WSU Master Class: New Ideas About Dark Matter with Justin Khoury

uh so the plan for today is i'm going to discuss with you uh the sort of broad understanding of what we in the field think dark matter actually is what we don't know about it and i'm gonna walk you through all the way to uh the stuff i'm working on at the moment that i'm thinking about as we speak let me begin by making the case for dark matter so i want you to imagine that you're part of a jury and that you're i'm going to present you evidence for why i think dark matter is there in the universe and then you can make up your own judgment at the end of the day so first of all let me tell you uh what we know about dark matter first of all we know that it's a form of matter uh that does not emit light okay that's why we think about it as dark it does not interact with ordinary light does not emit light does not absorb light it moves very slowly slowly compared to the speed of light so in the jargon we say that it's cold okay so it's a form of dark matter that's moving relatively slowly compared to the speed of light we know that it makes up a significant fraction almost one quarter of the total mass energy budget of the universe and it in fact that is six times more than ordinary matter so you see on the pie chart um and here i'm not going to go through the details of this but from combination of the cosmic microwave background on the left side and our measurements of the relative abundance of light elements that we know that the composition of the universe is given by this big pill which is about three quarters of dark energy stuff which is making the universe accelerate today that's not going to be the subject of this discussion dark matter which is roughly a quarter and finally ordinary matter which is which only makes up five percent so the stuff we're made of only makes up five percent so we know this now the first piece of evidence uh perhaps one yeah one of the key uh hints that dark matter is around is when we look at galaxies so here's a very short discussion of what a galaxy is what you see here is a spiral galaxy a quite massive galaxy so galaxies in general tend to have a mass between 10 to the 8 to 10 to the 12 the mass of the sun and the size of it is about 100 light years okay this varies from galaxy to galaxy but these are numbers you can keep in your in your mind now what is the hallmark of dark matter let's do a little warm up to start with let's think about the solar system so in the solar system most of the mass is of course concentrated in the sun at the center and what you see in the motion of planets is basically kepler's third law so you see that the planets which are closest to the sun rotate faster around the sun you see mercury very nearby which is spinning the fastest and the outer planets are moving more slowly so in other words when the mass is concentrated near the center what you expect if you were to show the velocity of the planets as a function of hot power they are from the center you see that it sort of monotonically decreases as you move further out so in other words now if we apply that logic to the galaxy here's uh here's here's the galaxy again most of the mass much like in the solar system is concentrated near the center and so what you'd expect if there were no dark matter is indeed what you see here so stuff near the center is rope is rotating quite fast but as you move further out things are moving quite slowly much like the planets what what we see instead is this okay and and you see the uh what you see is that as you go further out the stuff further out is actually moving along pretty rapidly in fact it's moving along yeah much more rapidly than you'd expect just based on the amount of matter that we see so as a curve what is plotted here on the left side this curve there is the velocity again as a function of distance you would expect it to fall off as you move out what you see is in fact that the velocity flattens out as you move out everybody has the same velocity so therefore what we imagine is that galaxies are surrounded by a dark matter halo that is doing this additional pull to make visible matter rotate fast all right that's piece evidence number one all right members of the jury let's move on piece of evidence number two are galaxy clusters so galaxy clusters are wonderful objects they're the most massive bound objects in the universe so there are clusters of galaxies they're contained many galaxies the mass of the most massive clusters range from about 10 to the 14 to the 10 to the 15 solar mass okay and their size is three million light years so huge objects despite the fact that we call them galaxy clusters in fact it's a little bit of a misnomer because in fact galaxies themselves only make up one percent of the total mass of a cluster the cluster is such a massive object that there's a lot of gas actually just moving around ordinary matter in the form of gas moving around at a quite high speed and and that makes up actually 10 of the total mass of the cluster and finally dark matter makes up the rest okay we think so in galaxy clusters uh we also imagine that there is a halo that a dark matter halo within the cluster and one way we see it again is one way to see it the first piece of evidence is that galaxies and clusters themselves move much much more rapidly than they should if there were no dark matter and second of all the you can also watch the gas the gas is moving so rapidly it emits radiation at a very high temperature so both of these things tell us there's dark matter piece of evidence number three is gravitational lensing so einstein taught us that in the presence of gravity everything should basically fall or should be influenced by gravity including light so not just ordinary matter falling towards the earth but also light should be affected the motion of light should be affected by gravity and this we observe in the form of so-called gravitational lensing so here in this video what you see is as i turn on a as i include a galaxy cluster you see that light emanating from the background galaxy is bent as it comes to the earth and at the end of the day in this particular case the bending is so strong that you see multiple images of this background galaxy at the end of the day you see there are two images so it acts the dark matter in the galaxy cluster is acting like a lens for um for for light another piece of evidence uh related to lensing but in some sense gives you an even more direct vision of the dark matter playing a role here is this bullet cluster all right so let me let me tell you a little bit about the bullet cluster because it's a fantastic object so in the bullet cluster what you see is the aftermath of two clusters colliding together the cluster remember has a lot of gas in it and so if you collide two clusters together the gas will be slowed down by you know the forces that gases feel okay so the gas you'd expect will be slowed down during the collision and this gas is observed here and is shown as the red spots dark matter on the other hand doesn't interact very much so you'd expect it in fact dark matter interacts so weakly you can think of it as just interacting to gravity it it will fly through during the collision and end up being segregated from the gas because it's not slowed down by the same interaction that gases this is shown here in blue blue is meant to represent dark matter the observation that we do is again gravitational lensing so what you see here the key piece of evidence is that remember most of the mass in ordinary matter of a cluster is in gas okay and the gas is in red whereas the mass which we observe indirectly through gravitational lensing is in blue and they don't line up okay so it tells you that there at least it's a very strong piece of evidence that there is matter that is not the gas okay and and that matter and moreover notice that the gas that the dark matter the blue spot is also lining up with the galaxies you see a bunch of galaxies and the galaxies just like dark matter pass through during the collision all right so here's the animation so you start with two clusters it's called the bullet because a small guy really acts like a bullet okay it comes in hits the big cluster with high speed you see that during the process there's a segregation between the gas and the dark matter and this is what we observe at the end of the day okay piece of evidence number four the cosmic web when you look on the larger scales of the universe so what people do is we do simulations we start with a universe that contains many many millions of dark matter particles put in a computer let them evolve through gravity and this is what happens under gravity of course dark matter particles will attract to each other and eventually this initially which was almost perfectly homogeneous evolves to this remarkable structure having these little clumps and this will be this is only dark matter simulation so these little clumps will eventually be homes for galaxies we think and you see also these beautiful filamentary structures connecting the different galaxies this simulation actually makes prediction therefore for how clustered we expect galaxies to be and then on the larger scales we can observe this fits beautifully with the data all right so let's do a summary remember you're part of a jury you have to make your judgment was that convincing was it not convincing okay so i showed you in summary uh three main pieces of evidence i've grouped a couple of them together so the first piece of evidence was the cosm actually that was the last one there was no preferred order okay i'm not i don't pretend to be a professional lawyer the first piece that i presented to you if you want was this cosmic web and gravitational lensing okay and here i've included also the microwave background which we didn't discuss i presented to you galaxy clusters and the third one was galaxies themselves you know you can make up your mind which of these is most convincing here's to give you an intuition for what is i think is most convincing cosmic web and gravitational lensing is like having dna evidence okay okay you find blood of the victim you match dna of the suspect whatever okay it's very for a cosmologist for a forensic expert it's like very clear that there has to be dark matter based on that galaxy clusters it's like finding blood okay in the car of the suspect at his house okay it's telling but galaxies on the other hand it's like finding a glove and it matches the person's glove but it doesn't quite fit you see and it's enough to make you ponder whether you've got the whole picture right okay so i want to talk about the conspiracy of galaxies what i want to convey is first of all i believe in dark matter i believe that dark matter is a particle but i believe that on the scales of galaxies our simplest notion of a dark matter particle that doesn't interact with itself very much and with ordinary matter that picture has to be revisited so what's going on with galaxies first of all galaxies are really complicated objects when you think about it because in galaxies you don't just have dark matter but you have stars you have gas you have all kinds of physics which is very complicated so that complicated physics should yield some pretty complicated looking objects but in fact galaxies are remarkably simple their rotation curves which i explained to you earlier the velocity as a function of distance away from the center as we've said is flat as you move out and that's pretty regular we see flat rotation curves all over the place with galaxies why is that more importantly and this may be a little subtle but it's a very important piece of evidence there is a scaling relation an empirical scaling relation with four galaxies which works remarkably well and we don't understand why this is known as tully fisher because of tully and fischer but really many people have contributed to this so let me tell you what this relationship is all about what is the relationship between v which is this asymptotic flat velocity that you see in galaxies the total mass in ordinary matter in the galaxy and that precise relationship is that the mass in ordinary matter is proportional to this asymptotic velocity to the fourth power why is this strange it's strange because if you think about it the velocity the velocity asymptotically should depend on how much dark matter there is right why does that have anything to do in such a precise way to how much ordinary matter there is it sounds like a conspiracy right the ordinary matter is a small component and yet it dictates precisely the amount of dark matter there has to be further out in this precise way and you see on the plot it's a very tight relationship okay and that's not understood but as i said it's a complicated process you have to model if you're going to try to model this in the computer to understand where this comes from what people do is the following you assume ordinary dark matter dark matter that doesn't interact with itself you can just think of it as just interacting through gravity doesn't interact with ordinary matter but you put on your computer that simple stuff with all the complicated stuff of stars of gas you know supernova exploding black holes this is just a partial list of what modelers have to put in their codes to actually simulate a real galaxy they have hundreds of parameters okay it's a very complicated system now what they do with the simulation is they actually choose those parameters in the best guess possible but in a way that they can reproduce this beautiful tally fischer scaling relation the key point is the following you see if you go back to this tully fischer relation what was striking about it is not the fact that there is a scaling relation but it's the fact that it works so well in other words it's so tight you would expect maybe on average it's this particular slope but you'd expect points further out representing the fact that there's complicated physics going on and it's not a perfect law whereas here it looks like almost it's perfect and so indeed when people do simulations they find that the dots are are the simulated galaxies so with all the complicated physics and what you should compare it to is the yellow band the yellow band is observations if you want so what you see is striking you see that the simulated galaxies are all over the place compared to this thin band which represents observations okay so there has to be a conspiracy in a sense to to explain this there are other problems and people disagree are there problems are they just again misunderstanding what the complicated underlying physics is uh let me mention a couple and as i say these are a matter of debates but one of them is the so-called missing satellite problem when you do simulations of dark matter only i showed you one with all these beautiful structures and these clumps what you find is the following you find on you see that on the left side these are this is a simulated dark matter story you see a big dark matter clump in the middle that would be the home of let's say the milky way galaxy you imagine this would be the home of the homemade kiwi but you see surrounding it are these hundreds of little clumps okay and you wonder those should also in principle be homes to smaller galaxies so when we look around the milky way we should see hundreds in principle of these small galaxies and we don't okay we observe 30 of them now it becomes a question maybe those guys these lumps are there but they were too feeble to hold on to their gas and to form stars so they lost their gas their own story you can say but and this problem one can make it even sharper uh with technical details but anyways it's a it's it's one of the puzzle that's been around for quite a while secondly you expect that around a milky way like galaxy notice that all these clumps are pretty much isotropic right they they're you know they look pretty much the same all around the milky way whereas what we observe is quite remarkable the satellite galaxies so on the bottom plot here i show you two our milky way galaxy at the top with the blue dots are the dwarf galaxies the satellite galaxies around the milky way those we observe and those around andromeda our nearest neighbor with its own satellites and what you notice about them you notice that the galaxies that we observe they're not isotropically distributed around the milky way or andromeda a number of them lie on these planes these thin planes okay and moreover the andromeda plane points towards the milky way this suggests at least there has to be more physics than the simple picture above of just these clumps individually happily forming around the milky way so these are things that some of us spend a lot of time thinking about it's uh again it could just be conventional physics at the end of the day but it's at least puzzling okay so here's the bottom line in my mind uh this is not just conventional physics it gives us strong hints about the nature of dark matter okay so if you want i think nature is singing for us loud and clear and we should be listening to what nature has to say about this i want to tell you about a crazy idea totally radical idea but at the end of the day i want to tell you how i think about this idea and it's called modified newtonian dynamics so this is a law that was proposed by marty milgram shown here israeli physicist who proposed it more than 30 years ago he studied galaxies and he proposed this rather bold idea so after all when we say in the galaxy there has to be dark matter we base that on the fact that we see stuff moving faster around the galaxy than it should but we only infer this based on gravity we don't see the dark matter directly we infer this so what if instead there was no dark matter but instead our law of gravity has to be revisited here's what he proposed he proposed okay let's imagine there's no dark matter and instead newton's law of gravity has to be altered when you go in a particular regime and here's how it looks like as an equation the statement is that when you transition to this new regime beyond newton it's when you go to very low acceleration okay so that you have to think about it right acceleration here 10 meters 10 meter per second squared okay this transition occurs at extremely low acceleration you see the number is 10 to the minus 8 centimeter per second squared that's all you need that's the acceleration you need to keep a star far away in orbit in the galaxy when you transition below that scale all of a sudden newton's law of gravity gets modified instead of being the newtonian acceleration you see this particular square root form it's the geometric mean of the newtonian acceleration and this new characteristic acceleration this sounds totally kooky okay but it's if you want at the end of the day it's a proposal okay so he goes ahead makes his proposal and starts looking at does gallic do galaxies actually obey this and amazingly they do okay so here are some fits actual fits of real galaxies their rotation curves and with this simple law there is no dark matter to mock around here you have a law which only involves the stuff you see and with that stuff you see you predict how these rotation curves should look like and you only have basically one parameter which is this characteristic acceleration and it works remarkably well okay furthermore this tully fischer relation that we observed is an exact prediction of this theory exact prediction okay and i could go in the details actually this sounds like a post-diction in the sense that you know you could say well you know he cooked up a law that fits those rotation curve not so he actually predicted that some galaxies in particular if you look at the bottom right corner that galaxy was was not observed at the time where he made his predictions when he came up with this this idea of mon and he actually predicted that such galaxies should exist now you can take different viewpoints on this the um the conventional viewpoint we have to distinguish empirical facts from theory as an empirical fact this works so even if you believe that dark matter is this conventional particle that doesn't interact blah blah blah your viewpoint is that i put in this simple particle i put in all the messy physics of stars and gas and all that crazy stuff this ought to come out it has to come out because it works with the data okay you see what i mean so you could say the extreme viewpoint is that no there is no dark matter this law is fundamental it has it's a fundamental law of gravity newton was wrong or the conventional viewpoint is there is dark matter it's a simple particle and somehow when you put it all the complicated physics this comes out but it has to come out as an empirical fact okay this is not all roses unfortunately that law that was proposed by milgram works beautifully well with galaxies but it fails on other scales which is what why a big portion of our community chooses to basically ignore it because it fails with these other pieces of evidence that i presented you as members of the jury in particular galaxy clusters doesn't do very well with galaxy clusters and when you get to the cosmic web or the cosmic microwave background also doesn't do very well on those scales okay and as i said this was to me the dna evidence for dark matter so if you're going to mess around with microwave background in this cosmic web as members of the jury you start feeling uncomfortable very uncomfortable but as i said it's true that i mean that empirical law works for galaxies i take the middle ground okay i mean i'm in the middle i don't think it's a fundamental law of gravity i don't think it's purely some magic having to do with stars and cooling and supernovae and stuff like that i think it's telling us something about dark matter that there's a property of dark matter that is such that this mon phenomenon arises in galaxies so i take the middle ground i think there's both dark matter and i think mon this modified newtonian dynamics is also there but it's due to the fundamental properties of dark matter itself so how do you do that right it's easy to say but how do you put the two together and in fact you know as as a scientist you already should have pause at that moment because if i tell you i have two phenomena mon and dark matter and i'm going to put them together you should start saying well okay wait a minute it sounds too complicated right why can't we just get away with one the proposal that i've been thinking about and this is what i want to share with you my research has been to think about the physics of super fluidity as precisely the ingredient that marries the two in in a natural way okay so this will be a unified approach to both this modified newtonian dynamics and dark matter through super fluidity what is a superfluid okay what is a superfluid superfluid the sort of poster child example is liquid helium here's liquid helium liquid helium you cool it it bubbles okay this is just boiling all kinds of things happen once you reach a particular temperature three degrees kelvin or so its behavior completely changes okay did you see it start it stop boiling entirely when the temperature is lowered at some point it completely stops let me tell you what's going on let me tell you what's going on okay normally when you take a fluid and you cool it eventually it forms a solid right so it happens with water and the reason is that you have molecules in the liquid moving around because of the temperature they're jitting or gendering around as you cool them their jitter slows down and eventually allows them to bind to bind into a crystalline structure very ordered structure and that's the ice phase okay however you know from quantum mechanics that even at absolute zero temperature atoms are not sitting still they will have a quantum jitter just due to quantum mechanics what happens with a superfluid is that that quantum jitter is enough to prevent this the substance from forming a solid does that make sense so the quantum jitter is enough to keep it into fluid even at zero temperature so liquid helium you can cool it all the way to zero temperature in principle and it would it will remain a fluid now you could say this sounds very nice thank you justin but why in the world does this have anything to do with dark matter right it sounds like a crazy why does dark matter behave like liquid helium in fact superfluids are not that strange of a beast of course we never really see them in our you know day-to-day experience because our day-to-day experience is at much higher temperature than three degrees kelvin but in principle there are two things you need to have a superfluid people do it for example just with cold atoms in the laboratory you cool down a bunch of atoms together and eventually they can form superfluid so really the two conditions you need for super fluidity are the following how how why would it be that dark matter can be a superfluid first of all you need a lot of these particles you need a lot of dark matter around in order to be dense enough in order to form superfluid so if you want the first condition to have super fluid is that you have a lot of atom you have a lot of dark matter i say atoms because i think of them actually as dark matter atoms but okay so you need a lot of them now we know the total mass of dark matter that we need let's say in galaxies so we know the total mass so if you want to have a lot of dark matter particles it means that each particle has to be light much you know quite light so when you work out the numbers it turns out that the mass of the dark matter particle has to be 0.1 percent the mass of the proton much lighter than what people think about when they think about weakly interacting massive particles wimps which are much more heavy than this but people talk about other types of dark matter particles that have masses in that range so it's not crazy they just happen to be light particles secondly they have to be they have to be cold enough and dark matter is pretty cold right it's one of the properties dark matter it just has to be in thermal equilibrium at a very cold temperature now when you work out the numbers as to what remember i told you for liquid helium it was three degree kelvin that transition if you work out what would be the critical temperature for transition to super fluid state for dark matter you plug in the numbers doesn't have to be doesn't have to come out with it could be any crazy number it turns out to be mili kelvin and that's interesting because in the laboratory with cold atom system mili kelvin is actually a reasonable number with lithium atoms i'm not saying dark matter is lithium atoms okay don't get me wrong it's something different but when you look in the laboratory and you take lithium atoms you find the critical temperature is 0.2 milli kelvin it's not a crazy number okay it's another hint that maybe these things are actually some kind of dark atoms again not made of ordinary matter but some form of dark matter atom okay so here's the picture we have in mind in that story we have in mind that in the milky way or around any galaxy there is our dark matter halo in blue but in this in the interior there is a super fluid bubble where the dark matter really behaves like a superfluid all right that's the picture now why does that have anything to do with mond or explaining properties of galaxies that's just a statement right that we think there's a superfluid phase one thing uh oh i should say one other thing uh we're about to propose that in fact mon has to do with this super fluid nature now as we've said earlier galaxies work beautifully well with mon galaxy clusters do not so that has to be explained right if i if i'm going to say something nice about galaxies i also have to explain something about galaxy clusters so here's the key piece of physics the temperature of the of the of the dark matter is set by how fast they're moving in the halo and so if in the dark in the in the ma in the massive object that we're talking about so in a galaxy you can work out that the dark matter motion of the particles gives them a temperature of order 0.1 milli kelvin so below this critical temperature you see so in galaxies you expect them to be super fluid at least near the center there's a superfluid inside galaxies galaxy clusters as we've said are much more massive so a dark matter particle in a gas cluster is whizzing around much faster because there's more gravity there right there you work out what the temperature would be for dark matter particles it's 10 meter kelvin above the critical temperature so in galaxy clusters you see galaxy clusters are hotter in this sense and therefore in galaxy clusters you're not in the super fluid state you're in this ordinary dark matter phase and that's what distinguishes galaxies from galaxy clusters okay now i haven't told you yet why mon comes out of the story right modified neutron dynamics now one amazing thing about super fluidity once you reach superfluid state like we saw with with helium liquid helium what you noticed in the movie was that initially the atoms of liquid helium were moving all over the place you reach the superfluid state and then they stopped why did they stop the physics of this is remarkable what happened and this is really quantum mechanics is the most striking manifestation of quantum mechanics once you reach this critical three degree kelvin temperature what happens with the superfluid is that at that moment the individual atoms of liquid helium are no longer independent entities does that make sense they lose their individuality now they have because of quantum mechanics they cannot individually whizz around which is why you see the boiling stop they start behaving coherently in unison if you want they have what are called collective excitations you don't excite yourself independently you have to all in the room create a wave like we see in the stadium and that's exactly what happens it's a collective motion just like the wave in the stadium these are known as sound waves these are the excitations of a superfluid or phonons now phonons here's a video okay you see that they're moving just like in the wave in the stadium moving collectively okay now there is one thing this is for a particular type of solid animation for a solid there's one thing that is not captured by this animation you see that the atoms as the wave is moving they're going up and down like you would do in a stadium the kind of wave we're talking about is not going up and down it's moving sideways we call this longitudinal wave it's actually how the sound from my mouth to your ear is propagated it looks more like this you see that the atoms in the movie each individual atom is moving back and forth right that's the red dot it's just moving back and forth but collectively there is a over density which is propagating in a particular direction that's the wave you see but each atom is only doing this unfortunately this animation didn't capture the fact that they're sort of coherent the other animation show that they're going so you have to in your mind combine the two okay so it looks more like an ordered thing that i showed you before but the atoms are moving this way as opposed to that way okay so these are the excitation of the superfluid so in other words in the galaxy once you're in the near the center of the galaxy i imagine that's my theory anyways that the dark matter particles are not whizzing around in the way you'd imagine them to be they're not whizzing around they're behaving like liquid helium like a fixed you know coherent bulk of stuff sitting there and moreover when you excite the dark matter you excite the dark matter it doesn't excite it by having a particle whizzing around it excites it with these coherent waves in the in near the center of the galaxy so completely different than what people that we normally think of dark matter as being individual particles now the key idea of this proposal is that the these phonons and this requires some mathematical details which i cannot show you but uh that the superfluid phonons are actually responsible for mediating a force and it's a bond force okay i will give you a gist for why this may be true but for the moment just take my word for it that mathematically these phonons can mediate a force on top of gravity which looks like mon precisely the square root form that we discussed okay now why does it have anything to do with monde and the reason is it's really uh what what got me the process by which i came about to this idea was really because of a mathematical analogy you look at the theory mathematically that you need to explain bond and it looks like the theory precisely of those sound waves of a superfluid in fact it looks eerily similar to the particular theory of a known cold atom system in the laboratory not exactly the same but similar in spirit okay so it's the mathematical structure that led me to think that wait maybe this month phenomenon is describing super fluidity and dark matter can certainly be a superfluid now as we said it naturally distinguishes between galaxies and galaxy clusters we've said it before so galaxies are very cold and therefore they will have a superfluid and therefore there you'll expect to have mon behavior as we said this explains these beautiful fits of rotation curves it explains why you have this conspiracy of tally fischer it comes about because of superfluid nature of dark matter whereas in galaxy clusters as we said galaxy clusters are much hotter there is no super fluid phase in the galaxy cluster and therefore no mod i want to qualify this because this will be important later on when i say there is no superfluid it's not quite true there is a little bit of superfluid near the very center of the cluster because there you don't have much mass and so therefore temperature can be cold this will be important later okay so near the center of the cluster you have a little superfluid bubble just surrounding the central galaxy in the cluster but for most of it it's all ordinary dark matter imagine our little bucket of liquid helium which we cool down so now it looks nice and still and imagine you start rotating the bucket now when you rotate a bucket of water water rotates with the bucket liquid helium does not actually it will remain still but at some point if you spin it too fast what will happen is something has to give okay it cannot stay still and what it does instead of starting to spin homogeneously as a as a whole solid as a whole liquid what happens is it start forming vortices little vortices inside the fluid and that's what represented in these sequence of pictures as you spin it faster and faster vortex is exactly like your toilet flushing right it's things moving around circularly and you see that you develop more and more of these vortices as you spin the fluid faster now the galaxy itself is spinning right the dark matter in the galaxy may well have its own spin and therefore if it's super fluid it should not spin homogeneously like a ball it should start forming vortices in fact just one more thing this is actually a beautiful experimental result with liquid helium by a group at university of maryland they show actually they're able to map these vortices you see them as lines what i want you to see from this graph is that these vortices are not just standing still boringly you know whizzing around they're actually they're actually interacting they're combining they're reconnecting etc it's a very dynamical system these set of vortices it's very cool okay so therefore the picture we have in mind as one possible observational signature is that in fact the galaxy the galaxy the dark matter in the super fluid bubble should have all these little vortices now if you ask great how come i don't see them okay first of all there are many of them if you estimate how many vortices there are there will be a hundred of them within the size of the solar system but they're very small in thickness they're like one millimeter so the mass that they have is very small okay so it's not clear whether you can easily see them for example you could say well if they're there they're massive i should see that i should feel their presence in the sources they're very feeble lines of density okay so we're still thinking about how you might go about finding those vortices but it is in principle there if you believe this idea now a very interesting and you actually saw this in the movie i showed you earlier about this beautiful cosmic web in blue and things coming together and so forth forming these filaments what you saw there if you looked closely was actually these little lumps right connected by filaments some of them were actually merging together they'd come together they merge form bigger lumps that's actually a very important part of our understanding of the formation of the structure in our universe is that small lumps can come together and form bigger lumps let me show you this and i'll tell you the physics of why this happens it's actually not so it's actually very interesting why these galaxies merge but i'm going to show it to you in the most dramatic way okay by showing you the future of our own galaxy the milky way as we know it you know there it's on a collision course with andromeda our nearest neighbor so you can ask let's look in the future okay billion years into the future we're on a collision course with our friendly galaxy nearby and here's what happens okay so here's a movie a simulation of what's going to happen with the milky way and andromeda and you see they're on a collision course but notice what's going to happen they come together they start sloshing together as this happens and eventually come back together slosh again slush again and eventually they relax into a single galaxy okay this is our fate okay and remarkably we'll survive this the uh this is our future now notice that this was a very interesting movie in fact if let's let's imagine this for a moment if the andromeda in the milky way were just two points right and they came together by gravity one thing that could very well happen is they would just slingshot past each other right there's no reason why they should merge to something together whereas in the movie what you saw is they come together they want a slingshot but they kind of like slosh around and they come together it's as if there was some friction force friction that's kind of sloshing them and bringing them together does that make sense if there were two particles you would expect them okay there's no reason why they would actually they might form a bound state they might start but why would they merge together at the center that usually think of friction that's a very interesting process okay and it has to do with the fact of course that they're not particles they're made up of a whole bunch of particles you have to think about a lump of particles coming together with another lump what happens then it's not two particles it's a bunch of particles together it's a process known a very well-known process dynamically known as dynamical friction it goes back to trend reseccar so dynamical friction it's a form of there's no friction fundamentally it's just gravity but it's a form of effective friction that comes about from the fact that there are many dark matter particles around in these galaxies let me illustrate to you it's very simple process so imagine at the top imagine you have a mass m which is passing through a gas of other particles this mass m could be itself a particle okay it doesn't but let's just imagine to illustrate it it's a particular body that's moving through this gas of particles at the first instance of course it attracts gravitationally the particles around it then it moves away but those particles then moved in because they were attracted to that body initially that's what you see in the second plot you see there's a little bit more particles on the right in the second slide and that's because the the object attracted those particles in but as a result now there's more dense particle there's more mass if you want on the right sorry on the right then on the left you see so as a result there's a force now a net force bringing the mass back to where it came from does that make sense let's think about it again so you start with the top this particle is moving around at the first instance it attracts the particles around it particles come in now the mass has moved but now there's an over density of particles that attracts it back right which means it slows it down and so on and so forth now in the second slide it attracts also neighbors brings them down attracts them back okay so this process is precisely the process which slows things down as there and it's what leads to the merger eventually we call this dynamical friction notice that it critically hinges on dark matter particles being individual entities being able to move around rearrange themselves independently so you see they come about and now as they come about now the sloshing is beginning and that's again the dynamical friction and it leads at the end of the day to the merger okay what happens with the superfluid the hallmark of a superfluid is that it has no viscosity what's viscosity viscosity is the resistance of a fluid to flow you notice you say honey is more viscous than water because it's harder to pour it out of the bottle that's because it resists to the flow in fact if you think about it and maybe you've had that experience imagine you took your bottle of honey and you punched a little hole with a with a with a needle at the top of your honey bottle you turn it upside down and you know what happens nothing will flow if the hole is too small and that's because of viscosity you see water would come out because it's less viscous but honey will just stick there okay notice what's going to happen with the liquid helium i'm going to about to show you a movie here's again our favorite bucket of liquid helium initially it's not going to be super fluid at the bottom of this bucket they did exactly what i said they punctured a bunch of tiny little holes just like honey when it's normal liquid helium nothing's going to flow but when it reaches critical temperature and becomes super fluid you'll see stuff starts to pour through the holes even though they're extremely tiny holes okay let's watch the movie initially it bubbles nothing goes in becomes super fluid and now you see at the bottom you see stuff pouring out right let's see it again you see initially nothing comes out cool it down stuff starts pouring out and that's the hallmark of superfluid the lack of zero viscosity it can flow through anything no matter how small the capillary no matter how small the tube it will flow through it okay that's a superfluid now what it means in practice of course i should qualify this it's no viscosity at absolute zero temperature if you're at a little bit non-zero temperature but below critical temperature there will be a little bit of viscosity okay in the fluid but in any case what we expect for us if you think about the dark matter fluid superfluid is when you had these galaxy mergers you expect there will be less dynamical friction less viscosity between the galaxies as they're moving together so it will change a little bit how galaxies merge you see it will change they'll still merge right because these things are not at absolute zero temperature and so forth but it will be less dramatic than what you saw with andromeda and the milky way less dynamic now why is that a good thing this is pretty subtle but let me give you some you know you guys are jury members you want to know evidence for this crazy idea so what are the evidence that leads me to think that less dynamical friction is a good thing less friction in the dark matter is a good thing here's a one of the dwarf galaxies satellite galaxies around the milky way it's known as fornax now here it is as you can see it's a very feeble galaxy has very few stars and compared to the milky way and around it i've circled some points okay these are clusters of stars we call them globular clusters they're in orbit around foreign acts now if you believe ordinary dark matter these little clumps of stars in fornax are swimming in the dark matter halo you see they're swimming dynamical friction just based on the little animation we saw earlier dynamical friction should have led these clumps of stars to go towards the middle towards the center of fornax precisely for the same reason that our galaxy is merged and they don't they just happily stay where they are they're not moving to the center they're somehow they're prevented they're orbiting but they're dynamical friction is not bringing them to the center now conventionally you could interpret this as maybe the dark matter halo doesn't extend as far or maybe the you know the dark matter distribution is different than we think it is but in my case this is explained quite naturally these clumps of stars are are swimming in the superfluid they're not feeling the viscosity they're not feeling the dynamical friction that would bring them to the center that's how i explain why these guys are staying where they are as opposed to being dragged to the center here's a galaxy cluster and remember i told you galaxy clusters don't have a super fluid except near the center okay here's a galaxy cluster what you see near the center you see this bright yellow region if you that's the brightest region of the cluster and it encloses the brightest galaxy in the cluster but notice if you squint your eyes you actually see three little dots within the yellow region you see that those are actually three galaxies they're so close to each other that if you believe they have this dark matter halo around them their dark matter halo are basically overlapping so now you can ask wait a minute you showed me a movie of two galaxies merging and when their halos were sort of overlapping they merged why these guys near the center of the cluster haven't merged they're in principle their halos are overlapping that's very interesting it's a puzzle people you know you can come up with but again in my case this is explained yes they have halos of super fluid but they're super fluid so the merger is very different they don't feel the same kind of friction that ordinary uh dark matter would give them okay anyways hallucinations hallucinations from a crazy theorist now some other very interesting observational properties of superfluids is when actually superfluids collide what actually happens so people do that in the laboratory so here's what happens you take two lumps of superfluid bring them together okay you see eventually they come out they come out and notice that they came out and they retained their shape that they had initially right so this is again a hallmark of the fact that superfluids pass through each other without any dissipation without any friction they're happily passing through each other and but what did you see in the middle when they were overlapping you saw fringes you saw these lines right let's look again at this movie you see they come together and when they're overlapping you see what we think what we call interference you see dark spots and bright spots and that really is due to the quantum mechanical nature of these guys it's the fact that they're quantum mechanical that you see these interference much like we see with the two-slit experiment as one of the evidences for quantum mechanics so when people simulate so people have done simulation not of my super fluid but a related type of dark matter superfluid this is again like the blue movie i showed you earlier with formation of galaxies i'll show it again in a second but you see again these lumps forming these are like the galaxies eventually you see them coming together and you see the wiggles no you see them the ripples around them those ripples are the same interference effects that we saw with the experimental lumps coming together okay let's see this again so you see they come together they form first you form the lumps you form the homes that will later be galaxies and then they start coming together because of gravity and as they come together they form these fringes okay could you see this well there is actually a known there there are known structures around elliptical galaxies there are these beautiful rings they have truth be told a conventional explanation within standard dark matter but this kind of interference that we're discussing between superfluid could have another contribution to these kind of shell-like structures that we see around elliptical galaxies perfect so i want to leave you with some thoughts i told you i brought you on a journey the first thing that i want you to take home is that dark matter as a particle is highly successful it's most successful in the dna piece of the evidence it's successful on the largest scales in explaining the cosmic microwave background properties the cosmic web gravitational lensings all the large-scale stuff works exquisitely well meanwhile this mon empirical law works beautifully with galaxies doesn't work well on the large scales is there but on galaxies it works amazingly well you should think of it not as a theory but as an empirical fact about galaxies so is there a way to marry these two ideas together without giving up altogether the idea of dark matter still keeping the successes of the dark matter but then incorporating this mon behavior not as a miracle of star formation and so forth but as a real fundamental property of dark matter and the idea that i presented to you is something that i've been working on actively is the idea of a dark matter superfluid state as i told you it's not crazy in the sense that we know with cold atoms you cool them you make them dense enough they form superfluid so if dark matter is of that type there's enough dark matter and it's cool enough you will form a super fluid state of dark matter and in this picture really dark matter as a particle and as a mon phenomenon it's really two sides of the same coin they're just different phases of the same substance it leads i think to eventually a rich spectrum of predictions in particular for these mergers of galaxies as we saw dynamics should be quite interesting and i want to leave you with some open questions that i think are fascinating and that i'm thinking about actively what what would be a precise analog i told you it looks like cold atoms that people study in the laboratory i told you mathematically it looks similar to the theory that describes those cold atoms so can we find an actual cold atom in the laboratory whose properties would be identical identical to the kind of superfluid i need for mon okay does that make sense if you can find such a such a actual superfluid in the lab you could in principle start within the laboratory start simulating galaxy collisions with cold atoms okay that's the ultimate dream can you find such an analog in the laboratory that will allow you to study galaxy mergers and so forth and of course the elephant in the room is dark energy right i didn't discuss dark energy but dark energy which makes up three quarters of the energy budget of the universe is itself a big mystery what is dark energy ideally it would be very nice you know if dark energy could somehow fit nicely holistically in this picture maybe not in this particular superfluid picture but in general as some other some other phase of the dark sector not as opposed to just another ad hoc component we add to the budget of the universe thank you very much you

Channel: World Science Festival

Views: 48,308

Rating: 4.8141336 out of 5

Keywords: Justin Khoury, quantum, gravity, master class, dark matter, non-Newtonian, gravitational force, galaxies, structure of the universe, cosmic microwave background, CMB, evolution of galaxies, World, Science, Festival, World Science U, New York City, wsf, wsu

Id: NCSYW_sevyc

Length: 52min 40sec (3160 seconds)

Published: Fri Nov 20 2020