(Sorry for the delay everyone. It has been a week.)
Did you hear the news? Nobel Prizes for black holes. We know there are stellar mass black holes and supermassive black holes, but how do you get from one to the other? How do black holes get more massive?
8-bit Community (Twitch)
A Black Hole Primer with Chandra (Chandra X-ray)
The Large Hadron Collider (CERN)
Neutron Stars (NASA)
White Dwarf Stars (NASA)
Classical Novae (Swinburne University)
Electron Degeneracy Pressure (Swinburne University)
Event Horizon (Swinburne University)
Can Light Orbit A Black Hole? (Universe Today)
Black Hole Bends Light Back on Itself (Caltech)
Stellar Black Hole (Swinburne University)
Quasar (Swinburne University)
Supermassive Black Hole (Swinburne University)
Extremely Metal-poor Stars: A Brief and Incomplete History (UCO Lick Observatory)
Birth of Massive Black Holes in the Early Universe Revealed (Georgia Tech)
What Is Hawking Radiation? (Science Alert)
Massive Compact Halo Objects (MACHOs) project (Australia National University)
Gravitational Lensing (Hubblesite)
Ned Wright (UCLA)
Cosmology Calculator (UCLA)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast, Episode 582, “Building Bigger Black Holes.” Welcome to Astronomy Cast, your weekly facts-based journey through the cosmos, where we help you understand not only what we now, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of science at Cosmo Quest.
Dr. Gay: I’m just the director of Cosmo Quest.
Fraser: The whole director, not just the director of science. I know.
Dr. Gay: I have to do way too much budgeting to simplify the name to “director of science.”
Fraser: You wish you got to do more science, less administration.
Dr. Gay: Yes.
Fraser: I know the feeling. How are you?
Dr. Gay: But, speaking of budgeting, for Cosmo Quest, we’re doing our hangout-a-thon next week, so, October 24th-25th – this weekend, for those of you listening on the podcast – we’re gonna do 36 straight hours of fundraising in the name of keeping the science going for fiscal year ’21.
Fraser: Now, normally, I would say that’s madness. What kind of maniac will attempt 36 hours? That’s impossible, and yet, you’ve done this many years. So, this is just same…time to do this again. It’s just like going to work for you.
Dr. Gay: Well, we’re adding something entirely new this year. We are going to do a scale model of the full solar system. We’re including the 34 largest objects in the solar system – so, there’s moons, there’s an asteroid because it turns out asteroids are tiny, there’s a bunch of Kuiper Belt objects, all of the planets, Sun – and we’re doing this in collaboration with the 8bit community here on Twitch, and this was the idea of Perinor, one of our dear colleagues who passed away a couple of weeks ago, and we’re doing this in honor of him, and we’re hoping that any of you out there who love to Minecraft, love science will please be part of this and donate in the process while we’re doing it.
Fraser: I might drop in and Minecraft with you guys for a little bit. Right on. I’ll get my kids to show up too. All right. So, did you hear the news? Nobel Prizes for black holes. Now, we know there are stellar mass black holes and supermassive black holes, but how do you get from one to the other? How do black holes get more massive? And, before you send us the email, the title is “Building Bigger Black Holes.” It’s just for the alliteration. It’s really about building more massive black holes – more massive. Actually, that’s not bad, too. Anyway, Pamela, how big is a black hole?
Dr. Gay: Well, it depends on how big they feel like being. So, theoretically, a black hole can range from something that is microscopic and created through a natural version of the Large Hadron Collider, and then range all the way up to billions of solar masses. Now, the reality is the only ones we’ve detected are from a few stellar masses to that billions of solar masses, and we’re still struggling to find things in between those two, but the situation is getting better than it was a few months ago.
Fraser: And, I think definitely, as I mentioned at the beginning there, it’s not entirely accurate to say how big a black hole is because we don’t actually know how big a black hole is. All you can measure about a black hole is its mass, its spin, and its charge.
Dr. Gay: Well, and, this is where “big” and “small” get to be confusing adjectives because you can say that room has a low temperature, you can say it has a small heat energy, and those are the same thing, so…
Fraser: So, when we say “big,” we just mean “massive,” right?
Dr. Gay: Yeah, we do, and here, it ties to the size of the event horizon, but I think we’re starting to get ahead of ourselves.
Fraser: Yes. Okay, great. So, I think we’ll talk about the idea of those microscopic black holes near the end of the show. Let’s talk about the traditional kinds of black holes that we know about right now.
Dr. Gay: Well, in general, you start out with a star that is initially more than 10 solar masses. Anything below 10 solar masses we think over the course of its lifetime is going to lose so much material to its environment through solar winds, mass loss events, that it’s gonna creep itself down to be small enough that it will die as a neutron star or white dwarf.
But, if you’re above that 10-solar-mass limit, when you die – if you’re a star – you will experience a supernova where some of that material gets blasted off, but what’s left in the core as it collapses down – it starts to say, “Hey, can I be supported by electron degeneracy pressure?” And the electrons are like, “No, I do not have the ability to support this.” It collapses down. Neutron degeneracy pressure tries to kick in. It goes, “No, neutrons also cannot support this.”
And then, we don’t really know what happens. It just keeps collapsing such that the mass is confined within a volume smaller than what light can escape from. So, here on Earth, we’ve used this description many times. We’re on a people black hole. I can jump as hard as I want, and I’m not leaving this planet.
Fraser: I love that description. Yeah, no person, no matter how strong they are, can escape the Earth by jumping.
Dr. Gay: But, light can go fast enough, rockets can go fast enough, so there’s a way to get information off of our world. Now, as things get bigger, they stop being escapable by rockets. They stop being escapable, in fact, by everything except for light, and at the point that even light isn’t going fast enough, they become a black hole. So, the definition of a black hole is something that, as you approach the surface, you hit a point where the velocity you have to go to get away exceeds the velocity of light.
Fraser: And, that always leads to people asking, “Oh, great. So, if you could go faster than the speed of light, could you escape a black hole?” And, unfortunately, the answer to that is also no, but mostly because a black hole has completely tangled up spacetime in a way that all roads lead back to the singularity. So, even if you then go faster than the speed of light – it’s kind of cruel, it’s kind of mean, but even if you launch your warp drive, you’re still ending up in the middle of the black hole.
Dr. Gay: Yeah. So, a black hole is just an exceedingly dense object. Even the Earth, if you crammed all of our matter into a small enough volume, would become a black hole. It’s just that “How fast do you need to go to get off the surface?” issue. Now, the surface of a black hole in this context is that place where the escape velocity becomes the speed of light. We don’t know anything about what goes on beneath that surface, so we’re just gonna ignore it here.
Fraser: A black hole – and, we’ve talked about this in the past as well – this idea that once the event horizon is just that speed limit, if you’re just outside the event horizon, then light can escape. If you’re just inside the event horizon, then light can’t escape, and that’s what defines it, and that’s what makes it look like a black hole, is that that’s the point where the light gives up, is that very moment.
Dr. Gay: And, what I love is that you can actually get photons of light in orbit around black holes, and there’s also evidence of light that has its path bent so that it goes around a black hole and then hits the accretion disk and reflects off, so you can get all sorts of weird things with light going on, and this is why, when we look at images like the Event Horizon Telescope, we’re not looking at a structure the way we think of them. What we’re looking at is how the light is bent around a place in space.
Fraser: Right. And so – and, we’re gonna talk about this more next week. One of the incredible discoveries – or, I guess, the evidence was starting to build that there were – we knew about these less massive black holes, these stellar mass black holes, the ones that have come from stars with 10 times the mass of the Sun through – a few of them had been found in the Milky Way. But, there was this growing discovery that there was actually another version of these black holes at the hearts of galaxies.
Dr. Gay: Yes, and this goes back to the discovery of quasars. There are, in the cores of many galaxies, a concentration of light such that that concentration of the light in the center is significantly brighter than the entire rest of the galaxy, so when you’re looking at a quasar that’s at a great distance, that disc surrounding the quasi-stellar object in the center – not visible. All you see is the starlike thing. But, when astronomers first looked at the atomic line spectra, they didn’t see anything that made sense until someone realized, “Shoot, all the lines are so red-shifted that this isn’t a star, this is a galaxy significantly far away that has all of its light shifted.”
So, in trying to understand what’s going on in the centers of these galaxies, for decades people were drawing overhead sheets with a monster in the center and saying, “There’s a monster in the center of galaxies,” and people would talk about, “Maybe it’s a massive black hole or something.” But, it wasn’t until the late ‘90s that we started to have the technological ability to say the amount of mass confined within this small of a volume says there must be a massive black hole here.
Fraser: And, we now know that the mass is not just a massive black hole, it’s a supermassive black hole. It’s not just several times the mass of a normal stellar mass black hole, not thousands, it is millions of –
Dr. Gay: Two billions.
Fraser: Two billions, possibly even up to hundreds of billions, I think even trillions at this point.
Dr. Gay: Millions and billions is what we’ve seen, so we’ll go with that.
Fraser: I’m gonna double-check while you’re talking, but please continue. So, yes, billions, and so, when you think about – if you have these black holes – one black hole over here, one black hole over there – how can you possibly get from something 10 times the mass of the Sun to something that has tens of billions of times the mass of the sun? How, Pamela? Explain it to me.
Dr. Gay: So, when we were first trying to figure this out, the idea was “Oh, maybe you have a swarm of black holes, and they merge together, and you get a massive black hole,” and trying to figure out the timescales that that would take and how to build something fast enough to allow massive galaxies to exist in the earliest moments of the universe was deeply confusing.
And, because human beings like to simplify things, for the longest time, there was this question of do galaxies grow, take a massive amount of material, and collapse it down to the entire galaxy all at once, or do they grow through a bunch of little things coming together and merging to build bigger and bigger things over time? And, this was not a “Some go through one, some go through the other” argument. This was a “Which of these two things is it for everything?” Human beings have problems.
And, it turns out the universe wasn’t into this either/or, it was into an and, and it was realized that if you have a massive amount of material gravitationally pulled in by a halo of dark matter, that that in-falling material can fall in with turbulence, and the turbulence has a way of distributing the energy such that a massive amount of material is capable of funneling into the center of this forming structure and forming the supermassive black hole simultaneous to the formation of the rest of a giant elliptical galaxy – a large spiral galaxy, maybe, but we can’t see that clearly, so I’m gonna go with giant elliptical.
And so, it’s this turbulent process that is capable in some instances where you start with a large dark matter halo of generating a supermassive black hole in a single go.
Fraser: And, I guess this was the question. Could you – did you need to have a star born, reaches – runs hot and fast and dies a million years later, leaves behind a black hole that’s 20 times the mass of the Sun, and then, two of them crash together, and now you’ve got a black hole with 40 times the mass of the Sun, and two of those crash into each other, and now you’ve got 80 times the mass of the Sun – by doubling with a lot of black holes –
Dr. Gay: You can get there.
Fraser: You can get there, but you’re saying that they don’t think that’s what happened. They think that there’s some kind of –
Dr. Gay: In all cases – and, this is the catch – is that doubling, which we’ll talk more about in a moment – you can do it, it will get you what you need, but the timescales weren’t working out. How do you have massive structures formed pretty much from the beginning of structures being formed? So, the thought is that for the largest systems, the inflow of material to form that entire system, the turbulence in it allowed enough material to collect in the core to cause that supermassive black hole to be formed simultaneous with the system.
Fraser: Now, we know that today, there’s a limit to the size that stars can form. At a certain point, the star – the solar winds blow, and it blows material off from trying to make its way into the star, and the star, whatever it is – a few hundred times the mass of the Sun is the upper theoretical limit of how big a star can be, and so, that defines the minimum possible black hole that you’re gonna get. It’s heavy, it’s 100-plus times the mass of the Sun, but it’s not –
Dr. Gay: It’s not a million to a billion times the mass of the Sun.
Fraser: It’s not a million times the mass of the Sun, yeah, so you do have to start that process. So, was there something different about the early universe that allowed maybe more massive black holes to form, or even just one big black hole at the center of a galaxy?
Dr. Gay: It wasn’t…the kind of difference that you might go, “Hey, I wonder if that could still exist today.” It was more a matter of as the universe went from this diffuse gas after the formation of the cosmic microwave background, there were places of higher density and lower density, and the higher-density places were able to pull in material, and the echoes of where those knots were in the early universe continued to be reflected in the modern universe. Over time, the things that started out high-density have only gotten higher and higher density; the places that started out low-density have only gotten emptier and emptier.
It was the highest-density places in that early universe that were able to say, “Okay, matter, come to me” and pull everything in, and other places that started out with still an overdensity, but not as massive a one, formed smaller galaxies, and this is where both ways of forming come in.
So, we also had small galaxies forming that it looks like the smallest galaxies don’t necessarily have unusual black holes in them, but they do have stars of all sizes, and that means some of these stars are going to form black holes, and as these systems interact, as these systems over time go, “Hey, I’m gravitationally attached to you” and they merge over time, those black holes can find each other as the interactions cause the heaviest-mass objects to fall into the centers of these systems, and this is where slowly, over time, you can have these stars coming together.
You can have these dead stars that are black holes come together and build – over generations, through a second mechanism – bigger and bigger black holes.
Fraser: Right, and I guess that’s – so, when I’m imagining this three-dimensional whirlpool at the beginning of the universe with the dark matter forming this – the agitator that’s spinning up and directing the materials – it’s infalling, and you’ve got black holes that have formed from various stars that are falling down this gravity well into the center, being directed into the center of this maelstrom, and they’re finding each other in a way that is much easier than if they were just drifting randomly across the universe.
Is that where we’re going here, that there is a – again, I’m imagining water going down a drain, except the water that’s going down the drain is containing rare earth magnets, and the rare earth magnets are finding each other and clipping into bigger and bigger magnets. I don’t know if that analogy holds very well.
Dr. Gay: I think that’s an excellent analogy, actually, and you have other reasons that drive things into the center of mass, from frictional interactions with gas and dust… All sorts of different things can cause your mass to get redistributed and increase the probability that massive objects will find one another.
Fraser: Right, but do the black holes have to go through this individual stellar mass black hole face, or can you have it all just turn into one black hole? Can one black hole just form –
Dr. Gay: Well, early universe, yes, modern universe, no.
Fraser: Right, okay. So, early universe – did they think there was even a limit in the early universe about how big these black holes could get?
Dr. Gay: Yes. Black holes have a built-in throttle where, as material falls in – angular momentum is a bear, and the angular momentum of the material that’s trying to fall into the black hole has to be radiated away.
The material can’t just “Hi, I’m falling straight into the black hole,” and this means that the material falling into a black hole in the process of feeding it, which is the other way black holes grow – that material that is feeding into the black hole – it’s going to get denser and denser and denser, eventually kicking in, having its own thermonuclear reactions, giving off its own light through thermonuclear-reactive processes, and the pressure from that light coming off that disc of infalling material will actually be sufficient to choke off additional material from falling in.
It will clear out the insides of a galaxy, and it’s this throttling process of the light saying, “I’ve got more force than gravity at this moment,” the light pressure overcoming the gravitational pull. It puts a limit on supermassive black hole growth.
Fraser: Because I know that astronomers did the math of saying, “Okay, if you just fed a black hole as much as it wanted to eat, you couldn’t get a supermassive black hole at the age of the universe that we see them today” –
Dr. Gay: That’s true.
Fraser: So, you can’t just feed a black hole to get the size that you want. You can’t – again, back to my bathtub analogy, you can’t have all of the water in your bathtub instantly drain out. It takes time. It spins around. So, you have to have some process where these individual black holes are being formed, and then they’re merging together, and fortunately, we now see evidence of black holes merging together, thanks to LIGO.
Dr. Gay: Yes, and so, now, we’re looking at you can have various size supermassive black holes in the beginning of the universe formed through a massive turbulent process. Those galaxies can then merge, forming the most massive supermassive black holes we see today.
You can also have stellar mass black holes – and, early stars were much more massive and very different from what we see today, and I’m not gonna pretend that we fully understand how these metal-poor stars lived and died, but all of the models that we have indicate they would have been far larger on average than the stars we have today, which means potentially more stellar mass black holes forming than we have today through generations of stars.
And so, you have this richness of massive objects dying. Those massive objects dying have the potential to then merge together and, at the same time, to gobble up the material around them. You have a black hole forming in a high-density star-forming region, it’s gonna suck some stuff in as it goes, and so, there’s myriad ways to grow these things over time. Mergers, sucking of material, accretion disks eating the universe around you are all viable options.
Fraser: Right. The thing that’s really interesting – and, we’ve talked about this in the past as well in shows is that did – do galaxies form or did the supermassive black hole at the heart of the galaxies form first? Which one formed first? And now, it really looks like they formed together, hand in hand.
Dr. Gay: And, this is something that’s cropping up in all sorts of different formation things. We’re now starting to think that planets formed at the same time that stars formed rather than coming later in the surrounding disks. It’s now looking like the galaxies’ greater structure, and the supermassive black holes, and the most massive systems in the early universe formed at the same time.
When you have a giant cloud of material collapsing, it does this fabulous job of forming all the structures together, and while the scales are very different, the idea of the cloud collapsing, spinning up, forming a massive object in the center, fragmenting and forming smaller objects on the outskirt – this kind of physics is scalable.
Fraser: Right. So, there’s one tiny modification to this that is by no means certain – this idea of primordial black holes contributing to this as well, this idea that there were folds of space in the universe – overdensities in the early universe where the matter was so dense that you could have black holes form naturally, and that would theoretically allow you to have black holes which were smaller sizes than we see – than the stellar mass, but also, theoretically, more massive ones.
You could theoretically get a black hole with a thousand times the mass of the Sun just forming instantaneously at the beginning of the universe, and then unleashed. And so, theoretically, some of these could also serve as the starting points of some of these larger anchors as well.
Dr. Gay: And, the messiness of this comes from the combination of not knowing for certain whether or not Hawking radiation is real and black holes evaporate, in which case the smallest of the primordial black holes would have just gone away, and understanding what was the potential size distribution of these primordial black holes. Microscopic ones are super easy to argue for the existence of, and they would all have evaporated. More massive ones –
Fraser: Anything below 1012 kg will have evaporated, so anything – it’s a tiny asteroid, will have already gone.
Dr. Gay: Yeah, and the larger ones are harder to argue for the existence of, looking at the acoustic waves that you can see in the cosmic microwave background as the echoes of what was going on prior to the formation of the cosmic microwave background. So, we just don’t know.
Fraser: Yeah, and it is one of the intriguing theories to explain dark matter, so apparently, if you have black holes of certain masses – I think they’re up to asteroid size, but then, also, tens to thousands of times the mass of the Sun – then that would explain the distribution of dark matter.
Dr. Gay: But, we haven’t found them –
Fraser: But, we haven’t found them, no.
Dr. Gay: – and the MACHO project looked really hard, so that idea seems eliminated.
Fraser: Yeah. There have been attempts – they’ve been able to rule out certain masses of primordial black holes as dark matter, but there’s other ones where it still could be the case, and there some really interesting surveys that are still being done using gravitational lensing to try to find them, and it’s this idea that won’t go away, but also, there’s no evidence that they exist at all so far, so every day that goes by, it’s less and less likely that it is primordial black holes that is causing dark matter, and possibly even less case that they even do exist at all. But still, can’t rule it out yet, and so, I won’t in my heart.
Dr. Gay: I’m very much Team It’s a Particle.
Fraser: I am also very much Team It’s a Particle, but I had a chance to ask Ned Wright, who’s one of the fathers of modern cosmological thinking, and I was like, “What’s the idea – the heretical thought that you enjoy trolling other scientists because they can’t say no?” He’s like, “I like the idea that black holes are dark matter because we just can’t – it fits the mass so far. We can’t rule it out yet, even though it’s ridiculous,” which I thought was quite entertaining.
Dr. Gay: And, Ned Wright is someone who – yes, he’s an amazing researcher, amazing scientist, but he’s someone who I think generations of researchers are now grateful to because he figured out all of the – it’s so harried to calculate all of the equations for a universe that has dark matter, dark energy, and curvature parameters, and all those things. He figured out all those equations, codified them, and then created the Ned Wright Cosmology Calculator.
Fraser: Yes, the calculators! I use them all the time.
Dr. Gay: Oh, I love that calculator. So, for me, he is always the person who created this web page that made me no longer have to cry doing calculations.
Fraser: That is awesome. Yeah, I use a very simple – for much simpler purposes than I think what you are trying to do, but that’s – I’m looking for back-of-the-envelope calculations, but I think it’s great. Awesome, all right. Well, I think next week, we’ll actually go deep into the actual Nobel Prize discovery now that you’ve got this groundwork, and we will talk about the different papers and the researchers who brought all those ideas to their modern day. So, thank you, Pamela.
Dr. Gay: Thank you, Fraser.
Fraser: Do you have some names for us this week?
Dr. Gay: I do, and this is where I have to say thank you to all the people out there who support us in doing everything we do. This show is made possible thanks to our patrons on Patreon.com/AstronomyCast, and it’s COVID times, I know a lot of you are struggling, and to those of you who are still finding it possible to keep supporting us month after month, you’re allowing us to pay the folks who keep our stuff going: Richard Drum, who does the audio for the show, Allie Pelfry, who does the video, Beth Johnson, who puts everything up on our website, and to offer them health benefits where they need it.
So, thank you for making it possible for us to offer part-time people healthcare here in the United States. So, I wanna thank Thomas Tubman, Claudia Mastrolani, Justin Proctor, Joe Wilkinson, David Gates, Jessica Feltz, Paul L. Hayden, NeuterDude, Matthew Horstman, Brent Crenup, Ihran Segyev, Mark Grundy, Arthur Latzhall, Andrew Harmsworth, Tim Garris, J. Alex Anderson, Bruno Lutz, Jeremy Curwen, Michelle Kellen, Mark Stephen Raznick, Dustin A. Rolf, and John. Thank you all so much for everything you do to allow us to keep going.
Fraser: Thank you, everybody, and we’ll see you all next week.
Dr. Gay: Bye-bye, everyone.
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