Where are all the medium-sized black holes?

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Speaker 1: Hey, Daniel, how do you like your coffee in my mouth? I guess usually or maybe sometimes in ice cream. I mean, like, do you like to drink the tiny, strong Italian coffees or the big week American style coffees? Hmmm, I guess I like the sort of medium sized coffee, you know, strong enough to be for grown ups, but not like a punge in the face. I see. And does that apply to particles too? Do you prefer tiny particles or massive ones? I don't know. I like them heavy enough to be rare and worthy of winning a Nobel Prize, but light enough that we can discover them at Collider's. And you're so middle of the road. Sometimes the middle of the road is exactly where the mysteries lie. Hi am Jorge, handmade cartoonists and the creator of PhD comics. Hi. I'm Daniel. I'm a particle physicist, and I usually eat a medium size portion of cookies for breakfast, for lunch, or all of the above. Every time you start eating cookies, you can't just have one. You try to avoid having the whole box, so I end up somewhere in the middle, somewhere like half of a box of cookies. Depends are you're doing arithmetic half or geometric half, you know, it depends on the day. Then you can do zenos paradox that can eat half the box, and then you eat the other half, and then the half the remaining half, and pretty you ate everything but a crumb. Yeah, well, that's a great way to avoid eating the whole box, right, Wisdom from the ancient Greeks. It's the Greek diet, it's the new Mediterranean diet. Only eat half of the remaining box of cookies. But welcome to a podcast. Daniel and Jorge Explained the Universe, a production of I Heart Radio, in which we take a bite of the entire box of universe cookies. We chew on the bits to talk about black holes, We think about all the little chocolate chips that are particles. We dig down into the crumbs and try to understand everything that's going out there in space, in the universe, in history, and in the future. We very humbly try to explain the entire universe to you. That's right. There are amazing mysteries all over the universe, and those mysteries range from gigantic size galaxy cluster billions of light years wide mysteries and really really tiny mysteries in the particles and the little quarks and tiny quantum fluctuations that make up who we are. Are you saying I'm just a quantum fluctuation that I could just disappear tomorrow. Well we can all disappear tomorrow, Daniel, depends on what you guys are working on at the large handring collider there. Well, if we make a black hole, will make a little itty bitty when I promise good, I'm glady that's an option. I'm so glad you're not tempted at all to just dial it up. We'll have a knob here in front of me, you know, So tell me what what size black hole should we be making here the late see? How about a zero sized black hole? That's no fun? And how are we going to learn the secrets of the universe? Keep you inside the tiny black holes? I'm okay where they are right now. But black holes are amazing. They are fascinating. They do contain amazing secrets of the universe inside them. But there are also really fascinating questions about how they get made and why they are the size they are or the size they aren't. Yeah, they seem to be everyone's favorite space mystery. Do you think maybe it was all in the name, Like if you had called them, I don't know, vacuum holes or gravity pits, do you think people would still be as interested in the I think they would. I think it's their fundamentally alien nature that makes themselves fascinating. We know they are really deeply, weirdly different from anything we have seen before, and I think that's what sparks our curiosity. You know, we want to go there and see it and explore it because we want to understand what's possible in the universe. As we say often on the podcast, it's the extreme situations that teach us what the rules really are because they show us the limits. Yeah, and as you say that, you can find them in all kinds of sizes out there in space. There are black holes that are humongous and warm the center of galaxies and help keep galaxies kind of together, and you can make them tiny little ones that evaporate right away. Yeah, that's right. There are these funny, two different populations of black holes, the ones that seem to come from the collapse of stars and the huge galaxy gobblers at the center of many galaxies, and the weak thing is that there don't seem to be any in between. Really, there's no like Goldilocks black holes. There's only two options, like VENTI and X are large. That's right. You either don't open the box of cookies or you eat the whole thing. The universe never goes for half a box. Yeah, so there are mostly two sizes of black holes in the universe. One you're saying it are the ones that come from the collapse of stars. Those are I mean, they're pretty big. There's still like the size of you know, many suns. Yes, these are small, only on the sort of like cosmic galactic scale, right, smaller than a whole galaxy. Still a normal is still more massive than our entire solar system. But yeah, tiny compared to other things. And then you have the huge ones, the ones that are how many, like millions of times the mass of the Sun up to millions or even billions of solar masses, so many stars compacted into a tiny little space at the center of some of these galaxies really just mind bogglingly massive and incredibly powerful objects. Yeah, and so black holes can be any size really in between, and you know, from tiny, two huge, But apparently you don't see a lot of ones in the middle out there in the cosmos. Most of the ones that you see out there are either kind of supernova size or gigantic galaxy center size. Yeah, it's even more dramatic than that. We have never seen any in between. There's not an example, a single example of a black hole in between those two. So we don't actually know if it's possible to have an intermediate size black hole. Maybe the laws of physics just don't allow it. So that's the question will be tackling today. So to be on the program, we'll be asking the question where are all the intermediate mass black holes? You've gotta made it easier for me, Daniel, and just medium mass. Well, that was going to be my next question is what would you have called these things? Because intermediate size black holes or intermediate mass black holes is quite a mouthful. Yeah, I don't know, just go with like the Starbucks rude and call them large black holes, which are really the medium sized black holes. That's true, but everything in the universe is enormous, right, so maybe the Starbucks naming scheme makes sense. Yeah, large, extra large, super large, and galectically large, cosmic large. That's right. It's like ordering fries and McDonald's there are no more small fries, Is that true? I think so. I think you can only order large or extra large. I will admit not having been to McDonald since I worked there when I was six. And yeah, you seem to know a lot about their menu, Danniel. I'm a member of society. I participate in society. I read memes on the internet to educate me about humanity. You turn your your nose up at fast food change, sure, like everyone else in society. But yeah, maybe I would call medium sized black holes. You know that kind of makes sense. Yeah, you know, as a middle child, I'm sort of sensitive to this naming. You know, middle seems to like be defined by the others. There's nothing like unique and identifying about being in the middle. You're like defining yourself by what you're not instead of by what you are. We just opened up a whole black hole of psychological trauma. There issues there and Daniel, your middle children out there. You know you know that well, you have like ninety seven brothers and sisters, right, so you you've got to understand and is this why you started a podcast just to err on your sibling rivalry issues. Yes, it's been a three hundred episode plan to get to this moment exactly. All right, tell me, Daniel, tell me about your childhood. I feel defined by my more massive and less massive brothers. I see now you're making swipes at their body weight there, man, I meant intellectual gravitas, of course, yes, of course, of course, but yeah, I'll rate that. Are talking about black holes, Daniel, and how there aren't any real middle of the ones out there in space. You mostly see big ones or smaller ones the size of a few sons, And so that's the question, where are all the intermediate mass black holes? So it's usually we were wondering how many people out there knew that these black holes are are missing from the universe and where they could be. So thanks to everybody who volunteered to answer my random questions with no preparation or reference materials. If you'd like to participate in future episodes, please write to me two questions at Daniel and Jorge dot com. We love your contributions and I promise you it's fun. So think about it for a second. Where do you think all the medium or intermediate mass black holes in the universe are. Here's what people had to say. That's a good question. I think that there's no proof that they even exist, but that they do. Maybe they are lurking somewhere in the intergalactic space where there's just no stuff for them to eat and growth. They don't don't form those Christian disks, And basically the only way that we can spot a black hole is by looking at the Christian disk and figuring out indirectly that there must be a black hole in the middle of it. But if there is no a Christian disk, that we may have a black hole that is somewhere out there, but we just have no chance of ever spotting it. So I would bet for intergalactic space. I know at the center of a galaxy you get really big black holes. I think they're called supermassive black holes. So they won't be there, but maybe be an intergalactic space. But I'm not sure. All right, not a lot of clarity here from our listeners. I like the idea that maybe they're floating out there in space and we just can't see them. This one president said, made they're lurking in intergalactic space. Yeah, it's a cool idea. If you're a black hole, can you do anything else in space except lurk? Oh? Come on, you could be very dramatic. You could swoop into us always system and gobble it up and destroy everything. Right, that's the opposite of lurking. That's like leaping. Yeah, but then afterwards you go back to lurking. Well, I like this idea because it suggests that you know, there are things out there and intergalactic space that we don't know about that we can't see. That it might surprise us that basically you could hide anything out there as long as it's dark enough. And you know, that's a reasonable point. And I guess the real mysteries that you know there there could be intermediate mass black holes out there, Like, there's nothing that we know about so far that would would prevent them. Theoretically they're possible, but you just don't see them. Yeah. Well, that's the problem with black holes is that you can't see them directly, right because they are black. All you can do is see their effect on stuff behind them or around them. And so if you had a black hole deep in the middle of space with nothing around it, how would you detect it, it it would be undetectable, it would be very effectively lurking. And so this is actually a good point. The question that needs to know how did it get made? How did you form a black hole out there in the middle of space with nothing around you. Usually black holes are made from stuff, and then there's other stuff around that didn't get black holeified, and you can use that to detect it. That's the mystery. And so I guess maybe, Daniel, let's start at the beginning and let's talk about how you would even define an intermediate black hole, like, what is there a technical range of masses that we qualify as intermediate Yeah, there actually is. Unfortunately for the psychology of intermediate mass black holes, they are defined by what they are, not they're defined by their bigger and larger siblings. Right, So it's a black hole that's between a solar mass black hole like one that comes from a collapsed star, and we can talk about it, but there's definitely an upper limit on how big that can be, because there's an upper limit of how big a star can be. So it has to be bigger than the black hole that could come from a star, but then smaller than black holes we see the center of galaxies, which is about ten thousand or fifty thousand times the mass of the Sun. So there's a huge range there between a hundred and like ten thousand solar masses. We call the intermedi a mass range for black holes. Oh, I see there's a limit due to the sizes of stars. Exactly what kind of black hole do you imagine when you think about black holes? Probably are the ones that come from when a star collapses, Right, you know that stars are formed from gases that are collected by gravity, and then you get fusion burning, which prevents further collapse. Right, gravity would just go directly to a black hole if the star didn't ignite fusion inside of it, which was pushing out with radiation to prevent the collapse. That happens for you know, several billion years or so until the fuel runs out and turns into heavy ash, which accelerates the gravitational collapse instead of preventing it, and then the whole thing collapses into a black hole. But there's a range. They're like, it can't collapse into a black hole if it doesn't have enough mass. So the lower limit on a black hole that comes from the Sun is like ten solar masses. Smaller than that, and it collapses into like a neutron star or something else because it can still resist the gravitational pressure. But what about larger than that, Like, can bigger star collapse into a bigger black hole? It can, but you can only get up to about eighty or ninety times the mass of the Sun. And the reason is that you can't have a star that's much bigger than about three hundred times the mass of the Sun. You try to make a star bigger than that, Remember we talked about this on an episode about the biggest stars in the universe. Then those stars aren't stable. They blow themselves apart because as the star gets bigger and the inside of it gets higher pressure, and that higher pressure means the fusion burns hotter, and so it produces more radiation. It's very nonlinear, and so a little bit more stuff means a much higher temperature inside, which means radiation blowing out. So then the star literally tears itself apart. So the biggest star you can have is about three hundred times the mass of our Sun, which means that there's a limit on how big a black hole you can get that comes from a star, I see, But the limit is smaller than the biggest star. So the biggest stars can be three hundred times the mass of our sign, but the ones that turn into black holes are only the ones that go up to about eighty times the mass of our Sun. No, no, it's because not all the stuff in the star ends up in the black hole, right it's tearing itself apart, and not all of it collapses in the black hole. You still get a huge amount of stuff outside the black hole that it's blown out. You know. Sometimes you get like a supernova before you get a black hole, and that shoots a huge amount of energy and mass out into the universe. So the process to go from star to black holes not ad efficient, which is why three hundred solar mass stars turned into about eighty or ninety solar mass black holes. You're talking about the kind of the limit of the size of the black hole that can come from a star. But the stars usually started off with three hundred times the mass of the Sun, and then they collapse into eight times the mass of the Sun, and those are very rare. Like stars that massive are very very rare because they're very unstable and short lived. Most stars are much much smaller than that. So most of the stellar black holes you see out there are in the lower end of the ten to eighty range. Eight is like really extreme and crazy and rare. So then that's like one kind of limit, like the biggest stars kind of defines what the biggest black holes that can come from the Sun with their masses exactly nicee. And we see a lot of those out in the space, right. Yeah, those are all over the galaxy. Every time you have a star that's that big, its path will lead it to become a black hole, And so those black holes are appearing all the time. They're not rare, they're not weird, that not unusual, and we see lots of those black holes around the universe. That's the kind of lower range of black holes, and then there's like the upper range of black holes. Yeah, the black holes that we see at the centers of galaxies are like completely different beasts. I mean, they are similar from the general relativity point of view in that they are very compact objects that are very dense that have incredible gravity, but they seem like monsters compared to these stellar black holes because they tend to have masses like a million or a billion times the mass of one son, and so these are just really enormous and they really like drive the gravity of a galaxy. They sit at the center of the galaxy and they are you know, like one one hundreds of the mass of the whole galaxy is in that single black hole and it's pulling on the whole galaxy and it's slurping everything around and it's gobbling stars at the middle. We have one like this at the center of our galaxy. They're pretty common, I guess, Like I think most galaxies have a supermassive black hole at its center. Like when you look out into space and you see, you know, millions and millions of galaxies, most of them have one of these giant black holes in the middle. We don't actually know the fraction of these galaxies that have one, And that's one of these questions, Like we know that black holes in the center of galaxies tend to be proportional to the mass of the galaxy. So bigger galaxy, bigger black hole the center. Smaller galaxy, smaller black hole the center. We don't know if they all have black holes, like we've seen a lot without them. We don't know the fraction of galaxies that have black holes, actually, but it's not rare. I guess that's what I mean. You see them often. Yeah, it's not uncommon. Absolutely, They kind of all over the place, yes, And so what's the range of size as we've seen for those? So up to billion, right, like ten billion solar masses is the biggest black hole we've ever seen. And as far as we can tell, there's no upper limit to how big a black hole can get. Like, you keep feeding that thing, it'll just keep getting bigger. The limit really is just like cann be around enough stuff and have enough time to gobble it. On the lower end of super massive black holes, the smallest one we've seen is about fifty thousand times the mass of the Sun. This is in a little mini galaxy we call these dwarf galaxies, about three hundred forty million light years from here. The galaxy is called r g G one eighteen. And that's the smallest black hole we've ever seen inside a galaxy. It's fifty thousand times the mass of the Sun. That's huge. Imagine fifty of our sons, Yeah, and then collapsing into a black hole like the gravity of fifty thousand suns is nothing to be sneezed at, or if you sneezed at it, your sneeze will get sucked up by that black hole right away. This is not we just gets slurped out of your nose. Probably we should call that black hole is in tight Hey, yeah, then will be a green black hole depending on the size of years these we'll see. All right, Well, it seems like there are these huge populations of black holes out there, kind of the the star sized ones and the gassy size ones, but maybe not so many in the middle. So let's get into whether or not they actually exist or whether or not we're just not seeing them. But first let's take a quick break. All right, Daniel, we're talking about intermediate mass black holes. Now. Is that the official names are an acronym for that, That is the official name. You'll see that in the literature if you search for them. And you know, I call them I m b H. But it sort of reminds me of I MDB, so I think maybe we need a better name for them. M I see. Well, they do sort of collect stars in a way, right, I'm sure there are films of gas clouds I'm really stretching in here. Yeah, well, we should go look this up on the Internet movie black Hole Database. I'm sure there is one, isn't There are dots and physicists kind of keep track of everything and polish it online. I don't know, but after this episode, I'm gonna go type in i mbhdb dot com and see if somebody owns that already. Oh man, maybe you should have checked that before sponsoring it on the podcast. Some family with kids are gonna look that up and then they're gonna be like what. Alright, we're talking about whether or not intermediate mass black holes exist because we don't really see them out there in space. We see the kind that are about the size of suns, a few hundred suns that came from suns. And we talked about black holes that are at the center of galaxies, which are huge tens of thousands or maybe millions or billions of times the mass of our sun out there in the middle of galaxies. But you don't see a lot of black holes in between out there. Yeah, there seems to be this weird gap there, and it makes us wonder, like, are there those black holes out there? But we just can't see them yet, or is there some reason why they don't even exist, like they're unstable, or they accelerate really quickly to becoming supermassive, or they fall apart in some way. Like anytime there's a puzzle like that, something we don't understand, where we see something we don't expect, or we don't see something we do expect, that's a clue. That's an opportunity to figure out something. It's a moment where we can learn something about the universe because we're ready to be surprised by the data. Right. Yeah, it would be like, you know, having a population of people and then seeing only two sizes of people, like not seeing you know, sort of a continuum and like a smooth range of sizes of people. Yeah, and it would make you wonder like how did all those tall people get so tall? At some point they must have been medium height, right, so where are all the future tall people? Why are there no medium height people who are growing into those tall people. That's basically the question we're asking about these intermediate mass black holes. I mean, for example, you wonder about these supermassive black holes. How did they get supermassive? They must have started from something. If they started from something very small, then you know, by simple arguments, they must have at some point been in that intermediate range. Right, Well, I guess that's one of the mysteries, right, Like, we don't actually know where these supermassive ones in the center of galaxies came from or how they came to be so large. That's one mystery, right, that's exactly the mystery. Like the argument had just made that supermassive black holes may have started from small things and then grown to supermassive, we don't actually know if that's true, and we see things about supermassive black holes that we can't explain using that idea. For example, we look really far into the edge of the universe to see old old galaxies, to see galaxies when they were very very young, right like just a billion years after the beginning of the universe, there were already galaxies, and in those galaxies we already see super massive black holes, like black holes that have two billion solar masses. So like, only a billion years have passed and already you've made stars and galaxies and supermassive black holes inside them. We don't know how it's possible to go from nothing to supermassive black hole in just a billion years. So it's a mystery. How did those black holes get started. We don't know. I see a billion years from the Big Bang, you're saying, we already had black holes that are billions of times the massive US Sun. Yeah, exactly, And we can see them because those super massive black holes have really big accretion disks around them, like this disc of gas and dust and other stuff that's getting swirled in it gets really hot and it glows. It sends off a huge amount of radiation. Those are called quasars. So we see these quasars in the very very distant universe from the very early universe, and if you model the formation of galaxies, it's really hard to get a black hole that big that fast. We talked about this in our episode about supermassive black holes, so you can dig into that if you want more details. But very briefly, there's sort of like a limit at how fast a black hole can grow. It's called Eddington limit. And as you pump more stuff near a black hole than actually the radiation from the stuff near it pushes stuff away from the black hole. So it's hard to feed a black back hole fast enough to have a grow to be that big that quickly. So it's a mystery. We don't know if these black holes actually started from something small and then passed through this intermediate mass black hole region and then became supermassive black hole, or if they somehow skipped it. Could they have just you know, drunken a lot of strong coffee or Italian style and black espresso, or maybe American style boxes of cookies. You know, there are other ideas to explain the supermassive black holes, like, for example, maybe there were black holes that begin in the very beginning of the universe, before there were even particles, before they were stars, before there was even really matter. Maybe there were these primordial black holes right because the Big Band was pretty crazy, I imagine, or at least how the universe was before the Big Bang, and you know, things were pretty chaotic and crazy, and so maybe why not, why couldn't you just have black holes form and that kind of primordial high energy soup. Yeah, you could have. And if you did that, then you could have gotten a bunch of really tiny black holes and intermediate mass black holes and maybe even supermassive black holes, or big enough at least to become a seed which later turned into these early supermassive black holes. And so it might be possible that the supermassive black holes were seeing were never intermediate mass, that they formed like suddenly during the Big Bang, already fifty thousand solar masses and then just grew to a billion masses. So the point there is just because you have really big black holes doesn't mean they were once medium sized. Right. It's it's kind of like having a birthmark, like a mall you're born with, not a mall of that shows up later. Yeah, and there's a lot of really fun stuff. We have a whole episode about primordial black holes, which is a really fascinating topic because they might actually be real and out there, and some people still think they might account for the dark matter. So it's a really fun topic. The problem with primordial black holes is that if you make them, you expect to make them out all sizes, like really big ones and then also really little ones and the smaller ones. We should see those, and we should see them evaporating, because from small black holes evaporate and they give off radiation and as they give off radiation, they get smaller, and as they get smaller, they give off more radiation, so they would evaporate very brightly and we should see that, and we haven't. So there's some skepticism about whether primordial black holes are a thing. You're saying, like, at the beating of the universe, we don't see evidence for those little tiny black holes for me. Yeah, we don't see evans for it. It doesn't mean that they didn't happen, but we don't see evidence for it. So if primordial black holes are the explanation for super massive black holes, you need some reason why they basically only got made on the larger size and they didn't make littler ones that we would also be seen. But that's not the only problem with that theory, right that Also the theory is that even if you had started with tiny black holes from the Big Bang, there's no way they could have grown that big to what we see today. Yeah, well, you need to make primordial black holes to be pretty big, so you need to start from pretty large primordial black holes so that they could be big enough to seed the supermassive black holes we see today. You'd make them at all ales in the Big Bang from tiny to really massive. And I see. And the problem is that we don't see evidence for those large seeds or what well, that would explain the supermassive black holes. But we would also expect to see the little ones, and we don't see the little ones anywhere. Like the little ones. We should see those evaporating all over the galaxy like they should last, you know, a few billion years, and they should be evaporating in these bright flashes of light as they disappear. But we don't see that. So that means that there aren't little primordial black holes. Doesn't mean there weren't really big primordial black holes, but it means there weren't really little primorial black holes. And that means that it's a little bit more complicated to explain. Oh, I see right, because we can see back in time. Right when we look at into space, we can sort of see back in time towards what was there close to the Big Bang. Exactly, we do literally see back in time because light takes time to get here from Earth. We're seeing now what happened a million or a billion, or five or ten billion years ago. As we look deeper into the universe. It's really pretty awesome that we can look backwards in time. But there are also other ways that you could form these sue for massive black holes without going through the intermediate mass black hole route right. You could, for example, even without forming them in the Big Bang, you could have this scenario where super massive black holes formed directly from clouds of gas and dust. It's called direct collapse. They never stopped and become a star and burn for billions of years. First they just go straight to black hole. Really, you can do that. You can just form a black hole without igniting and with fusion and everything. It's a theory. We've never proven it and we've never seen one, but it's a theory that people can't rule out. If you have enough stuff, like a really big blob of stuff, then it collapses fast enough the fusion doesn't have a chance to slow it down and repel it, and it just goes straight to black hole. And you can sort of simulate it, and that can actually happen, like if you like the clouds somehow takes up enough speed compressing that it starts to night in the middle, but it's too late. There's more stuff falling in and then poof, it becomes a black hole. Yeah, grand gravity wins. And again it's still just a theory, but the calculations check out out. We've never seen when we don't have direct evidence for it. But it's a way to potentially explain how these supermassive black holes got started and then had a chance to grow. And it would be another way to have really big black holes without ever going through this middle phase, this awkward teenage years of the black hole, when you head between a hundred and ten thousand solar masses. They could have directly collapsed to something really really big fifty tho solar masses and then just kept growing from there. Isn't there another theory that these supermassive black holes come from smaller black holes colliding and merging into bigger and bigger ones. For sure, But that takes a long time, right. If you think that these big black holes came from really tiny black holes, then they spend a long time in the tiny stage, right because the gravity is proportional to the size of it. And so if you start from just like one stellar black hole and then at another and at another, there's not enough time in the universe to get up to billions of solar masses. That's the problem, right, because I guess you know it thinks in space on just like collapse head on. Right, they have to circle each other for a while, you have to date, you know, you have to go grab some coffee, eat some cookies, and then eventually they collapse into each other. Right, that's kind of the idea, Like things just don't running to each other out there in space. Yeah, it's not like some huge cosmic kid out there is trying to build, you know, a supermassive black hole by sticking the black hole legos together. It's got to happen. And for that to happen, the things have to be in the right arrangement and they have to collapse into each other. And it gets easier as your black hole gets bitter because it's more powerful and it can gobble stuff, so like the sphere of its influence grows and where it can eat from grows, so then it grows more quickly. But that's why a black hole that starts from a really small object spends most of its time small before it accelerates its growth near the later phase. So we would see those and they would take too long to get to the supermassive black hole stage, so they can't explain all the really old, huge black holes that we see, right, and so that's the big mysteries that we know where the smaller size black holes come from. They come from stars, and then we see a bunch of the huge black holes in the center of galaxies, but we don't see the ones in between, and we don't know how the big ones got as big as they did, Yeah, exactly. We don't know those big ones were once intermediate size, and we can't see any examples of the intermediate sized ones, which seems like a head scratcher, right. Yeah, it's kind of like a double mystery, like we don't know how they got so big, and also we don't see any of the middle ones. Yeah, and we'd love to unravel that mystery. We'd love to understand how these supermassive black holes form. And so finding one like when it was a child might really give us a clue to how they formed, or if we could prove that they never were children, they were like fully born as supermassive black holes, then one fell swoop that would also be fascinating. That is a possibility, is that maybe they were made super big in the big bank as primordial black holes or through direct collapse. There are some mechanisms to make these things and skip that intermediate phase. Right, That would make sense, right, you know, to have their own proper origin story that doesn't involve growing. Yeah, it does make sense. But you know, galaxies come in all sizes. There are really big galaxies and there are smaller galaxies, and it goes down to pretty small like galaxies can be down to just a few thousand stars. And so if black holes inside galaxies are proportional to the mass of the black hole, and again we don't know that, we've just seen that relationship for larger galaxies, then it stands to reason that these smaller galaxies should have smaller black holes inside them. Well that's the mystery, and so let's talk about now how we could find them or are people looking for these medium sized black holes and is it theoretically possible to see them or that they can exist. So let's get into that, but first let's take another quick break. Alright, we're talking about the ignored middle child, i'll black holes of the universe, And I know this is a personal topic for you. Daniel I'm sticking up for these black holes. You're just trying to make them the cool ones. I think, Yeah, we're like, who cares about? The little ones are the big ones, They get all the attention. The cool ones are the ones that you know, sit in the back and plot their revenge with a podcast years later, decades, decades into making This is My Revenge. No, set the record straight. I love my brothers. They're wonderful and they're supportive, and we're all very good friends. Right, Yes, did that not sound sincere I meant it to. That sounded a little tacked on at the end, But hey, you know, I know how it goes your lawyer or a therapist. Is there supposed to be a difference? Really? All right, well, let's get into this mystery of the medium sized black holes. We can't see them anywhere, now, Daniel. Do you think we don't see them because we can't see them or because they don't exist? That's the question. We don't know the answer too, because frustratingly, smaller black holes are harder to see. Like, one thing we're pretty good at is figuring out what we're good at, and we're good at seeing really big black holes because we can see them affect how stars move in the center of galaxies, or sometimes they have like huge quasars. We also know that we're not very good at seeing smaller black holes, and so we haven't seen any. But we also know that we're not great at spotting them, so that leaves the questions sort of up in the air. Right. Yeah, even the ones that come from giants sons, they're kind of hard to spot in space, right, It's not like they glow, they're transparent invisible, and space is pretty dark, and maybe they're not heavy enough to kind of like have a big impact in the stars around them. Yeah, you have to infer them. You have to be lucky. So you have to see for example, gravitational lensing where the light that passes near them gets bent as if there was a huge invisible mass of stuff there, and you know, you have to know that it's really compact and dense, so it's not just like some big diffuse cloud or dark matter, or you have to see their impact on other nearby stars with nothing else to explain it. So it's usually just like a process of elimination. We see something happening in space and we can't explain it in any other way other than a black hole. It's pretty rare. You get very direct evidence of a black hole though sometimes, and that's usually in the center of a galaxy, when they're really big and there's like quasar emissions and other stars moving very close to them. Have we seen the smaller black holes, like, is there a catalog of them or are they still kind of theoretical that they exist? Now, we have definitely observed stellar mass black holes. Absolutely, we've seen their gravitational lensing, we have seen their effect on other bodies. In fact, we did a whole fun podcast episode about the history of how people became convinced that was not just a mathematical concept, but they were real and they were out there, and the first one was at the center of our galaxy is supermassive black hole. But then later we observed a few candidates whereas a stellar mass black hole m I see. And so we see those, and we sort of see them in our neighborhood kind of in our galaxy at least, but we don't see the ones that are bigger, which you would think we'd see, right because they are more massive and so they would have more more of an impact on the motion of stars around them, but we don't see them. We see them if they're big enough, right, A million mass black hole has a big impact on what's going on in the center of galaxy. And so we can look at those galaxies and we can study the motion of stars in their center, and we can infer the presence of a black hole, just like we do for our galaxy. The problem is that smaller galaxies have fewer stars and smaller black holes. So if you're looking at a smaller galaxy like a dwarf galaxy, and you're trying to understand does the motion of the stars tell me there's some invisible sorts of gravity there? Then these are small galaxies. It's sort of hard to just like get the angular resolution you need to tell the motion of the stars in these far away small galaxies. Oh, I see. So you're saying that maybe they are there in smaller galaxies, but we just can't tell. We know that we can't see them. We know that if they were there, we could not see them today. We just do not have the resolution in current tell lescopes to see these stars moving well enough to answer the question is there an intermediate mass black hole there, Like, we just don't have the eyeballs that we need. So they could be out there, then it could be out there. We know that we couldn't see them if they were there, so they could be out there. And we have big plans like the thirty meter telescope or the extremely large telescope. These are things that are being built now and will come online in tennish years. These will have much better resolution. They'll be able to like get crisper pictures of these small galaxies that are far away, to get a sense for the motion of the stars near their center and tell us if there's an intermediate black hole there or not. We just can't see them today, and we haven't been able to see them, right, Like, we know that they've been kind of hiding from us if they exist, which we don't know if they exist. Yeah, we know they've been hiding from us. But that's just one technique to see intermediate mass black holes, right is to look for the motion of the stars near their center. There are other techniques that people have been trying as well. What are those techniques? Well, another one is not to look at galaxies at all, but to look for other kinds of things. Is this stuff out there in the universe called globular clusters. We had a whole fun podcast EFFS just about what are globular clusters and how much fun it is to say that word. But these are weird spherical collections of stars, and because they're much smaller than galaxies, people think they might have intermediate mass black holes in them. The upside is that they're much closer than other galaxies. They tend to be like orbiting our galaxy. Like we have several globular clusters orbiting our galaxy, so they're nearby, so we can like take pretty good pictures and see the motions of the stars inside them and get a sense for whether or not there's an intermediate mass black hole inside. Oh, I see these clusters. They're busier basically, right, They have more stuff to them, more stars, and so you would be able to maybe see the effects of a medium black hole in them. Yeah, and they're much closer than these other dwarf galaxies which tend to be pretty distant, and so we would have a shot. The problem is that these globular clusters aren't as tightly packed sometimes as a dwarf galaxy, and so the speeds of the stars are slower, so it's hard to tell if there's an intermediate mass black hole. There are not. You need to take pictures over like decades to see the motions of those stars and infer it. And there's a few globular clusters out there where people think maybe there's a black hole there based on the measurements of velocities of the stars, but none of them really hold a to scrutiny. Like somebody announces, wow, look we found an intermediate mass black hole, and then another group looks at the same measurements and they say, no, we can explain this without a black hole, So it doesn't really hold up. You see, it's tricky. It's tricky because you have to kind of like see the footprint of the black hole, and it's not easy to do. And another technique is to look not for the motion of stars, but for the radiation from the accretion disc Right, we know that black holes are not on their own, and especially the massive ones in the centers of galaxies are surrounded by gas and dust that's emitting a lot of radiation, so these are called quasars. And we think that intermediate mass black holes should also have intermediate mass accretion disks and be intermediate as quasars, So we're looking for these as well. Really we've only seen like supermassive quasars or stellar size quasars. Yes, we've only seen supermassive quasars. Stellar mass black holes don't tend to have quasars because they don't have the mass to get their creation disk like up to that energy, so they tend to only be from the centers of galaxies. And the problem here is that we're looking for like really old galaxies before they got really really big, so like intermediate size galaxies, and these tend to be pretty far away. And if you're looking for the black hole, because it's a quasar, it tends to like drown out the rest of the galaxy, so you can't like really see what's going on in the galaxy. So we have these quasars we've identified as maybe candidates for intermediate mass black holes, but they're so far away we can't like see the stars and really identify whether there's a black hole there. Oh, I see, because there could just be like a bunch of stars, yeah, clustered together. So but that's one thing people are doing. But I think the most exciting is this brand new way we have looking at the universe first seeing things that's not based on light but based on gravitational waves, right, like the ripples and off space. Yes, out there in the universe, the ripples in space itself, because when anything accelerates in the universe, it creates a change in the gravitational field, and that change ripples through space. And if the thing is really really big, really really massive, and moving really really fast, then you can actually measure these gravitational waves billions and billions of light years away, and we have and the things that have generated the gravitational waves that we've been able to see here on Earth in observatories like Lego are the mergers. For example, of two black holes falling into each other, they accelerate around each other in this crazy death spiral, emitting a huge amount of energy as gravitational waves, and that we can use to figure out how massive were the two black holes that fell in and how massive is the resulting black hole, right because you're seeing like the actual ripples and gravity from these black holes, So just the ripple itself kind of tell how big the things are that are crashing into each other it's a very very detailed fingerprint because the ripples tell you how fast things were going, and from that you can back out things like how massive they must have been, how strong was the gravity, and so yeah, you can figure out how massive the individual black holes were before they collapsed, and the mass of the final black hole, which is not just the two black holes added together. Some of the energy from the original two black holes get lost in gravitational radiation, these gravitational waves, and so the idea is that you know, we can listen to when these black holes crashing into each other, and so you're saying, like, maybe one day we'll see or hear are the tag two intermediate black holes crashing into each other, and then we'll be able to say, hey, there's an intermediate one. Yes, absolutely, that's a possibility, because we can use this as a way to weigh black holes and we cannot otherwise directly see right, because remember, gravitational waves can pass through anything. No amount of gas or dust or radiation or whatever can stop them. So it's just another very powerful orthogonal way of looking at the universe, so we can get glimpses. Also, there's another angle there, which is maybe we can see two really really big stellar mass black holes merge. Like what happens when an eighties solar mass black hole and a seventies solar mass black hole get together, Well, they must form a black hole that's heavier, right, that maybe even over that threshold into the intermediate mass range. Here, it's an X rated event there that Yeah, and this has actually happened. And in two thousand nineteen they did see a merger that resulted in a black hole that weighed a hundred and forty two solar masses. And so this is really cool. You can look this up on Lego. They call this the black hole graveyard, and they showed like the masses of the black holes that formed the new black holes. And so we're now seeing more and more of these things, and they're sort of pushing up that limit. Like it's true that you can't make a black hole from a single star that's bigger than eighty but as you said earlier, as you start to combine these, they can creep up a little bit and get over that hundred stellar mass threshold. Right, And we've seen this, right, you're saying, you can go online and look at the data of a hundred and forty two solar mass black hole basically being born. Yes, you can see it. It happened billions of years ago, but just recently seeing the evidence here on Earth, so that's exciting. It sort of cracks that number. It doesn't really answer the mystery, right because it doesn't explain why we don't see them. At A thousand and five thousand and ten thousand, it's like just over the threshold of what you might consider intermediate mass black hole. But we're planning to build future crazy gravitational observatories that might be able to see even bigger ones from further away. I don't know if you've heard about the Lisa experiment. No, what is it. The Lisa experiment is a version of Ligo. This experiment we have mirrors dangling underground in Louisiana and in Washington and in Italy to observe the ripples of space. This is a version of it, but using three satellites in space placed like really really far apart, but precisely aligned relative to each other. Wow, that's pretty cool in space. In space, I know, it's names bonkers, right that you could have like two satellites and have them maintain an exact distance relative to each other. It's going to be very complicated and very expensive. But if they build it this facility, Lisa will be very powerful at seeing gravitational waves and you could detect them from intermediate mass black hole murders Lisa in space. It sounds like a song. He ripped that off from the Beatles, Lisa and the Sky black Holes. All right, Well, I think you convince me, Daniel. I think the middle children are pretty cool or they don't exist one of the two. Sometimes I feel that way. They're definitely lurking, They're big. Middle children are big lurkers. For sure. We are psychological lurkers. Yes, So it's a really fun question like why don't we see these things? How did the supermassive black holes get so big without going through this stage? Or maybe we don't see them just because we don't have the telescopes yet, And in twenty or thirty years we'll see a bunch of them and they'll answer a lot of our questions about how black holes get made because they are, I guess, in general, just a cool family. You know, the youngest siblings and the oldest siblings. They're all pretty cool in their own way and pretty impactful in the universe. Right, everybody contribute to the family. We love every part of it. All right, I guess we'll leave that question dankling. What happened to the intermediate black holes? Nobody knows. It's a black hole in itself of information, but we'll figure it out. Stay tuned for more updates from science. You might have to wait another three episodes until Daniel figures it out his therapist, but we'll get there, okay, all right. We hope you enjoyed that. Thanks for joining us, See you next time. Thanks for listening, and remember that Daniel and Jorge explained. The Universe is a production of I Heart Radio. For more podcast from my Heart Radio, visit the i heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.