Black Hole Mergers and Multi-Messenger Astronomy - Sixty Symbols

One of the great things, I think, about 60 Symbols, Brady, is that we've been doing them for a while now, and we've covered quite a few topics. And, of course, it's not that you do a video about a topic and then everyone who's researching into that topic says, oh, there's a 60 Symbols video, let's not work on it anymore. They carry on. There was an update by the LIGO Consortium. These are this amazing group of experiments, which were the first direct detection of gravitational waves. And the consequences of this has opened up a new field of astronomy, really, which we can call multi-messenger astronomy, which we can describe. Their first observation was in 2015, and then by 2017, its significance was recognized enough to get three of them Nobel Prizes. So we're heading on to six years on. They've released their third set of data. They've now got 90 events, which might not sound like a lot, given that you've been analyzing things for six years, but when you realize the majority of these events are from colliding black holes. So black holes, which have been orbiting each other and then collided together, merged, if you like, coalesce is the technical term, produce gravitational waves, waves, ripples in the space-time of our geometry, which then propagated out and arrived at these detectors. If I tell you it's taken over a billion years, basically, for each of these events to arrive at our detector, it kind of gives you a feel for, OK, these are very sensitive detectors. They're fairly rare events, but the fact that they've managed to go from one in 2015 up to, actually, they've reported on 35 in their last run, which was from November 2019 to March 2020, shows you that they're ramping up, that they're getting their detectors, sort of, they're homing in on how to get them as precise as possible. They're reducing all the possible errors and their sensitivities are increasing. And we're finding some really quite beautiful results emerging. And I thought I'd just sort of try and give an update on where I think we are with it, and maybe just a little reminder of what LIGO is. So LIGO, there's the LIGO and VIRGO, the two main detectors. LIGO has got two detectors themselves in America, one in Louisiana and one in Washington state. Each of the detectors are made up of two arms, which are about four kilometres long. They're called an interferometer. Laser beams are shown up and down each of these arms with incredible accuracy. So at the end of each of the arms, there's mirrors, and these mirrors are suspended in such a way, they're basically in free fall. They don't feel the seismic vibrations or the thermal fluctuations, or the experiments can handle all of these potential problems they have. But the laser beams bounce off the mirrors, and then they recombine them. So one set of laser beams has gone down one arm, and the other set has gone down the other arm, and then they bring them back and recombine them, and they look for path differences. And they set it up initially so that there's no path difference, and then the idea is, if a gravitational wave, a distortion of the space-time, comes by, it will distort one of these ever so slightly, and then these two beams will have passed a different distance from one another, and they'll pick up an interference pattern. And just to give you an idea of the sensitivity, these lasers are travelling four kilometres. The distortion that we expect to arrive from these gravitational waves, which, just to remind you, have typically come from a pair of black holes, which have been orbiting each other a billion years ago, coalesced together, it's been travelling for a billion years at the speed of light, passed through the detector, it distorts the detector by 10 to the minus 21 centimetres. The size of a proton is 10 to the minus 18 centimetres. This is a thousand times less than the radius of a proton. So the sensitivity of this experiment is quite staggering. And so there's one in Louisiana, there's one in Washington State, and there's one in Italy. There's a new one that has just come on board in Japan as well, making four sets of detectors. In the next decade there's going to be one coming in India. Let's stick with the two in the US. What they look for is, if this gravitational wave has come in, say, and hit the one in Louisiana, then they don't interact with anything, they just carry on through, distorting the space-time ever so small, and then it will eventually go through the one in Washington State. And so what the detectors look for is joint events. They look for the same signal. From that signal, they have two ways then of determining what they're finding. They have a bank of what you call templates. These are like images that they have already calculated, that they know what certain objects should do, what the signals should look like. So two black holes merging, they know what they should look like. Two neutron stars merging, they think they know what that should look like. A black hole and a neutron star merging together, they think they know what that should look like. So they then compare these images to these templates, and by then tweaking the parameters of the models for the templates, they can see which best fits the data. And that's how they've been able to give so much information about the black holes. But they've got another way, which I had forgotten about. And this other way is, imagine there's out there an object which we haven't yet discovered. And deep down I know, Brady, that the real reason you've got into this is that you do have your own star. Let's call it the Bradyon, the Bradyon star. It's a compact object. We've yet to see it. In particular, we don't yet know what happens when two Bradyons collide together. So they have this other way. So the LIGO people say, don't worry, Brady, we'll find it. And what they look for is a second way of analysing the data. They just look for the synchronicity of the events coming through. And if the events that go through Washington State and LIGO and Louisiana look very similar to one another, then they can take them out and begin to probe, look at those events themselves. They don't need the templates. And from it, eventually they'll probably have to wait for a second Bradyon star merger to occur. They'll be able to infer we've got some new thing out there. And that's one of the things they're doing. They look for unexpected events that we don't yet know what are forming. Aren't there particles called Bradyons? Have you got your own particles as well? There already are Bradyons. I've been wanting to ask you about them for years. Do that another day. We'd better look them up. So of course, we had the first detection in 2015, which was phenomenal in its own right. In 2017, they then discovered, and we have discussed this briefly, but I think it's worth mentioning, they discovered their first case of neutron stars merging together. And what was so important about that is when black holes merge together, the only thing that sort of produced is gravitational waves that propagate out. But neutron stars are full of neutrons. And so when they merge together, you not only produce gravitational waves from the as you distort the space time with the great masses of the neutron stars, but because they've got neutrons in there, you produce electromagnetic radiation as well. And so now what you can look for is detecting both the electromagnetic radiation and the gravitational waves. And one of the key results that came from this 2017 observation was that the gravitational waves and the electromagnetic radiation, the X-rays, arrived sort of on Earth. In fact, there was a satellite just above Earth detecting the X-rays, within 1.7 seconds of each other. So they've been travelling for 300 million years to get to us. So to arrive within 1.7 seconds over that distance means that these two waves are travelling at the same speed, or extremely close to the same speed. That had the knock-on effect of ruling out a whole load of cosmological models of modified gravity, which were trying to understand the acceleration of our universe by modifying general relativity. And in this one observation, whoosh! So there were some theories that were saying that the gravitational waves and the electromagnetic waves should have been travelling at different speeds. Exactly. And one of them was ours. The Fab Four was one of them. Oh, you lost a theory. Oh no. We made a video about that one. We did. You'll have to just bin that one now. It's a nice idea. I really think it's quite a nice idea. It may all collapse. That's the nature of research. So they've carried on, of course, making more observations. In the most recent set, they announced 35 new candidates. There's a beautiful picture you can see of the black holes. It's a diagrammatic representation of what's happening. Of these 35 candidates, 32 are black holes orbiting each other and coalescing. Within that, there's some really interesting features. One of them that's got people quite excited is that they're beginning to see black holes, large black holes, masses of order 60, 70 solar masses, coalescing together. And they're beginning to see these what are known as intermediate-scale mass black holes. And no one really understands how they form. But they're beginning to see them in more abundance, even though they've got 90 events or so. The rate at which they're beginning to see them is already pushing some of the recognized models of intermediate black hole formation to their limits. They're also seeing some very light black holes merging together. And there's a really interesting property of black holes which I haven't really thought about. Black holes, of course, are regarded as being a... So masses not much above a solar mass, a few, five, six solar mass black holes merging together. It turns out the black holes change the curvature of the space-time. That's why we talk about black holes as being a great laboratory to test general relativity because we can test the strong curvature regime. And you might think to yourself, that means you must look at the really biggest black holes. They're going to have the biggest impact. But actually they don't. The curvature of a compact object is inversely proportional to the mass of the object. So the smaller the mass, the bigger the curvature. So if you really want to test strong gravity, you want light black holes actually merging together. The really big black holes, the really massive black holes, by the time you're out at the event horizon of the black hole, the curvature is actually quite low. And so you're not really testing the very strong regime. So they're seeing evidence of that. And then the other thing that they're seeing evidence of, which I think is really neat now, is that for the first time they're seeing evidence of neutron stars and black holes merging together. Is that a merger? That seems not a very equitable merger. Is it more of a takeover? Yeah, that's right. It's a snack. You see, yes, I think it's a snack. I think you're seeing the black holes snacking on the neutron stars. So they've got one in particular, which they think is really fascinating. It's a 30 solar mass black hole. And there's a 1.2 solar mass neutron star that's just sort of been merged into it, eaten by it. And that's at the very border of when a neutron star can form. But the hope there is that that also, because it's a neutron star, will be able to give out electromagnetic radiation. So you'll be able to test the merger properly. What happens as the object goes across the event horizon? And what can you tell from the form of the waves that have been produced? Another thing that they're finding, which I think is really interesting, is that black holes have associated with them certain characteristics. The mass is one of them. The spin is another one. And its charge, does it have any charge, is another. What they're now finding pretty concrete evidence for in a number of the mergers is that the black holes that are merging have spin associated with them. And that's particularly interesting. Those are called Kerr black holes. And what's particularly interesting about that is that because you've got a bit more information now about the spin of the black hole, whether it's spinning quickly or spinning slowly, then you've got some information that you can compare with your theories. Because different theories will predict different spins of black holes depending on whether black holes formed by objects passing by each other really rapidly, then you can imagine they'll have a lot of spin associated with them or angular momentum, or whether they're just simply objects merging together when you won't expect much. And so by looking at the distribution of the spins, we're once again going to be able to test theories of black hole formation. Another element which they've got very good data on is, first of all, the distance scale out to these objects. And as I said, black holes are typically finding black holes at orders of gigaparsecs. Remember what I said, a parsec's three light years. This is of order three billion years the light's been travelling. The Earth was forming around the time when some of these coalescent objects were meeting one another. It's just so remarkable that we can get data like this. Another thing that is of interest to people who are trying to understand the nature of black holes and their merging is, if two black holes come together, what's the likelihood that they're near equal masses as opposed to one being completely dominant over the other? And they're able to get that kind of information out of this as well. So, you know, I hope you get the impression there's a lot going on there. And there's a lot for the theorists to sort of mull over and get into to try and understand this amazing area. And the future is just mind-blowing as to what we'll be able to do. As they upgrade the detectors, we'll be able to see deeper into the universe. If you see deeper into the universe, just imagine a cone coming out from you and going deeper and deeper and deeper. You see more and more of the sky, so you're going to see more and more events. And then you'll eventually begin to be able to probe early universe phenomena. That's the hope, that we might be able to see evidence of phase transitions in the early universe. Something that I would love is if we saw any evidence of cosmic strings emitting their gravitational waves. So that's one of the things Advanced LIGO still looks for. It's called the stochastic background of radiation. And now with the Japanese gravitational wave detector coming on board and the Indian one in the next decade, having more and more detectors just means that you've got better resolution as to where the gravitational waves are formed from. And this is going to have an absolutely amazing impact eventually. Because there's a thing out there called the Hubble tension. We've spoken about the Hubble tension, that's this idea, the universe is expanding. And if you measure the expansion rate using distant measurements, like the cosmic microwave background, you get one value. If you measure the expansion rate using supernovae, which are closer, you get another value. And these two values are not overlapping. Both of those processes rely on you understanding the cosmology of the system. This method with gravitational waves doesn't do that. All the information is wrapped up in the signal that we get. And so as long as you can pinpoint how far away the object is, and actually the polarization of the light, then actually the expansion rate of the universe will pop out. It will come out. It will be a direct measurement. If they can get enough data, enough objects to test, it will be, within a few years, a few percent measurement of the Hubble expansion rate, which will be a massive feature. And there's so many other things that are out there waiting to be detected. This whole period of gravitational wave and multi-messenger exploration is fantastic. Why is it called multi-messenger? Multi-messenger means when there's two sets of waves, at least two sets, different sorts. Gravitational waves and electromagnetic waves, those are the two usual multi-messenger. But it could also be neutrinos, for example, being emitted, which will get emitted when neutron stars collide together. You told me before about how subtle and small the signal received is, just this tiny, tiny deviation in this laser caused by the gravitational waves, and presumably also sometimes a little bit of light or x-rays sometimes as well. On the other hand, you tell me about the incredible detail that this is shedding on intricate events that are happening at the edge of a black hole as a neutron star is swallowed and things like that. Can you really glean that much information about subtle things happening at that kind of scale, just from such small data, from such a trivial little blip here on Earth? I've not heard the laser data being described as a trivial little blip. But it is. All it is is like a flicker. It's a flicker, a tiny flicker. It is. And I realise the repercussions of it, but it doesn't seem like a lot of information to reconstruct the collision of two black holes. Absolutely. A crucial question. It's the question, right? And the point is, remember I mentioned the template. So the template, what they're really telling you is you're solving general relativity equations for two black holes of some mass, maybe some spin, and you believe in general relativity. Either you have to believe for this to work, or you could modify it and do the same thing, and that's also done. And you solve the equations so that the black holes are merging, they're merging, they're merging. And what you're predicting out of your template, your solutions, are the solutions for the gravitational waves, the distortion of the space-time. And what you actually predict, over what time scale, the amplitude and frequency of that wave should change. You have a very precise prediction for a given set of masses. And there's a little chirp at the end, usually, that happens just as they merge together. So that's your template, that's your theory, right? You've done it on some big supercomputer. What the data is doing, what the LIGO observatories are doing, is they're detecting that. And they actually detect, you're quite right, it's over a relatively short period of time, but they detect the final infalls of the black hole. And so there's a precise, they detect a precise thing, and actually a blip at the end. And then what they can do, so that's the data, it's got some amplitude and some frequency over a very short time scale. The significance of the precision of the experiment is really in that it's just staggering that they can measure this and having subtracted out all the other noise, the train that goes by the detector, the crocodile that walks along the ground that must be producing a vibration bigger than the radius of a proton, I think, right? Proton neutral. And the fact that they feel they can do that is the stunning thing. The observation is of some wave, and then what they do is they have a whole bank of templates of different masses of black holes, and they basically compare. You told me about how some of these findings have actually shot down a theory that you had hoped may have some value. To what extent has looking through all this data got you working on new theories? How much is it affecting your work? Yeah, I mean, I have started thinking now about, I've worked on objects which can become packed. They're called oscillons, which can collapse and then live for a long time and oscillate. It has made me decide that I think we really need to understand the template of these objects because they're believed to have been relevant in the early universe. In fact, as the period of inflation ends, they could go through a period of falling, these oscillons. And we've done some initial calculations of oscillons orbiting one another, but I've never been involved in a project where we've taken it really seriously and tried to understand the template. And reading about this, learning about this, has made me think, yeah, actually I would really like to do that. And the other object I've always been interested in, in fact, my wonderful PhD student, Espina, just finished her PhD last year on its cosmic strings. And looking at the emission from costs of strings. And so we've also been working on that to see whether or not we can find the template that they can use for LIGO. But LIGO already have a very good bank of people working on the cosmic strings. So that's kind of already sorted. But the oscillons and those objects, not so. And I think I'd love to know about that. As you can see, we've named them John, Paul, George and Ringo. Okay, so we sent it off to the journal and it immediately came back without having been assessed by any experts. Just saying, there's no way we're going to publish a paper with all these references to the Beatles in it in view of the Beatles limited contribution to cosmology.