Could our galaxy have a dark matter center?

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Speaker 1: Hey, or hey, do you like a surprise candy center? It depends. Does it depend on what the surprise is? Absolutely, you know if it do leg yes, sign me up. If it's like marshmallow, I know. Thanks, Well, I guess that makes sense. Yeah. Also it depends on what is it surrounded by, Like what is it the center of? Like I don't want to surprise candy center in my taco or pizza. How about a surprise guacamole center in your taco? You mean like candy guacamole. I'm not so sure about that. How about candy guacamole in your dessert taco? Dessert taco? Now you're talking, can we put bananas in it? Banana avocado flavored ice cream? I'm not so sure. This was a great idea. Yeah, we went too far. Hi am r Hanmay, cartoonist and the creator of PhD comics. Hi, I'm Daniel. I'm a particle physicist and a professor at U c Irvine, and I will always try a weird flavor of ice cream. Really, have you tried a garlic ice cream? I have I tried garlic ice cream. I even tried pasta Fa Julie flavored ice cream. I've tried licorice ice cream. I've tried all the ice cream. Man, there's a lot happening in your laboratory there. I'm married to a biochemist. You know. She can really cook up some crazy stuff. That's long. She doesn't put a gut bacteria in your ice cream. That's what she researches, right, that's true. Well, there's that famous Nathan for You episode about poop flavored ice cream. What somebody made that? Or somebody made that accidentally. No, there's a company apparently which will create any flavor for you. So they created artificial poof flavored ice cream and just to see if people would try. Oh boy, let me try to are in flavored ice cream. No, I haven't been able to find that yet. Well, there you go. That's not too far from from some of these extreme flavors. But welcome to our podcast Daniel and Jorge Explain the Universe, a production of I Heart Radio in which we take a deep bite of the universe, hoping to enjoy its flavor. Whether it tastes like black holes or neutron stars, or swirling masses or tiny little particles, we are here to savor it with you. Because we want to know what the truth is out there in the universe, regardless of how it tastes, and so we don't shy away from taking a bite of any of the biggest questions in the universe. Where does the universe come from? What does it all mean, what's really going on on the tiny quantum scale, what's swirling around in the center of our galaxy? How does it all fit together? We talked about any end all of those questions and explain them to you. Yeah, because it is a wonderful and tasty universe. Now, Daniel, do you think we were taking bites out of the universe or are we just licking it before it melts? What is the neiver is going to drip onto in this analogy? That's my question. You know, a universe cone obviously hopefully a waffle cone. Maybe physics is like the napkin because we're trying to wipe up the messes. I see you're not trying to create the messes. I thought I thought physics was like the tongue doing the licking. I don't know, this is getting very strange. We are definitely mixing our metaphors here. But you know, the universe is delicious. The thing that I love about physics is that every time we do take a bite out of the universe, we find something amazing. It's pretty rare that you uncover some mystery and then go yawn, that was boring, And the universe has lots of impressive surprises for us. Yeah, even I hear it. It has Higgs boson flavored ice cream and also some cork ice cream as well. Or is all ice cream cork ice cream? Everything is cork flavor. That's right. In the end. The corks are the fundamental basis of flavor. And of course we do have flavor physics, which talks about different kinds of corks, but totally ignores the question of what they taste. Like, oh boy, we're getting inception here. We're like going down a few levels. It deploring the flavor of the particles that makeup flavor. That's right. And you know it's not just stuff here on Earth that you can taste. Astronauts report that space itself has a flavor. What what do you mean? It tastes like a vacuum. We talked about one time on the podcast here when you come back in from a spacewalk there are little volatile molecules from space that have stuck to your space suit, and when you enter the atmosphere they boil off and smell like barbecue. And sometimes people say, like a raspberry? Whoa really? Like? I guess there's like little tiny particles floating in space of raspberry and possibly barbecue. Yeah, and some of the gas clouds near the center of our galaxy are supposed to smell a little bit like cranberry, though nobody's ever been there. It's not raisins. Now, are you sure when they came in from the spacewalk the barbecue they're smelling is not themselves from being out out in the sun too long. Geez, smells like my hair is on fire. What's going on? It smells a cosmic ray fire. But anyway, it is a pretty interesting and amazing universe, and there's still a lot we don't know about it, even in our own neighborhood. There are big mysteries in our solar system and especially in our galaxy. Our galaxy is called the Milky Way, and it's big and it's amazing, has millions of stars, but there is a big mystery at the center of it. Because remember that everything we know about the universe and our galaxy comes from observations we have just made from here on Earth. Yeah, we've sent a few probes, maybe to the edge of our solar system, but mostly we have one eyeball trapped here in one spot in the galaxy, and we're trying to do something very difficult, which is make a map of the whole galaxy, of the whole universe, having seen it from just one spot. Imagine standing on the top of a mountain and then trying to make a map of the Earth. Some things are obscured from your view. Other things you have to just sort of sketch because you can't quite make them out. That's the challenge we face in trying to map out the cosmos around us. Yeah, it's pretty amazing how well we've done, you know, just from standing in that mount ten with a telescope, we've been able to basically figure out most of what the universe looks like, and how it's structured together, and how many stars there are out there, how many galaxies. But there are things that we can quite see from our point of view. Yes, sometimes we can be very direct and say, look, we see that thing. We know exactly what it is and where it is. Other times we have to sort of infer what we think is going on based on more indirect evidence, and that's how we've leveraged our powers of observation to tell us about how things look in places that we can't see. We have to make these indirect conclusions. But sometimes we might make mistakes because we might be folding in incorrect assumptions or guesses about what might be there. And people can come along with new ideas that give us new perspectives on what's going on. Yeah, you could be linking the wrong ice cream where all we know right there is no wrong ice cream. Man, We'll wait until you taste during ice cream or that poop ice cream that sounds like the wrong kind of ice cream. But yeah, it's a pretty mysterious galaxy that we live in. And so today we'll be tackling a question related to what's at the core of it. So on the podcast today we'll be asking the question, could our galaxy have a dark matter center? Mm? Hmm? Could our galaxy have a dark matter center? Now? Isn't dark matter all over the galaxy like it's in most of our galaxy made of dark matter. Yes, most of the mass in the galaxy is dark matter, and there is definitely dark matter hanging out at the center of the galaxy. But conventional wisdom is that the center of the galaxy also has a supermassive black hole as a like gravitational anchor something that's really holding the whole galaxy together, this incredibly massive object four million times the mass of the Sun. But we've never seen that black hole directly, and so some people wonder if it's really there or if instead it's dark matter doing the work. Interesting and yeah, because I guess we can't sort of look directly to the center of our galaxy. I mean, we can sort of look in the direction of the center of the galaxy, but it's so far away maybe, and there's so many things in between that it's hard to kind of see in there. It is, in fact completely obscured because the center of the galaxy is a busy place. There are a lot of stars between us and it, and there's also a lot of gas and dust, so it's not something that's very easy to look at directly in the visible light. And so we'll get into details of what we know about what's going on in the center of the galaxy. Why most astronomers think that there is a black hole there, and some new evidence that's casting doubt on that assumption. M I see. So the prevailing thought is that there is a supermassive black hole in the middle of our galaxy. But now the question is maybe there's not. Maybe it's just a whole bunch of super compact dark matter. Maybe. Yeah. In line with our podcast about dark stars, we are casting doubt on the existence of black holes everywhere. We are throwing shade on black holes because in the end, they are very difficult to see directly. It's always an inference. You're always jumping to a conclusion, saying there's a certain amount of mass and the certain amount of area, therefore it must be a black hole. But you know, that's not like finding the body in a murder. It's all circumstantial evidence. Yeah, And the problem with throwing shade at a black hole is that it never comes out. It just eats your shade and says, thank you, Can I have another. It's a pretty interesting question. What is at the center of our galaxy? Is it a supermassive black hole or is it dark matter? And so, as usual We were wondering how many people out there have thought about this question and think that maybe it could be dark matter. So Daniel went out there into the internet to ask people could our galaxy have a dark matter center instead of a black hole? So thank you very much to our cadre of volunteers. If you'd like to join them, please don't be shy, just email me two questions at Daniel and Jorge dot com. You know you want to, so think about it for a second. Do you think our galaxy has a dark matter center? Here's what people had to say. There is lots of evidence for a black hole from a high concentration of mass at the center of our galaxy because of the orbit of several star ours that are have been whipping around something very very small, very very fast. So that implies a density of matter foreign excess what we normally would predict for dark matter, and if the dark matter were to concentrate to that extent, it would then just become a black hole, since it is only gravity that matters. Anyway, I don't know why our galaxy would have a dark matter center instead of a black hole. I know that most galaxies do have a black hole at their center, as do most spots wear enough mass has just gotten together and hung out for long enough. Maybe it could be both. Could our galaxy have dark matter center instead of black hole? And I think we have a super massive black hole center of our galaxy, not dark matter. I don't really see how we could have a dark matter only center of our galaxy because would mean that all the regular matters being pulled into it, which in my head, would make it a black hole. Whether or not the dark matter falls into the black hole, it's kind of irrelevant because you've got a black hole there anyway, pulling in any barrionic matter. I do think that the galaxy can have the dark matter center because dark matter experiences gravity, it has it has mass, and it definitely experiences gravity. It accounts for like a very large amount of matter that we know, so we only know that normal matter is very less. And yeah, there is a possibility that there is a huge quantity of dark matter at the center of our galaxy instead of having a black hole. Turcky, Turcky. I have no idea, but it reminds me of something in in a video game I used to play. I'm just gonna leave this one as a shrug. All right, it seems that people are skeptical. Yeah, there's some great answers here. People thinking about what would happen to the dark matter if it got concentrated at the center of the galaxy. Really some great stuff. Yeah. I like the people who say it could be both, Like, yeah, why can't it have both both a super massive black hole and dark matter in it? Or how about a how about a dark matter black hole? A dark matter black hole be super awesome, Although you know, once material falls into the black hole, it's not really clear what its nature is anymore. Is it's still dark matter? Is it gotten converted into energy? Has got turned into something else? So once you're inside the black hole, it's just stuff. Man? WHOA yeah, because I guess dark matter could technically turn into regular stuff. Do you think it's made out of the same kind of like energy. We don't know, right, there might be some way for dark matter to turn into normal matter. We do know that there was a lot more dark matter in the early universe than there is today, and so we hypothesize that there's some mechanism by which dark matter can very slowly turn into normal matter. But it needs a lot of energy density for that to happen. So we think it might have happened in the early universe and then frozen out, and it might then again and have an inside black holes. Who knows, maybe threw a lot of shade to the black holes and lost some of its energy. All right, Well, let's tackle this question. Could it be dark matter at the center of our galaxy and not a supermassive black hole? Now, first of all, I guess why did we think or do we think that the center of our galaxy has a supermassive black hole. Well, let's not be too click baity. Definitely, the mainstream astronomy community is convinced that there's a supermassive black hole at the heart of our galaxy. I mean, they gave the twenty twenty Nobel Prize to two folks, Reinhard Genzel and Andrea Guz from u c l A, who've been observing the center of the galaxy for decades and trying to figure out what's going on there. But we've had a hint that there's something going on at the center of the galaxy for decades, since people sent up rockets into the upper atmosphere and heard this radio signal from the center of the galaxy. And then it was in the sixties and seventies that people figured out, oh wow, maybe this could be a black hole, and it was one of the first things identified as a black hole at the center of our galaxy. Mmmm. That's interesting because I think we talked about this in another episode where you know, we talked about how do we know there's maybe a black hole at the center of our galaxy? And I remember one of the ways is that you can tell from like how the stars around the center of the galaxy are moving, like they're moving faster, or their tighter orbits than you wouldn't have you didn't have a black hole. You can never see a black hole directly, right, It's just black. It's essentially invisible. So what you need to do to identify a black hole is to see its gravitational effect on stuff around it. And so the argument that something is a black hole is that you measure its mass by looking at how hard it pulls on things gravitationally nearby. But then the second crucial element is that you need to measure it's radius. Because not everything with gravity has a black hole, or the Earth has gravity. You can measure it's pull on the Moon doesn't make the Earth of the black hole. But if you can measure something's mass and it's radius and the radius is really really small, that means that the object is really dense, it's very compact, and below a certain radius, an object of that mass has to be a black hole as far as we know. Well, I so you're saying that we know that there is something really heavy at the center of our galaxy, is super duper heavy, but we don't know if it's actually a black hole. Like it could just be a giant ball of ice cream, right, it could be all that during and ice cream that people have been rejecting for all of those years. Yeah, nobody wants to lick yet. And so the tricky thing is the radius. And as you say, a great way to study this is to look at the effect on very close by stars, because they're mostly affected by the gravitational pull of this thing. And also as they pass close by, you can get a sense for how big the black hole is, because if something passes within one AU, for example, and survives, then you know that whatever is there has to have a radius of less than one AU, where a U is an astronomical unit, the distance from the Earth to the Sun. And that's exactly what they do, is they look at all the nearby stars. Is it's like a few stars that hang out really close to whatever this is at the center of the galaxy and they zip around it. So people look at the motion of those stars to try to measure the mass of this object, and they also look at the distance of closest approach to try to get a sense for what the radius of this thing might be. M So what you're saying that we can sort of look at the center of the galaxy, we can't see these stars orbiting around the very center of the Milky Way. Well, it's not easy and you can't see them invisible light, but infrared light can pass through a lot of the gas and the dust. So if you use filters on your telescope to only look through the infrared, you can see these stars. And in that way you can see what's going on in the center of the galaxy. And if you google you can watch these cool movies of these stars. And the movies take like more than a decade to make because these stars have like an orbital period of like sixteen years. But you can see them whooshing around this blank spot at the center of the galaxy, so it's really obvious that there's something very massive. They're interesting. So we do have sort of pictures of the center of the galaxy. You can s to c through that gas and dust, and it does show something pretty dense in the middle of it. That's right, So we can measure the mass pretty accurately to be about four million times the mass of our Sun. It's very very heavy. It's an incredible gravitational source. But the tricky thing is that we don't have a great measurement for the radius of this thing, because in order to measure the radius, something has to pass really really close and then survive, right, it has to like not fall into the black hole. There's a star that's called S two that goes really close to it. But the closest approach we've ever seen to this object is about twelve a U twelve times the distance between the Earth and the Sun. Wow, that that seems really small to me, right kind of right, like it's smaller than our solar system. Like this thing, whatever it is, the super dense thing at the center of the galaxy is smaller than our solar system. It is smaller than our solar system. But but it's four million times of the mass or sun. Four million times the mass of the Sun. But if you calculate how big should a black hole b, if it's four million times the mass of the Sun, then you can answer of like point one a U, like a tenth of an a U. So whatever this thing is, it's much smaller than sort of the radius we've been able to probe. Right, it might be a black hole that's really dense and at the heart of this sphere we haven't been able to see inside of yet. But that's the uncertainty. We don't really know if whatever this stuff is that has massive four million times the Sun is actually compactified enough to make a black hole, or if it's something else, something sort of larger and fluffier. Right, Like you could take four million times the mass of our sun in ice cream and put it in a giant ball, and probably would it be about that, you know, smaller than the size that we're observing. That could it be like you know, for a U, Yeah, twelve a U is a really large radius, and so it makes a huge volume. And so even though four million times the mass of the Sun is a lot of mass. If you spread it out through a sphere that's like twelve a U and radius, it's not actually that dense, So yeah, you don't have to be nearly as dense as a black hole. It's still a little bit denser than dury and ice cream, but you know, it's pretty close dense and flavor at least, all right, So that's kind of the mystery. We know there's something super heavy, four million times the mass of the Sun and in the center of our galaxy, but we don't know how dense it is, whether it's dense enough to be a black hole, or maybe it's just dense enough to be a big ball of ice cream or dust or cloud or who knows. So let's get into why would not be a black hole and what evidence we have for either answer. But first let's take a quick break, right, Daniel, we're asking the question what is at the center of our galaxy? And it could be a super massive black hole or it could be something else. All we know is that it's super heavy. It's four million times the mass of our Sun, and it's pretty small and smaller than our solar system, but maybe not small enough to be a black hole. So how did not be a black hole? Like, if you have that much mass four million times the mass of our Sun in that space, when it eventually collapse into a black hole anyways, Well, it depends a little bit on what it is. Right, Things don't always collapse into a black hole because they have ways of resisting that collapse. Like why doesn't this Sun collapse into a black hole immediately? Well, because it has fusion. That energy is pushing out and keeping the Sun fluffy. Why doesn't the Earth collapse into a black hole? Because of the strength of the materials, Right, they have this actual tensile strength to resist being crushed by gravity. And as things get denser and denser, they're all these thresholds to overcome. Like even neutron stars, which are crazy dense, are resisting collapse into a black hole because of the quantum degeneracy pressure. Like all these corks don't want to be on top of each other. And so in order to become a black hole, you have to have enough mass and already be compact enough to collapse to overcome all of these sort of thresholds. So it's possible to arrange this much mass in that small space that doesn't collapse into a black hole. Mmmmm, but I guess you know, if you had four million times the mass of our Sun in it, and if it was just gas or does wouldn't you know? Don't we have models that say what would happen to something like that, Like if it had fusion keeping it from collapsing to a black hole, we it would be shining and we would see it, right m m yeah, So clearly it's not shining. But there's also questions about like velocity. If something is swirling really really fast, then it's hard for it to collapse. That's why, for example, dark matter doesn't collapse into a black hole because dark matter has a lot of rotational energy. It's swirling around the center of the galaxy, and in order for it to collapse into a tiny point, it needs to lose some of that angular momentum, right, and angler momentum is conserved in our universe, So the only way to lose angular momentum is to like bump into something else and give your angular momentum away. And that's really difficult for dark matters to do because, as we've talked about before, it's not sticky, So it's possible that whatever is there is just swirling around for a while. But there are also other explanations, and people have a very specific theory for what could be at the center of the galaxy that's resisting this collapse and has the same mass. Well, again, maybe let's take a step back. You said the prevailing theory is that it is a black hole. So what makes people think that it is a black hole if we don't really have direct evidence of it. I guess it's just the leading candidate, you know, of the things that could have that mass and that radius and not give off any direct light, a black hole is sort of the least exotic. You can't come up with other things, but usually they require inventing new particles, things that we haven't discovered yet. So there's no way to arrange like normal matter as far as we know, into a configuration like that that's going to be stable and last a long time and not collapse into a black hole. Like even a big swirling cloud of gas that's spinning and resisting falling into a black hole eventually will collapse into a black hole because they'll bump against itself and lose its angular momentum. So you need some sort of like new kind of particle. I see. So, as far as we know, if you did have a giant ball of ice froam that big, it would sort of collapse into a black hole eventually, Yes, exactly, would be a Durian flavored black hole. Yeah, and that one you don't want to lick. I feel like we're going to get so much hate mail from Duryan lovers out there. Don't write his hate mail until you've actually tasted during an ice cream and can vouch for its deliciousness. All right, So then that's the leading candidate, because any other explanation requires a new theory or a new particle or something super exotic. You know, as far as we know, the universe dictates that something that big would fall into a black hole. So that's kind of why we think there is a black hole. It is, and it was very widely accepted until recently. What happened, Well, there's this really cool astronomical experiment. You know, astronomers don't get to do what particle physicists do, which is like design collisions. I get to like, say, I'm going to smash a proton into an electron or let's try shooting muans at this thing and see what happens. Astronomers don't get to do that. They don't get to build colliders and smash two black holes together. They just have to wait for stuff to happen in the universe in a place that they can observe it. So they have to get lucky. And about ten years ago, people realized that there was this huge cloud of gas called G two which was headed for the black hole. It was going to make a really close approach in our galaxy or in another galaxy. In our galaxy. It was heading right for the center of our galaxy. It was gonna make a near miss of the black hole. And they thought this would be really cool because it allows them to really study the black hole in a new way. Let's throw something at it and see what happens, right, and let them test the details of this encounter between the gas cloud in the black hole. So you could see this gas cloud like you can see it through the PLoud of gas in the center of the galase. You can see it like that. There was one nearby. Yeah, you can see X rays coming out of this thing. And so you can watch this cloud of gas and everybody got really excited because they were not sure what's going to happen. It sort of reminds me of when everybody saw that comment that was going to smash into Jupiter, Like we saw a comment coming into the Solar system, and then people calculate its trajectory and they're like, oh, yeah, it's gonna hit Jupiter like citing. Yeah, it's exciting unless you live in Jupiter. If you just bought real estate and Jupiter, I'm sorry your investment is a waste. But you know, astronomical collisions are a great way to learn about stuff. So people get really excited. And just like seven years ago or so, and they had all these calculations for what they expected to happen when this gas cloud passed near the black hole. Interesting like sprinkling dust into a swirling toilet bowl, right, kind of like what's gonna happen to it? Is it gonna stay together or is it gonna gonna get shredded apart? Yeah, And the calculation suggested that this thing should be totally shredded. I mean, a gas cloud is not really held together very tightly. It's just a big loose cloud of gas. And what happens when you get near a black hole is that there are huge tidal forces. Right. Remember, tidal forces are gravity trying to pull you apart, because if part of you it's closer to the black hole than another part, then the part that's closer is getting pulled on harder than the rest of you, And so the gravity essentially is trying to pull you apart because it's pulling on bits of you with different forces. So they expected that when this gas cloud came close to the black hole, it would be totally shredded by these title forces. But wait, how close did it get? Like I thought, we could only see up to like twelve AU. That's the closest that anything has ever come to the black hole. This gas cloud wasn't going to get that close. It was going to approach like a couple hundred a U, but close enough that you could get some information about the black hole. You know, you can get some like distant information about the black hole just by measuring the gravitational effect on nearby stuff. You can measure, for example, like the slope with a gravitational field. Right, but at that far away, wouldn't a black hole act the same way as a giant ball of ice cream. Yeah. Absolutely. If you stay outside of the heavy object at the center of the galaxy, then you can't tell the difference gravitationally between a black hole or a ball of ice cream or a giant space taco. As long as it has four million solar masses, they would all have the same gravity from the outside. You're absolutely right. So you can't use a gas cloud at like hundreds of AU to help understand what's going on inside the twelve AU radius where we think there's a black hole. But what if it's not actually a black hole, and what if it's not actually contained within the twelve AU What if it's something else much larger and fluffier, bigger than the twelve AU. If it's a big, fluffy mass that's hundreds of a U yde with a very very dense core, then it could still look like a black hole to the stars orbiting at twelve AU, but it would have a very different effect on the gas cloud. So I talked to Grant Weldon. He's an astrophysics grad student at u c l A who actually works in Professor Guests group. She's the recent Nobel lawyer who studies the center of the galaxy. He said, think of it like a fog that all the objects in the galactic center sit in. The idea is that if it's not a black hole concentrated within that twelve AU, but instead some bigger, more extended mass larger than the twelve AU, then it wouldn't have the same time tidal force effect on the cloud because the gas cloud G two would be passing partially inside this new fluffy blob. So the title force calculations that assume a big mass within the twelve au would be wrong. But I guess the point is that this gas cloud survived like it didn't get shredded as if it would if there was a black hole in the middle of the galaxy. Yeah, everybody was expecting this thing to get totally torn apart by this black hole or whatever is there. But instead it derived. It passed right by, and it mostly held its shape. You know, it got a little bit twisted up, but it was not totally pulled apart the way that you would expect for a gas cloud passing by this massive gravitational object. So that was a real mystery. People were like, hold on a second, what's going on. This isn't described at all by our calculations. Wow, maybe the gas cloud was made out of drain and the black hole was like, I don't want any I don't want to lick. This thing totally explains it. Break when you're getting rejected by clouds of gas, Wow, you must really be stinky. So then that threw some shade on the theory that maybe there's a black hole at the center of the galaxy, like, maybe it's not a black hole exactly. People start to think, maybe we should consider some alternatives. Are there other ways we can explain what happened to this gas cloud other than a black hole, because that would be then consistent with all of our observations. And that's when people started drilling into this question of how well do we know the density of this object? Is it possible that there's something actually much larger they're not actually a black hole, but something really much more extended, fluffier and broader and not actually a black hole. Interesting? Right, Yeah, I guess everyone just assumed this was a black hole, but now you have this evidence that maybe may people go, await a minute, how do we actually know that? Yeah? And so people have been creative and there's this really fun theory of a new kind of dark matter. It's called the dark nose. So this would be little particles that make up dark matter. They call them darky nose, and these are fermions, and fermions are a kind of fundamental particle. They're sort of like electrons and quarks. They have this really important property that they don't like to be on top of each other. They don't like to share the same state, you know how Electrons, for example, if you put one around an atom, it fills up a state, and you put another one there, it can't go into the same energy level, has to go into the next one. That's because their fermions, they don't like to ever occupy the same quantum states. The idea is that these dark matter particles, these dark enos, are fermions, so they can coalesce gravitationally, but they resist collapsing into a black hole because of this quantum degeneracy pressure, this polity exclusion principle that keeps them from getting too dense. WHOA wait a minute, you're saying, maybe it's not a black hole, and so therefore your go to explanation is to invent a totally new kind of matter. Another new kind of matter, Like, are there other explanations using regular matter that could explain this giant ball of stuff in the middle of our galaxy. Well, we talked about a few minutes ago. Any kind of normal matter is going to eventually coalesced into a black hole, and whatever is there has been there for a long long time, and so it would have time to collapse into a black hole. So you need something which has a new property of not collapsing into a black hole. So you have to give it this like you know, quantum de generously pressure or something in order for it to survive this crushing gravity. Didn't you mentioned like a neutron star maybe, or some other kind of dense object. Neutron stars can only be up to like two times the mass of the Sun. If it gets any bigger than they collapse gravitationally. So this thing is four million times the mass of the Sun. Neutron stars are a really very special case of avoiding a black hole. But if it's two million neutron stars kind of orbiting around each other like a beehive, you know, like I guess that would also eventually collapse to it would also eventual collapse and neutron stars are not black, right, they do emit. They're super hot and they glow. They emit X rays and they spin also, and we can actually see hot spots on the surface of neutron stars because they emit X rays. Is a telescope on the International Space Station called the Nicer Telescope, which looks just at those kind of objects. So in order to explain this like maybe new fuzzy fluffy stuff at the center of the galaxy, they had to use these new particles with this special property. Okay, So so then you're saying, this possible explanation for this heavy mass that's maybe not a black hole at the center of the galaxy. So it's a dark matter. But now you're sort of positing what dark matter is, and you're saying dark matter is maybe made out of a certain particle called the dark keynote. Yeah, and this would give dark matter the property that it needs to avoid collapsing into a black hole once it's already gotten into this little area of twelve a U and having four million times in the mass of the Sun. Right, because I think we talked about this in another episode, that dark matter can itself turn into a black hole, Like, if you get enough dark matter in a small enough space, it will create a dark matter black hole exactly, because anything can create a black hole that has mass. The cool thing about black holes and the cool thing about gravity is that they talked to anything that has mass, anything that has energy. So there are a really cool way to probe things in the universe that are otherwise totally invisible to us, that we might otherwise have no way to interact with. So if dark matter falls new black hole, it just adds to the mass of the black hole. It is challenging, however, to create a dark matter black hole, because it's hard to get dark matter that dense. Dark matter tends to be diffused, and it tends to be sort of fluffy, and it's hard to squeeze it down because there are no other interactions. It's not sticky like other kinds of matter. I see. So then you're saying the possible explanation is not really a new kind of matter. It's just like, maybe it's helping us pinpoint a specific property of dark matter that would allow it to become a dense black hole object without turning into a black hole. Sure, yeah, I think people in the dark matter community would say that this is a new kind of matter, and it's definitely not like the number one candidate for dark matter otherwise. And these particles are not like everybody's favorite dark matter theory, but they do have the properties you need that if you put them together and create this object of dark nose at the center of the galaxy, it's fluffy enough that it can explain why this gas cloud survived. I see, And it doesn't collapse into a black hole. You're saying because of its quantum properties, like it has this exclusion principle that lets it not collapse. Yeah, precisely. And you know it's a little bit cooked up, right. They created this idea. They've cooked up these dark he nose just to solve this problem. So it's not like something we otherwise already thought existed. We have good evidence for. Oh look, it also explains this, right, it's a little bit more descriptive. So you've always got to be a little bit more skeptical when somebody has cooked up a new idea just to explain one particular observation. Really, you've got to test it in other places. You've got to see like a symphony of results that are all telling you the same story. Yeah, I'm always skeptical when there's a physicist doing the cooking. So that's a possible explanation for what could be inside of the center of our galaxy. And let's get into what the answer is. What does the evidence say, is there a black hole at the center of our galaxy? Or could it be a big ball of this dark keno dark matter? But first, let's take another quick break. All right, what's at the candy center of our galaxy? Is it marshmallows? Is it? Is it guacamole? Or is it a new kind of dark matter called a dark keno which um ironically taste kind of like, how do you know what darkness tas like? I've been there, you know, they offer free samples. Alright, I'm packing my bags, let's go. So, yeah, we thought for a long time it was a black hole at the center of our galaxy, but maybe it's not. And so the only other thing that could explain it is something called dark keno, which is a kind of dark matter that doesn't collapse when it gets that compacted to What does the evidence say, Daniel, where's it thinking in the physics community. I think the physics community reacts to this idea like that's cute, but probably not. It hasn't really persuaded a lot of astronomers, And one reason is that it's not so easy to understand how you would make such a big ball of dark matter. As you've talked about a few times, dark matter is big, it's fluffy, it's diffused, it's swirling around the center of the galaxy, and so we don't know like how you would get it to collapse. Like if I gave you a serving of dark matter that had in the mass of four million sons, it wouldn't be easy for you to force it into that dense in area. So we have no story for how this thing could have formed, right, because as far as we know, dark matter is not very sticky or not sticky at all. Right, Like, if we have a bunch of dark matter in space, it's not gonna come together in clumb naturally. It's gonna get attracted to itself, but it's gonna keep flying past itself and keep flying past itself to the point where it would just be a fluffy cloud maybe forever. Yeah, And people have done simulations to test this and say, well, what if dark matter are these kind of dark eno fermions, and you have a big galaxy sized blob of them. What happens and you just don't get this kind of core. You don't get this like really dense ball in the center. That would explain the data. Now, if you happen to have that dense ball in the center of the galaxy, it can explain what happened to this gas cloud. But nobody understands how you would make such a dense blob of dark matter. It doesn't arise naturally in our simulations, I see. But these simulations are based on what we know about dark matter. But we don't really understand dark matter. So could dark matter have some sort of property we don't know about that would make it clump like that? It would need to have some sort of other self interaction right some way to get sticky, so that it tends to clump someone to like bump into itself and lose its angular momentum so it doesn't just orbit forever and it falls into itself somehow. But nobody has a theory of dark matter like that that's also consistent with everything else. You know, we're pretty sure that dark matter doesn't have a very strong interaction with itself. We've seen huge clouds of dark matter pass through each other, you know. The Bullet cluster, for example, was a collision of two huge galaxy clusters, each of which had a lot of dark matter, but the dark matter just passed right through itself to the other side. Even though there is some gravitational attraction between the two clouds of dark matter, that's really pretty weak. So we're pretty sure that dark matter does not have any very strong interactions with itself, and so it's really hard to explain how you would get a dense clump of dark matter at the center of the galaxy. Just like, no way to get it there, right, even if you give it these interesting quantum properties that it doesn't work out like you said, simulated, it still doesn't clump. But remember that also we have questions about how black holes form, right. We have a pretty good theory for why black holes conform at the center of galaxies, but we don't really understand how they got so big. So it's sort of a similar question. You know, if you simulate black hole formation in galaxies, they get big, but they don't get as big as the supermassive black holes that we see, So that's also an open questions. Just's just a lot we don't understand about how the centers of galaxies form, So you can criticize this theory for like, well, yeah, maybe that's what it's there, but how can you explain how it got there. We can't explain how black holes form at the center of these galaxies either, WHOA I see. So just because you can't explain how it got there doesn't mean it's not there. Because we don't know how the black hole. Even if it is a black hole, we don't know how the black hole got there. That means it could still be dark matter. It could still be dark matter. I think prevailing wisdom is still that is probably a black hole. And people have been creative and trying to find other ways to explain why the gas cloud wasn't torn up even if it is a black hole. Like Andrea Guez, the u c l A professor who won the know About prize, she suggested that maybe it's not actually just a gas cloud. Maybe inside this gas cloud there's like a string of stars that have the gravitational like strength to hold the gas cloud together, and that's why when it passed near this object. It wasn't actually torn to shreds. It has like more gravitational self consistency than people imagine. I see. Maybe that's why it didn't get treaded, Like it wasn't just my gas in that cloud. Yeah, and she has a very specific prediction for like a binary star system inside this gas cloud that could really hold it together so when it passes by, like keeps its consistency, it doesn't get pulled apart by the black hole. I see. Now, what about some of our listeners said that what if it's both, Like what if there's dark batter and the black hole in the center of our galaxy? Like, could it be a mixture of the two things, like some sort of super compact dark matter thing and maybe a black hole or not a black hole, or a bunch of intron stars or something like that. Yeah, it could basically be you know, all of the above, right, And remember the current theory, the prevailing wisdom is not just that it's only a black hole. We do think that there's a lot of dark matter at the center of the galaxy. We think that's the densest place to find dark matter in our galaxy is at the center. But the current thinking about the density of dark matter is that it's not that great, right, dark matter, there's a lot of it in the galaxy, but it's also spread out through space a lot more evenly than normal matter, because again, it doesn't clump. And so for example, in the volume of the Earth, we think there's like less than a kilogram of dark matter, like with less than one squirrels worth of dark matter. So there is almost certainly some dark matter at the center of the galaxy, and almost certainly some dark matter in that black hole if it exists, but it's not contributing significantly because it's not that dense. It's sort of spread out everywhere through the center of the galaxy. M m. Yeah, dark matter is pretty squirrely like that hard to catch. But we also see dark matter sort of clumping in our galaxy, right, Like dark matter is not a totally diffused cloud. It does sort of clump in the middle, but you're saying it doesn't clump enough to maybe explain what's going on. Yeah, what we're talking about here is like an enormous spike con density. Yes, dark matter is denser at the center of our galaxy, but this object, whatever it is, with four million times the mass of the Sun in a very small area is much denser than the typical dark matter density at the center. Yeah, it's four million sons basically in the space of our sources them. Yeah, that's a lot of ice cream. It's a big scoop. It's a big scoop of stuff. All right. Well, it sounds like a black hole is what most people think is at the center of our galaxy, But there are still a lot of questions about that, and there's even maybe a little bit of uncertainty whether it is a black hole. So what are we doing about it? How? How are we going to answer this question? Well, we have an awesome new telescope, this event horizon telescope, which is actually you know, like a collection of telescopes that all work together to make an effective telescope like the size of the Earth. And it recently took a picture of a black hole, right M A D seven, this black hole the center of a distant galaxy. And that was really cool because you could see essentially the size of the event horizon. You could see a black circle at the center surrounded by gas and dust that was emitting a lot of light, and that told you essentially the radius of the event horizon because you can see where the lights stopped. That's what's really awesome about that picture. So if you could train the same telescope at the center of our galaxy, you get a much better measurement of the radius of the event horizon if there isn't a black hole there, and then we could get a clear idea for what's really going on, because that's the thing we don't understand very well. It's like, what is the radius of this thing, whatever it is. Is it big and fluffy like a blob of dark matter or ice cream, or is it really compact less than a tenth of an au like a black hole would be. I see, yeah, I guess having a picture of another black hole at the center of another galaxy basically says like, hey, this is normal, Like, look, there are black holes at the center of galaxy, so probably ours has one to probably And this is challenging though, because it's very difficult to take these pictures, and it's harder in fact, to get the picture of the center of our galaxy than this other distant galaxy m A D seven. We could just sort of look at the center of it we're not buried in the galaxy having to look through most of it. We could look at it sort of from the side, from the top, you know, whereas our galaxy like we're right in the middle of it, so we have to look through all of this gas and dust to see the center of the galaxy. Right, And m AT seven also is targeted because it's like huge monster black holes, like really enormous. Where's the black hole the center of our galaxy? It's big, but it's sort of smaller on the scale of supermassive black holes. Right, Yeah, it is that I guess, closer than this other galaxy, but it's still you know, like fifty thousand light years away, right, Yeah, we're about twenty six thousand light years from the center of the galaxy, which seems like a lot, right, but astronomically speaking, it's really not that big. A number other galaxies are millions of light years away. Oh yeah, I guess it's just it's closer, but it's smaller and it's a dustier and it's also a dimmer black hole. Right, it's a dimmer black hole, and there's a more variability, Like the fact that it's smaller means that there's much more like variation in the brightness of this black hole, so it's not as easy to look at it to stitch this picture together. Now they've taken the data, like they turned the telescope towards the black hole effectively in two thousand and seventeen, and since then they've been crunching it through the computers to try to make this picture. So the only thing standing between us and a picture of the black hole the center of our galaxy is you know, enough computers, right, Yeah, we need more computers, but I think even if they we do get a picture, sometimes we're not sure if it's a black hole either, right, Like you were saying before, even the picture of the black hole we have in the other galaxy, it might not be a black hole, right, Yeah, there are other ideas for what could be that dense and that gravitationally powerful. You know. In the end, black hole observations are always a little bit indirect because you can't actually tell if there's an event horizon there or if it's just something that's not emitting and is very very dense. You know. It's sort of like we have a list of things that can do that, and black holes are the only thing on the list, So we assume that whenever we see something that dense, that gravitationally powerful, it must be a black hole. But then people come up with other ideas like dark stars, these very time dilated collapsing stars that could also do that. We don't know that's a thing in our universe or not. But in the end, until we go and visit the black hole, we won't know for sure if there really is an event horizon there. Yeah, so back your bags, Daniel, we're sending you all right. I'll bring my ice cream spoons. Yeah, make it a big scoop, all right. Well, it's a big mystery at the center of our galaxy, and it sounds like scientists are, you know, licking away at the data to get to the juicy candy center to find out the flavor of truth hopefully tastes like butter scotch or do legit. It tastes like hard work and a lot of GPUs. Let's be honest, it just tastes like coffee. That's what physics tastes like. Yeah, from all the coffee consumed to do it, chalkboard dust and coffee. That's the flavor of physics right there. Hey, you should come up with a candy for that and sell it chalk and coffee choffee. Yeah, it'll be good for your bones. Alright, Well, it's stay tuned. Well. We are slowly but surely looking closer at the center of our galaxy to find out what's at the center of it and what's inside could tell us a lot about how the galaxy got formed and even how the universe got structured and got to the shape it is today. And we're doing everything we can and to try to understand what's out there in the universe from this tiny little rock floating out in space. And every time we build a new kind of eyeball or figure out a new way to look out into the universe, we learned something new about what's out there. Yeah. I just hope we do it before it belt, but we hope you enjoyed that. Thanks for joining us, See you next night. 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. Ye