212 | Chiara Mingarelli on Searching for Black Holes with Pulsars

0:00:00.3 Sean Carroll: 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 pay-off of that equation is that you discover the existence of black holes. Basically, neither Einstein or 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. Einstein died in the 1950s. In the late '50s and '60s, scientists really began to understand what black holes are.

0:01:02.1 SC: 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 inspiraling 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.

0:01:46.4 SC: With that in mind, we're very happy today to have Chiara Mingarelli on the podcast. Chiara is a astrophysicist, physicist/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. Chiara is an expert in what are called pulsar timing arrays. And this is just a fun idea, it's one that 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.

0:02:19.8 SC: 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.

0:02:46.8 SC: And what happens is, these gravitational waves pass by pulsars, which are very tiny neutron stars rapidly spinning, and these pulsars turned out to be really, really good clocks. They emit their beams of light in very, very regular pulses. So if you had 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 Chiara'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 gonna learn something about it, hopefully in the near future.

0:03:55.3 SC: And we get to talk about 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 theoretical astrophysicist and likes to write down equations, most physicists out there are actually looking at data, collecting information. Chiara, 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.

[music]

0:04:49.7 SC: Chiara Mingarelli, welcome to The Mindscape Podcast.

0:04:51.5 Chiara Mingarelli: Thank you, it's great to be here.

0:04:53.5 SC: We talked about black holes before in the podcast and even gravitational waves, but there's never enough talk about black holes, so...

0:05:00.6 CM: I agree.

0:05:00.6 SC: I always like to ask the black hole-oriented guests, how do you define black holes? What do you think about? It'll be different for a quantum gravity person than an astronomer, I presume.

0:05:12.4 CM: Yeah. So, that's a great question. When I think about black holes, I think about water coming out of a water fountain.

0:05:19.6 SC: Okay.

0:05:20.1 CM: And I think about the water going up and then falling back on itself, and that a black hole is gonna 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.

0:05:34.5 SC: Right.

0:05:37.4 CM: 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.

0:05:43.8 SC: Oh, wait, so what is the analogy? Why is it a fountain?

0:05:46.7 CM: 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...

0:06:04.7 SC: Okay.

0:06:05.8 CM: 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 gonna just take off and go away. It's gonna be like water going up and then coming back down and on itself.

0:06:18.7 SC: So this is... So that's the view from inside the event horizon?

0:06:21.4 CM: That's right.

0:06:21.9 SC: But we're not gonna get, 'cause we're astronomers now, today...

0:06:24.0 CM: Yeah.

0:06:24.2 SC: For today's episode, we're looking at the outside of the black hole, right?

0:06:26.0 CM: [laughter] Yes.

0:06:29.3 SC: And we've come a long way. I don't know how, how has our thought about black holes empirically, like, in the universe...

0:06:36.2 CM: Yeah.

0:06:37.5 SC: Changed since you started thinking about these things professionally?

0:06:41.2 CM: Right. Let's see. So when I was starting to think about black holes when I was a kid, right, I would save my baby-sitting 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...

0:06:55.7 SC: Classic. Who didn't?

0:06:56.0 CM: Yup. Well, my friends didn't. [laughter] I had a few friends. And so, the black holes are just kind of like this... They were considered very theoretical, right?

0:07:09.9 SC: Right.

0:07:10.7 CM: There was sort of evidence from Cygnus X-1, there was x-rays that were coming out of this compact source, and it could have been a black hole creating something, so getting material from a star around it and then that material getting hot and ionized gas coming off of it, emitting x-rays. 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 wanna say. So now we have evidence of black holes merging, we have wave forms from ripples in the fabric 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 almost science fiction-y to something that's very hard science.

0:08:02.2 SC: And just be clear, 'cause 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.

0:08:15.9 CM: That's right. Absolutely. So we see normally coming from an accretion disc around the black hole, so a material that's 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 and 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...

0:08:43.7 SC: Yeah.

0:08:44.8 CM: Terminology...

0:08:45.6 SC: They're black.

0:08:46.3 CM: And think that the light is coming out of the black hole. It is not.

0:08:47.4 SC: Right.

0:08:48.4 CM: It's close to the black hole, but it is not coming out of it.

0:08:50.8 SC: Well, people have probably heard of the idea of Hawking radiation...

0:08:52.2 CM: Right.

0:08:52.4 SC: But nothing that you're doing has anything to do with Hawking radiation. We're never gonna see Hawking radiation from the black holes that you care about.

0:08:58.7 CM: No. That's right. Yeah, Hawking radiation is not something that we care about right now. Maybe you know when Boltzmann brains...

[laughter]

0:09:09.7 SC: That's gonna take a while. Yeah.

0:09:11.0 CM: Start waking up and looking around, there might be some evaporating supermassive black holes.

0:09:14.5 SC: Did you see that episode of Star Trek? The most recent episode of Star Trek: Strange New Worlds...

0:09:18.9 CM: No.

0:09:21.8 SC: Had a Boltzmann brain. Yeah.

0:09:23.3 CM: You must have been delighted.

0:09:23.4 SC: No, they got it completely wrong. I mean, it's a great show, I love the show, but this was like a god-like Boltzmann brain, that's really just not, probably. I mean, it could be, I don't know. Anything is possible.

0:09:33.3 CM: Maybe if you wait long enough.

0:09:34.7 SC: I don't think that was the simplest conclusion for them to leap to that it was a Boltzmann brain, but anyway. So, the people were weirdly, for a long time, sort of in the 20th century, skeptical that there were black holes. Like...

0:09:46.4 CM: Yeah.

0:09:46.5 SC: I think that my childhood cutting out pictures of black holes phase was in the '70s, and we already knew about Cygnus X-1.

0:09:53.7 CM: Yeah.

0:09:54.6 SC: 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?

0:10:13.0 CM: Yes. I think that nature makes black holes in lots of different ways, right? And if we think about the history of black holes, how there are these... I don't know what to call them in layman's terms, but if you just think of them as like these 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 is it really possible to have something that's like a black hole? And so, I understand why people were skeptical, because as scientists, sometimes we make a lot of approximations...

0:10:50.8 SC: Yeah.

0:10:51.6 CM: And sometimes it's fine, and sometimes it's not fine. And so I understand people being careful. But then going back to these 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.

0:11:09.8 SC: That's true.

0:11:09.9 CM: And what does that mean? [laughter] How can we believe...

0:11:12.6 SC: Don't ask that, 'cause we could talk about that, but it'd be a very different episode than we have coming up.

0:11:16.1 CM: Right? Okay. 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?

0:11:22.3 SC: It's the arrow of time, that's the answer.

[chuckle]

0:11:23.5 CM: Okay.

0:11:23.7 SC: But you should interview me for your podcast.

0:11:24.7 CM: Did you... Do you know about that, Sean?

0:11:25.3 SC: I do know a little bit about that, yes.

[laughter]

0:11:27.5 CM: Did you write a book on that, Sean?

0:11:29.4 SC: But you've already... You've mentioned that there's different populations of black holes, right?

0:11:34.6 CM: Yes. Yes.

0:11:35.2 SC: So there are ways that nature makes them, but there're different ways, 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?

0:11:47.1 CM: Right. So that's a fully loaded question, so maybe I'll start with how nature makes black holes and how nature 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 universes, 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 baryons that went into creating those black holes, that it's just a kind of blemish in the curvature of space-time. 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...

0:12:31.6 SC: And also, by the way...

0:12:32.2 CM: Yeah.

0:12:32.9 SC: Zero evidence that that actually happened...

0:12:34.6 CM: Yes.

0:12:35.2 SC: But it's something we can think about.

0:12:36.7 CM: Exactly.

0:12:37.7 SC: And they could even maybe be the dark matter, who know?

0:12:37.9 CM: Yeah. Well, exactly. It could possibly be, because you could possibly get some of them that are as massive as the LIGO sources.

0:12:44.6 SC: Right.

0:12:45.1 CM: And so the first detection from LIGO was to roughly 30 solar mass black holes. And some people think that those could 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.

0:13:08.3 SC: Yeah.

0:13:09.1 CM: That it's very unlikely that that's the answer. But it's curious.

0:13:14.3 SC: Well, black holes exist...

0:13:15.7 CM: Yes.

0:13:16.0 SC: And they're black, so that's good.

0:13:17.6 CM: Yes. [chuckle]

0:13:17.6 SC: But it is hard to make the right number of them to be the dark matter, is my impression.

0:13:21.8 CM: Exactly. And there's a lot of things that you would have to discount, like the lack of lensing events. Right? That...

0:13:26.4 SC: Oh, okay. Sorry.

0:13:29.1 CM: Yeah.

0:13:29.2 SC: Say more about that.

0:13:29.4 CM: Well, that 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.

0:13:55.1 SC: So there still could be primordial black holes, but maybe not enough to be the dark matter.

0:14:00.7 CM: Yeah. Or maybe not enough that are in that mass range.

0:14:01.8 SC: Okay.

0:14:03.3 CM: Right? It's possible that... I mean, there's a lot of theories about different masses of 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 a hundred solar masses, that's really hard right now to get those to explain dark matter.

0:14:23.9 SC: Okay. So how do you make them?

0:14:24.3 CM: 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. [laughter]

0:14:53.4 SC: That's okay. You can use those words and we can assume people know what it means, but anyway, neutrons...

0:15:00.1 CM: Basically the neutrons, you need a lot of pressure to get them to 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 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 one or 10 or a hundred solar masses, they can merge, and the final mass is the sum of the two black holes minus 5% for gravitational waves.

0:15:49.7 SC: Okay. So we're making... If the thing that is collapsing weighs roughly less than the sun, it'll be a white dwarf for a neutron star; if it's enough bigger, it'll make a black hole.

0:16:01.7 CM: Yeah. That's it.

0:16:02.3 SC: Okay. So we expect to have a bunch of black holes that are more than one solar mass.

0:16:06.0 CM: Yes.

0:16:06.6 SC: And then if they merge, they can get up there.

0:16:08.8 CM: 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 mass of the sun to make a supermassive black hole.

0:16:22.1 SC: Okay, what are those?

0:16:23.1 CM: So supermassive black holes are around a hundred thousand 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 that just directly collapsed into a black hole. 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 "schoop," and then created a supermassive black hole and there you go. So, that's kind of mind-boggling. But 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 holes, so maybe 10,000 solar masses, a thousand solar masses, and those all quickly merge to make a supermassive black hole.

0:17:20.1 SC: How would we know? Is this something we're trying to discover with telescopes?

0:17:24.0 CM: I'm so glad you asked that. Yes. So one of the ways that we can find out what the... 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 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 gonna be a huge LIGO-type instrument in space, and that will be able to detect those kinds of gravitational waves.

0:18:08.9 SC: Okay. Okay. We will be able... Alright, this is getting... It's interesting how many things come together at once, right? We 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 'em.

0:18:28.1 CM: So there's at least one supermassive black hole in the center of every massive galaxy. And my own research is studying supermassive black hole 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 Stephan's Quintet.

0:18:50.9 SC: Stephan's Quintet. Yeah.

0:18:51.9 CM: It was breathtaking.

0:18:53.0 SC: Five galaxies, yeah.

0:18:53.7 CM: 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 galaxies get bigger by merging with other galaxies. And so, if that's true, then there's 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.

0:19:20.7 SC: So maybe to explain this a little bit, because the black holes... Let me ask it this way.

0:19:26.8 CM: Yeah.

0:19:27.4 SC: 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. But they're not.

0:19:36.6 CM: They are not. They are a significant fraction of the galaxy, significant, maybe 1%, around 1%, of the mass of the galaxy, but 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 seeded, we say, but they formed in situ? That seems really hard to do. So, that's another open problem.

0:20:11.9 SC: How big is a supermassive black hole? How many light years is the black hole... Center of our own galaxy? Do you know?

0:20:19.8 CM: So, let's see. So, we have a fun trick.

0:20:22.8 SC: Okay.

0:20:23.3 CM: The relativist's unit is to use seconds for everything, which is light-seconds. So, one solar mass is 4.9 times 10 to the minus 6 seconds.

0:20:33.4 SC: Okay.

0:20:34.1 CM: 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.

0:20:49.4 SC: 10 light seconds?

0:20:49.7 CM: 10... Yeah, exactly. If you're a photon, it'll take you less than that, so... Yeah.

0:20:56.7 SC: But why I'm asking is, that's very tiny compared to the size of a galaxy, right? Even if it's 1% of the mass or less, it's much, much, much less than 1% of the size, 'cause 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?

0:21:17.6 CM: 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 timescales. So I've done a few calculations which show that supermassive black holes will merge in something like two or three billion years.

0:21:41.5 SC: Okay.

0:21:41.6 CM: But that's a...

0:21:42.6 SC: It's a long time. [laughter] Yeah.

0:21:42.7 CM: Sizeable fraction of the age of the universe, which is about 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 black holes are eventually slowed down in the merger process by interacting gravitationally with gas and stars. And so, the technical term for this, for experts that might be listening, is dynamical friction.

0:22:12.7 SC: Yup.

0:22:13.3 CM: 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 separation, but they will not merge unless something else acts on them.

0:22:44.0 SC: I see. So, it's easy... Let me just repeat it to see if I got it right.

0:22:47.1 CM: Yeah.

0:22:47.4 SC: So it's easy to see why black holes would sort of sink toward the neighborhood of the center, because there's friction, right?

0:22:55.2 CM: Yeah, exactly.

0:22:56.0 SC: But because they are so tiny, astrophysically speaking, they have to get really close, and we don't know how they do that?

0:23:02.8 CM: Well, we have a few ideas on how they do that. And so, this is called the final parsec problem, for anyone who 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's still stars. And so if you have some stars that are crossing the orbit of the supermassive black hole pair, every time that a star interacts with those two black holes, it'll carry away some energy in 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 disc 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.

0:24:08.8 SC: Yeah, okay.

0:24:09.4 CM: Yeah. But to add a fun breaking news headline to this, some theorists have found in large hydrodynamical simulations that the gas discs can apply positive torques, which means that the black holes get further away from each other, instead of negative torques, which make them merge. And apparently, it really depends on the properties of the gas disc around them. So, we think that for realistic discs, they probably merge, but you can make them not merge... [laughter]

0:24:44.6 SC: Yeah. Okay.

0:24:46.1 CM: In a super computer. 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 knitting 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.

0:25:07.9 SC: Well, this is really fun because it is a glimpse into where the frontier of astrophysics is these days, right? 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.

0:25:28.1 CM: That's right. That's right. And if we find gravitational waves from supermassive black holes, then we know for sure that they've overcome this final parsec problem. And then the question becomes, "Well, how did they do that?"

0:25:42.3 SC: 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.

0:25:50.7 CM: Yes.

0:25:51.2 SC: But we don't know whether they were made directly or they were assembled gradually, and all of these things, gravitational waves will help us sort out.

0:25:56.8 CM: Yeah, so, as you're saying, it'll help us sort out the formation scenario of the supermassive black holes. But even today, 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.

0:26:13.6 SC: Got it.

0:26:14.2 CM: So the first gravitation wave story was at 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?

0:26:28.9 SC: And it's an interesting reminder that the universe is still kind of young, it's still evolving. When that picture came out of Stephan's Quintet, it's five galaxies interacting with each other, I think that probably people see pictures of galaxies and figure that it'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.

0:26:48.5 CM: Absolutely, yeah. And the black holes are merging, hopefully there are stars being slingshot around, there's gas being funneled to the center, there's... Everything is very dynamic. But the timescale is not a human timescale. And so we see it as being static, basically.

[chuckle]

0:27:05.3 CM: But if you just hit fast forward, you'll see really beautiful physics happening. And that's some of the power of these super computer 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? 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."

0:27:26.7 SC: Oh, yeah. 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?

0:27:36.7 CM: Exactly. 'Cause that's all that we've got, right?

0:27:39.0 SC: Yeah. Yeah, we're not gonna wait around for a billion years to watch what happens, right?

0:27:42.0 CM: No, no, no.

0:27:43.3 SC: 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, right?

0:27:50.8 CM: That's right, yeah.

0:27:51.5 SC: 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.

0:28:00.1 CM: That's part of the picture, absolutely. But there's also the center of the Milky Way, right? And so there are... Andrea Ghez and her group at UCLA have very famously measured the mass of the black hole at the center of the Milky Way, and that 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 can 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.

0:28:42.4 SC: And we don't see a lot of photons coming from the center of our galaxy, right? It's a pretty quiet black hole.

0:28:49.4 CM: Right now, it's a pretty quiet black hole. There is evidence, though, 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. We can only say this is consistent with the existence of jets at some point in the past, but you can't rewind the universe to check.

0:29:15.2 SC: But we do see that distant galaxies are often 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 beam being beamed right toward us from a black hole.

0:29:29.3 CM: That's right, from a supermassive black hole.

0:29:31.5 SC: From a supermassive black hole.

0:29:32.2 CM: That's right, yeah.

0:29:33.8 SC: 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.

0:29:40.1 CM: That's right, yeah.

0:29:42.1 SC: [laughter] 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, is it not?

0:29:52.2 CM: 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. That gas has become stars, and SOA, so even if you wanna 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 merging supermassive black holes, it's mainly stars.

0:30:15.3 SC: This makes me ask a question, then, because I know we're gonna get this question, what about the dark matter here? 'Cause 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, or is it just irrelevant?

0:30:32.4 CM: 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 superradiance.

0:30:53.9 SC: Okay.

0:30:54.6 CM: 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 matte-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.

0:31:15.5 SC: So sorry, just to get this straight, this is because axions... The axion field exists in the universe, or is this literally because there are dark matter axions, if there are at all, axions are still hypothetical, but is it a bunch of... Like a cloud of axions near the black hole are helping it radiate?

0:31:34.0 CM: Yes.

0:31:34.6 SC: Okay.

0:31:35.9 CM: 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, emit 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.

0:32:03.8 SC: [laughter] I can imagine it, but I'm certainly not an expert. But let's...

0:32:05.6 CM: [laughter] Okay.

0:32:05.9 SC: Let's be kind to our listeners and explain a little bit about gravitational waves. We've been using the terminology, and I'm sure 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?

0:32:19.9 CM: 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...

0:32:33.4 SC: A little bit.

0:32:35.1 CM: A very little bit.

[laughter]

0:32:35.9 CM: 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 in 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, 'cause 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...

0:33:13.5 SC: Hertz per second.

0:33:15.4 CM: Yeah.

0:33:17.6 SC: Yeah.

0:33:17.7 CM: Yeah. That's right.

0:33:19.5 SC: Just gotta be... You know, I'm sure most people know what a Hertz is, but just checking.

0:33:21.3 CM: Yes, you have a very educated...

0:33:22.8 SC: Yes.

0:33:23.6 CM: Listening population. And so, you could actually hear those, if your ears could hear gravitational waves. They would go "ooo-wooo" when they merge 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 gravitation 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 the fraction of a size of a proton over a few miles.

0:34:06.4 SC: And the miles are the distance between that the sort of LIGO laser is moving.

0:34:09.6 CM: Exactly.

0:34:11.2 SC: Okay.

0:34:12.4 CM: Exactly. And so that's the strain. But if you think about...

0:34:15.4 SC: So sorry, wait, let me just make sure they understand this. 'Cause 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...

0:34:36.0 CM: Exactly.

0:34:36.4 SC: So that's distance divided by distance, that's what you mean by that.

0:34:38.7 CM: Exactly, yeah. So it's that change in distance over distance, is the strain.

0:34:43.7 SC: Got it.

0:34:43.9 CM: And that is something like the fraction of the size of a proton that the gravitational wave changed over a few miles.

0:34:50.8 SC: Right. Okay.

0:34:51.6 CM: Which is...

0:34:52.0 SC: Not very much.

0:34:52.8 CM: Crazy small. Exactly. The fact that...

0:34:54.9 SC: They should give a Nobel Prize to the people who did that.

0:34:57.1 CM: They should. Absolutely, absolutely. If we were time travelers, we could go back to 2017 and make sure that that happened.

0:35:04.6 SC: And so LIGO is the famous experiment that did win a Nobel Prize, yeah.

0:35:10.3 CM: That's right, that's right. And so, changing the distance, thinking about distance changes makes a 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, it sends a flash of radio waves to the Earth, like a light house. You get this really stable flashes, so we know exactly when those flashes should arrive.

0:35:56.5 SC: So the stable just means it's a good clock.

0:36:00.5 CM: It's a almost perfect clock. Before 2012, they were better than atomic clocks.

0:36:04.6 SC: Okay. Pulsars.

0:36:05.6 CM: Pulsars. Amazing, very... For the experts, it's a millisecond pulsar, but if you're not an expert, pulsar is fine. Just to be safe.

0:36:14.4 SC: So is millisecond a short period of time for a pulsar, or a long period?

0:36:17.4 CM: It's very short.

0:36:18.5 SC: Okay.

0:36:18.9 CM: Yeah, it means that it spins around about a hundred times a second.

0:36:22.1 SC: Okay.

0:36:23.5 CM: 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 a hundred times a second, and they would fit into the island of Manhattan.

0:36:40.8 SC: [chuckle] It'd be a bad idea, though. We don't want that to happen.

0:36:40.9 CM: It would be... We do not want that to happen.

0:36:42.9 SC: Okay.

0:36:43.8 CM: Absolutely not, but that's how small they are.

0:36:44.6 SC: A few miles across.

0:36:45.4 CM: They're very... Exactly, they're very small.

0:36:48.0 SC: And a hundred times a second.

0:36:49.0 CM: That's right.

0:36:49.4 SC: So, whenever... I like it because whenever we say they're very small, compared to what? They're much smaller than the Earth, but they're more massive than the sun. [chuckle]

0:36:57.3 CM: Exactly, exactly. But they are smaller than Manhattan.

0:37:00.9 SC: But the fact that they spin around a hundred times a second is impressive.

0:37:04.5 CM: Right. Exactly.

0:37:04.7 SC: And they don't fly apart, 'cause gravity is so strong.

0:37:06.7 CM: Right. That's right.

0:37:07.4 SC: How many of these do we know about?

0:37:11.2 CM: 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.

0:37:30.8 SC: Please.

0:37:32.5 CM: 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 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.

0:38:23.5 SC: [chuckle] Doesn't mean a lot.

0:38:23.6 CM: Right? 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.

0:38:31.6 SC: Okay.

0:38:31.7 CM: And that is something like 100 nanoseconds over a decade.

0:38:37.0 SC: Oh, okay, so small, a tiny amount.

0:38:37.5 CM: 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.

0:38:48.2 SC: Okay, wait. I'm very confused now.

0:38:49.8 CM: Okay.

0:38:50.8 SC: Let's catch our breath.

0:38:51.6 CM: That's bad, that's bad.

0:38:55.0 SC: What... So the idea is that there are all these different ways of detecting gravitational waves, 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.

0:39:13.9 CM: That's right, yes.

0:39:14.0 SC: And LIGO is sensitive to...

0:39:15.6 CM: Tens to hundreds of Hertz.

0:39:18.6 SC: Hertz. And do you know how many meters or kilometers that corresponds to? [chuckle] No, I don't know either, so. Okay, they can look it up. [chuckle]

0:39:25.3 CM: It's also complicated, because that my experimentalist colleagues will be very proud of me, I've known 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.

0:39:42.4 SC: Right. Okay, good. So just to translate that, to check that I understand it, the actual LIGO arms are about four kilometers apart, or at least 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?

0:40:01.1 CM: Potentially. Yeah.

0:40:01.6 SC: Yeah. Okay, good. So, what is relevant... I think you're right. What's 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?

0:40:16.0 CM: For LIGO, it's tens to hundreds of Hertz.

0:40:18.5 SC: Of Hertz? Okay, so tens to hundreds of cycles per second.

0:40:21.0 CM: That's right.

0:40:21.6 SC: Whereas your pulsar timing arrays?

0:40:24.0 CM: Yes, are sensitive to 1 to 100 nanohertz.

0:40:30.0 SC: And a nano is? 10 to minus...

0:40:31.3 CM: So 1 nanohertz is about 30 years. Like 1 over 30 years.

0:40:39.7 SC: One over 30 years. Right.

0:40:40.4 CM: Yeah, exactly. So, it would take 30 years for one full wave cycle to go by.

0:40:46.8 SC: And that roughly corresponds to the fact that we're looking to pulsars gathered throughout the galaxy that are light years away from us?

0:40:53.1 CM: 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 gravitational waves that are coming from supermassive black holes are very low frequency or have these very long wavelengths, so something on Earth can never detect a gravitational wave that has a period of decades.

0:41:19.5 SC: Decades. Right.

0:41:20.4 CM: You just can't do it.

0:41:21.4 SC: So the LIGO detectors look at these inspiraling black holes over tens of solar masses?

0:41:28.7 CM: That's right.

0:41:29.3 SC: 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 inspiraling or thousands of solar masses that are inspiraling, and LIGO just wouldn't know.

0:41:39.5 CM: That's right. Yeah.

0:41:40.4 SC: Okay.

0:41:41.0 CM: 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 millihertz frequency regime, which is right in between LIGO and pulsar timing arrays.

0:41:55.2 SC: Good.

0:41:55.5 CM: And so, they would be able to detect these thousand solar mass black hole mergers, but again, LIGO will not be able to detect that, and neither will pulsar timing arrays.

0:42:05.5 SC: But LISA is scheduled to be launched in, you said, 2030...

0:42:08.2 CM: Four.

0:42:09.6 SC: Four?

0:42:10.1 CM: Right now.

0:42:10.5 SC: And it's never gonna happen, obviously. I mean, it will happen, but it's not gonna happen in 2034, because no satellite's ever launched the year they plan to launch.

0:42:18.1 CM: 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 are some reasons that we might wanna launch LISA earlier. So number one, there's always this pathfinder mission, and LISA's pathfinder mission was... It performed extraordinarily, better 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.

0:42:50.4 SC: Okay.

0:42:53.2 CM: 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, at the very least, be alive in space at the same time, you would have a huge science case for looking at these electromagnetic or light signals from merging supermassive black holes. And there might not be another opportunity to do this in the near future.

0:43:20.9 SC: Got it.

0:43:21.5 CM: So, there's a strong case that's being made right now to move up the LISA launch date so that it can coincide with Athena.

0:43:31.7 SC: Okay. And LISA is the set of, basically, lasers in space bouncing back and forth?

0:43:35.4 CM: Lasers in space. Exactly. It's called a constellation, 'cause 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 LIGO-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.

0:44:02.5 SC: Okay.

0:44:03.0 CM: And so this is exciting not 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.

0:44:36.1 SC: Right.

0:44:36.8 CM: And then you get to stretch back up again. Now, you can...

0:44:38.0 SC: That's a plus.

0:44:38.6 CM: Exactly. That's the plus. So you can rotate that by 45 degrees and you get cross.

0:44:44.3 SC: Two polarizations. Right.

0:44:44.4 CM: There's two polarizations.

0:44:45.4 SC: 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.

0:44:53.6 CM: Plus and cross.

0:44:53.8 SC: That's the theory, anyway.

0:44:54.0 CM: 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, it's not a circular polarization.

0:45:23.2 SC: It's not. I know what you mean. Yeah.

0:45:24.0 CM: That technically means something else. I just mean it's like a [chuckle] a circle pattern that's breathing in and out.

0:45:30.0 SC: Well, I guess the point is that the regular gravitational waves, they stretch in one direction, but also squeeze in the other.

0:45:35.9 CM: Yeah.

0:45:37.3 SC: And you're saying with these fancy non-general relativity hypothetical waves stretch in every direction and squeeze in others.

0:45:45.2 CM: That's right. Yeah. But they don't exist on their own. It's an addition. It's like you have plus and cross and breathing.

0:45:49.8 SC: Right. Okay. And so we got there by saying that LISA could potentially disentangle this.

0:45:57.9 CM: 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. Because these gravitational waves then interact with your antennas, and your antennas are gonna be more or less sensitive to different orientations of your gravitational wave sources.

0:46:26.5 SC: So it sounds like so far what... So, we gotta separate out what we've done from what we're hoping to do, right?

0:46:32.3 CM: Yes.

0:46:33.1 SC: LIGO has detected things.

0:46:34.2 CM: LIGO has detected things, absolutely has, unequivocally.

0:46:38.7 SC: LISA has not detected anything yet, because it hasn't flown yet.

0:46:43.1 CM: Exactly.

0:46:43.2 SC: 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 consistent with what we've seen so far.

0:47:01.8 CM: Yeah, 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 wave form. And those can be very subtle and sometimes are only detectable in the inspiral part of the gravitation wave signal, so before the woop-chirp at the end of the light, you can get small changes that are happening in the inspiral 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 things like clouds passing over ahead of your detector or...

0:47:44.9 SC: Very sensitive detectors.

0:47:45.9 CM: Newtonian noise. You have earthquakes, you have trucks driving by, you have alligators that can crash in...

[chuckle]

0:47:54.2 SC: Because Louisiana has one of the detectors.

0:47:58.3 CM: 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 cosmic...

0:48:13.2 SC: That's another gravitational wave observatory?

0:48:15.0 CM: 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.

0:48:27.5 SC: Okay. You did say earlier, speaking of predictions of general relativity, that the gravitational waves move at the speed of light.

0:48:33.5 CM: Yes.

0:48:34.4 SC: How do we know that? Do we know that?

0:48:35.8 CM: Well, if there's any justice in the universe, they should have. But there might not be.

0:48:43.0 SC: Have you been in the university lately? There's not a lot of justice. [chuckle]

0:48:45.5 CM: Way to keep it real, Sean. Way to keep it real.

[chuckle]

0:48:50.2 CM: 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.

0:49:01.5 SC: So not a black hole.

0:49:02.2 CM: 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 in a million billion...

0:49:24.3 SC: Almost at the same time.

0:49:24.7 CM: So we know that... It's pretty good. It's pretty much the same time.

0:49:30.4 SC: So when the black holes merge, we don't see anything with electromagnetic waves.

0:49:37.8 CM: 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 that really blew my mind, is that gravitational waves, it's another spectrum. It has nothing to do with light. [chuckle] And it's... People also call it "gravitational radiation." That's another word that's used synonymously, that took me a long time to understand as well, that 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, but they're different. They're intrinsically different.

0:50:24.1 CM: You can have one source, like a light bulb, but that's emitting multiple frequencies, multiple different wavelengths. You can look at it with an infrared camera, you can look at it with optical, with your eyes. But with a gravitational wave source, it's really gonna be restricted to its own part of the gravitational wave spectrum. You're not gonna have two merging black holes of any mass that are gonna give you simultaneously different gravitational wave signals. It's just a continuous way of generating these gravitational wave signals.

0:50:54.3 SC: And in part that's just because gravity is a much dumber force than electromagnetism. 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 gonna radiate. Nothing complicated about it.

0:51:09.0 CM: Exactly, yeah.

0:51:09.7 SC: Okay, and so...

0:51:10.8 CM: Although I object to calling it dumb.

0:51:12.8 SC: It's pretty dumb.

0:51:12.9 CM: I feel like you're hurting the black hole's feelings. So if someone's gonna speak up on their behalf, I will. [chuckle]

0:51:17.9 SC: 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?

0:51:28.2 CM: One.

0:51:28.8 SC: Only one. Okay.

0:51:29.6 CM: Yeah. Just one.

0:51:29.9 SC: Special.

0:51:30.2 CM: It's very special.

0:51:32.9 SC: But there, you get both gravitational waves and an explosion that is very visible in light.

0:51:38.3 CM: Yes, that's right.

0:51:39.1 SC: That's why we can tell that the speed of them coming to us is the same.

0:51:41.7 CM: 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 platinum if you could... [chuckle]

0:51:58.9 SC: Yeah. Speculators go wild, but yeah, they can't get it.

0:52:00.2 CM: Travel out there. Exactly, yeah.

0:52:02.6 SC: Okay, good. So general relativity, once again, pretty good shape.

0:52:08.3 CM: Yes. It's in very good shape.

0:52:08.4 SC: Okay, good. Buy my book. I wrote a book about it. Yeah.

0:52:11.8 CM: [chuckle] I did. [chuckle]

0:52:13.2 SC: Good. Very good. That's one. So sorry, we have LIGO, we now understand that LISA is gonna happen in the future. And LISA, what is the main target for LISA going to be?

0:52:22.8 CM: LISA will look at intermediate mass black holes and supermassive black hole mergers. And it will also look at things called extreme mass ratio inspirals or EMRIs. And that means that the ratio of the two masses will be something like a thousand or 10,000 to one and...

0:52:45.0 SC: So a tiny black hole falling to a big black hole?

0:52:48.3 CM: 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.

0:53:01.6 SC: Sure.

0:53:03.7 CM: [laughter] People like me who studied supermassive black holes say, clearly, we wanna 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 pulsar timing arrays.

0:53:16.0 SC: Right. Okay, so their frequencies are too low to be in LISA's band.

0:53:19.9 CM: Yeah. Not only that, but they don't exist. Because there's another thing that we haven't spoken about yet.

0:53:25.6 SC: Okay.

0:53:26.0 CM: 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 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 emerge 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.

0:54:15.2 SC: Okay. Well, that's only given human ingenuity not yet up to the task of finding a way to look at that band, right?

0:54:22.7 CM: Mm-hmm.

0:54:23.5 SC: But okay. I'm actually a big fan of the tiny black holes falling into the supermassive ones, because that lets you map out the space-time metric around the big black hole...

0:54:33.3 CM: Fabulous.

0:54:33.6 SC: And that will really test general relativity.

0:54:38.3 CM: Absolutely.

0:54:38.6 SC: And that'd be great.

0:54:38.9 CM: Yes. I look forward to you doing that.

0:54:41.8 SC: I'm a big fan of LISA, for exactly that reason. I think that it almost went away. I remember, I was on a NASA panel that really pushed for LISA, but it was decided that it was a little speculative, the technology, we didn't know, and that it wasn't until LIGO found gravitational waves at all, then we said, "Oh, wait, we gotta do this now." [chuckle]

0:55:00.9 CM: Right? Well, there's also the elephant in the room. That's the James Webb Space Telescope that took all the money.

0:55:05.8 SC: I know.

0:55:07.4 CM: [laughter] Maybe it's not polite to talk about that now.

0:55:08.6 SC: No, I think that it's...

0:55:09.5 CM: That there's a finite amount of money.

0:55:10.9 SC: 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. [chuckle]

0:55:18.7 CM: It's true.

0:55:19.4 SC: They don't generally, when they cancel one big science project, give the savings to other science projects.

0:55:23.7 CM: Yeah.

0:55:24.8 SC: So it's true that there are priorities, we needed to decide what to do. But I think that scientists often think that their project is fighting against other science projects, when it's really usually not the case.

0:55:36.7 CM: You're right, you're right. In that sense, it's not a zero-sum game.

0:55:41.0 SC: Not a zero-sum game.

0:55:41.9 SC: We could always get more money. So that's a really good point. And the case of LISA, I think that we owe a huge debt of gratitude to the European Space Agency, which then took on the...

0:55:51.5 SC: They kept it alive.

0:55:53.6 CM: Entire project and kept it alive. And after the LIGO detection, as you mentioned, NASA has now rejoined as a junior partner.

0:55:58.4 SC: Yup.

[laughter]

0:55:58.9 SC: That's what you get.

0:56:02.1 CM: 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, so.

0:56:15.1 SC: Right. Three-armed LISA. Can't wait. [chuckle]

0:56:15.9 CM: Three-armed LISA in the 2030s, maybe.

[laughter]

0:56:17.0 SC: Well, but right here and now, so just to finish the retinue here, we have LIGO that has already won a Nobel Prize. LISA is in the pipeline. But we have your pulsar timing arrays, which is going right now. Right?

0:56:30.2 CM: Not only is it going right now, but my colleagues have been timing these millisecond pulsars for decades.

0:56:37.8 SC: Okay.

0:56:38.5 CM: 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 millisecond 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 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.

0:57:23.6 SC: Yeah. [chuckle]

0:57:24.6 CM: 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.

0:57:38.4 SC: Who knows? Right.

0:57:39.3 CM: Your 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 hundred of them that are timed by the International pulsar timing array. So not only NANOGrav, but also the European Pulsar Timing Array, the Parkes Pulsar Timing Array, and the new Indian Pulsar Timing Array. And so, we're collaborating and we're trying to create a new dataset which joins together all of our data for these pulsars, because your sensitivity to detecting gravitational waves scales is the number of pulsars and as the square root of the time, so you should always add more pulsars before sitting and just waiting.

0:58:29.5 SC: [laughter] And the good news is, this is my naivete as a theorist showing through, but you don't need to build anything to do this. You just use existing telescopes, right?

0:58:39.2 CM: 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, Sasin 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.

0:59:05.6 SC: Wow. Wow.

0:59:06.6 CM: Isn't that... It's so clever that you can use spacecraft and time delays from spacecraft. But then he was very disappointed that 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 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 100-nanosecond timing stability over a decade. But back in the day, in the early '80s, you actually had to read physical journals, and so [chuckle] there's a little bit of overlap where...

1:00:00.5 SC: Didn't get an email. Yeah. Right.

1:00:01.2 CM: And there's a little footnote in the bottom of that paper saying like, actually, maybe it's possible.

1:00:04.9 SC: Maybe there is one. Wow.

1:00:06.2 CM: Yeah.

1:00:06.7 SC: That's very good timing.

1:00:06.9 CM: 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 one of us very clever apes thought about doing that.

1:00:29.6 SC: Yeah.

1:00:32.4 CM: 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 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 NANOGrav. And so, we received a Physics Frontier Center Award for $15 million about six or seven years ago, it was the first one, and it was just renewed two years ago for $17 million. And so, this is the biggest investment in pulsar timing arrays directly on the planet. So we are very grateful for this money to enable the experiment and to pay for students and post-docs and researchers and telescope time.

1:01:34.1 SC: But LIGO was a billion dollars.

1:01:35.8 CM: But LIGO was at least a billion dollars. And so yeah, I think that there's this kind of interesting psychology that happens when you have an investment that's that big and something.

1:01:46.5 SC: And there was at least a chance 10 years ago that you would have found gravitation waves before LIGO.

1:01:51.8 CM: Oh, it was a neck and neck contest, we thought. And in fact, [chuckle] there's another funny story about that. So, in 2015, people... In September 2015, there was the very first gravitational wave event, and of course, scientists get very excited. And so, people accidentally leave pre-prints on a printer...

1:02:18.1 SC: Sure.

1:02:19.1 CM: 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.

[laughter]

1:02:27.8 SC: He lived a long time just to make this moment happen, he's not gonna ruin it with loose lips at the end. Yeah.

1:02:32.5 CM: Exactly, exactly. But everyone else is very excited, and I heard about it at the Red Door.

[chuckle]

1:02:39.5 SC: The Cafe at Caltech. Yeah.

1:02:40.8 CM: Exactly. When I was a post-doc there. At the same time, there was a signal in the NANOGrav 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 NANOGravs see this random gravitational wave background? No one knew what the answer was, 'cause no one know what nature did. No one knows how many merging supermassive black holes there are, what the amplitude of the background is, and no one knows what the merger rates are for real of binary black holes. Before the first LIGO detection, this varied by orders of magnitude. Every year, you would get new papers that had different wild estimates. So people just kind of throw their hands up in the air and they're like, the merger rate is whatever you think it is. Who knows?

1:03:42.8 SC: Then it ended up LIGO was pretty lucky with a bunch of black hole mergers.

1:03:46.4 CM: LIGO was so lucky, so it's LIGO in terms of these ground-based experiments, but we should also be careful, there was [1:03:52.2] ____... The Virgo detector in Italy, and that helps with triangulation when you can see, there's two LIGO detectors in the US, there's one in Italy, and there's also one in Japan called KAGRA. KAGRA is cool, 'cause it has sapphire mirrors.

[laughter]

1:04:06.9 SC: Okay, that's cool.

1:04:08.0 CM: It is cool. It doesn't look like a sapphire. You can't put a ring on it, but it is very cool.

1:04:15.1 SC: Yeah.

1:04:18.5 CM: But yeah, LIGO got really lucky with the first detection, because it was screaming loud. And I did some of my PhD 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, [chuckle] for this signal. And the first one was so loud, you could see it by eye. And no one would believe it. They are like, did someone shake the mirror? Like guys, for real, did someone just put this in? Were we hacked? What happened? And it was just a screaming loud gravitational wave signal. So, at that time, there was also a signal in the NANOGrav 11-year data.

1:05:10.4 SC: This is the pulsar timing array.

1:05:12.0 CM: This is the pulsar timing array. And we were like, oh, my gosh...

1:05:16.6 SC: We may get there first.

1:05:18.4 CM: Are we gonna scoop LIGO, which would be so fun in the sense that our little experiment that had very limited funding was now competing... It was like a David and Goliath kind of situation. And so, internally, we were like, maybe we should have a joint press conference.

[laughter]

1:05:36.2 CM: Imagining what we're gonna do with pulsar timing arrays. And then LIGO starts kind of slinging mud in a collegial way, of course, but saying like, "It's not a direct detection. You're not making anything that's direct. We have a waveform," and we're like, "That's not super true. 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 GR is right, so it's the same thing. You do not have a more direct detector." So, anyhow, there was a lot of weird backroom conversations, but in the end, this 11-year signal was likely due to solar system ephemeris errors, because 11 years is roughly the period of Jupiter.

1:06:25.7 SC: Oh, I was gonna say it was a sunspot cycle, but no, it's a... 'Cause you don't care about sunspots because it's gravitational waves, but you do care about your location in the solar system.

1:06:33.1 CM: You do care about your location in the solar system, and that's because if you think about how you're timing pulsars, the Earth 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 barycenter. The barycenter's where you can balance the solar system on the tip of your finger. That's the point you want all of your time of rivals or your TOAs to be at that point. You can trust everything at that point. If there's a mistake in how you calculate that point, what happens is that your pulsar arrival times will circulate the orbit, the true barycenter, and create the 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 quadruple. It'll have a dipole signature, but not the quadruple.

1:07:30.8 SC: So that's good. This is sort of a check that you haven't messed up.

1:07:33.3 CM: Yes, unless there is so much dipole signal that it leaks [laughter] into the quadruple in your data analysis, and that's actually what we found. We found that there was this... It could be something like eccentricity, there was some sort of error in the position in massive Jupiter that was perturbing the solar system barycenter, and once my colleague at JPL, Michele Vallisneri, wrote this software to correct for this, the signal went away.

1:08:03.6 SC: Okay. So, has NANOGrav detected something, are they announcing they're detecting? You don't have to tell us any secrets, but publicly, have pulsar timing arrays found any gravitational waves yet?

1:08:13.8 CM: So publicly, there is a lot of excitement about the last round of papers that have come out from NANOGrav, from the European Pulsar Timing Array, and from the Parkes 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. 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.

1:08:51.1 CM: So, if you have 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 build-up 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.

1:09:38.3 SC: 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...

1:09:48.9 CM: Totally different.

1:09:49.8 SC: 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, is one big smoosh on the sky?

1:10:00.1 CM: That's right. But if we wanna get technical, it is a foreground. It is the signal that we're looking for, but someone 30 years ago called [laughter] it a background, and we've been calling it that ever since.

1:10:12.0 SC: Fair enough. Yeah.

1:10:14.9 CM: But for anyone that studies these things, you are right, yes, it is a foreground.

1:10:18.1 SC: And they're from supermassive black hole mergers? What kind of mergers are we talking about?

1:10:24.8 CM: 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...

1:10:39.3 SC: Super early universe?

1:10:42.0 CM: 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, that you would be in a kind of ekpyrotic style universe.

1:11:04.1 SC: Right. And crazier things like cosmic strings would give you gravitational waves.

1:11:09.4 CM: 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 s to the minus two-thirds dependence. The problem is that a primordial background would be minus one; and cosmic strings would be like minus seven-eighths; supermassive black holes is minus two-thirds. So everything is about minus one.

1:12:04.3 SC: Yeah, it's similar. [laughter]

1:12:06.5 CM: Everything is about minus one.

1:12:09.4 SC: So it's not like LIGO where there was a big press conference that I was at, they announced the thing, it's something where it's gonna creep up on us, there's already been papers saying maybe we're beginning to see the hints of this.

1:12:19.8 CM: Yeah, so, right now, we think that it's a hint potentially of a gravitation wave background signal because there's two parts to a detection with pulsar timing. So the first part is this amplitude. You see the same amplitude and all the pulsars that you're timing. That rules out anything else that could possibly be talking to all of these pulsars in the galaxy at the same time.

1:12:41.9 SC: Right.

1:12:42.9 CM: There's nothing else. They have different noises until we cross-correlate all of the pulsars and RRA, 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 gravitational 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...

1:13:31.2 SC: But it could be something other than...

1:13:31.9 CM: But it could be something else. 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.

1:13:56.2 SC: You have to explain what a correlation function is.

1:13:58.2 CM: 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 a fist bump in the air with both of your hands at the same time, you go up and down, they are positively correlated, do you expect those pulsar signals to be positively correlate.

1:14:24.5 SC: Right, when you're seeing one, you'll expect to see the other.

1:14:28.4 CM: Yeah, you see them both moving at the same time, so they're positively correlated. But if your pulsars are separated by something like 80 degrees, then they're gonna be anti-correlated, so one is coming closer to you, the other one is moving away, but it's a negative, or an anti-correlation.

1:14:43.5 SC: Because of this plus or cross-polarization. If you're stretching the space-time in one direction, you're squeezing it in the other.

1:14:48.5 CM: 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 gonna be the smoking gun, and after we find that, that's when we'll have a big press release and make a huge [1:15:09.4] ____ about everything.

1:15:11.0 SC: Okay, very good.

1:15:12.6 CM: 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 NANOGrav, but also, Europeans found the same signal and the 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 NANOGrav, but in the southern hemisphere, it's very difficult for anyone in the northern hemisphere to time those pulsars. So, it's curious. Now we've found a consistent amplitude, and it's also curious as to what that tells us.

1:15:54.7 CM: If it does come from supermassive black holes, it means that the final parsec problem that we talked about doesn't exist. That 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 gravitational 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, we believe that in the universe, we have these hierarchical galaxy mergers, eventually a third galaxy is gonna show up with its own supermassive black hole, you're gonna have this three-body interaction and the least massive black...

1:16:41.6 CM: Yes the least massive [laughter] 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 super massive black holes is 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 the 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."

[laughter]

1:17:35.6 CM: This has gotta be something in the telescope back end, there's something weird happening here.

1:17:42.2 SC: Which it could be, I mean, this is not crazy talk, right?

1:17:44.2 CM: Exactly, no, it's not, but we were just... We just were not ready to admit that what we're seeing was potentially the signal, 'cause it was so loud. And people in my field have been writing papers for years about 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... That's why I got interested in this whole problem in the first place, is like, why haven't we seen anything yet? But it was there, potentially.

1:18:21.9 SC: Yeah.

1:18:22.0 CM: But maybe not, but what we really need to do is find those spatial correlations that come from our correlation function that we use. So a lot of fields of science use these correlation functions.

1:18:32.5 SC: Sure.

1:18:32.9 CM: And you have a prediction for what the value should be, you measure what it actually is.

1:18:36.9 SC: And so just to take a step back, 'cause 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. 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, and spiders feel vibrations in their webs, and it just reminds me of that, like we're feeling the vibrations in the web of pulsars.

1:19:10.9 CM: Exactly.

1:19:11.9 SC: And the wavelengths of gravitational waves that are doing what we care about are tens of light years, so, visible light is very tiny wavelength, microwaves are a centimeter or whatever, and this is tens of light years' wavelength, and we might be able to be detecting. We might be able... We might be detecting it already.

1:19:30.7 CM: That's right, that's right. And that's why it takes so long to make one of those detections, 'cause you have to, for an individual source, wait for 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 a square root of the time. So, you can try to add more pulsars, but you're not guaranteed that if you get telescope time and point it at the sky, that you're gonna find the pulsars that you need.

1:20:01.2 SC: Right.

1:20:01.6 CM: 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, 'cause you're the bucket of your experiment to 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 wanna have very long time spans, you wanna 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 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 gonna be particularly useful for finding the individual sources.

1:20:51.3 SC: 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. I presume that then the sort of spatial pattern would be much easier to proceed, but it's sort of a cacophony from all directions. Is there a 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 the SK, sky"?

1:21:26.4 CM: Sure. 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 wanna 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 right now to look for the individual sources.

1:21:53.6 CM: So, the individual ones could be relatively nearby. There's been a lot of research done to try to understand if we're gonna detect 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 the cosmic merger history of all of the supermassive black holes, that'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 signals, 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 that we can see what's underneath.

1:22:48.2 SC: Right, okay.

1:22:49.2 CM: And what's underneath will be likely either an individual source or it could be an isotropy in the gravitational 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 arrays, where you have parts of the sky that have more merging supermassive black hole binaries in 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 blemish in the gravitational wave background by leaving some excess power in that part of the sky.

1:23:25.3 SC: Will you be able to get anything about the epoch of most of these mergers? With this question of how do the mergers happen, so how are we gonna do science to use pulsar timing arrays to help answer that kind of question?

1:23:42.0 CM: 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 mass is, and then number two, what's the number density of supermassive black holes that you have? And so the number density tells you, 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 super massive black hole. 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 a red shift in the universe?

1:24:42.4 SC: It goes very quickly from, we discovered something new and completely unanticipated, or at least unprecedented, I should say, to, this is an every day tool we're gonna use to understand the universe better, right? [chuckle]

1:24:53.5 CM: Yeah, exactly. I think unanticipated is not quite right, as in it's been anticipated for 15 years at least. It's been so long. In fact, the first paper that is written on using this cross-correlation search in 1983. So it's been a while.

1:25:11.5 SC: So do you... Closing thoughts, do you recommend that young people who are interested in the frontier of astrophysics think about this kind of thing as something to learn more about?

1:25:22.0 CM: So young people interested in the frontiers of astrophysics should do with 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 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. [chuckle]

1:25:56.5 SC: Oh, yeah. But now it's extremely cool.

1:25:58.7 CM: And now it's extremely cool, so I think... 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 just be like, find the coolest thing you can think of, and do that thing.

1:26:13.1 SC: Cannot think of a better place to end that that. Chiara Mingarelli, thanks so much for being on The Mindscape Podcast.

1:26:17.2 CM: Thanks, Sean, it's a pleasure.