. So what happens, we think, is that your two galaxies interact gravitationally, their galaxies start to merge. And then it takes a while, but the block holes are eventually slowed down in the merger process by interacting gravitationally with gas and stars. And so the technical term for the,

for experts that might be listening is dynamical friction. And the black holes will then settle in the gravitational center of this newly formed galaxy. But unless they're interacted upon by other forces, they can stay there forever, basically in a stable orbit. It'll take many times the age of the universe for these supermassive black holes to merge by only emitting gravitational waves. And so they can get to within about a light year, separate. but they will not merge unless something else acts on them.

I see. So it's easy. Let me just repeat it to see if I got it right. So it's easy to see why black holes would sort of sink toward the neighborhood of the center because there's friction, right? Exactly. But because they are so tiny, astrophysically speaking, they have to get really close and we don't know how they do that?

Well, we have a few ideas on how they do that. And so this is called the final parsec problem for anyone. He wants to read about it. And so the black holes, the solution to this merger problem is that you realize that the black holes are not alone, right? That there is gas and there are still stars. And so if you have some stars that are crossing the orbit of the supermassive black hole, you know, pair, every time that a star interacts with those two black holes, it'll carry away some energy and angular momentum. And so every time a star gets slingshot out and interacts with the black holes orbit in that way, you get a little bit less mass. And so energy that's in the system and it slingshots it out. If you have this happen enough times, then you can get the black holes close enough, such that they merge within the age of the universe. You can also have a gas disk that develops around the two black holes, and the gas can torque the black holes and make them merge in that sense. In nature, it's probably a combination of the two.

Yeah, okay. Yeah. But to add a fun, you know, breaking news headline to this, some theorists have found in large hydrodynamical simulations that the gas disks can apply positive torques, which means that the black holes get further away from each other instead of negative torches, which make them merge. And apparently it really depends on the properties of the gas disk around them. So we think that for realistic disks, they probably merge. But you can make them not merge on a supercomputer. So it's all of these different competing effects.

But if you can get the black holes to within a thousandth of a light year, then they do merge by emitting gravitational waves quite rapidly. So 25 million years with respect to the 2 billion years that it took them to get to the center of the galaxy. So really the last part is just noise. Well, this is really fun because it is a glimpse into, you know, where the frontier of astrophysics is these days, right? Like, we know these supermassive black holes are there. We don't know exactly why, but we're also not just stuck speculating, right? We have some combinations of simulations and telescope measurements that will help us figure this out.

That's right. That's right. And we, if we find gravitational waves from supermassive black holes, then we know. know for sure that they've overcome this final parsec problem. And then the question becomes, well, how did they do that? I see, because we don't know, I mean, in some sense, they did overcome the problem because they exist, right? The supermassive black holes exist. But we don't know whether they were made directly or they were assembled gradually and all these things. Gravitational ways will help us sort out. Yeah. So as you're saying, it'll help us sort out the formation scenario of the supermassive black holes, but even today, like if you had a merging pair of supermassive black holes, you'd know that

they had to overcome this final parsec problem that comes from galaxy mergers. So the first gravitation wave story was about the formation of supermassive black holes. And then the second story is now it's much later in the history of the universe. Now there's black holes in the centers of galaxies. The galaxies are merging. What do the supermassive black holes do? And it's an interesting reminder that the universe is still kind of young. It's like still evolving, right?

When that picture came out of Stefan's Quintent, it's five galaxies interacting with each other. I think that probably people see pictures of galaxies and figure that's more or less a steady state kind of configuration. But it's really not. These galaxies are moving and bumping into each other and tearing each other apart. Absolutely, yeah. And their black holes are merging. Hopefully there's stars being slingshot around. There's gas being funneled to the center.

there's everything is very dynamic, but the time scale is not a human time scale. And so we see it as being static, basically, right? But if you just hit fast forward, you'll see, you know, really beautiful physics happening. And that's some of the power of these supercomputer simulations that you can speed up mergers and then actually try to get snapshots of galaxies that you see today in different parts of the merger process to be like, does this fit? Like, have I seen this part of the merger in space? And then you kind of look at pictures of space and you're like, oh, yeah, there's that galaxy there.

So you're using pictures of different galaxies at different stages of their life as a proxy for the trajectory or history of a single thing. Exactly. Because that's all that we've got, right? Yeah. Yeah. We're not going to wait around for a billion years to watch what happens, right? No, no, no. So we kind of knew or we had strong feelings that these supermassive black holes existed long before any of this gravitational wave stuff came along.

That's right. Yeah. And is that I honestly don't know the answer to this. I presume that's because we knew that there were quasars and things like that, and we're just trying to explain that. So that's part of the picture, absolutely, but there's also the center of the Milky Way, right? And so there, Andrea Gess and her group at UCLA have very famously measured the mass of the black hole at the center of the Milky Way.

And that's Sagittarius A-Star, which was recently imaged with the Event Horizon telescope. And so by watching stars orbit around this central compact object, without giving it a name, they could figure out what the mass was just by doing some very simple Kepler's laws calculations. So if you know the mass of the star and then you know roughly what its orbit is and you can watch several orbital periods, you can get a really good handle on what the mass of the central object it's orbiting is. And we don't see a lot of photons coming from the center of our galaxy, right? It's a pretty quiet black hole. Right now it's a pretty quiet black hole. There is evidence, though, that at one point in its history had some jets.

There's some gas that people have been able to see, which would indicate that at one point there were jets coming from Sagittarius A-Star. But this is very speculative, right? We can only say, like, this is consistent with the existence of jets at some point in the past, but you can't rewind the universe to check. But we do see that distant galaxies are often, like, very bright. That's what a quasar is, right? It's a tiny speck in space that is giving off way too much light.

And eventually we realized it was sort of a jet being beamed right toward us from a black hole. That's right. From a supermassive black hole. A super massive black hole. That's right. Yeah. And so those were all over the place in the earlier universe and now we're entering our adulthood and we don't have as many quasars. That's right.

Yeah. Yeah. And it does, yeah, yeah, the universe is changing a little bit. And so it's at least a consistent story that our galaxy, used to have a quasar. Yeah, that's right. And also, if you think about it, in the early universe, there was a lot more gas. And today, there's a lot more stars, right?

That gas has become stars. And so even if you want to harken back to the final parsec problem, it's possible that earlier on in the universe, it was solved through gas interactions, through these torques. And that today, for nearby emerging supermassive black holes, it's mainly stars. This makes me ask a question because I know we're going to get this question. What about the dark matter? here because you're a grown-up astronomer, you know, there's dark matter in the universe more than there is ordinary matter. Does that play any role in making black holes? Is it,

or is it just irrelevant? It's a tough question. There are different kinds of dark matter. There's not just one kind of dark matter. So one kind of dark matter that I think is very popular right now because just like everything, there's different fashions and trends in theoretical physics and astronomy. But there's something called super radiance. Okay. And so what happens is that there's these particles which are created around the supermassive black holes.

And the creation of this particular kind of dark matter-like particle, this axion, spins down the supermassive black hole. And so if you watch one long enough, you could actually see it spin down. So sorry, just to get this straight, this is because Axiom's, The axiom field exists in the universe, or is this literally because there are dark matter axions, if there are at all? Yeah. Axions are still hypothetical. Yeah.

Is it a bunch of, like a cloud of axions near the black hole or helping it radiate? Yes. Okay. Yeah, there's a cloud of axions that are around the supermassive black hole. In fact, it happens around stellar mass black holes. It's just these clouds of axions can exist around black holes. And they can, in fact, I believe, emigms. gravitational waves as well, kind of like a large gravitational atom, where to go from one state

to the other, they emit gravitational waves, if you can imagine that kind of funky gravitational atom system. I can imagine it, but I'm certainly not an expert. But let's be kind to our listeners and explain a little bit about gravitational waves. Like we've been using the terminology, and I'm sure that they've heard the terminology before, but let's try to explain what a gravitational wave is. When I say, let's, I mean, could you do that, please? Of course.

So gravitational waves are ripples in the fabric of space time that travel at the speed of light. And gravitational waves change the distances between objects. So you and I are sitting. A little bit. A very little bit. A very little bit. So, you know, by the fraction of the size of a proton for a LIGO style gravitational wave. So you and I are sitting at opposite ends of the room, for example.

So we would still be standing in place, but we would get closer together and then further away, and then closer together and then further away without actually moving, because it's the space time between us that's changing. And so with LIGO, the LIGO gravitational wave detector can detect gravitational waves that are at hundreds of hertz. And so if you could... Hertz per second. Yeah.

Yeah. That's right. It's got to be, you know, I'm sure most people know what a hurts is, but just check. Yes, we have a very educated listening population. And so you could actually hear those if your ears could hear gravitational waves. They would go when they emerge and make that chirping sound. And so it makes sense to think about the change in distance over distance when you're thinking about those kinds of gravitational waves. And that's really how we think about the strength of a gravitational wave.

The technical term is the strain, but it's just how strong that gravitational wave is. How much does it distort the fabric of space time? So you can think about a change in distance over distance, and for LIGO, this is, you know, the fraction of a size of a proton over a few miles. And the miles are the distance between that the sort of LIGO laser is moving. Exactly. Okay. And so that's the strain. But if you think about...

So sorry, wait, let me just make sure I understand this. because the point is that there's this uniform stretching of space, almost uniform, but what that means is the further away a laser moves before it bounces back, the more the distortion of space is. And the invariant thing is the distortion divided by the distance. Exactly. That's distance divided by distance. That's what you mean by that.

Exactly. Yeah. So it's that change in distance over distance is the strain. Got it. And that is something like the fraction of a size of a proton. that the gravitational wave changed over a few miles. Right. Okay.

Not very much. Crazy small. Exactly. They should give a Nobel Prize to the people who did that. They should. Absolutely. Absolutely. If we were time travelers, we could go back to 2017 and make sure that that happened.

And so LIGO is the famous experiment that did win the Nobel Prize. That's right. That's right. And so changing the distance, you know, thinking about distance changes makes a lot. lot of sense for LIGO in that sense, but there's other gravitational wave detectors. And the one that I work on is called a pulsar timing array. But it's the same idea. You look for these space time distortions. But with a pulsar timing array, what you do is that you look at a series of pulsars. So a pulsar is a neutron star that we talked about earlier. But now its spin axis is

misaligned with its magnetic field line. So every time it spins around, and it sends a flash of radio waves to the earth, like a lighthouse. You get these really stable flashes. So we know exactly when those flashes should arrive. So stable just means it's a good clock. It's an almost perfect clock. Before 2012, they were better than atomic clocks. Okay.

Pulsars. Pulsars. Amazing. For the experts, it's a millisecond pulsar. But if you're not an expert, pulsar is fine. Just to be safe. So is millisecond a short period of time for a pulsar or long period? It's very short.

Yeah, it means that it spins around about 100 times a second. Okay. And just to blow your mind a little bit more, these millisecond pulsars are about one and a half times the mass of the sun. And they spin around 100 times a second and they would fit into the island of Manhattan. It would be a bad idea, though. You don't want that to happen. Absolutely not. But a few miles across.

They are very, exactly. They're very small. And 100 times a second. That's right. Whenever I like it because whenever we say they're very small, you know, compared to what? They're much smaller than the Earth, but they're more massive than the sun. Exactly, exactly. But they are smaller than Manhattan.

But the fact that they spin around 100 times a second is impressive. Right. And they don't fly apart because gravity is so strong. Right. That's right. How many of these do we know about? Oh, I mean, there are thousands of pulsars that we know of. there's potentially tens of thousands of them in the Milky Way galaxy alone.

Currently, there's only about a hundred of these pulsars that are good enough clocks to look for gravitational waves. But I haven't yet told you how we use them as gravitational wave detectors. Thank you. Maybe I will. So, yes, the pulsars are perfect clocks, basically, for all intents and purposes. And so you measure their time of arrival of the pulse at the Earth. you know when it should arrive, you've measured when they do arrive,

and any change in when the pulse arrives with respect to when it should arrive could indicate the fact that now that pulsar is sitting on the other side of the room and it got a little bit further away from me and then it gets a little bit closer. So the pulsar time arrivals will change a little bit. They could arrive early and then they can arrive late. And so now when we're thinking about the strain again, if you're thinking about a change in distance over distance, LIGO style, this is something like 10 meters per light year. But as humans, it doesn't really mean a lot.

It doesn't really mean a lot. And so it's, in my opinion, more intuitive to think about a change in time over time. Okay. And that is something like 100 nanoseconds over a decade. Oh, okay. So small. Exactly. A tiny amount.

But that change is still. a million times stronger than the change that a stellar mass black hole merger will give you in the LIGO detectors. Okay, wait. No, I'm very confused now. Okay. Let's catch our breath. That's bad.

That's bad. So the idea is that there are all these different ways of detecting gravitational waves. Yeah. But just like a telescope that there are optical telescopes, infrared telescopes, x-ray telescopes, different wavelengths they're looking at. Likewise, the gravitational wave telescopes are only sensitive to certain wavelengths. That's right. Yes.

And LIGO is sensitive to... 10s to hundreds of hertz. Hertz. And do you know how many meters or kilometers that corresponds to? No, I don't know either. So, okay, they can look it up. It's also complicated because my experimentalist colleagues will be very proud of me of knowing that there are power recycling mirrors in the LIGO arms, which effectively makes the arms much longer than they actually.

are, which enables you to detect stronger signals from these black hole mergers. Right. Okay, good. So just to translate that, if detected, I understand it. The actual LIGO arms are about four kilometers in a part, or at least, you know, in length. But what you're saying is that they can detect gravitational waves with wavelengths longer than that because they keep bouncing back and forth over and over again. Potentially.

Yeah. Yeah. Okay. Good. So what is relevant, I think you're right. What is relevant is the frequency in hertz, not the wavelength, really. The frequency is what we care about. Okay, so then say it again, now that I'm listening, what was the frequency for the LIGO?

For LIGO, it's tens to hundreds of hertz. Of hertz. Okay, so tens to hundreds of cycles per second. That's right. Whereas your pulsar timing arrays. Yes, are sensitive to 1 to 100 nanohertz. And a nano is... So 1 nanohertz is about...

30 years, like one over 30 years. Over 30 years. Yeah, exactly. So it would take 30 years for one full wave cycle to go by. And that roughly corresponds to the fact that we're looking to pulsars scattered throughout the galaxy. There are light years away from us. Exactly. And so there's no other way to detect that we know of right now, these very low frequency gravitational waves.

Because now, if you think about, well, for a few reasons, mainly these, these, these gravitational waves that are coming from supermassive black holes are very low frequency or have these very long wavelengths. So something on Earth could never detect a gravitational wave that has a period of decades. You just can't do it. So the LIGO detectors look at these in-spiraling black holes that were tens of solar masses. That's right. And that's just what they're sensitive to. I mean, there could be black holes out there that are single solar masses that are in spiraling, thousands of solar masses that are in spiraling and LIGO just wouldn't know. That's right. Yeah.

Okay. And so we talked a little bit about the Lisa detector earlier on when we were talking about supermassive black hole seeds, but in fact, Lisa is now sensitive to the milliehertz frequency regime, which is right in between LIGO and pulse our timing arrays. And so they would be able to detect these thousand solar mass black hole mergers, but again, like LIGO will not be able to detect that. And neither will pulse our timing arrays. But Lisa is scheduled to be launched and you said 2034 right now.

And it's never going to happen, obviously. I mean, it will happen, but it's not going to happen in 2034 because no satellite has ever launched the year they plan to launch. It's interesting that you should say that because normally I would strongly agree with you. Like vehemently, I would be in violent agreement with you. But there's some reasons that we might want to launch Lisa earlier. So number one, there's always this Pathfinder mission. And Lisa's Pathfinder mission was, it performed, you know, extraordinarily, better. It surpassed all expectations.

It was an amazing flight. So the technology is ready to go. There's an x-ray telescope called Athena, which is supposed to be launched in 2028. Okay. And this x-ray telescope would be the perfect instrument to try to follow up on supermassive black hole mergers that Lisa could see. So if they were to launch at roughly the same time or at the very least be alive in space at the same time, you would have a huge science case for, you know, looking at these electromagnetic or like light signals from merging supermassive black holes. And there might not be another opportunity to do this in the near future. Got it. So there's a strong case it's being made right now to move up the Lisa launch date so that it can coincide with Athena.

Okay. And Lisa is the set of basically lasers in space, bouncing back and forth. Lasers in space. Exactly. It's called a constellation because there's three different points and it makes a triangle. And what's cool about this is that in this triangle, you can make two independent ligol-like detectors. So you can take your equilateral triangle and make two independent right angle interferometers from it. And that means that as your triangular configuration is circling the earth and floating around, it can detect the polarization. of your gravitational wave. Okay. And so this is exciting, that only for detecting polarization information itself,

but because according to general relativity, there should only be two gravitational wave polarizations, which are plus and cross. So the plus configuration is when you and I are going, now let's just imagine a gravitational wave passing through our human body. You first are stretched up and you look like a supermodel. You're very tall, or a modern supermodel, I should say, very tall. very thin, and then you get squished down and stretched out, and then you look like a sumo wrestler.

And then you get stretched back up again. That's the plus. Exactly. That's the plus. You can rotate that by 45 degrees and you get cross. Two polarizations, right. There's two polarizations. So just like light, but the specifics of how they're polarized are different, but light is either

vertically or horizontally polarized, gravity waves plus and cross. That's the theory. That's the, yes. Well, so exactly. But there's some alternative theories of gravity. which predict more polarization. So something like a breathing mode. And that would be like a circle, kind of breathing out and then contracting back in on itself.

So imagine a sphere getting bigger and then collapsing like a long breathing. But it's this circular polarization. Well, breathe is not a circular polarization. That technically means something else. I just mean it's like a circle pattern that's breathing in and out. Well, I guess the point is that the regular gravitational waves, they stretch in one direction but also squeeze in the other. Yeah. You're saying that these fancy non-general relativity hypothetical waves stretch in every direction and then squeeze in everything.

That's right. Yeah. But they don't exist on their own. It's in addition. It's like you have plus and cross and breathing. Right. Okay. And so we got to there by saying that Lisa could potentially disentangle this.

Ligo could potentially as well. if you have enough interferometers, and if you're lucky with the orientation of your source, because you'll always have this problem where you have more sensitivity to some polarizations than others by how your source, your gravitational wave source, is facing the Earth. Right.

Because those gravitational waves and interact with your antennas, and your antennas are going to be more or less sensitive to different orientations of your gravitational wave sources. So it sounds like so far what like, So we've got to separate out what we've done from what we're hoping to do, right? Yes. LIGO has detected things. LIGO has detected things.

Absolutely. Unequivocally. Lisa has not detected anything yet because it hasn't flown yet. Exactly. And we've not yet figured out the polarization of gravitational waves. So it seems like mostly what we have is consistent with our expectations, but it's hard to do detailed tests of general relativity. If general relativity were modified a little bit, it would still be.

be consistent what we've seen so far. Yeah, this, I mean, this is a really interesting point. There's lots of different modifications that you can make to general relativity. And so so far, the polarization ones, there's no red flags, but you can also make modifications to the waveform. And those can be very subtle and can sometimes they're only detectable in the in spiral part of the gravitational wave signal. So before the whoop chirp at the end of the life, you can get small. changes that are happening in the in spiral when it's not chirping so much. But that part is really difficult to detect because it's very low frequency. And at low frequencies on Earth, you're dominated by, you know, things like clouds passing overhead of your detector.

Very sensitive detectors. Newtonian noise. You know, you have earthquakes. You have trucks driving by. You have alligators that can crash in to your arm. Because Louisiana has one of the detectors. Exactly. Yeah. So there's a lot of things that can go wrong. So right now, a lot of the very low-frequency information is lost in that noise. But future detectors, like the Einstein Telescope, which is being proposed, or, you know, cosmic. Yes, or Cosmic Explorer. These will all have very good low-frequency noise capabilities. So the plan is to bury them underground to do a better job of controlling those kinds of noise sources.

Okay. You did say earlier, speaking of predictions of general relativity, that the gravitational waves move at the speed of light. Yes. How do we know that? Well, if there's any justice in the universe, they should have. But there might not be. Have you been in the universe lately? There's not a lot of justice.

Way to keep it real, Sean. Way to keep it real. You are right. So there's only recently been verification of this prediction that gravity travels at the speed of light by a binary neutron star merger that was seen. So not a black hole. It was not a black hole. It was two neutron stars that were merging with each other.

And we saw the light and the gravitational wave signal from that system. And the light arrived two times into the minus 15 seconds after the gravitational wave signal. So that's two parts and a million billion. So we know that it's pretty good. It's pretty much the same time. So in the black holes merge, we don't see anything with electromagnetic waves. Of course, everyone has a theory where you could possibly see something. But in very straight GR, there's no expectation of seeing an electromagnetic counterpart.

In fact, it's one of the things when I first started studying gravitational waves, it really blew my mind is that gravitational waves, it's another spectrum. It has nothing to do with light. And it's, you know, people also call it gravitational radiation. That's another word that's used synonymously. That took me a long time to understand as well. Gravitational radiation is gravitational waves. But you can, it's much easier to think about electromagnetic radiation and gravitational radiation. They each have their own spectrum.

They, but they're different. They're intrinsically different. Like you can have one source, like a lightball. but that's emitting multiple frequencies, right? Multiple different wavelengths. You can look at it with an infrared camera. You can look at it with optical, it with your eyes. But with a gravitational wave source,

it's really going to be restricted to its own part of the gravitational wave spectrum. You're not going to have two merging black holes of any mass that are going to give you simultaneously different gravitational wave signals. It's just a continuous way of generating these gravitational wave signals. And in part, that's just because gravity is a much dumber, force than electromagnetism. I mean, there's not positive and negative gravitational charges. It's just lumps of matter and energy, and they're doing something at a certain frequency,

and that's where they're going to radiate. Nothing complicated about it. Exactly. Yeah. Okay. And so. Although I object to calling it dumb. It's pretty dumb.

I feel like you're hurting the black hole's feelings. So if someone's going to speak up on their behalf, I will. It's straightforward. Let's say that. There's not a lot of cancellation of subtle effects. But for the neutron star, neutron star mergers, so how many of those? have we found? One.

Only one. Okay. Yeah. That's just one. It's very special. But there you get both gravitational waves and an explosion that is very visible in light. Yes, that's right. That's why we can tell the speed of them coming to us is the same.

That's right. And we've been able to monitor the remnant afterwards to see like how the light is evolving, to see what materials were produced when the two neutron stars merged. In fact, I think it was on the front page of the Wall Street Journal because it would have created vast amounts of gold and plight. Platinum, if you could. Speculators go wild. Trouble out there.

Exactly. Yeah. Okay. Good. So general relativity once again pretty good shape. Yes. It's in very good shape. Okay, good.

Buy my book. I wrote a book about it. Yeah. I did. So good. Very good. That's one. So sorry.

We have LIGO. We now understand Lisa is going to happen in the future. Yeah. What is the main target for Lisa going to be? Lisa will look at. intermediate mass black holes and supermassive black hole mergers. And it'll also look at things called extreme mass ratio in spirals or emiris. And that means that the ratio of the two masses will be something like a thousand or ten thousand to one. So a tiny black hole falling into a big

black hole. Exactly. And then those create really interesting gravitational wave signatures. So it can look at those. It can look at these intermediate mass black holes. And it can look at supermassive black hole mergers. So the point of Lisa depends on who you ask. Sure. So people like me who study supermassive black holes say, clearly we want to look for the baby supermassive black holes, the ones that are a million times the mass of the sun. Because the billion solar mass ones you find with pulse are timing arrays.

Right. Okay. So their frequencies are too low to be in Lisa's band. Not only that, but they don't exist because there's another thing that we haven't spoken about yet. Okay. There's something called the innermost stable circular orbit of a black hole binary system. And, well, of any black hole, really, but the same thing holds true for black hole binary systems.

And that's just the last stable orbit that any kind of body can orbit around, right? And so if you have two merging billion solar mass black holes and say they're roughly the same mass, they will merge at a millionth of a hertz, at 10 to the, the minus six hertz. And so what that means is that it merges in the space in between pulsar timing and Lisa. So those black holes will never make it to the Lisa band. They merge first. So their isco frequency for the experts who might be listening is 10 to the minus six hertz. And so you're right in between experiments. Okay. Well, you know, that's only given human ingenuity, not yet up to the task of finding a way to look at that band, right? I'm actually a big fan of the tiny black holes falling into the supermassive ones.

Yeah. Because that lets you map out the spacetime metric around the big black hole. Fabulous. That would really test general relativity. Absolutely. That would be great. Yes. I look forward to you doing that.

I'm a big fan of Lisa for exactly that reason. I think that, you know, it was, it almost went away. I remember I was on a NASA panel that really pushed for Lisa, but it was decided that it was, you know, a little speculative, the technology we didn't know. And it wasn't until LIGO found gravitational waves at all. Who said, oh, wait, we got to do this now. Right.

Well, there's also the elephant in the room. That's the James Webb Space Telescope that took all the money. I know. Maybe it's not polite to talk about that now. But there's a finite amount of money. There is a finite amount of money, but it's not a fixed amount of money. I mean, Congress can decide to pay for two things. It's true.

Generally, when they cancel one big science project, give the savings to other science. projects. Yeah. So I kind of, I mean, it's true that there are priorities when you have to decide what to do, but it's, I think that scientists often think that their project is fighting against other science projects when it's really usually not the case. You're right. You're right.

In that sense, it's not a zero-sum game. We could always get more money. So that's a really good point. In the case of Lisa, I think that we owe a huge debt of gratitude to the European Space Agency, which then took on the entire project and kept it alive. Even after the Lago detection, as you mentioned, NASA is now rejoined as a junior partner. Yep. That's what you get.

But that's also great because the European Space Agency couldn't afford the full Lisa. So it would have only had two arms. And now that NASA has rejoined, it's back to full three-armed Lisa. All right. Three-armed Lisa. Three-armed Lisa in the 2030s maybe. Well, but right here and now, so just to finish the retinue here, we have Ligo that is already, won the Nobel Prize. Lisa is in the pipeline. But we have your pulsar timing arrays, which is going

right now, right? It's not only is it going right now, but my colleagues have been timing these millisecond pulsars for decades. Okay. So some of the pulsar timing baseline span almost 30 years. And nanograv, which is the North American Nanohertz Observatory for Gravitational Waves, has been operating for the last 15 years timing these. milliseconds pulsars in a very strategic way to detect gravitational waves. And so as a gravitational wave detector, pulsar timing arrays are also really unique because, you know, we talked about how one pulsar will have an advance or a delay in its arrival time. But the galaxy is full of pulsars, right? And so by using pulsars in this way, you're turning the whole galaxy into a gravitational wave detector,

which is really kind of mind-blowing, but that is exactly what we're doing. And so if you were to see this advance or delay just in one pulsar, you can't really conclude anything because your pulsars are thousands of light years away and there's gas in the galaxy, there's dust, your, you know, wavelengths are affected in different ways by these processes. You can have things that scatter. So you have to look for the signal, not only in one pulsar, but a whole array of pulsars. And so right now, there's about 100 of them

that are timed by the international pulsar timing array. So not only nanograph, but also the European pulsar timing array, the Parks Pulsar timing array, and the new Indian Pulsar timing array. And so we're collaborating and we're trying to create a new data set which joins together all of our data for these pulsars because your sensitivity to detecting gravitational wave scales as the number of pulsars and as the square root of the time.

So you should always add more pulsars before sitting and just waiting. And the good news is, this is my naivete as Ethereum is showing through, but you don't need to build anything to do this. You just use existing telescopes, right? Yes, I think that's been a blessing and a curse for pulsar timing arrays because it's such an ingenious idea, which actually had its inception in the early days of space travel. So Sazen wrote this paper in 1979 describing how you could use the Doppler shifting of signals

from potential probes leaving the solar system to look for gravitational waves. Wow. Isn't that it's so clever, right, that you can use spacecraft and time delays from spacecraft. But then he was very, you know, disappointed that, you know, you don't really have the timing precision that would enable that kind of detection. And then there's a serendipity that happened with pulsar time. timing as well that in 1982, the very first millisecond pulsar was discovered. And in 1983, there was this paper that came out very early on, like in January, that said,

if only we had pulsars that were good enough, we could create a pulsar timing array. But unfortunately, none of the pulsars are precise enough. None of them have this kind of, you know, 100 nanosecond timing stability over a decade. But back in the day in the early 80s, you know, you actually had to read physical journals. And so there's a little bit of overlap where. And there's a little footnote in the bottom of that paper saying, you know, like, actually, maybe it's possible. Wow. Yeah.

And that was in early 1983. Yeah, it's all about timing. But back to your comment about this being a cheap experiment. I think it has been a blessing and a curse. The idea is beautiful. I think it's really an insight. It's a fantastic idea. It's my favorite.

It still thrills me to this day to think that, like, one of us very clever apes thought about doing that. It's also a curse in the sense that it is very cheap. And I think that that makes people take it less seriously. I think that if, you know, the National Science Foundation had invested billions of dollars in this experiment over the last few decades, that it would have a much higher profile than it does right now. Of course, that kind of investment also enables lots of public outreach and a huge machine behind the experiment. But in fact, in the last few years,

the National Science Foundation has invested heavily in nanograph. And so we received a Physics Frontier Center Award for $15 million. About six or seven years ago was the first one, and it was just renewed two years ago for $17 million. And so this is the biggest investment impulsar timing arrays directly on the planet. So we are very grateful for, you know, this money

to enable the experiment and, you know, to pay for students and postdocs and researchers and telescope time. But LIGO was a billion dollars. But LIGO was at least a billion dollars. And so, yeah, I think that there's this kind of interesting psychology, right, that happens when you have an investment that's that big in something. And there was at least a chance 10 years ago that you would have found gravitational waves before LIGO.

It was a neck-and-neck contest, we thought. And in fact, there's another funny story about that. So in 2015, people, you know, in September, 2015, there was the very first gravitational wave event. And of course, scientists get very excited. And so people accidentally leave pre-printed. on a printer, paper drafts are lying all over the place, and people are, of course, just calling their friends and telling them, unless you're Kip Thorne, who didn't tell his wife. Well, he lived a long time just to make this moment happen. He was not going to ruin it

with loose lips at the end. Exactly, exactly. But everyone else is very excited, and, like, I heard about it at the red door. The cafe at Caltech, yeah. Exactly. When I was a postdoc there, at the same time, there was a signal in the nanogram of 11-year data that looked a hell of a lot like a gravitational wave background signal. So I haven't told you yet what a gravitational wave background is. But if we just go with the fact that there was a signal in the data and it was basically a race against when does LIGO see the first binary black hole merger and when do pulsar timing arrays or nanograv see this random gravitational wave? background. No one knew what the answer was because no one know what nature did. Like, no one knows how many merging supermassive black holes there are, what the amplitude of the background is. No one

knows what the merger rates are, for real, binary black holes. I mean, before the first Lego detection, this varied by orders of magnitude every year. You would get new papers that had different wild estimates. So people just kind of threw their hands up in the air. And they're like, the merger rate is whatever you think it is. Like, who knows? But it ended up, LIGO was pretty lucky with a bunch of black hole mergers. LIGO was so lucky. So it's LIGO in terms of these ground-based experiments, but we should also be careful. There was also there's the Virgo detector in Italy, and that helps with triangulation.

When you can see, there's two LIGO detectors in the U.S. There's one in Italy, and there's also one in Japan called Kagra. Cagra's cool because it has sapphire mirrors. Okay, that's cool. It is cool. It doesn't look like a sapphire. Like, you can't put a ring on it, but it is very cool. Yeah.

But, yeah, Ligo got really lucky with the first detection because it was screaming loud. And I did some of my Ph.D. work on Ligo. And I know for a fact that people have been working for many, many years on creating very sensitive data analysis pipelines to tease out the hint of a gravitational wave signal and have very sophisticated Bayesian analysis techniques to look for the evidence in every sense of the word. for this signal. And the first one was so loud, you could see it by eye. And no one would believe it. They were like, did someone shake the mirror, like guys, for real? Did someone just put this in where we hacked?

Like, what happened? And it was just a screaming loud gravitational wave signal. So at that time, there was also a signal in the nanograph 11-year data. This is the pulsar timing. This is the pulsar timing array. And we were like, oh my gosh, are we going to sco go, which would be so fun in the sense that, you know, our little experiment that had, you know, very limited funding was now competing. It was like a David and Goliath kind of situation. And so internally we were like, maybe you should have a joint press conference imagining like what we're going to do with pulsar timing arrays.

And then LIGO starts kind of slinging mud in a collegial way, of course. But like saying, like, it's not a direct detection. Like you're not making anything that's direct. We have a waveform. And we're like, that's not super true. Like we're looking at the change in times of pulsars and you're looking at the change in distance and signals. And we know that, you know, GR is right. So that's the same thing.

You do not have a more direct detector. So anyhow, there was a lot of weird. kind of backroom conversations, but in the end, this 11-year signal was likely due to solar system ephemorous errors because 11 years is roughly the period of Jupiter. Oh, I was going to say it was a sunspot cycle, but no, it's because you don't care about sunspots because it's gravitational waves, but you do care about your location in the solar system. You do care about your location in the solar system, and that's because, you know, if you think

about where, like how your timing pulsars, the Earth is. is moving throughout the solar system, throughout the year, the pulsars are moving on the sky. So you want to take your time of arrival stamps for your pulsars and transform them to the solar system Bari Center. The Bary Center is where you can balance the solar system on the tip of your finger, right?

That's the point. You want all of your time of arrivals or your T-O-A's to be at that point. You can trust everything at that point. If there is a mistake in how you calculate that point, what happens is that your pulsar arrival times will circulate the kind of like orbit the true berry center and create this signal that's present in all of your pulsars. But it won't have the gravitational wave shape to it that we expect, which is a quadrupole.

It'll have a dipole signature, but not the quadruple. So that's good because it's sort of a check that you haven't messed up. Yes. Unless there is so much dipole signal that it leaks into the quadrupole. in your data analysis. And that's actually what we found. We found that there was this, you know, it could be something like eccentricity. There was some sort of error in the position in massive Jupiter

that was perturbing the solar system Barry Center. And once my colleague at JPL, Mikhaili Velasneri wrote this software to correct for this, the signal went away. Okay. So has nanograph detected something? Are they announced a detection? You don't have to tell us any secrets. but, I mean, publicly, have pulsar timing arrays found any gravitational waves yet?

So publicly, there is a lot of excitement about the last round of papers that have come out from nanograph, from the European pulsar timing array, and from the Parks Pulsar timing array, and then together from the international pulsar timing array, everyone has found a signal that has an amplitude that would signify that it comes from a gravitational wave background. right? The amplitude is commensurate with what we would expect theoretically to come from the cosmic merger history of supermassive black holes. So maybe I should take a second to just dig into that. So if you have, you know, one source submitting gravitational waves, you can detect that one source on the sky. But now imagine you have galaxy mergers that are happening all over the place. And then they are not only happening all over the place, but they've been happening for a long time. So you now get, a buildup of signals in each one of the frequency bins that you're sensitive to. And so this creates a stochastic or random gravitational wave background. So you not only have one signal, but you have potentially tens of thousands of signals.

So you don't measure just one merging supermassive black hole, but you measure the amplitude of all of this gravitational wave signal that's interfering with itself. for lack of a better word. So just to be clear, we have the cosmic microwave background, which are photons, and they literally were all bumping into each other and bumping into atoms, and it's all over the place. This is totally different. Totally different.

And you call it a background just because it's coming from many individual sources that are sort of like, as far as our detectors are concerned, it's one big smush on the sky. That's right. But if we want to get technical, it is a foreground. It is the signal that we're looking for. but someone 30 years ago called it a background, and we've been calling that ever since. But for anyone that studies these things, you are right.

Yes, it is a foreground. And they're from supermassive black hole mergers? What kind of mergers are we talking about? Yes. So if the sources are astrophysical, then yes, it would come from supermassive black hole mergers. However, people are very creative. And it's possible that there's also gravitational waves from inflation. So we call those.

Exactly. Primordial gravitational waves, which could either be part of the signal or the whole signal. If it were the whole signal, then we would be in a very strange universe where we would have a big bounce and a big crunch. I know that you know all about this, Sean, but you would be in a kind of, a eq-parotic-style universe. And crazier things like cosmic strings would give you gravitational waves. And cosmic strings also give you gravitational waves and a gravitational wave background. And so, in fact, what we have right now is that there's this amplitude of a gravitational

wave background that we've found. But right now, the way that you distinguish between what's generating the background is how that amplitude evolves as a function of frequency. So as you go to higher and higher frequencies, how does that amplitude vary? And right now, we don't have enough measurements in different frequency bins to say exactly how that signal is evolving. So we can't say for sure that that signal would be from supermassive black hole binaries. That would have a very finite like F to the minus two-thirds dependence. The problem is that a primordial background would be minus one and cosmic strings would be like,

minus 7 eighths, super massive black holes is minus two thirds. So everything is about minus one. Everything is about minus one. So it's not like LIGO where there was a big press conference that I was at, you know, they announced the thing. It's something where it's going to creep up on us. There's already been papers saying maybe we're beginning to see the hints of this. Yeah. So right now we think that it's a hint potentially of a gravitational wave background signal because there's two parts to a detection with pulsar timing. So the first part is this amplitude. You use. see the same amplitude in all the pulsars at your timing. That rules out anything else that could possibly be talking to all of these pulsars in the galaxy at the same time. There's nothing else,

right? They have different noises. And so we cross correlate all of the pulsars in our array, because as you do this cross correlation, anything that's not common and the pulsars falls away and only the common signal is left afterwards. And so this cross correlation is important for two reasons. you get what this amplitude is of the gravitation wave background. And as you said correctly, this is something that builds up very slowly as a function of time. And so we call this red noise. So what we technically right now call the signal that we found is a common red noise process. And that just means that it is a low frequency signal that's in all of the pulsars.

We're not sure what it is. it looks promising. But it could be something other than what you're meant. But it could be something else, right? We have to be very careful. Now, the second thing that you get from doing this cross-correlation search is this correlation function. It's kind of a two-point correlation function.

So basically, when you correlate any two pulsar pairs, general relativity tells you what this expected correlation function value is for any given pulsar pair. You have to explain what a correlation function is. Yeah. So if I have pulsars that are separated by a certain angle on the sky, they can have, say, let's just start with a positive and negative correlation. So if the pulsars are positively correlated, it's like doing, you know, a fist bump in the air with both of your hands at the same time. You go up and down. They are positively correlated.

You expect those pulsar signals to be, you know, positively correlated. When you're seeing one, you'll expect to see the other. Yeah, you see them both moving at the same. same time. So they're positively correlated. But if your pulsars are separated by something like 80 degrees, then they're going to be anti-correlated. So one is coming kind of closer to you, the other one is moving away, but it's a negative or an anti-correlation. Because of this pluser cross-polarization. If you're stretching space time in one direction, you're squeezing it in the other. That's right. It's not, it's that but integrated over the whole sky. And so

that, which we call spatial correlations, That has not been found by anyone yet. And that's going to be the smoking gun. And after we find that, that's when we'll have a big press release and make a huge, you know, faub, but everything. Okay, very good. Yes. But right now we have one piece of the puzzle, which is this amplitude, which is the same in all the pulsars. And in fact, now we have, as I explained a little bit earlier, the signal that comes from nanograph,

but also the Europeans found the same signal and the Australian. Australians found the same signal, and we do not use the same telescopes. We do time some of the same pulsars in the northern hemisphere. So there is some overlap between Europe and nanograph. But in the southern hemisphere, it's very difficult for anyone in the northern hemisphere to time those pulsars. So it's curious that we found a consistent amplitude. And it's also curious as to what that tells us. If it does come from supermassive black holes, it means that the final parsick problem that we

talked about doesn't exist. None of them, none of them stalled. None of the black holes got hung up. They all managed to merge very fluidly. Because if there is a hang-up, if they do stall, then this decreases the amplitude of the gravitation wave background by about 30%. And so the only way to get two black holes that have stalled to eventually merge in the

absence of anything else is to realize, you know, We believe that in the universe, we have these hierarchical galaxy mergers. Eventually, a third galaxy is going to show up with its own supermassive black hole. You're going to have this three-body interaction and the least massive black hole. Yes, the least massive. Wait. Yes, gets ejected from the system and the remaining two merge. So they always merge.

Sometimes it would just take them a very long time to merge. And so if there is that kind of stalling or if there's not enough stars, if there's not enough gas, that decreases the amplitude of the background by about 30%. But if what we're seeing right now really is from supermassive black holes, it's completely inconsistent with any kind of stalling. So the universe finds a way to make supermassive black holes merge on a very reasonable time scale. In fact, it's so reasonable that this signal was first seen in the international pulsar timing array data three years ago, maybe four years ago now.

And we were like, that can't be right. This has got to be something in like the telescope back ends. There's something weird happening here. So it could be. I mean, this is not crazy talk. Exactly. No, it's not. But we were just like we just were not ready to admit that like what we're seeing was

potentially this signal because it was so loud. So we were, you know, and people in my field have been writing papers for years about like, why we haven't seen the gravitational wave background yet? Like, is it because they're stalling? Is it because supermassive black hole masses have all been overestimated? Are they not so massive? And on and on and on.

And, you know, I've, that's why I got interested in this whole problem in the first place. It's like, why haven't we seen anything yet? But it was there, potentially. Yeah. But maybe not. But what we really need to do is find those spatial correlations that come from our correlation function that we,

So a lot of fields of science use, these correlation functions. And you have a prediction for what the value should be. You measure what it actually is. And so just to take a step back because this is very, it's fascinating stuff. And like you very accurately conveyed, it is a triumph of human ingenuity to figure this out. It's like almost like we have a spider web spread throughout the near regions of our galaxies connecting us to these pulsars. and we're feeling the vibrations, right?

That's how I had Ed Yong on the podcast recently. He was talking about all the different ways that different animals sense the world. Yeah. Spiders feel vibrations in their webs. And it just reminds me of that. Like we're feeling the vibrations in the web of pulsars. Exactly.

And the wavelengths of gravitational waves that are doing what we care about are tens of light years. So, you know, visible light is very tiny wavelength. Microwaves are a centimeter or whatever. And this is tens of light years wavelength. length and we might be able to be detecting. We might be detecting it already. That's right. That's right. And that's why it takes so long to make one of those detections, because you have to, for an individual source, wait for, you know, one wave cycle to go by.

And for the gravitational wave background, what you do is that you get more and more sensitive to the background as the number of pulsars you include in your array and then as the square rate of the time. So you can try to add more pulsars, but you're not guaranteed that if you get telescope time and pointed at the sky that you're going to find the pulsars that you need. Right. So then you just keep timing the pulsars that you have and you also keep trying to find new ones. But then because you need such long time spans to get to very low sensitivities,

because the bucket of your experiment or the lowest frequency that you're sensitive to is one over your total time. And so the larger your total time, the lower the frequency you can get to. And so you want to have very long time spans. you want to have as many pulsars as you can. That's why the international collaboration is so important because you not only increase your time spans, but you increase your number of pulsars,

and you can also increase, you know, the density of the data points that you have, because people have been timing different pulsars at different times. So if you can combine all of that data, you get this denser data stream that's going to be particularly useful for finding the individual sources. Well, okay, good.

That was my next question, because it would be a different thing if all of these waves that we were detecting came from one source, right? Then I presume that then the sort of spatial pattern would be much easier to perceive, but it's sort of a cacophony from all directions, right? Is there hope of eventually disentangling that and saying, okay, here are the locations of the loudest sources? How feasible is it to imagine going from, oh, there's a whole washed out background to,

oh, I'm beginning to perceive there's a bright spot in this sky. Yeah. Okay. So if you want to detect a gravitational wave background, you do this cross-correlation search and you look for this correlation function that should exist. But if you want to look for individual sources, what you actually do is that you look for right now, we look for sinusoidal waves in the individual pulsar timing data. And so we don't look for cross-correlations in the pulsar data.

and the pulsar data right now to look for the individual sources. So the individual ones could be relatively nearby. There's been a lot of research done to try to understand if we're going to detect like a single source first or the gravitational wave background first. Almost everyone agrees that it's the gravitational wave background because in the background you have, you know, the cosmic merger history of all of the supermassive.

massive black hole. So it's a lot of gravitational wave power that's going into those low frequencies. But if you have one black hole binary system that's relatively nearby, then that will swamp your other signal. So like which one is it? So it looks like we might have seen the first hint of something happening for the background, which means we now have to find a way to subtract it. To get rid of that noise so we can see what's underneath. And what's underneath. And what's underneath will be likely either an individual source or it could be anisotropy in the gravitation wave background, similar to the cosmic microwave background, how they have those beautiful maps of hot spots and cold spots. You could have something similar for pulsar timing rays where you have parts of the sky that have more emerging supermassive black hole binaries and other parts of the sky.

Or maybe there's one that's nearby that's not quite detectable on its own but might leave a huge blemings. in the gravitational wave background by leaving some excess power in that part of the sky. Will we be able to get anything about the epoch of most of these mergers? I mean, with this question of how do the mergers happen? So how are we going to do science to use pulsar timing arrays to help answer that kind of question? Yeah, that's a great question. So when you're computing the amplitude of what you expect, the gravitational wave background to be.

The two main ingredients are number one, what the black hole masses. And then number two, what's the number density of supermassive black holes that you have? And so the number density tells you, you know, I have a certain volume. How many supermassive black hole binaries do I have in that volume? And so that's a number that you can play with. And as you go out further and further and further, you'll have more and more and more supermassive black holes. And so given the fact that when you try to theoretically estimate the amplitude of the background, you have those two big ingredients, when you actually detect the gravitational wave background, you can try to tease out those two quantities. So what's the minimum black hole mass that's contributing to the gravitational wave background?

And what's the number density of these as a function of distance or redshift in the universe? It goes very quickly from you discovered something new and completely unanticipated or at least unprecedented, I should say, to this is an everyday tool we're going to use to understand the universe better, right? Yeah, exactly. I think unanticipated is not quite right. It's been anticipated for 15 years at least. It's been so long. In fact, the first paper has written on using this cross-correlation search in 1983. So it's been a while.

So do you, closing thought, do you recommend that young people who are interested in the frontier of astrophysics think about this kind of thing is something to learn more about? So young people interested in the frontiers of astrophysics should do whatever they think is the coolest thing that they can think of. And for me, when I was a kid, it was black holes. And I started working on the LIGO experiment when I was a graduate student. And then I thought that maybe pulsar timing arrays were, a place where I could make more of a mark because it felt like LIGO was already very saturated. It was a very mature field.

And I was like, this is a bit of a gamble, but like, what if I can make some sort of big contribution to this new field? So I've been doing it before it was cool, Sean. Oh, yeah. But now it's extremely cool. But now it's extremely cool. So I think, you know,

but the only reason that you can ever make it through a PhD is if you really love what it is that you're working on. And so my advice would to just be like, find the coolest thing you can think of and do that thing. Not think of a better place to end than that. Kiara Mangarelli, thanks so much for being on the Mindscape Podcast. Thanks, Sean. It's a pleasure.