Into the Impossible With Brian Keating - Chiara Mingarelli: Hints of the Gravitational Wave Background from NANOGRAV! (#123)

Episode Date: March 8, 2021

Prof. Chiara Mingarelli is a gravitational-wave astrophysicist, looking to understand how supermassive black holes in the centers of massive galaxies merge, if at all. She does this by predicting their nanohertz gravitational-wave signatures, which will soon be detected by pulsar timing array experiments. With pulsar timing data, She looks for both individual supermassive black holes in binary systems, and for the gravitational-wave background which should be generated by their cosmic merger history. She an assistant professor at the University of Connecticut, and an associate research scientist at the Center for Computational Astrophysics (CCA) at the Flatiron Institute. Before joining the CCA she was a Marie Curie International Outgoing Fellow at Caltech and at the Max Planck Institute for Radio Astronomy. PRESS RELEASE: In data gathered and analyzed over 13 years, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has found an intriguing low-frequency signal that may be attributable to gravitational waves. NANOGrav researchers studying the signals from distant pulsars – small, dense stars that rapidly rotate, emitting beamed radio waves, much like a lighthouse – have used radio telescopes to collect data that may indicate the effects of gravitational waves, as reported recently in The Astrophysical Journal Letters. NANOGrav has been able to rule out some effects other than gravitational waves, such as interference from the matter in our own solar system or certain errors in the data collection. These newest findings set up direct detection of gravitational waves as the possible next major step for NANOGrav and other members of the International Pulsar Timing Array (IPTA), a collaboration of researchers using the world’s largest radio telescopes. “It is incredibly exciting to see such a strong signal emerge from the data,” says Joseph Simon, lead researcher on the paper. “However, because the gravitational-wave signal we are searching for spans the entire duration of our observations, we need to carefully understand our noise. This leaves us in a very interesting place, where we can strongly rule out some known noise sources, but we cannot yet say whether the signal is indeed from gravitational waves. For that, we will need more data.” Gravitational waves are ripples in space-time caused by the movements of incredibly massive objects, such as black holes orbiting each other or neutron stars colliding. Astronomers cannot observe these waves with a telescope like they do stars and galaxies. Instead, they measure the effects passing gravitational waves have, namely tiny changes to the precise position of objects - including the position of the Earth itself. Support the podcast: https://www.patreon.com/drbriankeating And please join my mailing list to get resources and enter giveaways to win a FREE copy of my book (and more) http://briankeating.com/mailing_list.php 📝 🏄‍♂️ Find me on Twitter at https://twitter.com/DrBrianKeating 🔥 Find me on Instagram at https://instagram.com/DrBrianKeating 📖 Buy my book LOSING THE NOBEL PRIZE: http://amzn.to/2sa5UpA 🔔 Subscribe for more great content https://www.youtube.com/DrBrianKeating?sub_confirmation=1 ✍️Detailed Blog posts here: https://briankeating.com/blog.php 📧Join my mailing list: http://briankeating.com/mailing_list.php 👪Join my Facebook Group: https://facebook.com/losingthenobelprize 🎙️Please subscribe, rate, and review the INTO THE IMPOSSIBLE Podcast on iTunes: https://itunes.apple.com/us/podcast/into-the-impossible/id1169885840?mt=2 🎙️Listen on all other platforms: https://wavve.link/into A production of http://imagination.ucsd.edu/ Learn more about your ad choices. Visit megaphone.fm/adchoices

Welcome everybody to this episode of Into the Impossible with Professor Brian Keating. In this episode, Brian interviews Professor Keira Migueli. She's a gravitational wave astrophysicist based at the University of Connecticut and the Flatiron Institute's Center for Computational Astrophysics. You can watch the interview on the YouTube channel at Dr. Brian Keating, DR. Brian Keating. Enjoy. Any sufficient advanced technology is interested. from magic.

Thanks for joining the Into the Impossible podcast. Yeah, thank you for inviting me. So I've, of course, followed your career for many years and been fascinated by the stuff that you do. I first want to say to everyone who's listening to Clubhouse, you can watch the fun on YouTube, Dr. Brian Keating. We are streaming from there. And if we have time, we'll take some questions.

We're on a little bit of a tight time schedule today, but that shouldn't deter you from asking any burning questions. I'll certainly have time to take maybe one or two. But today we are going to talk about this interesting discovery that has huge implications and a little bit of frustration for me, Kiar, because as you know, I build a gravitational wave detector that's a huge telescopes and so forth that detect the signal via the cosmic microwave background, we hope.

And here you are outdoing me building a galaxy-sized, detector of gravitational waves. First of all... Go bigger, go home. That's right. So can you introduce yourself who you are, where you are, how you spend your time on

planet Earth, and tell us about the lovely University of Connecticut, which as I was mentioning, is where the alma mater of my beloved mother, Barbara Keating. So, Kiarra, tell us who you are. Who am I? I'm a gravitational wave astrophysicist. I'm originally from Ottawa, Canada,

and I've done my studies all over the planet. So I did my undergraduate degree in Canada and then graduate work in Italy and then the UK. Then I lived in California for a while. And then Germany, the New York, and now I'm an assistant professor here at the University of Connecticut. And you have another kind of this part-time job or maybe sometimes it's a full-time job.

And that's as a research fellow or scientist. at the Flatiron Institute at the Center for Computational Astrophysics in New York City, which is a project of the Simons Foundation. Tell us about that and what do you do there? So at the Center for Computational Astrophysics or CCA, as you said, I'm an associate research scientist. So I think about how supermassive black holes merge and all the different ways that they can merge and emit gravitational waves as they're merging. And then try to figure out, how all of this happens using these supercomputing facilities there that are really one of a kind. So it's a huge resource and benefit to my research to work there.

I also am really plugged into the New York community there. So not only is CCA a wonderful place in of itself, but it also hosts really big communities. So we have colleagues at CUNY, Columbia, NYU, Rutgers, Princeton. It's a real nexus of people coming in and out, or at least it used to be. Those are my last memories there, which were almost a year ago at this point. Right now we're all working remotely, but hopefully we can go back soon. Yeah, that's right. The city has become a very different version of what it used to be,

but I have many, many fond memories of seeing you work in your cubicle, along with all my friends at the CCA. It's really a fantastic organization, and really unlike any other organization of its kind, to my knowledge. So it's very, it's very impressive how far it's come in such a short period of time and how it continues to grow. So I particular, as I said earlier, I wanted to talk to you for a while maybe over a year. And then I was like, well, this announcement that was made just about a month ago really set the astronomical community a buzz. And I felt like it would be wonderful to have you on the Into the Impossible podcast to tell us a little bit, first of all,

starting from the very basics, what is a pulsar? Why are they interesting? How do you build a gravitational wave detector from a network of pulsars? And then what the discovery that you guys announced back at the AAAS meeting is all about? So first of all, what is a pulsar? Why should we care?

How does it affect Joe Bagget, don't? No, no, I'm not going to say that. I hate when they ask me that question. Don't dump it down, but please, yeah, what is a pulsar? Why are they so fascinating to you? Sure. So a pulsar is a neutron star. And a neutron star is a dead core of a star that was about 10 to 25 times the mass of the sun.

So when the star dies, it goes supernova. And what's left over is this neutron star in the center. So there's so much pressure during this implosion, during this supernova explosion explosion event, that the protons and the electrons get squished together to make this ball of neutrons. Now, there's not enough mass for it to continue to collapse into a black hole, But there's enough so that about 1.4 times the mass of the sun is squeezed into a space that has the diameter of the island of Manhattan, for example. So something that's the size of Manhattan has 1.4 times the mass of the sun in it and spins around hundreds of times per second. Those are the objects that we use as gravitational wave detectors.

So the way that we can do that is because the spin axis of the pulsar is misaligned with a little bit of a little bit of. its magnetic field line. So if it's, this is the spin axis, and then the magnetic field line is this way, then every time it spins around, it sends a flash of radio waves towards the Earth. And each one of these flashes is very regular. So we know exactly when the flashes should arrive at the Earth.

And of course, we measure when they do arrive at the Earth. And the difference between when we expect the signal to arrive and when it actually arrives could signal the presence of gravitational waves. Now, that's because gravitational waves change the distance between the Earth and the pulsar. And so gravitational waves stretch and squash the fabric of space time, right? In this kind of plus fashion or in a cross fashion, there's more, if you believe, other theories of gravity or extensions to GR. We can probably talk about that later.

Yeah. So the important part, though, is this time advance or delay, depending on if you're being compressed or stretched out. And that's what we can find using these ultra-stable millisecond pulsars. And so how did the idea of nanograv, how did that come to be, what was sort of the impetus for it, and how is it designed, quote-unquote, to do what it's doing? Well, the history of nanogram and of pulsar timing really goes all the way back to, I mean, to the discovery of pulsars in 1967 by Jocelyn Bell and her collaborator.

She was one of the first people to discover pulsars and then millisecond pulsars, which are the ultra-stable version of these, were discovered in 1982, so in fact, not that long ago. The idea, though, of using a regular signal and trying to change differences in the arrival time in the signal, goes back to 1975. when some scientists at JPL proposed to do this with the Pioneer 10 spacecraft. And that was an amazing idea. They were the ones that proposed like, oh, you know what, this spacecraft is going really far away. And actually, if we can detect changes in the arrival times of the signals from this spacecraft, then we might be able to look for gravitational waves. And so by using these ultra-stable milliseconds pulsars together with this idea of,

of changing arrival times, we can come together with this idea of pulsar timing arrays. But, I mean, it really took a lot of people, a lot of time to get the idea off the ground. The work that I think is the, that forms the basis of our current collaboration is the 1983 paper by Hellings and Downs, who really had the idea of cross-correlating all of these millisecond pulsars to look for a gravitational wave signal that's common in all of them. So nanograv in itself started about 13 years ago, and the idea of pulsar timing arrays, of course, is much, much older than that. So nanograv is only one of the pulsar timing arrays that exists around the Earth.

There's also the Parks Pulsar Timing Area in Australia and the European Pulsar Timing Array, and there's some emerging Pulsar Timing Arrays. Now the Indian Pulsar Timing Array, South African Pulsar Timing Array, array, Chinese pulsar timing array. And so together we share data and are forming the international pulsar timing array. Oh, wow. Now, the results that you announced last month at the American Astronomical Society's meeting, those were acquired recently. Are they from a decade ago? I remembered something like 12 and a half years plus more years. So can you clarify how much data was actually obtained to make this announcement,

and then where does the future see us going in this particular direction? Yes, so at the American Astronomical Society meeting, we announced that we discovered the first hint of a gravitational wave background. So this took 12 and a half years of data, and that means hundreds of people have been timing these millisecond pulsars.

In this instance, there were 45 millisecond pulsars that we used. And their length span from three years to the full 12 and a half years, which means people have been going back to these pulsars roughly every two weeks for 12 and a half years to time them to look for gravitational wave signals. So I'm not sure you can be too upset with us, Brian,

for having a hint of a signal after 12 and a half years of doing this experiment and timing every two weeks. Right. Now my, my, my, Soon to be five-year-old always wants to give me a hint of what I'm going to get her for her birthday. What does it mean a hint? What does that mean? Is that like a scosh or a little tidbit?

How do we quantify what a hint means and why do you use that particular claim rather than saying detection or evidence? This episode is brought to you by Netflix. Most valuable promotions in Netflix are hosting a blockbuster triple headliner Saturday, May 16th. Rhonda Rousey returns to face fellow woman's MMA pioneer Gina Carrano in the main event. Plus co-main's Nate Diaz versus Mike Perry and the best heavyweight in the world, Frances Ngano versus Felipe Lins. Watch Rhonda Rousey versus Gina Carrano, live only on Netflix. Saturday, May 16th at 9 p.m. Eastern Center time, 6 p.m. Pacific time. Right. So detection and evidence for our very technical words that have very strong meaning in the scientific community.

and we're not that confident yet. And so what we found in the 12-and-a-half-year data is a signal that's present in all 45 of the millisecond pulsars, and that signal has the same amplitude in all of these 45 millisecond pulsars. Now, we've tried to figure out if something else could be generating the signal. And so far, we've come up short. So we can't think of anything else that could be generating the signal, but that doesn't mean that there isn't something else.

So what we expect when we do eventually make the detection is that there are two parts to the detection story. The first is seeing this common amplitude, this common low-frequency signal in all the pulsars. Now, these millisecond pulsars are scattered about in our own galaxy and they're thousands of light years away from the earth and from each other. So having a signal that's present in all of these pulsars at the same time is really exceptional, right?

And one of the only physical processes that could be generating that would be this gravitational wave background. And we believe that this comes from the cosmic merger history of supermassive black holes. So the first step is this common amplitude. Now, the second step is looking for this signal that you get when you cross-correlate all of the millisecond pulsars. Now, according to Einstein's theory, of general relativity, there is a quadrupolar type shape.

So you start up and it goes down like a cosine, but it doesn't quite come up again to the same level that it started. So there's this distinctive shape, and we call it the Hellings and Downs curve. And when we cross-correlate all of these millisecond pulsars, we expect this shape to emerge from the data, these spatial correlations. So if two pulsars are nearby to each other,

they'll be positively correlated. And that means that if they move, they move like that. this at the same time. Now, if they're negatively correlated or anti-correlated, like two pulse separated by 100 degrees on the sky, then they would move like this with respect to each other. So they'd be anti-correlated. And so this hellings and downs curve tells us how all of these different pulsar pairs should be correlated if they're under the influence of a gravitational wave background. We have not seen that part of the signal yet. So we've seen part of it, what we believe to be part of it. But the second part is what we definitely need in order to have a smoking gun kind of

detection of the gravitational wave background. Now, for anyone who's studied statistics on the call or has ever placed a bet at a roulette table, right now we believe that the evidence for there being some common signal in the pulsars, and it's not just some intrinsic noise in the pulsars, that the odds of that are about 10,000 to 1. So I'm definitely putting my money on that, right? But the odds of that common signal also having these spatial correlations that we expect from a gravitational wave background, right now is about three or four to one. So good odds, but not really convincing enough to have a press conference

and, you know, say that we've made the ultimate detection of a gravitational wave background. So we think that we're getting close because we've seen part of the signal. But we need about two and a half or three years more data in order to really be sure, in order to see those spatial correlations that are emerging that are predicted by general relativity. Oh, very good. So just reminder to folks that are listening on Clubhouse. We're also live streaming and recording to YouTube. You can catch that there with some more information about key.

And we are, for those of you on YouTube, then I want to ask a question with your verbal abilities. I will take questions in just a bit. I do have a question on YouTube from one of my listeners, American Magna. He is, or she is asking, how has the signal changed at all, even by the same amount in the last 12 years? Has there been noticeable variation or time variability or has it been relatively consistent over the last decade plus? What a great question. So the signal itself has a different amplitude at different frequencies, right? And we call that a spectrum.

And we haven't been able to get our detector down to the level where that signal exists until now, where we've discovered part of the signal, this correlated amplitude, but not yet the spatial correlations. And so it's this interplay between what's generated in nature, right? gravitation wave background that comes from potentially the cosmic merger history of supermassive black holes and our ability to get enough pulsars that are sensitive enough to bring our instrument down to a sensitivity where we can detect that signal. So I think I'm going to take another question.

Those of you on Clubhouse, please feel free to raise your hand. I'm happy to take you up on stage while I am also asking a all my questions of my friend Kiara. There's another question of, can you reveal more about the properties of these mergers? You know, someone's asking, could you see hawking radiation? Are there other features in kind of the black hole or merger history that would maybe have some impact

on our understanding of GR or perhaps of our understanding of at least the statistical behavior of these objects? Learning more about their intrinsic properties rather than them as a collective. Brian, you have a very intelligent audience. This is a fantastic question. Well, they rise to the occasion. I didn't plant this question. No, it's not. No, it's another Mingarelli. I don't know. It must be some other, you know, just a coincidence.

No, no, I'm just kidding. I was going to say, is it my dad? That's my dad. Okay. Right. So absolutely, yes. Now, I was telling you a little bit earlier that you have this amplitude of the gravitational wave background that's a function of its frequency, right? So it should be some sort of, you know, tilted line like this, right? We call it a power law. It really just means that the amplitude scales, like the frequency to the minus two-thirds. Now that's true if black holes are circular when they're emitting these gravitational waves and that nothing else is interacting with them. However, we know that supermassive black holes in these binary systems are in the centers of galaxies and they're surrounded by gas and dust and stars. So if stars cross the orbit of

the binary and are slingshot out, that's going to take away some energy from the binary. And the energy is not gravitational wave. So in fact, it changes the separation of the binary in a different way than gravitational waves do. So if stars are the dominant way that these supermassive black holes are merging at very low frequencies or very wide binary. separations. That'll change the shape of this strain spectrum. That's what we call it. But it's really just this kind of behavior that we think should be a straight line and makes it turn over at very low frequencies. Similarly, if your black holes have an accretion disk around both black holes, we call it a circum binary disk, but it just means that there's an accretion

disc around both black holes. They're in the middle. That can torque the binary. And that also takes away energy and angular momentum from your system and causes the black holes to merge more quickly than just gravitational waves alone. So both of these processes can leave imprints and what we measure when we measure the gravitational wave background amplitude as a function of frequency. Very nice. So next we're going to take a question from the audience at Clubhouse. And we have Eric and Miguel Yeti Tears, aka Yeti Tears Miguel Tully. And first we have Eric, Eric, do you have a comment question or topic for Kiera? Just have one question.

How do you confirm that dark matter isn't responsible for some of the effects and delays that you're saying? That's a great question. So dark matter wouldn't have the same structure that we see when we cross-correlate the pulsar. So I was talking a little bit earlier about this quadrupolar type shape we expect to see when we're cross-correlating the millisecond pulsars. Dark matter wouldn't create that distinctive shape. However, you'd be correct in thinking that dark matter has a gravitational field. And as a pulsar pulse traverses the galaxy, that could cause a timing delay, right, from that one pulsar on its way to the Earth.

That's absolutely right. But that time delay would not be the same in all of the millisecond pulsars your timing. and it wouldn't have that spatial structure that Einstein tells us should be there. So for an individual pulsar, absolutely, that's a really interesting way of trying to map the Milky Way and its gravitational potential. It's a really interesting idea. But we're sure that what we're seeing right now is not due to dark matter because it doesn't have that, well, because dark matter wouldn't have that quadruolar type shape. And it would be different for all of the pulsars in this.

the array, depending on what the local structure would look like. Save now at Whole Foods Market. It's the summer splash event with great everyday prices on 365 brand ground beef for the grill and ice cream for dessert. They have yellow sales signs on ready to cook beef or chicken kebabs too. Level up with savory marinate, spices and rubs, and complete your cookout with a crowd-pleasing cherry pie and their balsamic chicken salad. Available at the prepared foods counter.

Get Summer Splash Savings Now at Whole Foods Market. Ah, very good. Thank you. Thank you, Eric. Great question. Yeah, very, we have the brightest minds and the multiverse. And the only thing I pride myself on, Kiara, is that I give, I invented Clubhouse streaming for scientific podcasts, interactivity, as well as taking it live. I mean, there's lots of great people that do wonderful conversations with folks all over the internet, especially in science.

I'm not the only one, but I love the fact that my audience can interact with my guest, because how often is it you get somebody as great as you? In fact, Kiara, you'll be interested to know that someone by the name of Michael P. says the following, uncontroversial statement, Kiara is awesome. Smiley emoji with sunglasses, Kiara. You should be so thrilled. I hope you can feel that. I don't know who Michael P is.

You have any sunglasses here. I have my regular glasses. So let's just pretend these sunglasses. That's right. You must be a nerd like me. Physics professor. Physics professor. That's right.

Just like I aspire to be someday. Okay. Ibrahim is asking a question about the computational requirements. This is a cool question. I was interested too. What kinds of computation? Why are you using the CCA, the full power of the CCA or are you using it? What kind of computing power?

If I want to do it at home on my Motorola flip phone, Can I do it? Your Motorola flip phone. Oh, my goodness. I don't think that your Motorola flip phone could do it. Maybe if you had a Blackberry back in the day. Oh, you Canadian you. Exactly, right?

Go REM. So let's see. Computing resources. So I use computing resources differently, like in my own personal research, than what we use for these big gravitation wave background searches. And that's also different from what other people at CCA use. the computers for. So in my personal research, what I try to do is create 100,000 versions of the local universe and try to figure out the statistics of which local galaxies can host supermassive

black hole binary systems, for example. So my computations are what we call embarrassingly parallelizable. So, you know, one realization might take five minutes, but if I have 10,000 cores, everything can take five minutes, right? It just depends on how many times I can run this simulation over and over and over again to get this large number of statistics. Now, with the nanograph calculations of the gravitational wave background, and I should say this paper was led by a postdoc called Joe Simon, there you have these large Markov chain Monte Carlo analyses that are taking place, and not only do you have 45 milliseconds pulsars that you need to solve for their intrinsic red noise, for example,

but they have lots of underlying parameters. Each pulsar has almost 10 different parameters in itself to solve for and the amplitude of the gravitational wave background and that frequency dependence that I was telling you about. That goes like frequency to the minus two-thirds for supermassive black holes, but you can leave that minus two-thirds as a dependence to solve for to say, well, what if I'm completely agnostic, right? What if it's not minus two-thirds?

What if it's something else? I'm going to also solve for that parameter. So you have these huge MCMC chains that can take weeks to even longer, potentially longer, to converge on an answer. So that's how we use supercomputers in that sense. And at CCA, I have some colleagues who work on large cosmological simulations, like illustrious, Eagle, Simba, and those just basically take as much computing resources as you have, right? That just kind of just spreads out and takes everything that you possibly have available.

So I like to tell my students that for us, you know, our work is going to take, you know, for three to four weeks per year, for my personal research that I lead, I'm going to saturate all of the CCA computers. and then we analyze that data for the rest of the year, and then we come back. So we have a nice mix of people who use the resources in different ways, and that way everyone gets along. Right, and no computing resource is wasted. Question from Jose, could you detect longitudinal gravitational waves in the background if they exist? Absolutely.

So these would be extensions to general relativity, and they would have different correlation structures. So you're clearly an expert. So I'll tell you a little bit more information. So general relativity have these plus and cross polarizations, right? Which means that's how gravitational waves can change the fabric of space time. You can also have breathing modes, right? In these extensions to GR.

So you have plus and cross and a breathing mode. You can also have in other theories gravitational waves that extend space time along the way that it's traveling. And that would create something that doesn't look like a typical quadrupolar hellings and downs curve. So Sydney Chamberlain, K.J. Lee, J. Lee, J. Siemens have all studied these results and can show you exactly what these different correlation curves look like. Very good. And we're getting towards the end. We have about eight minutes left before I have to break off into one of my telecons. You know, K.R., I thought as an astronomer, I'd be on telescopes all the time. I'm just on telecons all the time. It's very frustrating. I know you can

sympathize with that. So where do we go from here? When I looked at this Emmy worthy or Oscar worthy video that Yukon Huskies put out last month, this is just so spectacular. It's on Kiera's website, and you guys should watch it on YouTube. But the implications we might be able to discuss or detect with nanograv, you might be able to detect the primordial gravitational wave background. Now you're infringing. Then we would have beef, right? Yeah, that's right. Now you're infringing on my test.

I don't care Canadian or not. That is a shot across my bow. I would not be sorry if we detected a primordial gravitational wave background, just to be clear. I would not either. I would not either. Let a thousand pulsars in CMB modes bloom. Can you tell me, though, how would that work?

And how would you distinguish it from the merger block? background that you may have indeed detected. Right. So this, the way that the amplitude of the background changes as a function of frequency can tell you what's sourcing or generating the gravitational wave background. So a primordial gravitational wave background, say these, you know, quantum fluctuations in the early universe that are then through inflation, you know, the whole size of the universe can induce gravitation. waves that we can detect. That has a different spectral dependence.

So instead of going like frequency to the minus two-thirds, a primordial background should go like frequency to the minus one. And for the experts, that's that it's flat in Omega-GWs like we expect. But you could have other kinds of cosmologies. If you believe in bouncing universes, ectoporosis, or string gas cosmologies, other kinds of exotic physics, then you can have something called a blue-tensurial. or index and you could potentially detect a signal that is visible in the pulsar timing

rate part of the spectrum and not necessarily visible in or could still be hidden in terms of the LIGO gravitational wave background. So instead of having something that's flat, it could tilt up and be blue in that sense. So if things tilt up, they're called blue. If they're tilt down, they're called red. But yeah, you could have these blue tend to be blue tenders. or indices that could be really interesting too. But this is far from standard cosmology, as you know.

So there is a question by Ivan on YouTube that ask a question that's kind of painful for me to suggest, but as you know, there have been claims of detections of the primordial gravitational weight background that were later found to be contaminated by dust. I happen to know a little bit about that. Yes, yes, Kiar, it's true. That was in the introduction to my PhD thesis, by the way. I was so excited. I wrote like a whole section on Bicep 2.

I was like, this is so cool. We have to retract your PhD then, K.R. I'm sorry to tell you. No. The PhD Poulouse will come and take it away from me. Ivan is asking, could there be any local change in the ISM, maybe the dispersion measure or something like? Could that explain the signal?

Is that a big systematic? It's a big systematic. It's something that we think about a lot. In fact, in nanograph, there's an interesting. media mitigation group. All they do is think about this. So the interstellar medium could indeed be a correlated signal between all of the pulsars or many pulsars in a particular line of sight or just depending on if there's like a blob of something that's affecting multiple pulsar lines of sight. It doesn't have the quadrupolar type shape, right? But it couldn't do some sort of common time delay.

And so something like that, you know, we can't rule out at the moment, even though we think that that's really unlikely. And so again, we're not claiming a detection until we can find this quadrupolar type correlation that we call the Hellings and Downs curve. But yes, it's a galactic scale, gravitational wave detector. We can't turn off the pulsars and turn them on again and see if they behave differently. We don't have a nice vacuum cylinder that goes all the way to the pulsar to make sure that they're there's no gas and dust along the way. So we can learn a lot about the interstellar medium. But as your very bright listener points out, it can also be a pain.

Yeah. So last question from YouTube. I'll take one from Clubhouse, one more from YouTube, and then we have to break off. Actually, I have one question as the host of the Into the Impossible Podcast. So find us on YouTube. If you're on YouTube, give it a thumbs up. Give me a subscribe. Tell me what you learned from this episode.

and hopefully we'll get Keira back if we get enough ground swell. I promised her I wouldn't tell her how many hundreds of people are watching her right now. Okay, last question from Ibrahim on YouTube is also very interesting. Could you tell anything about the effort to map the Milky Way's gravitational field and whether or not nanograv is participating in that effort to map out, presumably influence of dark matter and other features in our galactic gravitational potential? It's a really interesting and exciting project. Right now it's not one of the key nanograph projects, although there are a few scientists who are involved in doing that.

But these are more of our pet projects right now, not one of the flagship results that we're really putting a lot of person power into like gravitational wave background detection. And then after we detect the gravitational wave background, it'll really be a background. In fact, it's a bit of a misnomer. Right now it's the foreground signal. It's what we're looking for. And hopefully one day it'll be a background signal that we're trying to subtract. Right. And then we can look for things like an aisotropy in the gravitational wave background.

And this will look a lot like one of the CMB maps. Maybe you even have one of those beach balls in your office, Brian. I do. And then we can find individual supermassive black hole binary systems or when we can eventually constrain how that, that amplitude evolves as a function of gravitational wave frequency, then maybe we can say, you know, it's 90% supermassive black holes and 10% cosmic strings. Or maybe it's, you know, 80% primordial because that's just the way the universe is and the rest of it is from like

this puny population of supermassive black holes. But really, you know, it's it's all like perosis. Very good. Lindsay Forbes is a good friend. of the show. It just says, Dr. Kiera, you explain this complicated stuff so well, and I love the way you use your hands to explain the signals. Thumbs up. So that's a tip to folks to, yeah, she's, I don't know,

you might be part Italian or you might be Jewish. I don't know. We use our fingers and hands a lot, at least in my culture. Kiara, thank you so much. There's one from Ernesto. I have to take it. He's a good friend of the show as well on YouTube. Is there a noisy or random nature to the gravitational weight background pattern?

or does it manifest some regularities? How, what a great question. So the gravitational wave background itself is stochastic, or at least we believe it to be stochastic, which means random, right? And so this happens because supermassive black holes over their cosmic merger history, they evolve really slowly at these very low frequencies.

And so there's a pile up of sources. They're all in this one to 100 nanohertz range, where we have our experiment at the same time. And then eventually they take off and merge. But for example, if you have two billion solar mass black holes that are merging and their equal mass, let's say, they are about 25 million years from merging at one nanohertz, right? 25 million years to get across the pulsar timing array band. Now compare that to LIGO, where their sources are in their band for a fraction of a second,

25 million years. So you can get a lot of super massive black holes at low frequencies over 25 million years. And that's what creates this background. Now, even though it's a stochastic signal, it has things like an amplitude and this spectral dependence. And that means that we can define some global properties of the background. Even though we can't resolve individual sources, we know things about the ensemble. of super massive black holes that we're looking at. So this amplitude, this frequency dependence, for example. Very good.

Okay. Miguel, do you have a quick question on Clubhouse? Otherwise, I will finish up. Or maybe Rockwell, if you have a question for Kiara, I'm inviting you guys up. And Azada, if you would like to ask a question, I'll invite you up. So this is clubhouse, we've got to get Kiara on Clubhouse because, as Michael is saying, on YouTube, definitely have her back. This is a great mind bending.

This is great mind-bending stuff. She is bending my mind more than gravitational waves bends space time. Wow, Michael. Well, that's about one part in 10 to the 15, so it's not actually a big compliment. You just insulted her. Michael. That's one part in a million billion, Michael. Just to let you know.

Try harder, Michael. Let's see, Miguel, you're up there. You got a quick question for Kiara before I asked the final question to her. How are you, Chiar? Go ahead. She is listening. Just wanted to know what is your most cherished piece of memorabilia from your career. Oh.

Can I go get it? It's in the fireplace, man. Tell me. Oh, yeah. Go get it. All right. She is, for those of you not watching on YouTube, take the opportunity. Go to YouTube.

Yeah, I see you there. Thanks, my brother. So Kear is back. Here she is. And memorabil. Go to YouTube, Dr. Brian Keating, if you want to see it live. Oh, my gosh. is that? Yeah. So the first year of my PhD, I worked on experimental gravity. I was looking for

compactified extra dimensions predicted by string theory. And this is at the University of Birmingham. And I'm clearly a theorist, but when I got to Birmingham, the experiment that I was supposed to use to look for, compactified extra dimensions, was not built. So my then supervisor told me to design an interferometer that could make nanometer surface corrugation measurements so that we could make sure we weren't detecting the Casimir force, that we were detecting torques from extra dimensions. So as a good theorist, I was terrified and then decided to get to work. So I built and designed a Machzender interferometer that could do nanometer surface corrugation measurements. And part of that was doing a machine class where I machined all of the components for my interferometer. And this is

one of the components that I made. So I made absolutely everything here. The screws, the threading inside. I used a lathe to machine all of the aluminum. This holds a laser diode right here and that you can screw into place with this screw. There's a lens here to focus your beam. And then you can take off the front to put in a new lens. And then this, you know, attached to the top of my optical tabletop experiment. And I ate lots of Thor snacks while I'm Thor lab snacks. Those of us who build things know exactly what Kira is talking about. Yeah, I think that's the most cherished thing that I have in terms like a physical thing.

You know, I have a rule against inviting theorists into my lab. But now I know I will be safe to invite you into the laboratory. I actually had Sir Roger Penrose in the lab last year. And we're still cleaning up the debris. This is pre-Nobel Prize. So afterwards, now we can't even fit his ego. No, I'm just kidding. He's a little.

All right. So we've got all these compliments. They want to have a show just with you for another hour. We were going to make that happen. Kiara just reassured me in humanity. Gave me hope that they're intelligent, hardworking people trying to understand the secrets of the underlying universe. That's really beautiful, Ibrahim. Thank you so much. Lindsay says you got to get you back. Javier says, extraordinary talk for the non-specialist.

Thanks. I want to ask you one last question if you are willing in the remaining minute that we're over. I want to ask you, Keira, a question I ask all my guests from Nobel Prize winners to billionaires to Olympic athletes, to astronauts. I had an astronaut on recently. I'm going to ask you this question. And I apologize for not alerting you ahead of time. But maybe you watch some of the video homework assignments I sent you. I didn't get to the end of them. I wish I did now. All right.

Now you've got your homework set up. I want to ask you, the title of this podcast is Into the Impossible, and it derives his name from Sir Arthur C. Clark, who had all these famous aphorisms, one of which is any sufficiently advanced technology is indistinguishable from magic. Another one is, for every expert, there's an equal and opposite expert. and the third one kind of relates to advice. I want to hear your advice to your former self. And it says it goes as follows.

The only way of determining what is possible is to venture beyond the limits of the possible into the impossible. And that's the name of this podcast. I want to ask you as a younger person, you know, maybe 15 years ago, 10 or 15 years ago, what advice would you give to a younger version of Kiara to help her have the courage to go into the impossible?

What would I tell myself? I would tell myself to hang in there that nobody has all the answers and everyone is struggling just as much as me. And that even people who you think have it all together and that are brilliant, they're just people. There are some people who are more famous than other people. There are some people who are smarter than other people, some people who are kinder than other people. but no one is better than you and everyone struggles and everyone you know has has a whole history of things that you don't know anything about that's affecting them every single day so i would tell myself to hang in there and do your best and you can always be proud of things even if it doesn't work out right like like my tabletop mock sender interphromer I didn't complete the experiment. I started working on gravitational waves instead,

and it's going pretty good. The world is richer for that, and the universe as well. Kiara, I want to thank you so much. Dr. Professor Kiara Mingarelli, the University of Connecticut stores, Go Huskies, my mom's alma mater, and also of the Flatiron Institute, the Center for Computational Astrophysics,

where... Team Simons! Kiara, we all are huge fans, obviously. We've got to have you back. Maybe in a few months, you'll grace us again with your presence. And I want to thank you so much from the bottom of my heart for going into the impossible with my audience. Oh, thank you so much for having me. I'd be happy to come back.

Any sufficiently advanced technology is indistinguishable from magic. Hello, I'm Stuart Volko, producer of Into the Impossible. If you enjoyed this episode with Professor Brian Keating, please let us know by subscribing, commenting, sharing, and most importantly, rating and leaving reviews. It really helps keep our universe expanding. We appreciate hearing from you and read every review and comment. We're always open to your suggestions for future episodes. Watch our YouTube channel at Dr. Brian Keating, and join our premieres every Tuesday at 8 a.m. Pacific

time for live chats. Follow Brian on Twitter, Medium, and support us on Patreon at Dr. Brian Keating. That's DR. Brian Keating. For free access to exclusive content, please visit Professor Keating's website and sign up for his informative newsletter at briankeating.com. Into the Impossible is produced with the Arthur C. Clark Center for Human Imagination in the Division of Physical Sciences at the University of California, San Diego. Eric Viri, Director, Brian Keating, co-director, Patrick Coleman, Associate Director, produced by Stuart Volko and Brian Keating. For more information on the Arthur C. Clarke Center, go to imagination.ucsd.edu.