Why A Medium-Sized Black Hole Is Surprising Physicists

17:08 minutes

two black dots resembling black holes coming together in the middle of a purple swirl
Still from a numerical relativity simulation showing the gravitational waves just after the merger, with the trajectories of the initial black holes and the horizon of the final black hole just visible in the center. Credit: LIGO/Virgo/D. Ferguson, K. Jani, D. Shoemaker, P. Laguna.

If you’re looking for a black hole, they normally come in two sizes. There’s the basic model, in which a large, dying star collapses in on itself, and the gravity of its core pulls in other matter. Then there are the supermassive black holes, millions of times the mass of our sun, that tend to be found at the center of a galaxy.

But recently researchers reported that they had evidence for two colliding black holes that created a surprising offspring. Their collision formed a middle-weight black hole, around 142 times the mass of our sun. 

Daniel Holz, a member of the LIGO team that spotted the collision, and a professor of astronomy and astrophysics at the University of Chicago, joins Ira to talk about what the observation means for theories of how black holes form and grow.

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Segment Guests

Daniel Holz

Daniel Holz is a professor in the Departments of Astronomy and Astrophysics, Physics, and the Enrico Fermi Institute at the University of Chicago in Chicago, Illinois, and a member of the LIGO Scientific Consortium.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow.


Hi, can I help you? Oh, you say you’re looking for a black hole? Well, you have come to the right place.

What size are you looking for? I’ve got your basic model here. It’s a big star collapsing in on itself. And the gravity of its core pulls in other matter, making it still more massive and more attractive.

Now right here is your supermassive black hole, millions of times the mass of our sun. That’s the kind of black hole that tends to be found at the center of a galaxy. Oh! You say you want the middle-sized one, yeah, I know, like, the Goldilocks size?

Well, I just so happen to have one that came in today. Because researchers just reported that they have evidence for colliding black holes of the middle size, a middle-weight black hole, maybe 142 times the mass of our sun. And that was surprising. Oh, now you want to know more? OK.

Joining me to talk about that finding and how it changes what we know about black hole formation is Daniel Holz, professor at the University of Chicago and the Kavli Institute for Cosmological Physics and a member of the LIGO Scientific Collaboration. Welcome back to Science Friday.

DANIEL HOLZ: Thanks for having me. It’s so great to be back.

IRA FLATOW: Did I get those sizes correct?

DANIEL HOLZ: Yeah, yeah. That’s right. We’ve got a whole panoply of black holes. And the ones right in the middle, these Goldilocks black holes, what we call intermediate mass black holes, are somewhat mysterious.

IRA FLATOW: Let’s talk about that. Because this was published in new papers. What did LIGO see or detect here?

DANIEL HOLZ: So LIGO detected what we affectionately call GW190521, or 190521. It’s just a date, May 21, 2019. And on that date, our detectors were on. And we heard a thump.

And that thump corresponds to two quite big black holes, by our measure. The larger black hole was 85 times the mass of the sun. And a smaller black hole was 66 times the mass of the sun. And those black holes orbiting each other and merged and emitted gravitational waves. And we detected those gravitational waves.

IRA FLATOW: Now why were people so surprised about this black hole?

DANIEL HOLZ: Well, there are a number of reasons. One is we’ve detected quite a few black holes at this point. And we’ve detected maybe 14 binary black holes that we’re confident about. And then there are many more that we’re still analyzing.

All of those are less massive. The most massive component of those binary black holes usually comes in at around 50 solar masses. And so so far, from that set of detections, it seems like there is a limit around 50. So then suddenly getting one at 85 is kind of a surprise.

There are also theorists in this game. And the theorists have said that there should be no black holes between about 55 and 120 solar masses. And so that’s a very strong prediction from astrophysical theory. So the fact that these two things, that the observations so far and the theory gelled, was something very satisfying for scientists like myself.

You know, we have our theory. Then we go and do the observations. And everything fits.

And then along comes this 85 solar mass black hole colliding. And that doesn’t fit! That’s not supposed to be there! So that’s why this is a very interesting and exciting event and somewhat disappointing event.

IRA FLATOW: Exciting and disappointing, does this mean you have to go back to the drawing board about how black holes form?

DANIEL HOLZ: Yes. So we’ve got to add to the story or change the story. We’re not sure exactly what’s going on now. The simple story, which is that you have a star, the star in this case may be two stars.

Each star collapses and makes a black hole. And then those black holes merge. So the stars are orbiting each other. Then you end up with black holes orbiting each other. And then the black holes eventually merge.

That story can’t account for what we’ve just detected. So we need a more complicated story. We think that a star, when it collapses and makes a black hole, that a star can’t make a black hole this big.

So either we’re wrong about that, which means our understanding of stellar evolution and stellar death, that there’s really a fundamental problem there. And that could be the case. And that’s exciting.

But we’d really have to go back and understand where things are wrong. And that involves a lot of nuclear physics, a lot of stellar astrophysics. There’s a lot of science there. And something might be wrong there. So that’s one option.

Or maybe that’s not the way these black holes are being formed in the first place. Maybe, for example, the black holes are being made out of smaller black holes. And then the smaller black holes find each other and merge and make larger black holes. And then that process repeats itself. And so this big black hole, the 85 solar mass black hole, is actually made out of two smaller black holes.

IRA FLATOW: If you were a betting man, which explanation would you put your money on?

DANIEL HOLZ: Well, so I’ve worked quite a bit on the stellar case, where it’s just two stars and the black holes aren’t made out of smaller black holes. People might expect me to bet there. And I am tempted to.

To be honest, what I’m doing is trying to come up with maybe even crazier explanations. And my current favorite explanation is actually something I’ve just been working on with a student at the University of Chicago, a graduate student, Maya Fishbach. And we’ve just finished a paper where we say maybe the black holes aren’t in this mass gap in the first place. Maybe what’s really happened is that there’s a black hole, the more massive black hole is even more massive.

And if it’s at 120 solar masses, then it’s big enough that once again, we can form it from stars. So I’m going with the even more extreme hail Mary, where we save our theory by making the bigger black hole huge. In that case, we’re OK.

IRA FLATOW: I like that. Go where no physicist has gone before.

DANIEL HOLZ: Exactly. Desperate times call for desperate measures.

IRA FLATOW: Just wondering what the new collision would sound like or look like if you compared it to the original collision sound we’ve all heard a few years ago, that chirp. Was it, like, 100 times louder? Or was it a different kind of sound altogether?

DANIEL HOLZ: So it’s a different sound. And it’s not that the volume, actually, this one was slightly softer. The first detection was really loud by our standards. This one isn’t quite as loud.

But what makes it absolutely distinct is that it’s much lower frequency. So while the others you know, like, the first detection, and then we had that binary neutron star detection, those sound more like real chirps that you can hear and goes whoop. This one is really just a thump.

It’s a thump. Very short thump, bump. It’s a fraction of a second. It lasted about a tenth of a second in our detectors and then was gone.

IRA FLATOW: Now does that sound itself, then, give you some weight toward one theory or the other?

DANIEL HOLZ: Well, what the sound tells us is that it was two unusually massive black holes by our standards. That’s really what that sound– because it’s so low frequency and so short, it tells us, boy, these are big. But it doesn’t give us any additional detail.

We’ve been analyzing it, hoping to tease out some information, for example, if the black holes are spinning and how they’re spinning. Each black hole can spin like a top. And by testing whether they’re actually spinning and the relative orientation of the two tops of the two black holes as they merge, that gives us information about how they’re formed. And so if we can get that information, that’ll help us figure this out.

IRA FLATOW: I’m wondering how you come up with numbers for the sizes of these black holes, like the 85 number. How do you estimate how big the black holes involved were?

DANIEL HOLZ: So it’s really by analyzing the sound very carefully. What we do is we analyze the frequency and the way that frequency evolves. And so we only get a few cycles. You can think of it as we’re only measuring the last few orbits before the black holes crash into each other.

But the gravity is very strong there. These are extreme objects, the most extreme objects in physics. And they’re going around each other at close to the speed of light. And those last few cycles before they crash into each other give us a lot of information.

And so by analyzing those carefully, we can figure out the masses. But as I alluded to before, we don’t get the masses really pinned down that well. We know the final mass quite confidently. And that’s at something like 140 solar masses.

So above this magic 100 solar mass delineation between what we call stellar mass black holes and intermediate mass black holes, we know the final black hole is above that. But the individual components, we don’t constrain as well. And they might be kind of in the middle, or as I mentioned, they could be further apart. That’s harder to get from the data.

IRA FLATOW: So what you need to know, what kind of data, what kind of theory that predicts the data, what kind of information do you need to have to pin down which option that you mentioned before is correct?

DANIEL HOLZ: There are a number of things. One is we’ll continue to analyze this data. And we’ll try to tease out as much as we can from this particular event. But the main thing is our detectors will– well, right now our detectors are off. But we’re going to turn back on in just over a year.

And we’ll be at increased sensitivity. And this is both the LIGO, the two LIGO detectors and the Virgo detector. And soon we’ll have the KAGRA detector in Japan joining our network. And eventually, we’ll have another LIGO detector in India joining our network.

And all of these will be even more sensitive and will detect many more binary black hole collisions. And if we detect many of these, and they all seem to look the same, it’ll be a clue, yes, you really have these black holes in this mass gap where they shouldn’t belong. They’re just there.

Because we’ll detect enough of them that, as a population, we’ll be able to infer these properties. And if we don’t detect them, then we know that this was kind of a one off and maybe it’s something else. We don’t know. We have to wait and see what we get.

IRA FLATOW: I’m Ira Flatow. And this is Science Friday From WNYC Studios. In case you just joined us, we’re talking with Daniel Holz about the weird world of black hole physics and some strange new discoveries.

You know, from the way you talk and from other astrophysicists I’ve talked about, the real joy is in the hunt, isn’t it, in finding these things?

DANIEL HOLZ: Yes. Absolutely.


So we’re constantly– and we have– and I’m sure you’ve read this. We have a whole system, our cell phones, and the second something happens, we get pinged. And we all run to our computers and look at what it is.

And is this an event? And if so, what are its parameters? And is it exciting? And should we trigger optical telescopes to do follow up?

And you know, all this, and this has just been the way we are for the last few years. Where we’re constantly waiting for the next thing to come into our detectors so that we can learn something new. And you know, we’ve been very fortunate. The universe has just been throwing black holes our way. And it’s been absolutely fascinating.

IRA FLATOW: Are you saying that you could turn an optical telescope, let’s say like the Hubble or something else, onto that spot and try to get an image of it?

DANIEL HOLZ: Yes. That’s exactly what we do. And of course, for black holes, no light comes out. And so you probably don’t expect to see anything. But who knows?

Maybe black holes aren’t completely black, which would be the discovery of the century. Or maybe black holes have other stuff around. And when the black holes merge, that stuff gets really hot, or the black holes end up plowing through another star that’s nearby. You can imagine scenarios where there is light.

Or maybe in some cases, you’re not observing black holes, but you’re observing either one or two neutron stars. Neutron stars definitely are expected to emit light. And with the discovery in 2017, which is what I would say is probably the most exciting discovery we’ve made so far– this was of two neutron stars colliding– in that case, we pointed telescopes and we did see light, including the Hubble. We pointed– essentially, every telescope in astronomy pointed at this object. Because everyone was so excited about it.

And we saw a lot of light. We saw a gamma ray flash. And then we saw just a bunch of optical light that came up. And then much later, we saw Radio, so light in the radio band.

We saw light across the entire frequency that light comes in over a period of weeks and months. And that was amazing. Now I should say, for this particular event, for the one we started the show with, this very massive event, some people pointed telescopes because there are groups that point telescopes every time we have an event, just in case.

And this group, called the Zwicky Transient Facility, ZTF, saw something a few weeks later that they say may be associated with the binary black holes. It’s very speculative. But they saw an active galaxy, what we call an active galactic nuclei, and AGN, they saw this galaxy get bright.

It kind of put off a flare. And they say, we think that flare may be associated with the black holes. Because it’s kind of in the same direction and kind of at the same distance.

IRA FLATOW: Just so that people don’t think this happened yesterday, I mean, this collision, this happened in a galaxy far, far away, many, what, billions of years ago, right?

DANIEL HOLZ: Yes. Yes. This is very, very far. This is 5 billion parsecs. So it’s something like $15 billion light years away. So it occurred when the universe was much, much younger.

In fact, we usually measure distance by this quantity called redshift, which tells us the relative size of the universe. And the universe was about half the size it is today. The observable universe was about half the size it is today when these black holes merged and emitted the gravitational waves in the first place.

IRA FLATOW: Well, it sounds like you have Christmas in September here, Dr. Holz.

DANIEL HOLZ: Yes. It’s really– it’s been remarkable. Just that this event has happened, we’re all just still struggling to understand what it’s telling us.

IRA FLATOW: Well, we’ve run out of time. We could go on forever. And I would like to pick this up somewhere in the future. You’ll come back and talk to us more about when you know more about it, right?

DANIEL HOLZ: Yes. I hope so. And I’m sure we’ll detect other things along the way.

IRA FLATOW: Thank you. My guest has been Dr. Daniel Holz. He’s a professor at the University of Chicago and the Kavli Institute for Cosmological Physics, member of the LIGO Scientific Collaboration. And I wanted to thank you, again, for taking time to talk with us today. And good luck. Happy hunting!

DANIEL HOLZ: Thank you so much, Ira. It’s always a pleasure to be on your show.

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