Daniel and Jorge Explain the UniverseDaniel and Jorge talk about whether galaxies have to have black holes at their hearts.
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Good job.
Thanks, Hey Daniel, have you started planning for your retirement?
Retirement? Why would I ever retire me?
You to be a physic this forever till you're dying breath.
I don't see myself burning out of questions.
Well, I guess eventually the Sun is gonna expand blow up and burn the Earth, right, so at some point you have to stop.
I guess so, And eventually the galaxy will collapse into a black hole. But that doesn't mean we have to stop.
You're gonna be doing physics inside of the black hole.
Absolutely I'll be writing papers about the singularity that nobody can ever read.
Oh, because you're inside the black hole, right, unless I'm like closer to the center, in which case you could still technically send me the paper.
Yeah, and I could never get your comments back. Hey, that sounds pretty good.
Actually, it's a win win situation or a lose lose situation. It's a wind lose wind loose situation.
I'll be waiting for peer review literally forever.
Hi.
I'm Jorgem, cartoonist and the creator of PhD comics.
Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I do plan on doing this.
Forever, forever, even after your death.
I'm hoping my ideas live on.
Are you going to program an AI to do research for you and make your children feel guilty after you're gone.
I'm going to upload my intelligence into the cloud so I can continue to live as some sort of weird mixture between biological and mechanical intelligence. But even that eventually will fall into the galactic center and be gobbled up by the black hole.
M But I guess why wait, I mean, we could technically generate this podcast with AI right now.
How do you know we haven't already? Who do you think he's writing these outlines? Oh?
My god, am I talking to an Ai right now?
Maybe I'm talking to an Ai right now? Maybe AI are listening to this podcast.
He did seem smarter than usual.
That's the real Daniel.
And the real Danger. But anyways, welcome to our podcast Daniel and Jorge Explain the Universe, a production of our Heart Radio.
In which we use are still limited biological intelligence to try to understand how the universe works. We want to know everything that's out there. We want to know how it works on the microscopic scale to weave itself together into reality that we experience and that we want to understand. We cast our minds out into the farthest reaches of space and hope to understand what they might encounter out there.
That's right, because it is a very perplexing universe full of interesting mysteries for us to go out there explore and try to uncover, and try to understand and try to explain to others.
Some mysteries have lasted for decades or centuries or millennia, but others have succumbed to the onslaught of human intelligence. We have sometimes actually managed to crack puzzles of the universe and understand what's going on at the atomic scale or the subatomic scale, or how space bends to form black holes. So there are when our limited intelligence is enough.
Why I make it sound like we're masters of the universe? How far along are we are? We up to like one percent of the universe by now?
I think that's probably a pretty generous estimate. But it's promising. You know, we are making progress and we are understanding things, and the understanding is coming faster and faster. So far, there's no evidence that we're going to hit some wall of understanding.
Especially with the AIS. Could we train AIS or just ask AIS to do research for us?
I use AIS in my research all the time. We have AIS answering questions. The big challenge is understanding what they're doing. Are they doing physics for themselves or are they doing it for us?
What do you mean for themselves? Like for their own curiosity? Oh my god, are we at that point already?
Well? We have AIS generating YouTube videos that are mostly watched by other AIS to generate response videos, And now we have AIS generating physics research, and at some point AIS are going to be reading that physics research and summarizing it for themselves.
HU.
So the AI science community is going to have as much politics and drama as the real physics.
Community, or maybe even more right, more drama.
But it's all artificial drama.
So all drama is artificial, and.
It happens faster, I guess which is good.
Though there might be some real drama going on at the center of our galaxy as the black hole there keeps sucking things in that's swirling around it.
Yes, we discussed on this podcast. There's a lot going on at the center of galaxies, including some very big mysteries.
That's right, as we peer towards the center of our own galaxy, we notice a very very large black hole sitting there, gobbling up anything that comes near it, and while we are safely swirling around it at tens of thousands of light years away, eventually we think that everything might fall into that black hole flush down the gravitational drain.
Down physics toilet. Is that really the case eventually? Like, there's no prediction where we don't get sucked in to that black hole.
There are some variations if the universe keeps expanding at a faster and faster rate, and something called phantom energy takes over and creates enough space between us and the central black hole to distance us from it. But if dark energy stays at the level that it is, the prediction is that the universe ends up with a bunch of isolated galaxies, each of which collapse into their own black holes.
Wait wait, wait, did you just say phantom energy.
Phantom energy exactly like.
The Phantom Menace named after that movie exactly. Not a coincidence, m I guess we'll have to dig into that in another episode. But it is interesting how as you said that every galaxy has the black hole.
It is pretty cool to have a black hole the center of our galaxy, and it makes us wonder if that required for every galaxy? Is that a necessary product of a galaxy? Do you have to have a black hole to form a galaxy? Do galaxies form black holes? Do black holes create galaxies?
It's a deep question, and so today on the podcast we'll be asking the question, does every galaxy have a black hole at its center? You mean, like like a tasty snack or something like? Are there any defective galaxies that they didn't get a black hole in the middle.
You mean like you'd be disappointed to bite into a galaxy and be like, oh, I didn't get a black hole in the middle.
I know that's the tastiest part. It's got the richest flavor, so deep.
That's interesting because black holes have mass, but I wonder if they actually have calories. Like, if you eat a black hole, do you lose.
Weight or is the secret to a new diet?
I guess really, the black hole eats you and it gains weight, so you become part of.
The black hole. Something gets eaten and it's you, which I guess is a diet in a way, you lose a lot more than weight.
Though, definitely, do not take health advice from either of us.
That's right, from either physicist or cartoonist. Not the healthiest people in the world or the universe.
We are totally unqualified to dispense health advice. I know that does and stop most podcasters, but it does give us pause.
So, yeah, this is an interesting question. Does every galaxy have a black hole at its center? I guess a lot of galaxies have black holes at their centers.
We certainly have seen a black hole in the center of our galaxy and a few others. But it's a really interesting question both about how black holes get formed and how galaxies get formed. It seems like there's a fascinating dance between the two.
I guess kind of the deeper question is like, do you need to have a black hole at the center to have a galaxy?
Yeah? Exactly does it count as a galaxy if it's just a bunch of stars?
So, as usual, we were wondering how many people have asked this question, whether every galaxy has a black hole? So Daniel went out there into the wilds of the internet to ask folks, does every galaxy have a black hole at its center?
Thanks very much my enduring gratitude to everybody who participates in this segment. But remember it's open to everyone. If you've been listening to pod and learning for years and feel ready to answer a random questions, please don't be shy right to me to questions at Danielanjorge dot com. We want to hear from you.
So think about it for a second. You think every galaxy has a black hole? He's what people had to say.
Man, who knows if black holes even exist? Like what if they're just all dark stars, like you guys talked about earlier on the show. They could be wormholes or something that we haven't even thought of yet. So no, not all galaxies.
A galaxy is a cluster of a bunch of objects in space swirling around a massive object in the middle. And the only massive object you can have is a black hole or a star, I guess, and you can't have such a big star. So I think every galaxy has to have a black hole.
In the middle, probably because there's got to be something like holding it all together. But since space is really big, maybe there's some galaxies that don't and they're just like really dense packed like stars spinning around each other.
Guests that just because you're asking the question, the answer is no. But I don't know. I would for you guys to explain it to me.
All right, some people we have some conspiracy theories, apparently as listeners, some galactic conspiracy theorists.
Well, we are guilty of promoting those black hole conspiracy theories right when we talk on the podcast about how black holes might be dark stars or fuzzballs or something else.
Even weirder, are you seeing we're ground zero for the black hole ism movement.
Black hole truthers. Yeah, this is the dark Horse podcast for black holes exactly.
Maybe if we switched our podcast name to like universal conspiracies or conspiracies about everything, maybe our ranking will go up.
It might go up, we might get a bigger audience. But I don't think we'd get a better audience.
I mean, isn't physics, after all, just like the Hunt for the Conspiracy of the universe. You're like, there's something going on here, There's some sort of plan or structure. We got to find out what it is. We got to find out the truth. Man, isn't that what physics is all about?
Yeah, but conspiracies are usually built on wild speculation and a lack of evidence.
Isn't that every other episode we discussed on the podcast.
Yeah, but we don't trust strong conclusions based on our lack of knowledge. We talk about what we don't know and how we might actually figure it out. We're trying to build evidence. We're looking for data, and we're happy to disprove our ideas if the universe shows us we're wrong.
It sounds like something someone involved in a conspiracy would say.
And you've just proven that you can make a conspiracy about anything with no information.
But anyways, this is an interesting question. Does every galaxy have a black hole at its center? And, as you said, there is one at the center of our galaxy. The last podcast we're recorded talked about how there's a big one at the center of our galaxy, along with a whole bunch of other stuff going on down there.
We certainly think that there is a black hole at the center of our galaxy.
What'd you say? We think, Yeah, we don't really.
Ever know for sure. I mean, our galactic center is the one that's closest to us, so it's easiest to study. It's also shrouded in gas and dust, so it's complicated to study. And in the end, all of our evidence for black holes is always a little bit indirect. Usually the observations tell us that there's something there that's very massive, something that's very small and also has a lot of gravity, but we've never actually observed an event horizon directly. It's always a little bit indirect.
I guess it's kind of hard to see a black hole because they don't emdlight themselves.
They might emit light. There might be gentle hawking radiation from black holes which would be very powerful direct evidence for black holes, but that's not something we've ever seen. The story of black holes for the last few decades is an increasing belief that black holes probably are real. As we identify these very massive objects and we put limits on how big they can be, we see things approaching closer and closer to the black hole the center of our galaxy, which tells us more and more about how small it has to be. And there are only a few things out there that could satisfy all of those constraints that are that massive, and that black holes of course the most classic example, but now we have a few other candidates, dark stars, fuzzballs, et cetera, et cetera.
Yeah, it's still a big mystery talking about the ones that are at the center of galaxies. Let's maybe break it down for folks and talk about black holes a little bit in general, and the different kinds of black holes that are out there and that could be at the center of different galaxies.
When we hear about black holes, we're often thinking about stellar black holes. Like a star burns for billions of years until eventually its fusion peters out and it loses the battle with gravity. So the gravity collapses the star into a black hole, meaning that there's a region of space where there's so much mass and energy that space becomes so dramatically curved that every path within the event horizon leads towards the center. That any particle that passes into that event horizon will eventually find its way to this center of that region. There are no paths out. Space within the event horizon is so bent that every timelike path for a particle leads towards the center. So that's a stellar black hole. And those are awesome, which is why we call them stellar, but also because they collapse from individual stars, and they can be quite big, like ten times the mass of our Sun, but they're small compared to the kinds of black holes we're going to talk about today.
Yeah, those are called super massive black holes.
Yeah, there's basically two categories of black holes. The stellar mass black holes that are like a few tens of masses, and then we jump up to really really really big black holes, things like ten thousand or one hundred thousand times the mass of our Sun in one huge black hole. But they get much much bigger than that as well. There are black holes that are millions or even billions of times the mass of our Sun, and these are the ones found at the hearts of galaxies.
Yeah, you can sort of see them even in distant, faraway galaxies, right, Like when you look at little Fuzzz out there in the night sky with special telescopes, you can actually kind of see the black hole in the middle. Right.
Yeah, that's a really interesting and complicated question, like how do we see these black hole holes in the milky Way. We have actually a lot of really interesting and cool ways to look at the black hole because it's so close. Like some of the best evidence we have is a picture of the black hole released by the event Horizon telescope, which is actually a picture of the accretion disk around the black hole, the hot gas that's swirling very very close to it and radiating light because it's so hot, so we can see that. We can also see stars orbiting very very close to the black hole the center of our galaxy. There's this one star in particular S two with zooms super close to the black hole and whips around it. We've actually seen an entire orbit of that star all the way around the black hole, which won a Nobel Prize very recently. So the black hole the center of our galaxy we've observed very closely. Black holes and other galaxies are harder to spot because those galaxies are further away.
Now.
For the one at the center of our galaxy, even that one, we're not one hundred percent sure it is a black hole, right, It could still be something else super massive there, or are we pretty sure it's a super massive black hole.
I think both of those things are true. We don't have direct evidence that it's a black hole. We know that there's a lot of mass there because we see the influence of that object on the nearby stars and all the gas and dust. We know that it's not emitting any light itself, so it's compact, it's dark, and it's massive. So most physicists, I think are pretty convinced that it's a black hole. But that's just sort of like the best idea we have. There are these alternative theories that it could be a very rapidly collapsing region of space which is going to bounce back and turn into a white hole, for example. So there are other theories there, but I think the mainstream physics community is pretty convinced that it's a black.
Hole, all right, So then how do we see them in other galaxies.
There's sort of two ways to see black holes in other galaxies. One is when they're feeding. When black holes are eating stuff, when they have like a lot of gas swirling very close to them and falling in, it tends to get hot and that gas radiates and it can generate very very powerful beams of light. These are things we call quasars, and we can see these from like across the universe. There's some objects that are super duper far away but incredibly bright, and when they were discovered decades ago, people couldn't believe that they were actually that far away because they look super bright already here on Earth. And if they're also very very distant, that means that at their source they're incredibly bright. And that's exactly what's happening. The gas surrounding the black hole is getting very very heated up, and as it falls in, it radiates it. Radiation gets channeled by the magnetic field of the black hole, and you get these very powerful beams emitted from above and below the black hole, and we can see those from very very far away. So those are quasars, but we can only see that for black holes that are like actively feeding and growing, and that tends to happen sort of early in the life cycle of the universe. Quasars peaked about ten billion years ago, so that's good for seeing like young super massive black holes very very far away.
So not every super massive black hole is a quasar, right, There are some that could be out there just sitting there being big.
Exactly most of the super massive black holes we think are not. We think that a lot of them stop sort of quazing billions of years ago and are now more like dark relics, huge black holes sitting at the centers of galaxies but not being quasars anymore. But we can still spot those by looking at the motion of stars around them. So pick them nearby galaxy and look at the stars that are near the center of it. You can't resolve them individually. We don't have telescopes that can say here's a star, there's a star, there's a star. But we can look at stellar populations at the hearts of other galaxies, and we can measure their velocities, we can measure their brightness. It's really complicated and very computationally expensive, but we can build a model of how fast those stars are moving around the center of that galaxy, and from that we can infer the mass of the black hole. How heavy does the thing have to be at the center of the galaxy to support this super fast radial motion by those stars.
All right, well, let's get into whether every galaxy has a black hole and what would it mean for a galaxy to not have a black hole at its center. But first, let's take a quick break.
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All right, we're asking the question does every galaxy have a black hole at its center? Like, does every galaxy have a dark soul? Or something?
I'll leave you the philosophical and moral implications of it. But from a physics point of view, it's a trend that we're noticing that galaxies have these black holes, and so we wonder if it's a rule. And it's basically our strategy in physics is like, look out of the universe, fine patterns, wonder if those patterns reflecting like deep rules of the universe. Figure out what those rules have to be. So, you know, we're still in like step two here.
Hmmm, Now I guess what does the data say? Like, you know, we can see traillions of galaxies out there in space. Can we tell what percentage of them have black holes? Or does every galaxy we've seen have a black hole in it that we can tell?
Yeah, that's a great point. You're right that we can see zillions and zillions of galaxies out there, like the Hubble deep field or the James Web deep field. Whenever it focuses on some random patch of space and resolves it. You can see evidence for so many galaxies in every tiny little corner of space. So we know there are lots and lots of galaxies out there, very few of those. However, have we identified a super massive black hole, in a few very distant ones that happen to be koasars, where like, okay, that's definitely a super massive black hole, and some nearby ones where we can watch the stellar populations and infer that there must be something very very heavy the core. But most of those we can't do either because they're either too far away or too quiet. But every galaxy that's close enough for us to watch the stars, we have seen a super massive black hole at its center. So there's lots of galaxies out there, only very small number can we check for a black hole, and every single time we've checked, we've found one.
Mmmm.
So like Andromeda isn't Andromeda the closest galaxy to us.
Andromeda is the closest galaxy to us, and we think it has a huge black hole at its center, and it's close enough that we can pretty well model the velocities of the stars at the center of that galaxy, and so we're pretty sure there has to be something super massive and very dense at the heart of Andromeda, very likely a black hole.
All right, So then the data kind of suggests that, like if you look at the galaxies that we can't look at, that we do see a black hole in the middle there or something that is probably a black hole? But why else would we extrapolate that to every galaxy in the universe.
Yeah, that's a good question, and to underscore sort of our lack of knowledge here, we really identified black holes in an approximately like one hundred to two hundred galaxies. So of the zillions of galaxies that we've seen, only a very small number have we been able to do this check. And so you're always tentative when you're extrapolating from like a few hundred examples two trillions of objects out there. On the other hand, it's every single one that we've seen so far, right, so we wonder if it really is a theory.
So there's no black hole nearby, then we can check that they didn't have a black hole in it, or would we know or I guess what are you saying.
I'm saying this no galaxy nearby that we can check that hasn't had a black hole in it, And that includes really large galaxies like Andromeda and also even dwarf galaxies, like there's a dwarf galaxy called RGG one one eight and they recently found what they call a teeny tiny super massive black hole in it?
Where is that the actual scientific name? Teeny tiny supermassive.
That was the title of the press release. I don't know if that ended up in the paper or not.
Doesn't that just average to a small black hole.
It's only fifty thousand times the mass of the Sun, So for a super massive black hole, it's pretty teeny tiny compared to the Sun. It's pretty much a monster. It depends on your perspective. But there's something else going on here that makes us suspect that maybe every single galaxy has a black hole in it. It's not just that every single one we've seen has a black hole in it, but we see this very tight pattern. We notice this very close connection between the mass of the supermassive black hole and the mass of the galaxy. And that might not be a surprise. You figure, like, look, more stuff means a bigger galaxy, means more stars means bigger black hole, right, And that's true. But if you run those simulations, you get kind of a scatter like you get some bigger black holes and some smaller black holes. But what we notice when we plot like the mass of the black hole versus the mass of the galaxy is a much tighter correlation than you would expect just from like more stuff means bigger black hole, you get this very very compact line that suggests that there's like some feedback between the black hole and the galaxy.
So you mean, like when we look at a black hole in another galaxy, we can tell its size first of all, And if you compare the size of the black hole to the size with the galaxy, it's like it's like almost one to one kind of.
And if you just made bigger and smaller galaxies and guess how big the black holes would be, then the same size galaxy shouldn't always give you the same size black hole. Depends a little bit on like where the stuff is and how much falls in, et cetera. But what we notice is a very very close connection, as you say, like one to one, that the mass of the galaxy in the mass of the black hole track very very closely.
Meaning that there's no small galaxy with a big black hole, and there's no big galaxy with a small black.
Hole exactly, And two galaxies with the same size have basically exactly the same size black hole at their heart. There's almost no variation there, and that tells us that there must be some sort of connection, that there's something about how the black hole is forming and how the galaxy is forming that connects these two things. I mean, even from a spatial point of view, it's kind of weird that like this dot at the very very center of the galaxy. I remember, these things are very massive, but they're also very very small, So it's weird that this dot at the center of the galaxy is influenced by like the mass of the whole huge galaxy that's like one hundred thousand light years across. So there must be some sort of connection between them, some information passing back and forth, some process that's controlling both of them. And if that's the case, and that makes us think, oh, there must be a connection between the two. And so probably every galaxy does have one of.
These things, because, like you're saying, if it's sort of inevitable for a galaxy to get a black hole in the middle when it forms, then there probably isn't any galaxies without black holes.
Yeah, exactly. And if there's some feedback mechanism something which is controlling both of these things, that it probably generated both at the same time, it controls the stars that form in the galaxy and also controls the mass of the black hole. There must be some process tying these things together, and being weirdly vague about that process because we don't know what it is. There's a bunch of theories about how the black hole might form and grow and then the radiation from the black hole stops it from growing, and the same process might control how stars are formed and how they collapse from blobs of gas and dust into stars. So there's a bunch of different ideas out there, but they're all very vague and nobody can really agree about it. Where at the point where we're just like, it seems like there's something going on here, but we don't know what it is.
Although this relationship between the black hole and the galaxy size comes from data of galaxies that we found that have black holes, right, Like, maybe there's still the possibility that one and in a thousand galaxies doesn't have a black hole.
Yeah, it's certainly possible that that's the case. There's a small number of galaxies here. Also, people might remember there was recently this crazy idea that super massive black holes or the hearts of galaxies are connected to the cosmic acceleration, that they're really like bubbles of dark energy. And a crucial thing that people notice that fueled that idea was that there's a connection between the expansion of the universe and the size of these black holes. That the black holes seem to be more closely connected to the cosmic expansion than their own galaxies. So that might seem like it's in contradiction of what we're saying here today. Today we're saying, oh, the black holes at the hearts of galaxies are very closely connected to the size of their galaxies, and a few weeks ago we said, no, they're not. They're more closely connected to the universe. And the way to untangle it is to remember that there's two different kinds of black holes that we're seeing. We're talking about black holes we see very very close to us, and in those the black holes very tightly connected to the mass of the galaxy, versus black holes that are very far away, very old black holes those are from quasars, So those are the ones that seem to be connected to the cosmic expansion. So, long story short, there's a lot we still don't understand about super massive black.
Holes even how they form, right, Like, that's still a big mystery.
Absolutely, we have no idea how these things even got to exist. If you just start from like a big blob of stuff and watch it form a galaxy, like in simulation, you get a black hole at the center, it's not a mystery, but it's not this big. Like we look back in time by looking at old light and looking at really early galaxies, and we notice that they have huge black holes at their hearts, like already billions of times the mass of the Sun in like the first billion years of the universe. These are again the very distant, very old black holes we see from quasars, and we can't explain how that happens. In our simulations. That just doesn't happen so quickly. It takes much longer for these black holes to get so big. So there's something else going on to form these black holes that we don't understand.
So we really have no idea what could be going on. I mean, it kind of seems like maybe there's a simple explanation in there somehow, Like you know, if you start with a big cloud of gas, and yeah, the galaxy is going to be bigger and the black hole in the middle is going to be bigger, Like what's the big mystery there?
Yeah, and that would give you a correlation that would say that in general, masses of galaxies should be connected to the masses of the black hole. And we see that. But again, we see a much tighter connection than you would expect just from that simple excepent We see that galaxies with the same mass have basically exactly the same mass black hole. There's like no variation there. So the connection is just tighter than what you would expect from that simple argument. So there must be something else going on. Is it true that we have no idea? I mean, people definitely have ideas, and I read like ten papers about ten different ideas for what could be controlling it complicated theories about how the gas gets blown in or out, or gets heated up or cool down. We have lots of ideas, we just don't know which one might represent reality.
Interesting And now, is it possible, I guess, for a galaxy to not have a black hole? Like why couldn't that happen?
Yeah, that's a great question. And there's sort of two questions there, right, Like one is, could you make a galaxy without a black hole. Is it possible to pull all those stars and all that dark matter together without making a black hole? And we think the answer to that is no, That every time you get enough stuff together, whatever is pulling that together is going to form a black hole at its center. That's just inevitable.
Why do we think it's inevitable?
Well, I guess for a couple of reasons. To summarize, One is every galaxy we've seen so far has a black hole at its heart. Right, We've never seen a galaxy where we're able to check whether there's a black hole and haven't found one, though there's an asterisk there, which we'll get to in just a minute. And number two is this connection between the sizes of them that tells us that there's probably some mechanism that's controlling both the mass of the galaxy and the mass of the black hole together. So back to the asterisk, there actually are a couple of galaxies nearby that we've looked at that don't have a black hole at the center, and that points to another potentially fascinating story, which is whether it's possible for a galaxy to form a black hole and then eject it like can a galaxy form, make its own black hole and then lose that black hole?
Wait? Wait, wait, wait, we're seeing that there are galaxies we've seen that don't have a black hole in them in the middle.
There are a small number of galaxies we've seen that have a black hole, but it's no longer at the middle of the galaxy.
Wait, what where is it like at the edge or really far away from it? What do you mean?
A few variations. In one case, we've seen a super massive black hole that's like displaced from the center and has a pretty high speed away from the center. So there's this possibility that galaxies could form make a black hole at the center, and then through some dynamical process, some like interaction with other galaxies, their black hole could get like kicked out of the center. And we've actually seen this in a few galaxies.
What are the other ones like that don't have a black hole in the middle.
So there's the one example called CID forty two, which is about four billion light years away. It has a super massive black hole, but it's near the center, but it's sort of displaced from the center. And then there's another observation just a few weeks ago where they see a streak of light shooting out of the galaxy. The streak of light is like two hundred thousand light years long, and at the end of it there's a black hole, and so it looks sort of like the black hole was ejected from this galaxy. It's like a runaway black hole and left this streak of stars in hot gas in its wake.
Well, it can actually see like the skid.
Exactly, and so we think that this might happen sometimes when galaxies merge. We know that galaxies merge, that that's a very normal thing. We think the Milky Way is formed by a bunch of galactic mergers, and Dromeda is so big because it's a combination of a bunch of baby galaxies that all got merged together. And the normal thing to happen when galaxies merge is that the black holes also merge. I mean, you have two clouds of stars, each with a black hole at their center. They're all going to orbit each other. Eventually, the black holes that their hearts are going to orbit each other, and then because of friction and gravitational radiation, they'll eventually collapse into a single black hole. That's like the normal thing to happen, but there's some variations there. When two black holes collapse into one, they also emit a lot of radiation. Like the mass of the two black holes doesn't one hundred percent go into the mass of the final black hole. It loses some mass and it generates a bunch of gravitational waves. That's how we see these black hole mergers. We talked about Lego and Virga and all these observations that see the these ripples in space time generated by these black hole collisions. You get those ripples because the black holes are accelerating as they orbered each other, and that radiates a way energy, so it loses some mass. Now, sometimes that radiation is in every direction, like it just sprays gravitational waves everywhere, but sometimes in special circumstances that gravitational radiation tends to be in one direction rather than another, and then it acts sort of like a recoil. It's like shooting a gravitational wave gun in one direction and the black holes get pushed back in the other direction by conservation of momentum.
So you're saying you can have a galaxy without a black hole, But the ones we've seen so far that are like that there's evidence that it had a black hole in the middle at some point.
Exactly, so it might be possible to have a galaxy without a black hole by building a galaxy with a black hole and then like ejecting it getting rid of the black hole.
I guess one thing that maybe important to understand is that even though a galaxy can have a super massive black hole in the middle with the mass of millions of our suns, it's not like the black hole is anchoring the galaxy, right, like to a galaxy with tondreds of millions of stars, like one little black hole in the middle is not super important, maybe in the same way that the Sun is important in our Solar system, right.
Yeah, you're absolutely right. The Sun is like ninety nine percent of the mass of our Solar system. Black Holes, the hearts of these galaxies are a tiny fraction, much less than one percent of the mass of the galaxy. Like in the Milky Way, we have billions of stars and so billions of solar masses, and our black hole is only five million solar masses. So it's a tiny fraction of the mass of our galaxy. Even though we're talking about really big objects. Galaxies themselves are much much bigger. So it's more like if our Solar system lost Jupiter. Right, Jupiter is a big planet, that'd be kind of a big deal personally, but it wouldn't affect the dynamics of the Solar System the way would if we lost the Sun. Right, that would be much more dramatic.
Right, These super massive black holes are really more like little pimples in the middle of galaxies. Right, It's not like the galaxy is there because of the mass super massive black hole. It's more like maybe just a feature that pops up when you're making a galaxy.
Yeah, I think we don't really understand that. I mean, I think the causal relationship is probably complicated. It seems like it's probably a necessary outcome when you form a galaxy that you get a super massive black hole. So in that sense, it kind of is necessary to have it. But you're right, you can get rid of the black hole and the galaxy can still hold itself together, So in that sense, you don't need it anymore.
Like maybe the black hole needed the galaxy to form, but maybe the galaxy didn't need the black hole to form.
Yeah, but then the black hole stays a black hole. When it's been ejected, it's still a black hole out there wandering an intergalactic space.
WHOA.
So one way to get rid of your black holes to generate a bunch of gravitational radiation in one direction to kick the black hole. For that to happen, you typically have to have sort of like a low mass black hole in order to get enough acceleration to like get out of the galaxy. Another scenario is to have sort of like a three galaxy dance. Remember that when three objects interact, it's much more chaotic, Like, the three body problem is not something we know how to solve, whereas the two body problem is simple. So if you have three galaxies that are merging at the same time, then one of those black holes can get kicked out instead of merging with the other two. So that's another scenario that can create a runaway black hole, essentially kicking a black hole out of a galaxy.
Like if three solar systems similar to ours came together, it would get so chaotic and so scrambled that it could actually like shoot off the Sun or Jupiter out into space.
Right, yeah, exactly, And because of conservation momentum, if you're going to kick one black hole out in one direction, then the other two are going to get kicked in the other direction, and so the whole system might end up without a black hole at its center.
All right, let's take a little bit more into what this all means and maybe how a dark matter plays into it. But first let's take another quick break.
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All right, we are asking the question does every galaxy have a black hole at its center? And I guess we've figured out the answers. No, right, some galaxies have a black hole near its edge if it gets kicked out.
Yeah. We think that probably every galaxy has a black hole at its center during its formation or at some point in its history, because we think probably the process that makes these galaxies, it gathers together all this mass and funnels it into this gravitational well that remember, in the end, is made by dark matter. Right, The reason these galaxies exist is because there's a density of dark matter there which is pulling in all this other matter to make this dense stuff that we can make a galaxy. Probably that process inevitably makes a black hole at the same time as it makes a galaxy. But then you can lose that black hole during the life of these galaxies as they merge its they come together as they dance around each other, those black holes can get ejected from the hearts of those galaxies. So yeah, you could end up with a galaxy without a black hole at its center.
I feel like we said earlier in the episode that every galaxy we've seen has a black hole at its center, but it sounds like that's not really true. That there are galaxies out there we've seen that we can measure it that don't have one in the middle at all. Have we seen any with no black hole at all.
We have not found a galaxy without any kind of black hole. Either they have a black hole at their center, or we've identified a black hole that's been ejected from the center. That's a very small number of cases so far, or they're too far away for us to tell what's going on at the center. Then there's one particular galaxy which is a bit of a mystery called Able two two six ' one. It is a really big galaxy and we have not yet found the super massive black hole there. We looked at it in the X ray and not seen the sign of it. It's still sort of an open question whether people are going to find evidence of the black hole or not, but it's one one that has a question mark next to it. Remember this is like a recent and active field of research. People are developing new techniques to try to study these galaxies to try to see if they're black holes in them. It's definitely a question. A lot of people aren't working on, so it's evolving rapidly. But either we've seen a black hole at center, or we've seen a black hole like leaving its center, or there's this one question mark galaxy able two two six 's one, or we're just not able to see it. Because remember, the galaxies have to be close enough for us to study their centers in order to be able to see these black holes, or they have to be active enough for us to see them emitting. Like another way to see these black holes is to watch them like burp as they eat something.
But then you're only seeing the rude black hole.
That's right. But occasionally, even a quiet black hole, even when it's not surrounded by a big disc of gas and dust and emitting like constant beams of light, occasionally a star might wander close to it and get eaten, and the tidal disruption there will give you a very bright flare. So we've seen those kinds of things as well, like otherwise quiet galactic centers that suddenly emit a big, bright burst of light. We've even seen that from our own galactic center sometimes. So that's another way you can observe supermassive black holes.
It's like the final scream of a star before it gets eaten.
Exactly. Another thing people are doing is trying to identify these super massive black holes out in intergalactic space. Like people are curious how often do supermassive black holes get kicked out into space? Remember we talked about like rogue planets before, planets that used to be orbiting a solar system but then due to the gravitational chaos of their system, get thrown out into space. And it turns out there's a huge number of them out there. People have discovered rogue planets by micro lensing, looking for moments when these planets pass in front of a star in the background and that star's light gets distorted by the gravity of the planet, And by doing that we can spot a bunch of these things and then extrapolly to how many there are. The same way we can look for super massive black holes in intergalactic space by looking for these micro lensing events.
Hmmm, sounds kind of scary, the idea that there are rogue super massive black holes out there roaming space, possibly in our direction.
Yeah, it's certainly possible that they're out there. We've seen some of them getting ejected from their galaxies, but we think that might be kind of rare, and when we're talking about like a one percent level event based on our recent observation, so of course with a big uncertainty. And there are many fewer galaxies than there are stars, of course, so the number of rogue super massive black holes out there could be big. But space is of course really really vast, so these things would be pretty hard to spot.
I guess it's hard for even a super massive black hole to leave a galaxy, right Like, as we said before, a galaxies huge, got billions and trillions of stars, it's really massive. It's probably pretty hard for a black hole to get the escape velocity needed to leave a galaxy and come towards us exactly.
That's why ten to happen mostly for lower mass super massive black holes, not necessarily even teeny tiny ones, just ones on the lower edge because a lot of them are going to have this asymmetric radiation from gravitational waves or some sort of chaotic merger event. It's not totally symmetric, but most of them are massive enough that they get pulled back to the center of the galaxy and eventually settle down. But the lower mass ones can get going pretty fast, like this CID forty two galaxy. They estimated the velocity of the galaxy to be half a percent of the speed of light, which is like very very fast moving black hole.
It's in a hurry to get out of there.
He wants to go somewhere exactly.
I guess the question is how does this all tie in too dark matter? Because I know dark matter is very important in the formation of galaxies. Right, It's almost like, I know we've mentioned this before, the galaxies form around where dark matter is. Does that effect sort of how black holes might get formed.
It definitely affects where galaxies and black hole get formed, right. Galaxies basically trace out where the dark matter is in the universe. If you look at the large scale structure of the universe, you see these filaments of galaxies and these sheets of galaxies, and that's because that's where the dark matter is. We can't see the dark matter directly, but we can see that it's gathered together all this matter and made all of those galaxies. And so every galaxy has a huge dark matter halo around it. Like the Milky Way is about one hundred thousand light years across, but there's a big blob of dark matter that's like two hundred thousand light years across that the Milky Way is embedded in. Now, if we shot our black hole out of our galaxy, then it would pass through that dark matter halo and it would gobble up a lot of dark matter along the way. And so as these supermassive black holes leave the galaxies, they can increase their dark matter fraction.
They can get even darker than black Yeah.
We think that these black holes already have some dark matter in them because dark matter is everywhere. But dark matter also isn't sticky, and so it's very easy for dark matter just like rotate in orbit forever around the galaxy and not fall in We think will probably fall into the center of the galaxy because eventually it's like dynamical friction. Our star will get pushed by other stars on all that jostling ends up kicking somebody towards the center of the galaxy. But dark matter doesn't do that, right, because dark matter just passes right through itself. So normally dark matter can swirl around the center of the galaxy not getting eaten by the black hole. But if the black hole runs free, then it's basically like plowing through a buffet of dark matter.
But maybe when as the galaxy was forming, maybe the black hole did eat a lot of dark matter. Or maybe I wonder if the black hole in the middle of galaxies is maybe mostly made out of dark matter.
It's hard to know, right, And what does it mean to be made of dark matter? Because once it goes past the event horizon, who knows what happens? Everything's dark matter, everything's something else, some black hole state of matter. Right, we don't know if dark matter gets annihilated it turns into something else, or if it retains something of it's nature. We just don't even know because we don't know the particle properties of it. So, like, what are the conservation laws for dark matter of the universe, might like keep track of how much dark matter there is and not allow things to convert. We just don't know. We're so clueless about it.
So some black holes might be darker than others.
Absolutely, some might be darker than others. We know that some galaxies are darker than others. Some galaxies have a larger dark matter fraction than others. That's for sure, all right.
Well, what does it mean for our understanding of galaxies and how they form?
It means that we're still the very beginning of the journey of understanding how galaxies come together, which is sort of shocking because we've been setting galaxies for so long, but every few decades we'd learned something new and surprising about what's going on with these galaxies, how they come together, what their history is, and what their future fate is. Right, and we know that the galaxies and sort of our local group are gravitationally bound together and so eventually going to end up falling together and probably forming one super galaxy while they're being separated from the rest of the universe by energy, And that super galaxy is going to have a super duper massive black hole at its center unless that gets kicked out and shot into intergalactic space. So it tells us a lot about our potential future because as we orbit our galaxy, and merge those other galaxies. We also eventually fall towards the center of this blob, and if there's a giant black hole waiting for us there, this is really only one way for the story to end.
But even if we kick out that black hole, eventually the galaxy is going to collapse anyways, right, and maybe it will collapse into a new black hole.
Yeah, exactly, you could definitely form a new black hole even if you kick out the original one. There's no noncompete clause in galactic formation.
And when is this supposed to happen, like tomorrow or trillions of years?
Oh, we're talking about billions and billions of years for sure, So we are much more a threat of the Sun expiring before our system collapses into the center of a black hole. But if we want to continue on for billions and billions of years, we definitely need to plan deep into the future. And also, we're just curious about how black holes were work and how galaxies work, and we want to understand it because it could be that there's some other deep insight into the way the universe works waiting for us. Every time in science we're like, don't really understand how something works, and we dig deeper, we discover something fascinating underneath that we didn't even expect to find, so satisfying our curiosity and like trying to understand this in great detail is good path to like opening up some surprising new doors. So we're doing our best to improve our future prospects for understanding these things.
Mmm, sounds like a problem for the AIS. You could just kick back, let to figure it out.
Well.
One of my favorite ways to study these things in the future is with our space based gravitational observatory. So LIGO is this gravitational wave observatory that has two arms that are kilometers long filled with lasers, and they're planning to build one in space called Lisa LSA I mean much much bigger, and so it's going to be able to observe gravitational waves at lower frequencies, which is what's generated by the collisions of supermass. They have black holes, so we might be able to observe gravitational waves from galactic mergers and understand this process in more detail. What happens when two super massive black holes really do come together?
WHOA, which is what happens when two galaxies collide right exactly.
Yeah, yeah, and so we could see the signature of black holes getting injected from the hearts of their.
Galaxies, ejected or smushed.
Smushed or ejected, we don't know. Both outcomes are possible.
But when we hear the black hole get injected, well, black.
Hole injection comes with a very powerful, very directional gravitational wave signature, and so they think they might be able to distinguish that from just like a normal combination of two black holes into one. But it all depends on this crazy set of satellites in space shooting lasers in each other, measuring tiny changes in their relative distances.
Well, it sounds like the future is all black holes. So we biters start to understand them and get used to them and figure out how they form.
Right, absolutely something we'd like to understand, not just because we're curious about the universe, but because they might control the fate of our galaxy yep, or it.
Might be just something for our AI replacements to figure out. We might be long gone.
I'll read about it from my lounge chair at the center of the black hole.
All right, we hope you enjoyed that. Thanks for joining us, See you next time.
Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of iHeart Radio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases. Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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