Could dark matter be a superfluid?

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Speaker 1: Jorge, I have an idea for our Daniel and Jorge explain the universe food truck. Oh nice, wait, are reactually doing that? I thought it was a joke. Well, it was a joke, but then our listener Tim Lazarov road in to ask when our food truck is going to be in his neighborhood. May should be more like a food spaceship it would be more appropriate. Well, that goes perfectly well with my idea. You see, normal food trucks sort of drinks right fluids to wash down their taste treats. But our food spaceship should serve super fluids like zero viscosity beverages nice, so they go down easier. Or do they have Bose Einstein condensation on the outside. I'm thinking we should test it on some undergrads, you know, make sure it's safe before we sell it to the public. I'm sure the FDA is all over that. I don't think they have a physics food division yet. That's the p d A Physics and Drugs Administration. Hi, I'm or hand me a a cartoonists and the co author of Frequently Asked Questions about the Universe. Hi I'm Daniel. I'm a physics professor and a particle physicist who does research at CERN. And every time I'm in a lab, I'm always tempted to taste everything that sounds like a terrible Daniel, especially if you visit a virology lab. Yeah, but you know they got like weird glowing goo. You're like, m I wonder what flavor that is. I'm never gonna do it, of course, I hope, but you know, the curious mind wonders, right, Remind me never to invite you to my lab, otherwise you'll be licking everything like a dog or a two year old, like that kid in the movie that licks the flagpole and wonders if his tongue is really going to stick to it. We should get into the physics of a Christmas story will shoot your eyes out. But welcome to our podcast Daniel and Jorge Explain the Universe, a production of I Heart Radio in which we invite you to taste the entire universe to enjoy the flavor of knowledge as well as ignorance, to take a bite out of everything that we do and do not know about the universe. We don't shy away from the big mysteries. On this podcast. We talk about the smallest things, the medium sized things, and even the biggest things, and we attack all of them and try to explain them to you. Yeah, it is a delicious looking universe, like a giant cosmic bouffet of amazing ideas and incredible phenomena that are there for us to dig into and fill our bellies with amazing knowledge. Yeah. And if you're not a curious person, then you wouldn't take a bite to these weird things. And I think that's why when I'm in some kind of laboratory and I see something weird bubbling in a flask and I wonder, I wonder if that would make a good soda for our food truck, It's that same curiosity that inspires me to try to take a bite out of the whole universe because I want to know. I want to understand. It's not enough to just say I bet that green bubbling thing tastes something like lemon lime soda. I want to actually know the truth. I think there are easier ways to find out what something is, Daniel, rather than bringing it in your mouth and in your body, Have you tried asking what it is? It seems like the polite thing to do. But then how would they know, right if they haven't tasted it. Maybe some questions are not meant to be answered, What like, are you curious about how sanide tastes? I am curious? Actually, you know, I like almond pastries and so hey, maybe you know a nice cyanide after flavor is not the worst thing in the world. But yeah, I'm curious about all this stuff, even the stuff that might kill you to find out the answer. It's just this deep desire to know these truths. Well, speaking of dark matters like tasting poison, there are amazing mysteries out there, including one that is maybe one of the biggest mysteries in the universe. At least it's the second biggest mystery in the universe by percentage. Boys, right, that's right. If the universe was laid out as a buffet, most of it would be dark energy, but a huge, heaping pile of it would be dark arc matter. Most of the actual stuff in the universe, the matter, the things, the bits and pieces that move around in our universe, are not the things that make up me and you and weird beakers of bubbling green goo in chemistry laboratories, it's something else, something different, something we do not yet understand, but we have a cool name for it. That's right, we have a cool name for it. It's dark matter. And that is an interesting analogy, Daniel. I guess the universe is like a giant buffet, and five percent of it is like regular food, right, chicken and bread and pasta slad, And about twenty seven percent of that buffet is a big giant mystery, some kind of weird dark stuff. And I guess you would be right in there tiling it on on your plate. Yeah, I don't know if I'll go back for seconds. You know, I got to try the first serving, but yes, serve me up some dark matter. I want to know what it is. Does it in the end just taste like chicken? Would it taste? I guess it would just go through your tongue, wouldn't it. That's right. Dark matter sounds like something black and heavy, but actually it's invisible and intangible. Dark matter would pass right through you like a cloud of neutrinos, because we don't think that it interacts with normal matter in any way other than with gravity, and gravity is a very very weak force, so you couldn't even like pick up a spoon of dark matter. You had like a blob of dark matter, and you dropped it, it would fall to the center of the Earth, right, And we don't even know if it is matter. We don't even know if it is stuff. All we know about it is its effect on the rest of the universe and the rest of the stars and galaxies out there that we can see. That's right. We have never confirmed the particle nature of dark matter. We don't even really know a hundred percent that it's stuff. And we've talked about it on the podcast a lot of times and we've said that we are pretty sure it's matter, but there are some questions that remain. There are some things that we see out there in the universe that the idea that dark matter is stuff is some kind of invisible new matter in the universe doesn't quite explain. And there are a few other our ideas, different hypotheses to try to explain it. And I know that it's one of the favorite pastime of our listeners because they're always writing the emails about maybe dark matter is actually this other thing, or what if dark matter it could be something else totally different. It is one of the biggest most accessible mysteries in the universe. That's right. Maybe it is a giant tree of some kind of cosmic buffet for some giant beings. Perhaps. Yeah, maybe it's just like weird pasta instead of squidding, because they put something else into it to make it like really dark or invisible. But I guess what you mean, right, Yeah, if there's some kind of animal, let's bray something that makes it invisible, maybe they've just inserted that into the dark matter pasta. Well, hopefully it's more like the dessert of the universe, because that would be pretty neat, right. A third of the universe is just desserts. Does that tell us about your diet? Or Hey, are you like desserts? Sure? Why not? I guess it all depends on your perspective. Vegetables can't be dessert, right, sure, yeah, I guess so I like a nice zucchini bread. Yeah, which, it's that's technically what whread you eat at the end? Right, sounds good. Well, I'm hoping that we can gobble of the mystery of dark matter one day, but until then, we have to think carefully about what we have seen, what we know, what we can explain, what we can't explain and what new ideas we might need to tell a complete and true story about everything that's happening out there in the universe. Yeah, it is an ongoing debate about what dark matter is, and it's an ongoing exploration of what it could be. So today we'll be talking about one possible idea for dark matter. So today on the program, we'll be tackling the question could dark matter be a super fluid? And if so, is it's sparkling? And will it kill you? I guess if you drink it question number two? Right, First, I want to know if it's carbonated, and then question number two is can I survive drinking it? Well, it dissolve all the carbon in your body is the second question. I guess you would want to know before you drink anything, unless you're Daniel whites It. Yeah, and it's a really interesting question. Other dark matter is a super fluid, and you might wonder like, why do we care? Why do we think it might be a super fluid. And for all the successes that dark matter has had in explaining the large scale structure of the universe and gravitational lensing and the wiggles we see in the cosmic microwave background radiation. We'll talk about it in a minute, But there are some things that dark matter as a theory of some weird invisible particle really struggles to explain in our universe. It needs a little bit of help. Yeah, and I feel like we maye skipped the question here, Like I feel like we never even tackled the question could dark matter be a fluid? Like do we even know if it could be a fluid or a solid or gas? You will, Actually, dark matter already we think is kind of a collisionless fluid, sort of like an ideal gas. You know, we think about it as these particles flying around in the universe, not interacting with each other at all, because again, the only interactions we think it has is gravity, and gravity between particles is basically zero. So already we think of dark matter sort of like a collisionless fluid or an ideal gas. So here we're talking about it having like special produced from being a super fluid like a superhero. But shouldn't be like a super gas then? Yeah, exactly, Maybe it gets bitten by a radioactive spider and then turns into a super fluid or super gas. Yea radioactive fluid spider, I guess it would have to be the marvel theory of the universe. Well, as usually, we were wondering how many people out there had considered this question whether dark matter could be a super fluid. So Daniel went out there into the wilds of the internet, or maybe the campus of UC Irvine, which one this time? Daniel, these are Internet answers, So thanks very much to everybody who participates in these and waits patiently for us to get to the episode. If you'd like to participate for future episodes, please don't be shy right to us. Two questions at Daniel and Jorge dot com. So think about it for a second. Do you think dark matter could be a super fluid? Here's what people had to say. Well, I wish I knew what a super fluid was, because that sounds like a really awesome thing to get to know. The dark matter could be a lot of different things still at this point, um, so sure it could be a super fluid. It could be a great soft drink that sadly just passes right through your body. I don't really think that's true because the other superfluids that we've created are all made out of hawks, they're just in a different arrangement, and I don't think quarks can, you know, display the behaviors that dark matter does. So no, I don't think that dark matter is a super fluid, but who knows, well, what's a super fluid. I believe that's material that doesn't lose energy when it's moving, So it sounds like document that would really hit that point because it doesn't even inter fear with itself. On the other hand, we know that doc meto is impacted by gravity, so that would speak against it. So I'm not really sure. But if I would have to make a bed, I would say, yeah, it's a super fluid. I don't see how can anything out in space be considered a fluid given how low density it is, and if it isn't a fluid, it could be a super fluid. No idea. That's fascinating. I would not have thought dark matter was dense enough to be a fluid in the sense that we understand it. Um my understanding that dart matter is going to be as diffuse as regular matter and its distribution through a nearly infinite universe, so that it's going to behave like a guess. I would think if I remember correctly, a super fluid doesn't as a fluid fluid without viscosity. I think it was viscosity. And well, we know dark matter as supposed to interact with matter only through gravity. But I suppose if it interacts any through gravity, that implies some form of attraction, which means, if we regarded as a fluid, it must have some sort of viscosity, which, if I remember the definition of a superfluid currictly means that can't be a super fluid. Sure, I guess it could be anything. We really don't know much about dark matter other than it exists and it has gravity. Um, so why not? Could be, could be super fluid, could be, could be anything? Really m M interesting answers some skepticism. Some people were like, I don't know, don't think so this one sounds weird to people. I think the idea of having like a fluid out in space sounds weird. People think of spaces like cold and mostly empty, maybe filled with tiny little crystals or particles flying around, But like a fluid is a weird thing to think about having in space, right, And it seems like a lot of people are like, maybe it could be a fluid, but a super fluid, I don't know if I would give it, you know, supernatural powers. Yeah, well, at least one guy is ready to taste it, though, you know he's thinking about super soft drinks. Hmm, interesting, super soda, super dark soda. Exactly. It's very massive. It's like Coca cola dark coke. Give us a call. We got ideas, special dark recipe. But a lot of people didn't seem to know what even a super fluid is. I guess that's not a common word even I'm not sure what it quite means. Yeah, super fluids are not the kind of thing you have experience with that you don't see them in your everyday life. The river that runs through the park in the middle of your town doesn't ever become a super fluid. You don't make super fluids in your kitchen. They're like a weird quantum state of matter that we only recently even we're sure could exist. And so it's a sort of a new theoretical idea, and whenever that happens, people have fun applying into like, oh, this is new and cool. Maybe this works also over here in this different part of physics where there's something we don't understand we have a new hammer. So let's see what else could be a nail. Yeah, it seems like maybe dark matter being a super fluid could maybe explain some of the things we can't quite figure out about it. And so Dania, let's step people through this. First of all, I guess let's recap what dark matter is and what is it that we don't understand about it that is making us consider this idea. So we think that the universe has a bunch of invisible matter in it because we see a lot of gravity out there in the universe that we can't explain from visible matter. You know, we know that stars are huge balls of hydrogen and they have a lot of mass, and so they have a lot of gravity, and the Earth spins around the Sun, for example, because of the mass of all that grab But if you look at a galaxy and you add up all of the mass from all the stars that you can see, you can't explain all the gravity that's happening in the galaxy. Like the galaxy is spinning like all galaxies do, and that spinning would tend to toss stars into outer space the way like ping pong balls on a Merry Go Round. If you spin, it would toss those ping pong balls out past the Merry Go Round, But the gravity of the galaxy keeps those stars in place. That's why this Milky Way is not just like throwing all of our stars away. But the galaxies are spinning really really fast, and in order to hold them in place, they need more gravity than we can account for. The gravity from the stars we can see doesn't give us enough gravity to hold the galaxy together. And on a bigger scale, we also need to hold galaxy clusters together. Galaxy clusters are big groups of galaxies, and we don't think those galaxies seem to have enough mass to hold themselves together. Galaxy clusters are also spinning. But if you add a bunch of invisible mass to the galaxies, then it all works because it's invisible, explains why you can't see it, and it's new matter, so it adds more gravity, and so it solves those problems if you add this weird new invisible stuff to the universe. The weirdest thing about it, though, is that you need much more dark matter than visible stuff. It's not like you just add a little sprinkling of dark matter. You need to take every star and add five stars worth of dark matter to explain all the missing gravity. Yeah, it's it's almost like dark matter is kind of like the missing piece. And what we see of the universe, right, and the way the galaxies stick together and the galaxy clusters stick together, they stick together more than they should given what we can see in them. And so one solution is that maybe there's invisible stuff out there that's holding it together. Yeah, and this is the kind of thing we're always trying to do, is reconcile everything we see. We think we understand how gravity works. Let's check it. Let's make sure that our explanation makes sense, that it works for this scenario. And that's how we discovered oh my gosh, it doesn't. And that was the clue that maybe it was something else going on, or there was something else new out there. But before you believe that, just like crazy new idea that the universe is filled with a huge amount of invisible stuff you just happened and never noticed before, you want other pieces of evidence. And so we have other clues that dark matter might be real. We see it affecting the way light moves through the universe because if it has mass, it changes the curvature of space, and so it can lends light. We see that it existed in the very early universe because it affected the wiggles in the cosmic microwave background radiation, these photons from the plasma that filled the universe very very early on. And we also know that affected the whole way that the universe form, the large scale structure of the universe. Galaxies we think wouldn't even exist if dark matter hadn't created little gravitational wells to pull stars into mostly all hangs together into a very nice story, right, But it's kind of interesting. I guess that it's a story we made up assuming that what we know of the rest of the universe is true. Right, Like, if we assume that the laws of fixed work the way we think they do, then you sort of need this invisible matter to just make what we see make sense. But that's only assuming that we're right about the laws of physics. Yeah, that's saying gravity works a certain way, and so in order to explain this missing gravity, we need more mass. But you're right, there are other ideas. People have also thought, well, maybe gravity doesn't work the way that we thought maybe there isn't any missing gravity. It's just signs that our theory of gravity is wrong. And people have tried to modify the theory of gravity to explain what we see, and this is called Mond modified Newtonian dynamics. The theory says that instead of gravity going like one over distance square the way Newton said, there's another factor there that when things have very low accelerations, gravity is a little bit different. It gets a little stronger. So if you tweak gravity in just this way, you can also explain how galaxies rotate without using dark matter. That's really interesting. So it could be that dark matter is not dark matter at all, Like maybe we just have the laws of physics a little bit wrong. But does that explain everything about the way the galaxies stick together and even the gravitational lensing. No, So Mond, this alternative theory gravity does in fact a better job at explaining how galaxies rotate than dark matter does. There's some things about galaxy rotations that dark matter just can't seem to get right, but it doesn't explain everything else that dark matter does. Like Mond does not do a good job of describing how galaxy clusters rotate and spin. Around themselves, and it's much more difficult for it to explain, like the cosmic microwave background and lensing and all sorts of other very strong evidence for dark matter. So mond is sort of a nice idea you can explain one thing actually better than dark matter can. But dark matter is sort of like a stronger idea across the board. But neither of them, I guess, is perfect. And let's maybe dig into that a little bit, like what is it about the idea that dark matter is invisible stuff that doesn't explain what we see out there? So by now we've seen a lot of galaxies, and what we try to do is understand like how am my dark matter is in a galaxy versus how much normal matter? And is that common across galaxies? Like do all galaxies have the same amount of dark matter and the same amount of normal matter? And we also try to understand how that could have happened. You know, if dark matter is this weird particle, this new heavy, invisible thing, then it would have clustered together and we can make models for how that would have happened informed galaxies. We look out into the universe, the galaxies we see don't really line up with what we expect for dark matter. So one thing specifically we look at is a relationship between how bright the galaxies are and how fast they are spinning. How bright they are is really interesting because it tells us like how much normal matter is there, how many stars, how fast they're spinning should tell us something about how much dark matter there is in the galaxy. And if you do a bunch of simulations, then you expect like a loose relationship there. You expect, like some galaxies to have a lot of dark matter and something to have a little bit, but there to be a lot of variation. What we see when we look at these galaxies, though, is that there's a very very tight relationship. It's like almost no variation, like the amount of stars in a galaxy and amount of dark matter and the galaxy has a very very close relationship, which we think is weird and we can't explain with our models. You mean, like when you look at the galaxies out there in the universe, they almost have the same proportion of regular matter and dark matter. I think that's what you're saying, right, Like, there aren't galaxies out there with a lot of dark matter, and there aren't a lot of galaxies with a little bit of dark matter, which is weird. But I guess to me, it's weird that you would think it's weird. Why wouldn't they all it's sort of the same. But they were all made in the Big Bang, you know, because there's a random element here, right, Like, how do galaxies form? Anyway? It comes from a quantum fluctuation in the initial seeds of the universe that gave you a slight over density in the dark matter that pulled together a little well and then grab some stuff. And you know, we do expect some relationship. We expect there to be a relationship between the amount of matter and the amount of dark matter, because in the beginning we think this stuff is mostly evenly spread out. But we also expect some variation. And when they do simulations to try to predict what kind of variation we see and we run all of our laws of physics, we see a much wider variation in our simulations than we see out there in the actual universe. That tells us like, maybe there's something wrong with this theory. That's something that we're putting into our simulations that isn't doing a good job of describing what we're actually seeing. I think you're saying that the ratio between dark matter and regular matter is too constant, like it's too consistent across the board of the universe, which means that maybe the problem is that what we expect to see in the universe is wrong or something. Yeah, because for that to happen, you might expect some sort of interaction between them, for them to like turn back and forth into each other, or interact with each other, some process that's keeping them so tightly coupled. But we think that dark matter and atomic matter don't interact except through gravity, so we don't have a process for making this happen. The alternative theory MOND actually predicts this perfectly, like Mon says, there is no dark matter, there's just normal matter, and gravity changes how things spin, and the apparent rotation of velocity of these galaxies should be very tightly connected to their brightness because the rotation velocity just comes from stars. So the Mond prediction is like bang on exactly what we see, whereas the dark matter prediction is sort of like scattered all over the place and doesn't do a good job of describing what we see. For astronomy nerds out there. This is called the Tully Fisher relation. M I think you're saying that maybe this idea of Mond, that maybe our laws of physics are wrong, does a better job of explaining the consistency of what we see out there in the galaxies. Like, it makes more sense that gravity works differently than we think it does, and then it would be for other to be a bunch of invisible mass. Yeah, it does a better job of explaining the relationship between galaxy brightness and rotation that we see in the universe. We see them much more tightly coupled and connected in the universe than we would expect if it was due to dark matter. If you've got like a random sampling of how much dark matter and how much normal matter, you expect there to be more of a spread. But Mond predicts a very tight relationship because there is no dark matter, So it does a better job of predicting what we actually see out there in the universe. This is like kind of a headache for dark matter as a theory. Interesting, like it has its failings. The idea that it's invisible matter, and this idea that maybe the laws of physics are wrong comes in and says, hey, I can fix that, but maybe it doesn't fix everything, which is why it's still not the prevailing theory. Yeah, exactly, But it's been like a real thorn in the side of dark matter for a long time and keeps a lot of people, I think, from accepting this idea that dark matter might be real. Well, there is a new theory, a new idea that maybe would make dark matter make more sense, and that's the idea that it maybe it's a super fluid. So let's get into what a super fluid is and whether dark matter could be one of these superpowered fluids. But first let's take a quick break. All right, we're asking the question, could dark matter be a super fluid? It? And we're asking the question because there are some things about dark matter that we can't quite explain out there. Even if we assume it's some kind of stuff, some kind of particles, some kind of fluid, it doesn't quite explain the ratio of regular matter and dark matter we see out there in the universe. Yeah, that's right, And so people are trying to be creative. They're saying, like, dark matter by itself doesn't quite work as a theory. Mond by itself has lots more problems than dark matter by itself, like, neither of them are perfect. Is there some way we could take dark matter and make it a little Mond dear right, to try to capture some of the things that Mond has. Remember, the key feature of minds that it changes the effect of gravity over some distances. So people were like, well, is there any way we could change dark matter or tweak dark matter so that it basically has the same effect as Mond, but only in these scenarios the hearts of galaxies where dark matter seems to be having a problem all right, And so the idea is that maybe dark matter is a super fluid, which is like a superpowered fluid that was I don't know, born in krypton or something inherited power ring from an am. The superfluid is not from cartoons and not just from science fiction. It's a real thing. A couple of Nobel Prizes have been won already because of superfluids, and they're called superfluids sort of analogy to superconductors, right. A superconductor is something that conducts electricity, so you can send energy down a wire if it's superconducting and lose none of it right, you don't turn any of it into heat, and so super fluid is similar. It's a liquid that flows, but without any internal resistance, so the bits just sort of slide by each other. It doesn't heat up at all as it flows, doesn't lose energy. Like if you take a bucket of water and you put your finger in it and spin it, you get a little vortex that's forming. But eventually that vortex will sort of peter out right, the energy will diffuse and the water will stop moving. In a superfluid, that doesn't happen. You start a vortex and it just spins forever. Wow, it's super interesting. I guess that's the idea of zero viscosity, which means no friction between the molecules of the fluid. But I guess maybe let's dig in a little. This might be interesting. What exactly is viscosity or what exactly is friction? Like? Where does that come from? So it comes from the interaction of the bits inside of it. Right, when we talk about like an ideal gas, we're talking about particles flying through space, but we ignore the possibility that they can bump off each other and exchange energy. In real life, the particles inside of gas can bump off each other, can exchange energy, and the same thing with a liquid. In the case of a zero viscosity liquid a super fluid, then the particles don't really bump off each other and they can like change places without losing any of their energy. What do you mean they don't bump into each other. They do bump, but they don't lose energy, or they don't bump at all, Like the things inside of the liquid don't interact with it with the self. When you're thinking about a liquid, it's sort of like an emergent object, and it's easiest to think about it like layers of liquid. Imagine like two layers of a liquid passing by each other, and think about whether there's like friction between those layers. Say, for example, say for example, you have a tube and you're pushing some liquid through it, right, then if it's a very vis gets liquid, it's going to flow more rapidly near the center than near the walls, because I get the fluid particles near the walls of the tube are hitting the particles of the tube, right, and so they lose energy. They're like bumping against the wall, and it's not just that they're bumping against the wall. Think about like, as the layers are passing by each other, are the particles grabbing at each other? Like what is friction anyway? Like if you run your finger along the surface, why is kinetic energy getting turned into heat? Because the particles in your finger are grabbing at the particles on the table. There's little deformities and there's bonds between them that are getting broken and reformed. And the same thing happens inside liquid. When you have like layers of liquid passing by each other, they have an interaction, then they can grab at each other and sort of like slow the next layer down and it's super fluid, that doesn't happen, and the layers can sort of like pass by each other without any friction at all. And why is it? I guess it just depends on the interaction between the particles. Right. It's not something you can do with the normal liquid very easily. It's a quantum property, right, It's not something you can really understand at an intuitive level just thinking about little balls. Instead, you need to think about these objects as quantum objects, which means you think about their wave functions and when these things get really really cold, then you have very little uncertainty on their temperature that their wave functions grow really really wide, because the Heisenberg and certainty principle tells you can't know something's momentum and its location very very well. So when you cool something down, its wave function grows very very large. So now instead of having just like a bunch of little particles bouncing around that you can sort of think of a particles, you have these overlapping wave functions between these objects, and they form like one big quantum state, and they tend to move like all together instead of interacting with each other, so they're like more tightly coupled to each other weirdly, which gives this super fluid state. I see, I guess that you're saying. I think the cold or something gets the bigger the wave function of the particles gets, which means that their things get fuzzier almost in a way, right like instead of a little tiny ball, suddenly it's more like a hazy blob. And it's kind of hard maybe for too hazy blobs to really drag on each other. Is that kind of what you're saying. Yeah, And it acts more coherently instead of individual particles which you can grab at each other. Now it's like a huge train of particles that tend to move together rather than bumping against each other. So it's like it's all much more coordinated. Now. It's not like a random crowd of people bumping into each other. Now it's like a tightly packed formation of a marching band walking down the street. But they don't bump into each other at all. They just sort of like flu and so this happens. For example, here on Earth, of you cool helium a lot, you get a Bose Einstein concert which is super fluid. The first demonstration of this was in super fluid helium. We get the Nobel Prize and Physics at Stanford for that. That wasn't actually a Bose Einstein condensate, but it is. A super fluid can flow without losing any energy. Another example of a super fluid is a Bose Einstein condensate. Another special state of matter rape a whole podcast about that again, where you're cooling atoms down in a trap to make them very very cold and overlapping. So they have other weird quantum properties as well, which include being a super fluid. We also think we have seen super fluid sort of indirectly inside the large Hadron collider. When we smash big atoms together, like lead nuclei and gold nuclei, we can make this state of matter called a cork gluon plasma. And one of the features of it, we think, is that there's a little bit of super fluidity, very very briefly at the heart of that thing while it exists. I mean, when you smash particles together, you get so much density of energy, and these particles really packed together that they behave like a super fluid for a tiny little bit. Yeah, for a tiny little bit, even though it's super duper hot. It's also so dense that these particles undergo this new phase change into a cork gluon plasma, which can also be a super fluid. So a super fluid isn't like one unique state of matter. It's like a description of a phase of matter, the way like some phases of matter conduct electricity and some don't. Some phases of matter are fluid and some are super fluid and others aren't. So these are just examples of places where we have seen super fluidity happening, Like really is a thing in the universe. We're sure about that. That's not a hypothetical thing, and we think it might even be what's happening inside of neutron stars, right, Yeah, the inside of neutron stars is a lot like a cork glow on plasma. It's very very dense, and the particles get squeezed together and their wave functions start to overlap. And we don't really know what happens inside a neutron star because it's some state of matter that we can't really access anywhere else, and all the forces come into play, including gravity and the strong force, and it really tests our ability to even do calculations or predicular what might happen. We think that weird states of matter like nuclear pasta might occur, and there might also be super fluid states inside the heart of a neutron star. But is there generally like a recipe for making super fluids? Like, what's the thing that makes all of these different examples? What do they have in common? They all have in common high density. So you squeeze these particles together basically so that their wave functions are overlapping, and that's how you achieve it. It's easier to do that if they're very very low temperature because they're way from are larger. But if you get high enough density, you can also achieve it even at high temperatures, like the inside of a neutron star. But the crucial thing is density. Wonever. It's like, you know, taking a bunch of water balloons. When you squeeze them together, they almost become super fluid. Haven't you been to a birthday party recently? He saw this happen. Well, like, if you have a bunch of water balloons out and they're really far spread, far apart, they sort of behaved like maybe like little particles, but if you sort of packed them together in a bucket, they kind of act like a fluid. Right, Yeah, that's true. I never thought about what it's like for a bunch of water balloons to slide out of a bucket. Have you like dumped a bucket of water balloons on somebody's head before? Yeah? Yeah, super super flat water balloons. Actually that sounds like super fun. But you're exactly right. When simple things come together, they can do new, weird things. And that's the whole amazing size of chemistry. Right. We have these phases of matter that come out of the way, these particles interact with each other or don't interact with each other, and generate these emergent properties electrical activity or shiny nous or liquid phases or other weird phases of matter. It's incredible what matter in the universe can do. The variety of things that come out just of the basic laws from the interactions at the microphysical level. It always amazes me. M Okay, So that's what a super fluid is. And now the idea is that maybe dark matter could be a super fluid. So it's the idea that dark matter is made out of little particles and then when you somehow get them really close to each other, they behave like a super fluid. Yeah. The idea is that dark matter is still dark matter. It's still some particle that's invisible and intangible and doesn't interact with us except through gravity. But you get enough dark matter together under the densest conditions, maybe it's forming a super fluid. And now we give it new properties like those water balloons in the bucket. It can do things when it's all together in those conditions that it couldn't do otherwise. And the idea is that this new super fluid state of dark matter might explain what's happening inside galaxies that currently dark matter as a theory can't explain. Interesting But I thought maybe dark matter didn't interact with itself. So isn't it already a super fluid that doesn't have any internal friction. Yeah, so currently we think that dark matter doesn't have any any interactions. So you might think, oh, yeah, dark matter out there in space is a collisionless fluid. Isn't that also a super fluid, right, Not technically, because they don't have overlapping wave functions. Like if you just have a really dilute gas of dark matter, like we think exists out there at the edge of the galaxy and beyond, that's not really a super fluid because the particles are just really far away from each other. To have a super fluid have these new phenomena emerge, you really have to have them close enough to each other, so the wave functions overlap. So it's more about the quantum overlap of the individual particles and less about the kind of frictionless flow of the fluid for dark matter. Yeah, And in this case, it's the overlap of those particles that generates new phenomena like frictionless flow, and in the case of dark matter super fluid, they think it can effectively create a new force. What emerges from a dark matter super fluid. It's sort of like a new force which effectively can change the way that gravity works to give you exactly the same behavior that we see in the mond theory of dark matter. To me, the question should be is dark matter a super duper fluid? All right, Well, let's get into how dark matter being a super duper fluid at the heart of galaxies could explain some of the things we can understand or explain about the current model of dark matter. But first let's take another quick break. All right, we're asking the question could dark matter be a super fluid? And we're asking the question because there are things about dark matter that we can't quite explain. I mean, we don't know what it is or what it could be. But even our idea of it as a particle, as a bit of matter, as bits of matter, doesn't quite explain some of the things we see out there in the universe. And so the idea is that maybe dark matter is a super fluid, which might explain these things. Yeah, and I think the kernel of the idea. What generated it is noticing that dark matter as a theory seems to have trouble in the densest situations. It works really well between galaxy clusters, it works out there in space for gravitational lensing of lots of galaxies. It works in the early universe, but inside galaxy is currently the densest places in the universe is where it struggles. So people thought, well, maybe when dark matter gets denser, it forms this new state, this super fluid, and then we can figure out how to give this superfluid new properties. Maybe you can solve the problems of dark matter inside galaxies without breaking with dark matter is already so good at explaining everywhere else, right, So the idea is that it only forms a super fluid inside the dense environment of a galaxy. Everywhere else it's just this normal, boring, old dark matter, normal boring, but it's still quite mysterious and elusive. They're saying that at the center of galaxies, where things are pretty dense anyways, right, there are sometimes black holes in the middle of galaxies. There are a lot of stars clustered together. The ideas that maybe dark matter is super duper compact at the center of galaxies more so than like at the edges of galaxies, and not just at the center, but yes, definitely denser at the center. But essentially we're thinking about galaxies as like a dense place in the universe. And as we talked about earlier, how do you make a super fluid. You need to get the particles close enough so their wave functions overlapped and make the super fluid thing happen. And so that can happen in a galaxy because there's a lot of gravity there, it gathers together a lot of dark matter. Another trick they pull is to make dark matter very very very very low mass. We know like how much mass the dark matter we need, but we don't know how much mass each particle has. So if you make dark matter out of really massive particles, you have fewer of them. If you make dark matter out of really low mass particles, you need more of them. So the folks who are working on this theory say that dark matter is really really low mass, then there's a huge number of them, right, And so they're imagining the centers of galaxies being swarmed with zillions and zillions of very very low mass dark matter particles. That come together into a super fluid, and when they do that, they get all sorts of new weird behaviors. Cool, well, it's getting too a little bit of what those behaviors are, Like, what do you think happens when dark matter is that close together that the wave funtionings overlap. So it's really hard to do these calculations because you're talking about like overlapping wave functions of lots and lots and lots and lots of particles. So when physics needs to do that, they try to describe these new behaviors in terms of something they're already familiar with. So the way they usually talk about it is in terms of sound waves propagating through this super fluid. I think about like shock waves moving through it. How if you pull on one part of it that would affect the other parts of it. And dark matter is this super fluid, so it's like weirdly tightly couple that acts like a big coherent blob instead of individual pieces. And so they talk about phonons, which are like sound waves moving through this super fluid, and they build up this whole theory which comes out looking a new force sort of like this dark matter when it enters this super fluid as a new way to interact with itself that it didn't have before. Well, I guess maybe one thing that's confusing me is that I thought dark matter didn't interact with itself, right like it The particles of dark matter don't usually or can't bump into themselves. That's what was part of the idea of dark matter. So why would bring them together really close together make them interact in a different way. But if they can't interact with each other, you're right, at a particle level, they don't have that kind of interaction. We're only talking about gravity, but now we're adding other weird quantum effects, and quantum effects when they all work together, can make it feel like there's a force. Another example is the poly exclusion principle. As someone that tells you that, like two fermions can't be in the same location. That's the thing that keeps some kinds of stars from collapsing. It allows you to resist the force of gravity is trying to push it in. It's not technically a force at the particle level, there's no force there, but this quantum behavior of the objects basically acts as a block to g so in the same way, this weird quantum behavior of dark matter when it's a super fluid acts sort of in a way to change gravity. It's sort of like there's a new force. It's not an individual new force on the particles. It's a way to describe what happens to all these particles when they do this new quantum thing. You can tell a story about it as if it was a new force. Okay, I think maybe I'm starting to get it. Like if I have two particles of dark matter, and I have them really far apart, then they do interact with each other, and not like they can't bump into each other, but they can attract each other gravitationally, like there's a gravitational force between the two particles of dark matter. I think what you're saying is when you bring them really really close to each other, so that wave function of these two dark matter particles starts overlap, then there are other effects that start to kick in, other quantum effects then maybe affect the gravity between them. Yeah, that's precisely it. And you can talk about those new quantum effects as a new force and introduce even like new particles for that force. Call these things phone ons, or you could just say, maybe that changes the overall effective gravity, right, it changes the impact of gravity because now you have to factor in this new, weird quantum effect. And the amazing thing that comes out of the math is that the change it makes to gravity is to make it look exactly like Mond predicts. Remember, Mind, is this change in Newtonian gravity that would beautifully describe everything we see at the hearts of galaxies, but fails everywhere else. It turns out if you make dark matter of super fluid, it changes the gravity within this dark matter to make it look just like Mind. But wait, I thought that you know the gravity The gravity is just between the two particles, right, Like the gravity between these two dark matter particles maybe changes when you bring them closer together, so that maybe they feel or not feel more or less gravity. But to someone standing far away from these two particles, why would they why would the gravity change for them? No, it doesn't. You're absolutely right. And so if you're outside of a galaxy, it doesn't matter whether the dark matter is fluid or not. But we're talking about inside of galaxy. We're talking about what's happening internally, how fast things are spinning the gravity that like one blob of dark matter is feeling on another blob of dark matter inside the same galaxy. So this affects how dark matter inside the Milky Way, for example, is pulling on other dark matter inside the Milky Way, which is exactly what keeps the whole galaxy together as it spins. So these quantum effects make the gravity stronger of the superfluid or weaker. It makes the gravity stronger, right, enhances their gravity. What do you mean, like, do you know what the quantum effect is or are just kind of postulating that there's maybe some quantum effect that would make the gravity stronger. The quantum effect comes from these overlapping wave functions, and when you put it together and you do the math, and you squeeze it theoretically sort of into the box of a force and say, how do I interpret this as a force. Then the calculations come out to predict a change in the force of gravity that looks just like the math you get from Mond. It's not just speculation. You can go directly from these quantum effects to calculating the new effective force of gravity, and it looks just like Mon's prediction, which is as we know something that works very very well well. But I think what you're saying is that the super compact dark matter forms a super fluid which has stronger gravity between the dark matter particles that are in the super fluid. But with something outside of the super fluid, would it feel this extra gravity or not? Well, something inside the galaxy would, but something outside the galaxy wouldn't. Right, So something outside far far away other galaxies in the cluster wouldn't feel any change in the effective gravity. And that's key because we don't want to change the predictions for dark matter in the cluster that already works really really well. The way we see the galaxies rotate around each other, and big clusters of galaxies rotate around other clusters of galaxies. That's very well described by the dark matter theory. So we don't want to change that. So the super fluid thing only changes what happens inside galaxies, not between galaxies. And so how would that explain what we talked about earlier was one of the shortcomings of dark matter, which is that the proportion of dark matter and regular matter is too consistent between galaxies. How would this explain it? Well? Conceptually, you can imagine that it gives us away for like dark matter and normal matters sort of talk to each other more intimately, so effectively it solves a problem by saying, you do still have some variation in how much dark matter and normal matter you have, but dark matter itself acts a little bit differently, so it changes how galaxies rotated, changes the effective gravity of that dark matter, and that's what determines how fast a galaxy can rotate without tearing itself apart. And remember the discrepancy we saw it was not actually directly in the dark matter density of these galaxies, but the rotation speed of the galaxies versus their brightness. So now we have a new way for dark matter and normal matter to side interact a little bit more strongly because this new force that's inside the dark matter super fluid, because we think that's sort of like couples the stars and the dark matter a little bit more tightly, makes it possible for them to have like more feedback mechanisms to potentially explain what we're seeing out there in the universe. I think what you're saying is that maybe there's like an extra effect here that comes from the super fluidity of dark matter that maybe makes it not random, right, because before the problem was that we expected the ratio of dark matter and normal matter to be is a little bit more random, more variation. But maybe this special effects kind of acts in a way that gives you less variation. Like if you have more dark matter, it acts in a way so that you have more regular matter as well. And if you have less dark matter, then maybe acts in a way to give you less regular matter. That's the kind of feedback effect we're looking for. We see out there in the universe, this is strangely tight relationship between the dark matter and the normal matter. We didn't understand that if the only relationship between the two was this fairly weak gravity. But if gravity gets a little bit stronger, it helps solve those problems. And more specifically, we see that the effective gravity inside these galaxies now follows exactly the prediction of mind, which, as we said before, predicts very precisely the ratio of these dark matter to normal matter inside the galaxy. So it all clicks very nicely into place. M M. It seems like a pretty super idea. This is a super fluid, but it also sort of constrains dark matter in a bit right like it it depends also, like it can only be a super fluid if dark matter is made out of really light small particles, right, And there are ideas out there for dark matter to be very very light particles. The most common idea is a whimp, a weekly interacting massive particle where the mass of the particle would be like a hundred times the mass of the proton. But there are other ideas where dark matter could be very very light. We've talked about it before on the podcast, the idea of an axion sort of like a photon with a little bit of mass to it, And so this idea is a little bit more like an axion than a whimp, which would also maybe make dark matter harder to eventually detect and study right directly, Yeah, a lot of our searches for dark batters, these big underground tanks that are looking for a dark matter particle to come and bounce off a zenon atom for example, are not will of sensing dark matter and very very low mass. But we have other experiments that are looking for very very low mass dark matter particles. But there are also other ways to test this theory. People think that if this is true, it will also affect like how galaxies merge. I mean, if you have two merging galaxies and they each have their own halo of dark matter, then what happens when they emerge. You will see the super fluidity effect because the halos won't merge as fast as they would otherwise. Like two halos made of a normal fluid, you'll expect a little bit of friction. Two halos made of a super fluid, they'll basically pass through each other and it'll be gravity that pulls them back, so they would like oscillate more times as they merge. If it's a super fluid, then if it's just a fluid, So if we can like study merging galaxies as a chance, we can see whether dark matter is a super fluid or a normal fluid. I think we're saying is that the dark matter at the outsides and the edges of the galaxy, which is not as compact or super fluid, would sort of become a superfluid once it crashes into another galaxy. I'm saying that we can test the super fluidity of dark matter by slamming into another blob of super fluid. Dark matter, if it really is super fluid, should basically pass right through. If it isn't super fluid, then we should see some friction between the two blobs of dark matter. And we can't do this very easily, but sometimes galaxies collide, right, huge galaxies slam into other galaxies, basically testing this hypothesis, doing this experiment of slamming one blob of dark matter into another. So if we can study those collisions, we might be able to tell the difference between super fluid collisions and normal fluid collisions because they should look a little bit different. Pretty interesting to think that dark matter, which we can see or touch, could be doing things that we can maybe imagine and even the dunk right and figure out, yeah, because we can't see the dark matter directly, but we can see it indirectly because of gravitational lensing and because of its impact on the other stars. So doing a lot of statistics and very careful measurements, we can get a sense for where the dark matter is and what's happened to it. And then we can check that against our calculations and see does it look like it's being a superfluid or a normal fluid? Right, And if you see that it has a cape on it, then you know, like Hey, it definitely has superpowers, right, that's right, And then it needs to stop by our food truck so we can promote our super fluid beverages, which we has not quite passed the p d A approval. Right, that's right. Don't go at drinking any super fluid yet, please people. Yeah, and don't invite Daniel to your lab because he will definitely put his own super fluids on everything. Hey, I'm a curious person. What can I say? All right, Well, another interesting idea about dark matter, the one that could explain what is going on out there, and another example of how this is still work in progress. We don't know what this thing is. We're trying to figure it out, and there are still new ideas coming up that could explain what's going on. Yeah, there are whole categories of ideas. Some of them even try to combine dark matter with mond and say, maybe dark matter is real but also gravity and used to be modified. Lots of people out there trying to make the best of both worlds, and this is like a cool alternative to try to capture all of the best bits of all of those theories. You could make a dark Mond to the theory, right, Yeah, and we could have a dark mond flavored coke. There you go, and you can make a dark Mont Sunday ice cream Sunday as well. All right, well, we hope you enjoyed that. Thanks for joining us, see you next time. Thanks for listening, and remember that Daniel and Jorge explained. The Universe is a production of I heart Radio. For more podcast from my heart Radio, visit the i heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows. Ye