How did supermassive black holes get so big?

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Speaker 1: Hey, Daniel, I have a question for you about how big things are. Is this a question about my pandemic snacking habit? No, No, I'm wondering. How do you think about the massive size of things? Like there are so many big things out during the universe. I know you mean, like how the sun is a million times bigger than our earth. Yeah, I'm wondering there are things out there there are even millions or billions of times bigger than our sun. Yeah, you know, some things out there are incredibly massive. Maybe they've been doing some pandemic snacking of their own. You mean, like eating whole stars between meals. I wouldn't recommend it, just three stars per day? Is there a doctor's recommendation? Yes, we are doctors. I am forhanda cartoonists and the creator of PhD comics. Hi, I'm Daniel. I'm a particle physicist and technically i'm a doctor. But please don't follow my medical advice. Yes, physicists don't make the best lifestyle examples. No, that's true, and particle physics especially is not a very practical degree. You can't collide the germs in your body. Oh that sounds like fun. Actually, I'm going to write that proposal. I think most people would love to collide a certain virus right now in your super collider. But welcome to our podcast. Daniel and Jorge explain the university production of I Heart Radio, in which we explore all the crazy and amazing things in the universe colliding them with your brain, from the very smallest particles to the weird stuff happening on our scale, to the very big, the very enormous, the very super duper mysteries of the universe. Yeah, the biggest mysteries out there and also the most massive mysteries. Because Massa's is such a fun thing to think about. You know, it makes stuff stuff if you think about it. I don't like to think about my mass very much. Actually, actually, I'm getting pretty fit in this pandemic. I've been doing more exercise and normal to say that I'm getting some exercise. But mass is incredible because on one hand, there's so much space out there that's just empty, like most of the universe is just emptiness. But then there are these incredible clumps of amazing density, these blobs of stuff. They know the size of a planet, the size of Jupiter, the size of the Sun, and then even monstrously bigger than that. Yeah, because there are things out there that are so massive and so dense that they actually turn into black holes. And you know, black holes, I think we people tend to think of them as one thing, like once you become a black hole, that's it. You're a black hole. But actually there's a big variety of black holes out there in the universe, that's right. Black Holes come in all shapes and sizes. Some of them spin, some of them have electric charge, Some of them are quite small, some of them are incredibly super duper huge. They come in different ship apes. Can they come in different shapes depending on the stuff around them, So it depends on what you call part of the black hole. They're mostly spears, but some of them have disks of stuff around them that are bigger and smaller. Some of them are on their own, and so I guess maybe they don't have different shapes, they have different clothing, They're dressed differently. Can you have like a banana shade black hole? Probably? You know, you get a decretion disc with stuff is swirled only on one side that would look a lot like a banana. So I'm gonna go with yes. It's likely that out there somewhere there's a huge gas banana getting sucked into a black hole. Official word here, ladies and gentlemen from your doctor. That's right. And so, yeah, some of these black holes can be really, really, really heavy. Some of them are millions of times more massive than our sun, which is crazy to imagine. Like imagine a million sons crowned into one place, I know. And when you think a million sons, you instantly think something that's super duper bright, right, But of course a black hole is not that bright. It's huge, it's massive, but it's not actually that bright. Right. All that mass works together to create this strange fold in space where space becomes like one directional. It's like a one directional portal in space where once you go in it, you can't come out. It doesn't matter how fast you're moving, if you're a massless photon or a very very fast neutrino or a blob of stuff, there's just no way out. It's not about your speed or about your energy, it's just space has become folded in this bizarre way, right, And so they're one of the most amazing mysteries of physics. Yeah, it's like one direction. The boy band also a black hole in music. I think that was a career black hole. But yeah, Harry Styles somehow managed to escape that that black hole. But but yeah, but that's that's like an average black hole. Like a black hole that's millions of times more massive than our sun. That's like a small black hole that doesn't impress you. I mean, it's impressive to me, but sure the universe can impress me even more. That's right. And the incredible thing about these black holes is that they have to form, right, You've got to make them. They gotta somehow get that big by gobbling stuff up. And there are black holes that are so big out there that scientists don't really know how they got that big. Yeah, there are black holes out there. There are billions of times more massive than the sun. Right, Like we take our sun and get a million of them together, get a thousand of those together. That's like a that's a super massive black hole. That's really pretty impressive. I mean, I think that that should be called super duper massive black hole, not just supermassive. And that's actually the official name for them, right, isn't it? Super Massive black holes like supermassive black holes is the official name used by actual black hole scientists. I like to call them super duper massive because it just conveys better how impressive they really are. Super duper mega awesome black holes, califragilistic black holes. Can't run out of enough superlatives. Alright, So these giant supermassive black holes, they're really big and nobody seems to know how they get formed. So to be on the podcast, we'll be asking the question, how do super massive black holes get so big? I mean, do they even lift? Man? Did they go to the gym? Like, how did they get so big? What? What? What did they do for the core? Exactly? Don't skip leg day black holes. You know, it's not all about being ripped. Are they doing the t X something something? I bet they're doing something more holistic, right, you know, they've got that nice, perfectly spherical shape of the event horizon, so it's definitely some core strength. I bet there's a lot of yoga, downward dog and all that stuff. Yeah, so they're super duper massive and really big and people don't know how they got that big. But we were wondering how many of our listeners out there know or think about how these black holes get so big. That's right, because they are really pretty incredibly big, and it's hard to make a black hole that big in the short amount of time we've had in our universe. So I sent this question to our friendly volunteers. Thank you to everybody who signed up to answer random questions, and if you like to participates for future episodes, please they're u with a line to questions at Daniel and Jorge dot com. Before you listen to these answers, think about it for a second. If someone ask you how does supermassive black holes get so supermassive? What would you answer? Here's what people had to say. My guess is that black holes become super massive by simply absorbing neighboring planets, stars, gas, any other type of matter nearby the black hole. When it comes into existence, it has to be large enough where it doesn't just evaporate due to hawking radiation, and it needs to be close enough to other sources of matter so that it can pull them in and gain their mass to become supermassive. The obvious answer would be that just by you know, gobbling up nearby stuff. So I think that all the supermassive blig holes are originating from the early stages of the universe when it was much then, and I think it was just easier for existing black holes to eat up the surrounding planets or other stars. If they start one size and gain more material, it makes sense that they would grow. The event horizon would grow. Certainly the more mass is in it, probably number one would be merging with another or more black holes. Black holes get to be super massive by emerging with other black holes, such as when two galaxies flide and their central black holes merge. Maybe by you know, as they keep absorbing more matter into it. All right, so some pretty good answers there somewhere are like, wow, I didn't know that they does seem really possible. Yeah, the general sense seems to be like, well, black holes get bigger by eating stuff. So you want to get supermassive, you gotta eat a supermassive amount of stuff. That's what black holes do, right. They can't do anything else. They can only consume, you know. It's not like they can poop stuff out or anything. And it's like that's all that's all they were intended to do by the universes is sucks stuff? It is that not enough for you? Like incredible folds in space gobbling up matter and incredible rates, and you're like, what else do you got I'm saying. I'm saying they have a limited range of skills there, but you know, they're a niche player. They're the best in the universe at what they do. And I think in terms that we're talking careers here, that's really the right way to go, right, stick to your niche, that's right. But I think the thing that's missing from these answers is an understanding of how you can get so big in the limited time we've had in our universe. Like, yeah, the universe is fourteen billion years old, but these black holes are so big that we don't understand how they got so big in that amount of time. Wow, that's weird. It's weird that we can't explain that, Like that, we have these things in the universe that are not small, they're big and significant, like they're helping to hold the galaxies together, right, and we don't know how they got to how they are. But this is a really important part of how we do science. We look at the stuff that's out there in the universe, and we say, do we understand how it got here? We have a model of the early universe. We think we know the laws that determine how things interact and how things grow and change. So we should be able to then look around and describe basically what we see. So if there's something out there that we can't explain that we don't think should be there, it tells us there must be something wrong in our understanding, either of the initial conditions how things started or of the rules for how things change. All right, so let's jump right into it. Daniel and I think we can assume that most of our listeners know what a black hole is, which is an accumulation of a mass so intense that it creates like a pocket or a hole in the universe out of which nothing can escape. I think we can assume most people know that, But maybe what people don't know is kind of how big or what how they can vary in size. So step us through, like, what's a normal black hole? So a normal black hole is the kind that you might be familiar with that comes when a star dies. Stars this big blob of gas, and it's a balance between the graph compressing it and the energy released from the fusion and the burning, and when the burning stops, then the star collapses because all that's left is gravity, and sometimes it collapses so much that you get a black hole. And these black holes tend to be around the size of the masses of stars because they were made by one star. And so the units are usually the mass of our Sun because it's a convenient thing. It's not like the official star or the universe, but it's the one we use to measure the masses of things. And so the typical range of a stellar black hole when they came from a star is a few times the mass of our Sun, you know, like a few up to maybe ten twenty. The biggest one we've seen in this category is like, you know, seventy or eighty times the mass of our Sun, because they come from when the stars go supernova, right, and that's kind of the only way they can form from stars, right, that's right, you have a gravitational collapse, the burning stops, everything collapses in then you sometimes get a supernova that blows out a huge amount of material and then what's left to the core or is a black hole. And so you don't always get all of the stuff from the star into the black hole. Right, You lose some stuff, it burned, it got sent away, there was radiation, there was a supernova blew some of that gas out into the universe. But at the core you get that black hole. So some fraction of the mass of the star turns into the black hole. So for black holes that come from a star, the mass of the black hole is a little bit less than the mass of the original star, right, and those stars can be as big as seventy or eighty times the size of our sun. Yeah, there are stars out there that are much more massive than our star, and so they can be you know, up to a hundred times the mass of our star, which leads to black holes up to like, you know, eighty times the mass of our sun. All right, So those are the kind of the regular black holes because they form from stars, and there are a lot of stars out there in the universe, and those can get pretty big. I mean, eighty times the size of our sun. That's not nothing, that's nothing to scoff at. You know, there's eighty million earths all packed into a tiny little blob. It's pretty impressive. But the incredible thing about the universe is that every time you turn around, there's something that dwarfs what you were previously impressed by. Like why you thought the Earth was huge. Look at Jupiter, you thought Jubiter was big. Look at the Sun, you thought the Sun was big. And it just keeps going and going and going, and there's all these different scales that each one blow your mind. That's the experience of, you know, understanding the depths of the scales of the And so that's one sort of category of black holes that we see or think are there in the universe, you know, the sort of like one to eighty times the size of our sun. But then there's sort of another big category of black holes, which is kind of like way out there, much bigger. That's right, there's not like an even distribution. It's not like black holes start from a few solar masses and go all the way up to millions and billions. There are two different kinds of black holes. The ones we just talked about that come from a star, and then this other category of really massive black holes that are like hundreds of thousands of times the mass of a star up to millians and billions of times the mass of our sun, and so these are what we call supermassive black holes, and there's like a gap there. You don't see any black holes that are like a thousand times the mass of the Sun or five dred times. There's just nothing there. It's like two distinct classes. Yeah, and it tells you that there's like two different ways to make black holes, are two different populations, that these things have a lot in common, but they're also really different and they probably have a different history. Now, are we sure that we haven't seen any in the middle sizes or is it that we just can't theoretically come up with them. We have not seen any in those middle sizes. That's right, And the problem is not that we can't come up with them. The problem is that we can't explain how these ones got so big. It's easier to explain smaller black holes than the ones we see because there's more time for to accumulate gas and to grow. The hard thing is to explain how you've got these really really big black holes. And these big black holes they're not just like floating out there in the universe or part of a galaxy the way like a stellar black hole is from a collapse of a star. These guys tend to be at the center of a galaxy. That's usually we'll we see them. We don't see them floating around on their own. We definitely do not. And every galaxy has one, and usually exactly one. Like, you don't have two supermassive black holes in a single galaxy. It's like, you know, this town is only big enough for the one of us. And that's kind of funny, isn't it. Like you only see one in the center of each galaxy, and most galaxies have them, that's right, And there's a reason you only see one, and that's because they're so big and massive. Like if you get two galaxies that collide and that they merge, then the two supermassive black holes at their centers will orbit each other for a little while, but eventually they'll merge and become one. So they can't really stay separate because they're each so powerful and sucking in the other on like Highlander, that can only be one, that's right. And the other thing that to understand is these black holes are a really big part of a galaxy. It's not just like, Okay, you've got a big black hole, but the galaxy is much much bigger, Like each of these black holes is on average about one one since the mass of the entire galaxy, which you know has like hundreds of billions of stars, but this black hole really dwarfs any other object in the galley. It's like probably millions of times bigger than anything else in the galaxy. Yeah, exactly. And so the biggest ones we've seen are billions and billions of times the mass of the Sun. And the amazing thing is that some of them are really really old. Like we've seen black holes that are billions of times the mass of the Sun and have been around since the universe was only a billion years old. What how do we know their age? Black holes don't have any wrinkles or you can't ask them. It's pearl cream, man. They just look great. Well, we see an old picture of them, Like if we're looking at something really really far away, we're seeing old lights. So the picture we see of them is really old. So the short answers we're seeing really really massive black holes very far away, which means that they happened a long time ago because I have an old picture because the light is taking billions of years to get here. It's it's an old picture and they're still there. Yes, so we know that after only a billion years of universe formation, that were already super massive black holes that were billions of times the mass of our sun. Man, all right, let's get into how we can see these black holes, and let's get into the mystery of how they got so big. But first let's take a quick break. All right, Daniel, we're talking about super duper awesome, ginormous black holes, and there's a big mystery about how they get so big, because they're much bigger than the regular black holes that we see floating around in space. And so I guess the question is how do we First of all, how do we see them? How do we see black holes? So black holes we can never see directly, so we have no like, really direct proof that black holes actually exist. We just have a lot of really good circumstantial evidence, and all of our evidence essentially is gravitational. Basically, the argument is always there's a huge amount of mass in a small amount of space, and nothing else can do that, so it must be a black hole. We see nothing else there, you know, like you can see a big swirl of gas surrounding some black object, and that gas is obviously under incredible pressure because it's emitting a lot of radiation. Or you can see other stars moving around this object in space on crazy orbits that would require intense gravitational fields that are only consistent with an object that's very small and very massive. And so then we conclude, okay, there must be a black hole there, and we can calculate its mass based on the orbits of stars around and but those are the regular ones. The one at the center of galaxy is the one that we know are really old and big. How do we like, how do we see it? Can you can you actually see the stars going around it in a galaxy so far away? In our galaxy, you can see the stars going around it. Like we have looked directly at the path of stars zipping around the black hole at the center of our galaxy, which is called Sagittarius A. So we know that one pretty well. In terms of other galaxies, like ones really far away in the old early universe, we see them because the radiation of the gas that they're squeezing around them, So this is disc of stuff around them that's swirling around, waiting to get sucked in and it's getting squeezed and pulled by the tidal forces, the gravitational pressure, and so it's emitting a huge amount of light. And those are actually some of the brightest things in the universe. Sort of weird, these really massive dark objects actually end up being some of the brightest things, and they're called quasars. That's how they were originally discovered. So we saw these really bright objects in the sky and we didn't understand what could be emitting so much radiation. Right, so we can actually see them. They glow, or at least that the stuff around them glows. Yes, then how do you how do you tell how massive they are? Yes, so the stuff around them glows, right. The black hole itself is black, with the exception of Hawking radiation, which nobody has ever seen. It emits no light, but you know, the stuff are round that it's entourage is very very bright. And that's how we see these really distant ones. But you can also just watch stars move around them and just track their path and do the calculation and say, for this start to bend so much in space, how much mass would there have to be in that black hole? And that's how you can estimate their mass. It also gives you a limit on their size, right because you can see the star moved near the black hole, so you know how big the black hole isn't all? Right, So there's this whole category of black holes that are super massive, much bigger than the regular ones that form from supernovas. And so I guess the big mystery you were telling me is that we don't know how they get started or how they get so big. What does that mean? How can we not know how black hole gets Do they just eat or stuff? Yeah, so it's sort of a two part mystery. Right. You need your black hole to get big, so you just feed it a lot, right, Well, if you start from a stellar mass black hole, like one the size of a few suns, then there just isn't enough time to feed that to make it big. Why not? You want to grow a plant in your backyard? Is a limit to how fast as they can grow? And the same is true for black holes. Really, why is there a limit on how fast it can grow? Can't just sucks stuff in at an incredible rate? There is there a limit. There is actually a limit, And because there's a feedback, like as the black hole gets more powerful, it starts to excite the gas around it, which gives off a lot of radiation, and so it actually pushes stuff away from it. So the faster you feed a black hole, then the heavier it gets, the more it pushes stuff away from it. So it's really delicate balance. This is called the Eddington limit. If you want to grow your black hole, you can either feed it a little bit at a time right and avoid that radiation. You dump too much stuff in it, it's gonna blow all of the fuel away. So you've got to find this right balance. You gotta feed it at just the right rate to get the maximum growth curve. Oh I see if you if you try to feed it too fast, it's actually going to create an explosion which is going to blow all your food away. That's right, because remember these quasars, these the brightest things in the universe. That's an enormous amount of radiation. So it's going to clear out all the space near it, which is going to prevent it for growing any further. So it's not like a physical limitation. Is no law that says the black hole can't grow faster. It's just a question of like getting that stuff into the black hole while its meanwhile pumping out a bunch of radiation. Right, But I guess doesn't that all doesn't assume that the stuff goes in kind of in a spiral. What is stuff just goes in directly? Yeah, if you have really cold gas, then it can go in, but it's got to get past all that radiation. Remember, you're surrounded by there's always going to be some warm gas, some stuff surrounding the black hole that's going to radiated. So it's it's like an environmental question. First they suck up all the cold gas, the stuff that's not gonna swirl, but then what's left is the hot gas, and so that takes longer to get in, all right, So then there's a maximum rate or speed at which you can black holes can eat. And so the mystery said, if you take that speed and you're multiplied by the age of the universe, that doesn't give you enough stuff. You can't get enough stuff into the black hole to explain how big they are. Now, that's right. If you start from really small black holes you go with the maximum speed of growth, you don't get to millions and billions of masses of black holes. They just stay too small. In our simulation, and so you need to figure out a way to make them grow faster, which we don't understand, or you need to start from bigger black holes in the early universe, so you sort of like get ahead, start right, like maybe they started big, that's right. The major mysteries on understanding how these black holes could have formed in the very early universe and started out really big. So there's a few ideas there for how you could have really big seeds of black holes in the early universe. All right, step us through. What are some of the different ways in which you could start with a really big black hole. Well, one of the ways is really theoretical and speculative and probably wrong, but it's also really fun. You guys have to make that call all the time. They're like, wow, this is obviously wrong, but it sounds fun. So I'm going to spend the next three months thinking about it. Yeah, it's the idea that we talked about a few weeks ago. It's called primordial black holes. That is, maybe black holes were made not by a collapse of stars, but some of them were made in the very early universe before we even had matter. Like these are called primordial black holes that were formed out of the early energy fluctuations just after the Big Bang. You mean like that, that's just how they form from the primordial soup of the universe. Like they just as the universe expanded and exploded. They're just happened to be some random, you know, blip in the quantum fluctuations that created a big black hole. Yeah, the pictures that you start out with this homogeneous, smooth universe immediately after the Big Bank, then there are quantum fluctuations. Things get a little denser here, a little less dense there, And in places where you happen to have a really big quantum fluctuation, you can get enough energy density to create a black hole. Even before the universe cools enough to make particles, and those particles turned into atoms, and those atoms turned into gas which collect to grow stars and eventually become black holes. You circumvent that whole process and just make a black hole boom right from the get go. So these are called primar real black holes, and we have no idea if they actually exist. We had a really fun podcast about them. But if they do, we expect they exist from the very very small sizes up to really enormous sizes, and so this is one way to get black holes really big in the early universe, right, And that may kind of makes sense to me, But why is it so speculative and why is it such a crazy theory? Well, nobody's ever seen one. We don't know if they're out there. We know the black holes are out there, we know that stars can form black holes, we know there are supermassive black holes, but nobody's ever seen a primordial black hole, I guess. I mean, how how do we know that the supermassive black holes are not primordial black holes. We don't. And it's possible that primordial black holes are the seeds of supermassive black holes. That would explain a lot, but it's a bit too convenient, and the theory is is sort of weird, And we would expect to see primordial black holes sort of at all masses, not just really really big and not just really really small, but also sort of in the intermediate range, and they would be doing stuff that we should be able to see, like sometimes they would pass through a star and cause disturbances on its surface, or sometimes if they were really small, they would decay with hawking radiation in these bright flashes of light at the edge of galaxies, we would expect to see some primordial black holes if they had been around them. We've never seen one, so that makes it a bit awkward. I see, if these came from primordial black holes, we would expect to see more of these primordial black holes floating around, don't But we don't. So maybe only primordial black holes were made on the larger sizes, which is a little bit awkward cosmologically to like come up with a model that only makes really really big primordial black holes, So it gets less and less attractive as an option the more you have to like tweak it and cram it into this box. All right, So then what's another possible reason that we have these massive black holes. Well, it could be that in the very early universe, stars were much bigger than they are today. So you could have had like horses, like courses used to be ginormals the size of a bus. Yeah, horses used to be three hundred times the mass of the sun. It turns out it's a big horse, of course. No, So these days stars are you know, range up to you know, maybe a hundred times the mass of the Sun. But it could be that the very early universe stars were bigger that the first stars, the ones made out of just hydrogen that came out of the Big Bang that had no metals in them, were able to form to be like really big three D four hundred even five hundred times the mass of the Sun. Interesting because just from the from having like the purity of hydrogen, or is it that the conditions of space themselves were different. It's from having the purity of hydrogen. Like if you have heavier stuff around, if you have carbon and nickel and heavier elements around from previous stars, that changes the way the gas is condensed. It makes it easier for gas to cool because you have these heavier elements. That's what's needed for a big blob of gas to turn into the stars that has sort of cool together. If it's hot enough, gravity will never attract it down into clumps, right, It needs to cool enough to form a star. And so if you have all hydrogen, it's harder to cool small clumps, and so they tend to cool in these big blobs, and so you get these really massive early stars. Now nobody's ever seen these. They're called population three stars and they're the very first population of stars in the universe really big, would have burned really fast, wouldn't have lived very long, but they could have then collapsed to form pretty massive black holes like two hundred three hundred times the mass of the Sun, which could form seeds that give us super massive black holes today. WHOA, So we have a hypothetical giant star at the beginning of the universe that somehow becomes a hypothetical medium sized black hole, but then grows to become the super massive black holes we see today. That's right, that's the idea, and it's pretty cool and it makes some sense and you don't have to like invoke any new magic physics or anything. But it's also doesn't quite work. One reason is, like, how many of these could there have been? Like, the theory suggests that we don't expect enough of these to explain all the supermassive black holes that we see. What do you mean you don't see enough? Well, we see a supermassive black hole in the center of every single galaxy, and we don't expect every single proto galaxy to form these supermassive stars like it happens sometimes, but not enough to give us all the supermassive black holes that we see today, So it might explain some fraction of them, but not all of them. And the other problem is that they're not quite big enough. Like three d four hundred times the mass of the sun. That's pretty good. It's better than starting from you know, five times the mass of the sun. But if you start from there, you've got to grow with the maximum rate the entire time to get close to the supermassive black holes. And it's really on the edge. And nobody thinks the black holes can grow at the Eddington limit at the maximum rate their entire lifetime. It requires like exact, delicate balance of gases being fed in just the right way to hit that maximum. So it's kind of precarious. And can you have even bigger stars or is or a limit to how big a star can be? Yeah, we don't think that these population three stars can get much bigger than that, and so that sort of limits the size of those seeds. Right, So then what's the third way in which supermassive black holes can form. Well, the third way is to skip the star formation and say let's have a big blob of gas in the early universe. And let's assume that there's a really like dense blob of dark matter in the center of it. Dark matter. We know there's more dark matter than any other matter in the universe. It's this invisible stuff that's around us, but it has gravity, and it's the most of the gravity in the universe, and so it plays a big role in how stuff clumps together. So if you've got a big blob of dark matter around in this early gas, it could have collapsed a huge blob of stuff down straight to a black hole like skip the whole star step just from the dark matter. Just from the dark matter. I mean, dark matter creates a gravitational well, right, so all this stuff falls into it and it doesn't have the outward pressure of of gas pushing out, and so stuff can just fall in. And if that happens, people have these models where you can form black holes in the very early universe that are already like thousands or tens of thousands of masses of the sun. Oh. I see, like the dark matter gives you that extra boost, like the secret steroid or something. Yeah, you're like no, no, no, dark matter, like the super different high protein diet kind of that's right exactly. I don't recommend the dark matter diet, but you will lose weight. So this gives you higher mass black holes, which is cool. It gets you like further up the ladder, gets you closer. You don't have to grow as much and as aggressively to get to the black holes we see today. But it's also less likely and so these things might happen, but there would be more rare. So they also can't explain all the supermassive black holes that we see out. Why do we think that they're rare? Is in dark matter everywhere? And couldn't these concentrations of dark matter happen all the time? Yes, dark matter is everywhere and there's a lot of it. But to make a big black hole out of a blob of stuff, you need a big fluctuation in the amount of dark matter, and so that's just less likely happened, like in the density of it. Yeah, exactly, it's like a statistical argument. You know, you distribute dark matter randomly through the universe, you're gonna get clumps. How big are those clumps? Well, to get really big clumps, it's going to be less likely. I guess a question I have is if you have a black hole with dark matter in it, the dark matter can leave either. That's right, dark matters also gravitationally bound. Everything is gravitationally bound, neutrinos, photons, dark matter. Because remember, it's not just like a powerful force holding onto you that you could maybe ignore if you're a particle that doesn't have the right charges. It's a shape of space, like inside the black hole. Space is one directional, So if you're inside the black hole, any direction you go brings you closer to the center of the black hole. So it doesn't really matter how fast or strong or weak you are. All right, Now, let's get into how they could possibly grow as much as they are now and if we could ever escape these super mess black holes. But first let's take another quick break. Alright, we're talking about super massive black holes and how they form, which is a big mystery. So the main idea that I guess you're telling me is that maybe super massive black holes started out big. That's right. They were always big. They were always be that's what you're telling me. It's like the baby came out huge, you know, that's why he's so tall. He came out tall. Oh, that's terrifying from the parenting point of view. So there are three possibilities. Maybe they formed huge in the primordial soup of the universe, or maybe they came from really big stars, or maybe dark matter was involved. And so does that mean that they came into being as big as they are now or would they still need to grow some to see what we see now? Well, the black holes that we see now are much bigger than those seeds we just talked about. Those seeds get up to like maybe tens of thousands of times the mass of the sun. The black holes we see out there, they get up to, you know, like five ten billion times the mass of the sun. And so you definitely got to grow it. It's like that's a good seedling. You know, if you're a gardener, you know, it's much harder to get something growing from a little seed up to a big, robust plant. Somebody gives you a seedling than boom, you can just water it and sun it. You have something giving you tomatoes, it's already going. Yeah, it's already going. And so these are basically like black hole seedlings, but you still got to grow them to get them as big as the stuff that we see and is there a mystery there or you know, if you if you start with like a ten thousand solar mass black hole, can you get up to those billions of super massive black holes? You can? But it's hard, like you gotta really ride the edge. You know. These are questions that are not like physical limits. It's not like there's some reason why a black hole can't get any bigger. In fact, there's no theoretical limit on the size of a black hole, but there are sort of environmental limits, Like you need to create the situation where a black hole has the stuff it needs to eat, because tends to just like gobble of stuff around it and then push the stuff away. So that's this balance we were talking about earlier, right, because black holes emit a lot of radiation and sort of in all directions, pushing away from themselves the stuff that they would need to eat to get bigger. Right, And also like you know, if it has to eat that much, where does it get all that food? Like if a black hole is ten million times the size of the Sun, it has to eat ten million sons. Yeah, precisely, And for example, the black hole the center of our galaxy, how much bigger could it get. It could only get about a thousand times bigger, because that's all the mass that's in our galaxy. If it ate every single star and blob of stuff in the galaxy, that would only give it a factor of a thousand. So if you see a black hole out there that's a million or a billion times the size of the Sun, that means it it ate that much. And so you know, I like, how much big or what's the galaxy? He sounds so judgmental, like somebody who you know, had to pop their bells after dinner, like, hey man, you shouldn't have had that sixth plate. I think if you eat a few billion sons, Yeah, yeah, exactly. And it's just sort of hard to arrange the stuff and the right conveyor belt to get it there. And so they've calculated what is the maximum effective rate that a black hole can grow. It's called this Eddington limit, and it's possible to sometimes have what they call super Eddington growth, but only very very briefly. And it's because the radiation that increases a lot and blows out the gas. The highest stable rate requires a special configuration. You've got to have just the right amount of cold gas falling into the black hole and just the right way. It's a bit tricky to arrange. Instead of get these seedlings up to the massive black holes that we see today, requires a sort of unusual, unlikely arrangement of stuff to fall into the black hole at just the right rate, like perfectly funnel into it with causing a big message. Yeah. And you know another way these things can get bigger is by merging, Like you can merge two galaxies. Two galaxies collides because you know, on the scale of like the universe, each galaxy is like a dot, and you can think of it like a particle and a gas flying around, and occasionally they get close to each other and they emerge, like our galaxy the Milky Way is going to merge with Andromeda in a few billion years. And Andromeda's black holes even bigger than hours. It's like dwarfs our black hole. And and so these two things are going to collide, and eventually those two black holes in the center will spiral around each other and via the emission of gravitational waves, become one and so will become an even bigger black hole. Wow. I guess one curious question I have is um, you know, why do we see them at the center of galaxies? Like, you never see a supermassive black hole not inside of a galaxy, do you know? You don't. And the reason is that they need stuff to grow. Like if you had the seed of a supermassive black hole just out there in the middle of the universe, it couldn't grow. It's got to be surrounded by stuff. That's why there's like a correlation between the size of the supermassive black hole and the size of the galaxy. Bigger galaxies have bigger black holes at their centers just because there's more fuel for them to eat, and smaller galaxies have smaller black holes because there's less fuel for them to eat. And so it makes sense, But it sounds sort of like a coincidence. Almost did these supermassive black holes? Are they one of the reasons the galaxy form? Or is it sort of a coincidence that you've got of galaxy forming and you also had a seedling for a massive black hole in the middle, and so that's why you have them in the middle of galaxies. I think it's the opposite. I think that larger galaxies are going to form supermassive black holes just because they have the fuel around. And so I think if you take a big distribution of galaxies of different sizes and you put a seedling at the center of each one of them, the larger galaxies will grow their black holes faster, and we'll get to bigger sizes. Oh, I see, So galaxies form for other reasons. But they all sort had a sprinkle of super massive black holes, the seeds of super massive black holes, And so the seeds that fell and the larger galaxies got bigger. Yeah, and the galaxies formed because actually of dark matter, right, dark matter forms these gravitational wells somewhere out there. Dark matter is not just smoothly distributed through the universe like some soup. It's got blobs and and chunks to it, and galaxies form in those blobs and chunks. It's the reason we have galaxies. If you delete dark matter from the universe, you don't even get galaxies by ten billion years. You've gotta wait a lot longer for gravity to pull just the normal matter together. So the reason you have these galaxies and those seed things together is because the dark matter sort of like forming the pot in which all this stuff is growing, all right, and all that points are kind of a maximum size of black holes. You're saying that the maximum size that a black hole can be is about ten billion sons. Yeah, so why is it? Is it just because we don't see galaxies that are bigger than ten billion times the mass of our sun? Yeah, it's not a theoretical limit, right, could in principle form a black hole that's larger. But if you ask, like, what's the largest size black hole we expect to exist in the universe right now, given the time they've had to form in the size of galaxies, then we think that's about ten billion times the mass of the Sun. So it's an environmental limit, not a fundamental limit. And these biggest black holes, the ones that are like billions of times of the mass of the Sun, we see them in galaxies that have very little gas means they've already sucked up most of the fuel. Okay, so the ones that are smaller have more fuel around them, they still have a place to grow, and so there's not really much more room for them to grow unless they merge with another supermassive black hole. Could you have had like a really big galaxy you know, could you have formed a really much bigger galaxy or is that rare? Yeah, that's a really fun idea. You know. It's a question of like how big could a galaxy get? And remember we talked about this in our episode last week about the sort of structure formation of the universe. Things sort of form bottom up and clump together, and there's a balanced there between like the initial velocity of this stuff and the gravity to pull it together. And so space is expanding and pulling stuff apart, and things have velocities which prevent them from clumping together and this gravity pulling stuff together. So what limits sort of the size of the galaxy is just like how much stuff you can get together in a gravitational well before dark energy takes over and starts pulling things apart. And so we think that there is a limit to how big galaxies could have gotten in the early universe. I see, which then limits the black hole. It does unfortunately, I know, thankfully, I guess maybe. I mean, if you were that black hole, you probably want to get bigger, or maybe you'd be like, thank you for putting that stuff away in the fridge so I don't eat too much. Yeah, you know, like, we'll slow down there, you know, I think you've had enough. You know, limits are good? All right? Well, um, so that's a big mystery, I guess. And and do we do we have to worry about these super massive black holes, like are they dangerous or maybe they help galaxies actually stay together? They do help galaxy stay together. And there is one really big one at the center of our galaxy. You know, it's like four million times the mass of the Sun, which is big and incredible, but on the scale of supermassive black holes, not that big. And it's like sixty million kilometers in diameter, which again seems massive, but compared to other black holes, is really not that large. For a black hole, the size of it, the distance from the center to the event horizon, grows linearly with mass, so you're like, you double the amount of stuff in it, the radius gets twice as big, which weirdly means they're not actually that dense. Like a supermassive black hole has about the density of water. What you mean where the event horizon starts, Yeah, exactly, Like all the stuff contained in the black hole, divided by the volume of the black hole has about the density of water and it's incredible amount of gravity because it's just so much stuff. But when you double the amount of stuff inside your black hole, you double the radius, which makes the volume go up by a factor of eight, and so the density actually goes down. So the largerer black hole, the less the density. It's like dark waters. I don't recommend drinking it. You know, get one of those filters or something. But in our galaxy, the black hole doesn't eat a star that often. It's like every ten thousand years or so a star will get sucked into the black hole. It's pretty rare really. Mostly it's just sitting there. But is it Is it eating gas? Like, are there funnels of gas going in? There are funnels of gas that it's still eating. But you know, the stars are mostly in stable orbits. The ones that we're going to fall in have mostly fallen in, just like our Earth is in mostly a stable orbit around the Sun. We don't expect to fall into the Sun at any time soon. Now, something could come along and disturb our orbit and give us a bump, and then we fall into the Sun. The same way. If two stars collide, one of them could end up getting gobbled up by the black hole. But mostly the star is going to escape the black hole. They're just gonna be in orbit around it for a long long time. And you can think like, well, what about in ten trillion years, like do those orbits eventually decay because of interactions with interstellar medium? But you know, we don't know the future of dark energy or anything for ten trillion years, so it's pretty hard to imagine what's going to happen that far into the future. Right, to think of black holes as pretty scary, But if you think about it, our sun is also pretty scary. You don't want to fall into the sun either. Our sun is pretty scary, and the center of our Earth is also scary. You don't want to get down there. Anything that's got a lot of gravity is pretty intense situation and you know, a good place to avoid. Yeah, you don't want to take that situation too lightly. You know, it's pretty massive, going to give it the right amount of gravity? All right, Well that's the mystery of super massive black holes. How do they form so big? How big were they when there were babies? Nobody knows? And who's been feeding them secretly at the maximum possible woman, No, it's like a fun question like how big could a tree get if you fit it at just the right rate, or with the largest cucumber or pumpkin you could grow in your backyard before like implodes into a pumpkin black hole? Right, yeah, because it's kind of weird to think that things can scale like that, you know, like you could have a technically a giant pumpkin, like just like once we had giant horses running around, that's right. And if you google giant pumpkin, you will be amazed at the size of these gourds that people actually can't throw in their backyards or in their farms. It's impressive. Could one of these collapse into a black hole? Daniel, I think we're pretty safe, all right. Well, we hope you enjoyed that. And the next time you look out into the universe once again, remember the big mysteries that are out there, and even the super massive mysteries. That's right, The biggest things out there in the universe are things that are not yet understood. Scientists are working on this, their eyes puzzled about it as we are, which means that their curiosity is your curiosity. And it also means that somebody out there, maybe you, maybe your kids will be the person to unravel the mystery of super massive black holes. That would be super massively cool. So thanks for joining us, See you next time. Yea, thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of I Heart Radio. For more podcast for my heart Radio, visit the I Heart Radio Apple Apple Podcasts, or wherever you listen to your favorite shows.