Public Lecture | How to Bend a Stream of Dark Matter and Make it Shine

hello everybody.welcome it's really great to see a very nice turnout here although I'm not surprised considering the fact that this big question of dark matter is something that keeps many people certainly many physicists awake at night as you probably know there are many different experiments that are being conducted in these days and those experiments have not really produced any confirmed results no conclusive results about what dark matter is all about but that keeps very bright theorists such as Sebastian awake at night thinking about what other ideas can we possibly have about dark matter so he's going to tell us about that in his lecture today Sebastian got his undergraduate degree in the UK and then continued at University of Michigan and became a postdoctoral fellow with us here at SLAC about two and a half years ago or so and he continues working on variety of theoretical physics problems but I think that the one that is most compelling is the question of what dark matter is all about so without much ado [Applause] okay so thank you Greg for the very kind introduction and thank you also to the rest of the organizers for the opportunity to take part in this great tradition of giving public lectures here at SLAC so as Greg said you know dark matter is a question that keeps me up at night there's another dark matter that keeps me up at night and that's coffee but in any case so as you can see from the title I'm going to tell you about dark matter and in particular dark matter that may shine so that might seem counterintuitive but hopefully by the end of the lecture you will come away thinking yeah okay that makes sense so without much ado let's get started so what am I and what do physicists at slack do so I'm a theorist right and the people here in the audience and watching are interested in knowing what people it's like - so I can only tell you what I as a theorist do but before I get there why don't I start with why so you might have this question why on earth do I do well why on earth do you do what do you do talking about me and the answer to that question comes in the form of more questions because if you can't ask answer questions by asking more questions we wouldn't stay in business so the first question that I think is very fundamental but we actually have pretty good answers to is what are we all right so what are we made up of and I will explain what we are made up of in due course over the introduction of this talk so here are little images of a proton and neutron and an electron which make up us in the form of atoms a second fundamental question is how did we get here so there's obviously lots of different answers to that some of you might have biked some of you might have driven but how in particular did we arrive here in the history of the universe is the question that I would like to get into so I will explain that in due course and in explaining that I will also explain the various forces of that we experience that interact they allow the matter above to interact and so I will explain that as well but there are unanswered parts to this question and I will explain what role dark matter plays in how we got to where we are today then there's more of a sort of existential question which is well why are we here and I cannot answer that I anyone here can answer that that would be great I'd love to hear the answer but I would argue that dark matter has some role to play there and so hopefully we'll go some way towards answering that question as well in this talk and finally by the end you should get a sense of where we're going right so what does this all mean what is this all for what's the point where are we going from here and I will tell you about an experiment that might be a way forward from where we are today and so as I said in the course of this introduction dark matter is going to play a crucial role in answering many of these questions although not really the top one because we pretty much know what we're made up of okay so let's start with what are we and in order to do this I want to start by setting the context that we're in and in particular a scale so first I will talk about the small starting with us so here's us a human order one meter tall maybe a bit taller a bit smaller doesn't really matter thinking pondering very hard about questions about fundamental issues in the universe okay and we live in a scale which is larger than us so for example here is 10,000 times larger than us is roughly the size of a city or in this case also a city shown next to the Large Hadron Collider which is one of the very large scale experiments that we as physicists have been conducting to try and understand all of these questions and some of the questions that I will be discussing today and of course we're here on earth which is ten million times larger than we are roughly speaking and here's a pretty picture of the earth but let's go smaller than us let's go smaller than a human so if you were to pluck a hair which please don't do it because that probably hurt and you looked at it head-on you would see the width of the hair and it's visible to the naked eye and that's roughly a hundred thousand times smaller than you are okay so this is a size that sort of makes sense to you you can observe it you can pluck it out look at it and see okay that is roughly the scale that is the smallest you can observe with the naked eye but that is by far not the smaller scale that we are able to access indeed protons one of the the fundamental building blocks that makes us up is actually 10 to the minus 16 meters in length so if you think about what that means that's 10 million billion times smaller than you are is the proton that is sitting up inside of you in many many of protons sitting inside of you and the Large Hadron Collider believe it or not actually probes length scales even shorter than that proton so it's colliding particles protons indeed and energies that probe length scales that are four orders of magnitude even smaller than the proton itself so that's why the Large Hadron Collider is actually associated with two scales and finally there's the sort of fundamental length scale of the universe which is the Planck length which is associated with gravity and this is the only place where this will appear here but that's just to give you an idea of how small can you possibly go okay so what about the big so still in the logarithmic scale we're gonna start from us the earth and we're gonna go bigger so a little bit bigger than us a hundred times bigger is the Sun okay so we observe the Sun it looks pretty small but if you sat next to it it would be a extremely hot and be very very large the Sun of course sits inside our solar system which is a thousand times bigger yet again than the Sun itself and so we can observe this with a telescope probably in your backyard you could set up a telescope and observe Saturn or Jupiter probably not Pluto if you still count it as part of our solar system as a planet but you could certainly start observing things here with a telescope now taking a big leap actually is the difference in size between the solar system where we live and the galaxy in which our solar system lives so that is a billion times larger than the solar system is even bigger than that however our galaxy happens to sit in a super cluster of clusters of galaxies so in each one of these super clusters which you can roughly make out that there's little dots of light here and there those each one of those dots of light would actually be a galaxy so there are big structures composed of many many galaxies that all seem to cluster together and through this talk I will explain exactly how these clusters came to be between the clusters and indeed the clusters themselves live in what we know as filaments and so these are sort of those pretty elongated structures of clusters of galaxies that you saw on the the title slide earlier and again and this is something that I will explain during the talk and then finally the observable universe which is why there's an asterisk there because we don't actually know that it's this size it's probably larger is 10 to the 27 meters long so that's 27 zeroes more than the size of us humans okay so that's the scale but what are we in terms of matter well we know that we're made up of protons and neutrons that bind together but they themselves are made up of fundamental constituents known as quarks so we're mostly made of up and down quarks but there are additional sort of heavier siblings of those guys as well now these combined in two nuclei here for example I'm showing the helium nucleus and then you have to add more charged particles to make stable elements so for example this is helium where I've taken two electrons and paired them up so this thing is electrically neutral and so these would be the the atoms that we observe and the electron just like the quarks also has sort of heavier siblings it also has a charge less cousin which I will discuss in very short detail later on now all of these particles interact with each other and these particles interact with each other via forces so we know of all these forces and we know of the most obvious one of course which is the the photon so the electromagnetic force the way that you are able to see me and then I am able to see you and that we're able to see stars is a result of exchanging photons that is the force carrier of the electromagnetic force it's also responsible for magnetism so if you were to take a big magnet next to this TV and destroy it that would be because of the exchange of photons the next force is the strong nuclear force and that's mediated by a particle known as the gluon so that proton and that Neutron that I showed you earlier is made up of those quarks and those quarks are bound together via this gluon they're exchanging gluons all the time to form this composite proton object then there's the weak nuclear force which is mediated by particles known as the W and Z bosons and the Zed was named because they thought that was the last particle they were going to discover yeah right so this one is responsible for for example radioactive decay so here you see a beta decay of a neutron into a proton electron and neutrino and this is what happens in radioactive processes all the time so of course the joke with the Z being last discovered particle is that in 2012 they discovered another particle which is another force carrier known as the Higgs boson and that was discovered at the Large Hadron Collider so the Higgs I want to explain in a little bit more detail and you can think of it as sort of being this big snow field with which particles can interact so the photon being massless you can think of as being like a downhill alpine skier it's you know Mikaela sheffrin going straight down that that slope extremely fast and not interacting with the snow field whatsoever that's because the photon is massless slightly heavier particles like the electron and the up quark interact with the Higgs boson a little bit quite weakly and so they're sort of more like a cross-country skier gliding across the snow in to acting with it just a little bit then heavier particles like for example the W and Z bosons interact quite a lot with the Higgs snowfield so that would be sort of like a guy with snowshoes who's sort of trotting through the snow making pretty good progress and finally the top quark which is the heaviest particle fundamental particle that we have discovered so far would be like some guy who did not plan at all took only as hiking boots and is finding it really hard going through that snow field okay so that's what we are so let's move forward to how did we get here and in particular the aspect of this question that I would like to discuss today is the history of the universe and that is cosmology that comes from a from Greek cosmos which is the universe and logy which is to do with the study of it so here is a time scale starting from just 10 to the minus 36 seconds after the Big Bang to now 14 Giga years later so we started with the Big Bang and this was followed by a period of very very rapid inflation so the universe came to be and then just blew up literally it inflated very very rapidly this was followed by a period where all of those fundamental particles were free and just moving around in what's known as the quark gluon plasma and this lasted for a whopping 10 to the minus 10 seconds after this the protons and the neutrons and also other particles known as maisons started to form and this period happened sort of 1 in 10,000 seconds after the Big Bang so things were starting to congeal things continue to congeal further and nuclei began to form about one second after the Big Bang so all of your hydrogen helium and lithium mostly comes from this era of what's known as Big Bang nucleosynthesis roughly one second after the Big Bang now we're getting to the meat of the things that I would really like to discuss in detail which happened 300,000 to 400,000 years after the Big Bang and that is this thing known as the Cosmic Microwave Background so the Cosmic Microwave Background I'll explain in a lot of detail but it's basically basically the most ancient light in the universe and I'll explain exactly what that means in just a moment this was followed by a period of structure formation which the Cosmic Microwave Background gave the seed for all of the structures like those galaxies and clusters of galaxies and filaments that I discussed to form over billions of years and finally we get to where we are today with pretty pictures of galaxies okay and so I would like to argue that in basically all of these stages dark matter and mattered okay but in particular I would like to discuss these last two stages of the Cosmic Microwave Background and the influence of dark matter on structure formation so as I just said the Cosmic Microwave Background is basically the most ancient light in the universe and it's light that started shining once it escaped from all of that matter about 300 to 400 thousand years after the Big Bang and as I mentioned earlier this provided the seeds for that eventual structure formation that grew into the vast structures that we observe today okay so what role does dark matter have to play in all of this story well before we actually answer that question we need to ask a different question so you see every question actually has the answer in the form of another question and this question is how do we actually observe things all right so you're observing me hopefully I'm observing you and we do this via lights so we're exchanging photons right where we're having a good time exchanging photons and this can happen either through something that's shining so for example a galaxy just emits lots and lots of photons and therefore you observe it because you can see all the lights coming off of it or it can be something like me or you the floor the table and it's reflecting or indeed a planet reflects light so we observe it because some very shiny object emits photons they bounce off of it and they come back and we observe those so there's lots of observational evidence in the form of light for dark matter the most famous one of course is that of rotation curves so this was due to pioneering work by Vera Rubin back in the 1970s and she was looking at galaxies and in particular tracking the motion of the stars and the galaxies and she was looking at stars in a galaxy not too dissimilar from our own and we're saying okay well what are the velocities of the stars in this galaxy as you get further away from the center of that galaxy and what she expected was something like this curve here but what she observed were these dots maybe not exactly these dots but dots like these that seem to indicate that there was much more matter in that galaxy that then what you could actually observe and there was just no way that the regular old matter was resulting in this what's called a velocity rotation curve of that galaxy and we know that because the velocity is related to the mass distribution so if you had the regular matter distributed in the regular way you would never get a curve that looked like this you also get additional evidence through light by looking at big clusters of galaxies and observing that again there seems to be a lot of matter missing from that galaxy from that galaxy cluster to explain why all of the different galaxies now not just stars are moving it's the speeds that they're moving and in fact it just so happens that in the 1930s a guy at Caltech Fritz Vicki had actually made this observation and had given the name Dark Matter to the thing that must have been there to explain these velocities but he thought it was just regular matter and it just wasn't shining for whatever reason or it's light was being absorbed he didn't make the connection that it might be a new fundamental type of matter okay so that's how we observe dark matter or rather don't observe dark matter via light how else do we observe objects well we use gravity and light paired up together so here we have a nice grid which is supposed to represent our space-time right so we live in space-time and we're going to make it two-dimensional and represent it as a grid we have galaxies and they're emitting photons and those photons are just streaming across that grid of space-time now if we took that grid Dhar galaxies but instead place the big heavy dark object in the middle of that space-time much like a rubber sheet if you put a big heavy object on it it would deform the rubber sheet so indeed a big heavy dark object deforms our rubber sheet of space-time and makes these wonky curves in our in our two dimensional space-time so now if we were to observe some distant galaxy which is on the other side of that big dark object from us and we observe the light being emitted from that galaxy or cluster or whatever it may be on the other side of that dark object that light gets deflected because it just follows the shortest path to us which because of the deformation of space-time actually is curved rather than straight and so this is the phenomenon known as gravitational lensing so if you take a dark object and you're a little bit off access from it you can get for example smearing so a star instead of looking like a a nice round point will actually get smeared out or you might get duplication or you might get the very fancy phenomenon known as an Einstein ring where it basically gets smeared all the way around the foreground object and so we can use gravitational lensing to infer where there is matter and then see whether that corresponds to actually observing matter or not so that brings me to the next piece of very strong evidence for dark matter which is the bullet cluster so this is an animation showing a simulation of what happened when two large clusters of galaxies collided so in blue is where lensing tells us most of the mass is where in red is where most of the light is shining in x-rays and what you observe is that the blue passed through very easily and didn't interact very much whereas the red slowed down and got sort of stuck in the middle and that's because regular matter interacts via all of those forces that we discussed earlier and they get sort of slowed down as they pass through each other whereas the dark matter isn't just not interacting and just passing straight through so what this looks like in reality is this so this is an of a tional picture of just the galaxies that we're observing in in the bullet cluster overlaid with the actual image in x-rays in pink here of where most of the luminous matter is the stuff that's actually actually emitting light and the the sort of gravitational map of where most of the matter is in blue and you see that they're not in the same place that can only be explained if there is dark matter in addition to regular matter that just is not interacting or interacting very very weakly as these clusters pass through each other the other way in which we're able to use gravity and light together to tell us about dark matter is actually from the Cosmic Microwave Background itself so that ancient light from the universe we've now been able to observe very very precisely using the planck satellite which was sent up by the european space agency so this is a map of the galaxy in the Cosmic Microwave Background this is actually a little bit untrue because if you were to just look at the map of the Cosmic Microwave Background what you would see is a uniform temperature of three Kelvin or minus 454 Fahrenheit and it would basically just look roughly green across this entire image so once you subtract that foreground and you subtract all the other four grounds that we know of what you're left is with these these small variations of temperatures so this is three plus or minus one part in a hundred thousand of three Kelvin so little blue spots a little bit colder than three kelvin little red spots are a little bit hotter than three Kelvin and this map actually contains a huge amounts of information way more than just being a pretty picture and indeed a lot of that information we've obtained in part thanks to the pioneering work of Jim Peebles in the 80s and later for which he was rewarded with a Nobel Prize this year so one cool thing about the Cosmic Microwave Background that I want to explain is that it's actually quantum mechanics in action so okay quantum mechanics sounds very potentially complicated but it also sounds cool so what do I mean by that well so now I've turned our space-time into a one-dimensional instead of that two-dimensional plane that I showed you earlier and for the most part it's flat assuming that there's no heavy objects deforming it however small quantum fluctuations in the early universe in fact during that period during that period of cosmic inflation that I referred to much earlier in the talk mean that if you were to blow up a part of your one dimensional space-time you would actually see this lots of small dips and mountains and dips and mountains of variations in that fundamental space-time itself and that's just due to quantum fluctuations so quantum mechanics tells us that it can't just be flat it has to have small variations and those small variations get blown up by cosmic inflation to turn into big dips and big mountains and of course in big mountains what of space-time you can think of this as like that deformation where that heavy object was sitting and so that is going to accumulate stuff and that stuff comes in the form of baryons of protons and neutrons and photons the carriers of light okay so it also is going to attract dark matter dark matter is going to sit in those troughs of deformed space-time and depending on the amount of dark matter and the size of the trough you will accumulate more or less and so this will result in observable differences in what the baryons do and the correlations in the temperature of that CMB map that I showed you that Cosmic Microwave Background map so if we go back to this map what our folks over a chi pack in the next building over do is they study this early light and compute for example temperature correlations so they take various different points on this map and they say okay what is the characteristic size of the variations of these temperatures and what they produce for us is a very nice plot but before I show you that plot I'm going to tell you what you would expect to see if you saw no dark matter so if there was no dark matter and everything was luminous matter this is what you would observe something that looks like this so most are the objects that you would see in the Cosmic Microwave Background would be worth roughly one degree in size so if you think about one degree that's maybe the size of a nearby planet in your in your telescope most and then there are other Peaks associated with other scales where there are smaller objects but they are generally less dominant than that first big peak so this is what you would observe if there was no dark matter but what they actually observe is the line that is underneath or really rather the the red dots that you see underneath there and you see in particular that okay I've normalized the first peak so that they look the same but the subsequent peaks look quite different and this is because of course we have dark matter and it's not just luminous matter in the universe and indeed from the data now removing that fake plot that I showed you earlier we can actually infer from the third peak the amount of dark matter that there is in the universe in combination with the second peak we can also determine how much regular matter there is in the universe or how much of you and I and stars and so on there is in the universe and that first very big peak actually tells us something about dark energy so I don't really want to get into dark energy during this talk because it's pretty tangential to what I want to say but if you have questions about it I'd be happy to answer them after the talk so people have studied in great detail what would happen to that plot that I showed you earlier if you were to vary different parameters in the universe and to try and understand what degeneracies there might be between the amount of dark matter or the amount of dark energy or the amount of regular matter or how curved the universe is and they can produce many other pretty pictures showing how if you introduce more or less matter you would get higher or lower Peaks or if you were to introduce more or less regular luminous matter that would also change the shape and so on and so forth you see that it's not super sensitive to dark energy so turning that plot back into the pretty pic of the colors this is what you would observe if you had different universes with different amounts of dark matter and regular matter in them so now I'm going to ask you as a pop quiz which one you think corresponds to our universe and if nobody answers I'm going to pick on somebody any B is our universe unfortunately B is not our universe a is our universe so a is ours B is almost the same as ours but with no dark matter C is if you took all of the dark matter and turned it into regular matter and DNA or other variations that I've put into this simulator where you can go and simulate your own universe now one thing to notice here is that our universe as we were mentioning earlier has various different objects so different hot and cold spots of different sizes they're sort of a big clump here of hot there's little clumps of hot over here big clumps of cold little clumps of cold etc you notice that that's quite different from the case where you would just have only matter instead of the dark matter where basically all of the clumps are roughly the same size and this will be important later when we discuss how this turns into structure formation so what does the data actually tell us in terms of numbers well it tells us that regular matter so you and I make up about 4% of the universe and of that most of it is actually not visible it's just gas in the interstellar medium very little is in stars a little bit less in neutrinos those sort of very light cousins of electrons that I was mentioning earlier even less in heavy elements like what is going into my computer or your phone so 0.03 percent of the universe is that 27% is actually dark matter so there's way way more dark matter than there is in regular matter in the universe and finally the rest of it is made up of that mysterious dark energy which I mentioned earlier so what you'll notice from this part of the pie chart which is the one that I'm particularly interested in is that we basically know nothing about what the matter in the universe actually is so 27 percent of the universe is a pretty good reason to stay up at night I would say okay so how does what does this have to do with cosmic structures in their formation well of course our deformations of space-time that accumulated dark matter allow us to track the the the evolution of these cosmic structures as a function of time so starting from that earliest light in the Cosmic Microwave Background we know where there were sort of over densities and under densities of stuff and therefore also of dark matter into which our baryons are going to fall and accumulate and that is exactly where the stars and the galaxies and the clusters are going to form so now I'm going to show a video showing an evolution of the evolution of such structures and you see at the very beginning so this is starting roughly half a billion years after the Big Bang in blue here is actually just dark matter so they haven't put in the stars yet you see that there's big clumps and little clumps much the same way that in the Cosmic Microwave Background we saw big clumps and little clumps and as the simulation keeps going and we're now sort of almost 2 billion years after the Big Bang they're going to add a new color and this is going to be the regular matter which you will see will end up being in exactly the same places as the dark matter so you can already start to see a little bit of it in this pink here and so if we just wait a little bit longer we should start to see more colors appearing it's also just a very pretty video to keep watching so here's the green this is all the stellar matter so all of the stars and the galaxies and here are some nice little animations of supernova explosions of stars going off and you see that those stars appeared in exactly the same places whereas where all the dark matter is so all of the structures that we observe in the galaxies actually tracked where the dark matter was way way earlier in the history of the universe so okay let's end the animation and move on to just the static picture so again a pop quiz which one of these do people think is a simulation and which one do people think is data you in the front row don't it ignore the colors which one do you think looks like simulation which one do you think looks like data the right one is data or simulation so this is actually exactly what someone said when I did the practice as well this is actually the simulation both of these are the simulation and these are actually the data but what you see is of course that our simulation involving just dark matter so there's no matter in this whatsoever very well replicates exactly what we observe in the galaxies and in the the universe that we are able to observe and so there are these beautiful structures known as the Sloan Great War and the CFA two Great War of these filaments of galaxies clusters in the universe and that looks exactly like what we get out of simulations using just dark matter no regular matter whatsoever if we had actually removed some dark matter we would get something very different so with dark matter here again is simulation you observe this sort of structure up here in the top right hand corner so you see that there's those big clumps and then there's smaller clumps and they're all sort of of different sizes if you were to remove some dark matter you start to get more or less similar sized clumps and now finally with no dark matter you observe that all of the clumps are about the same size so that is to be compared with what we saw earlier in the CMB pictures where again you had these hierarchies in the structures of the clumps and indeed you see that the CMB clumping here in the absence of dark matter looks quite similar to the clumping of late time structures that we observe and that's just because the matter tracks where the dark matter is okay so having gone through a lot of evidence for the dark matter and I have hopefully convinced you that Dark Matter exists I now I want to discuss what the dark matter actually is so it made up of and that is the main question that I try to answer in my research so let's go back and look at those particles that we discussed way at the very beginning and say okay well one by one can any of these actually be dark matter so the first thing is that dark matter is dark therefore it doesn't really interact with photons it might indeed not interact with photons at all and so it should have no charge under electromagnetism or at least very very small charge as you will see you later so that rules out a whole bunch of particles in the standard model as being possible dark matter candidates additionally we know and indeed I showed you the bullet cluster that Dark Matter doesn't interact strongly with other dark matter right it just passed straight through so therefore it shouldn't also be interacting via the strong force so that rules out the gluon and it again rules out all of these quarks that had already been ruled out to start with now you might notice that there are those cousins of the electron the neutrinos that are still there in the game unfortunately they're not heavy enough to make up dark matter and so those also get ruled out and you notice that there's still two guys left but these guys actually are not stable so dark matter existed way in the early universe it still exists today these particles decay extremely quickly so they don't exist for very long and therefore they can't be dark matter either so there's nothing in the standard model that can be dark matter so okay what are the candidates well if you've been to some of these public lectures before you've probably heard of some of these candidates in the past so here is another scale now in terms of mass in these fancy electron volt units so let's start with this range over here the proton weighs 1 GeV in this unit scale and so it's it's sort of roughly over here and so we have proton like dark matter candidates known as wimps which live roughly in that range and that's something that many people have gone after experimentally experiments here for example like super CDMS but also at the LHC they go after these wimp dark matter candidates now way up here on this broken scale you also have P BHS and here you have electrons so if we take the electron as its and its mass of roughly one in 2000 the mass of the proton we can also find dark matter candidates living around that mass range as well and very creatively we basically just took the neutrinos made new ones made them heavier and said okay that could be dark matter so those guys would sit roughly in this mass range over here between sort of one millionth and one thousandth of the mass of a proton if we were to take stellar sized objects and think about those as being possibly Dark Matter candidates so here around the mass of 30 times the mass of the Sun we could have primordial black holes as dark matter so this would be black holes formed by those quantum fluctuations in that early universe space-time that I showed you way way back in a few earlier slides and those could have developed into what we call primordial black holes and those could also be dark matter and those would live in a completely different mass range from the rest of these candidates then there's another Dark Matter candidate which is particularly well loved which is the acción and so here there was an image of a logo of an experiment that I will go into a little bit more detail on later on but this is where an action could live and this is now way way way lighter than the mass of a proton so these were the old candidates but there are actually new ideas about what dark matter could be and those are the ones that I actually want to focus on and in particular I want to question that assumption that dark matter couldn't experience electromagnetic interactions that it could not talk to photons it turns out actually that as long as you dial the charge of this dark matter way way down it can still be dark matter so the charge of this particle which I'm gonna label Epsilon would have to be much much smaller than the charge of an electron and these could be produced for example in the early universe via collisions of electrons and positrons that would just produce this Millie charge dark matter in collisions occurring many many times early in the universe so given this potentially new Dark Matter candidate you might ask okay well how are we going to go about observing it discovering it but we can borrow intuition from the known charged particles so again let's take our positrons so our positively charged electrons and think about what they do if we were to stick a big magnet next to them so if I took a big magnet and placed it in the middle of this room and then I fired positrons at it they would get deflected so if you remember your right hand rule from electromagnetism 101 you can see that they're going to go either towards or away from the board depending on the direction of the magnetic field and the charge of the particle and indeed this is the same phenomenon that they use just over in LCLs where they use magnets to accelerate electrons and positrons so okay we have this intuition that we know how to deflect charged particles so maybe we can deflect these Milli charged particles as well so let's assume that now we have a bath of dark matter all around us which we do because our galaxy lives in one of these clumps of dark matter and let's assume that we can somehow get that dark matter to flow through our magnet and therefore get deflected now get to what we're gonna do with that deflection in just a moment so that dark matter is gonna flow through our big deflecting magnet and it's going to move apart but you might say hang on a minute how do you get the dark matter to flow through your deflector in the first place and so then how are you going to be able to do that and then detect the deflection so that's where streams come into play and where the general flow comes from so here's our galaxy sitting in our big over dense clump of dark matter that formed very very early in the universe and that's sort of a slice on the side but if we were to look at it in slightly more 3d ish picture or at least as 3d as I could make it this is what we would see so we would see a disc of the galaxy that is encased in a sphere of dark matter so the galaxy gets flattened out into a disc because it has these interactions that allow it to sort of flatten out like a pancake so if you're imagining you're making a pizza and you twirl around the the dough it's going to flatten out into a disc that is exactly what happens with stars as well they 12 round and they flatten out into these beautiful structures with the spiral arms the dark matter on the other hand doesn't dwell around and flatten out it stays in that initial sphere and so that dark matter is just sit is just there as a sphere encapsulating our galaxies and we're sitting nicely on one of these spiral arms trying to observe it so what does this have to do with being able to chuck dark matter through our deflector well we're spinning right so the galaxy is spinning around its central axis and we're nicely sitting on this spiral arm and so that means we're also spinning around the center of the galaxy through the dark matter and we're actually spinning through all of this dark matter at roughly a hundred kilometers per second that's pretty damn fast so we're going to be able to leverage of the fact that the dark matter is not really moving relative to us but we're certainly moving relative to the dark matter and in fact was zipping through it all the time so that's the dark matter that's just sitting there in our spherical we call halo around our galaxy but there's additional structures of dark matter that came about due to late time structure formation so structures that formed much much closer to the time that we are now rather than billions of years ago so for example you can get structures forming due to smaller structures like here a dwarf galaxy that came in to contact with our galaxy collided with it and got tidally stripped so here's our us sitting on the spiral arm of our galaxy and let's see what happens if we chuck a dwarf into the galaxy so it interacts with our galaxy and it gets tidally stripped it gets stretched out and smear out across the sky and so you observe this stream of particles with its own accompanying dark matter which I've tried to show here in a slightly different shade of grey that it's also getting smeared out indeed we observe many of these streams and here is a pretty picture of one known with the fantastic name of GD 1 so here is an observation of a bunch of stars that came from one of these collisions of a dwarf with our galaxy that got tidally stripped and all of these stars got smeared out across the sky so I chose this one to show you because there's other interesting things that might have to do with dark mats are involved with it but I just wanted to motivate that there are also these streams of dark matter that are also moving through the galaxy at a very large speed and indeed there's actually a stream very near to us so now we return to that picture of our deflecting magnet and our dark matter that has a very very small charge with electromagnetism and now we flip that picture around right we're no longer chucking the dark matter into our deflector but rather because we on the earth in the solar system are orbiting around the center of our galaxy we are actually flowing through the dark matter itself and thereby deflecting it and so I've said all of this about deflecting dark matter and gone through this rigmarole of trying to explain why there is actually a relative motion of us and the dark matter but what do we want to do with this well when you separate charges what you actually get is light being emitted by these charges so charges don't like to be separated apart if you if you separate out an electron and a positron it tries to come back together because opposites attract and they do that by exchanging photons so that's exactly the same thing that would happen if you were to separate Milly charged dark matter so if you separate it out they try to come back together and they do this by emitting these photons and so we as the cannae observer will try and observe the effect of this but so what we did in a recent paper and here I'll show you a technical plot which I you shouldn't try to understand what is going on here except for the colors so that here is our central deflector which I pictorially represented with that magnet flowing through the Dark Matter and you see that as it flows through it induces sort of these separations of charges that will shine right so here you can see that they're in the wake of that deflector you get a big effect shown by this deep red color and it dies off as you get further away from the deflector and so what we're going to try and do is we're going to try and observe that separation of these Dark Matter charges so in practice what we're going to do is not use a magnet but rather use a big sphere so a big sphere of charge let's imagine we've taken a sphere with positive charges on the outside and negative charges on the inside and we're going to flow through that through our dark matter so now instead of that separation I showed you earlier the negative charge in the middle attracted all of the positively charged dark matter particles and the positive charges on the outside attracted all of the negatively charged particles again inducing this separation of the dark matter charges that are exchanging these photons and so what we're going to do with this is actually flip the sign of that sphere that I showed you a hundred thousand times per second and this will induce basically a flipping of the sign of the charges that are being separated from plus to minus the plus to minus and this will induce a large exchange of photons well not very large but large enough that we can observe it using an ultra sensitive antenna so this ultra sensitive antenna would be hooked up to a detector with the fantastic name of squid so a squid stands for superconducting quantum interference device and it's basically a device that measures very very small magnetic fields so these photons remember are associated with electromagnetism they are the particle that induced the exchange of the electromagnetic force and so that means that there's also magnetic and indeed electric fields associated with these photons being exchanged by those Miller charged particles and those can be measured using this ultra sensitive squid so it's not your garden-variety antenna but rather it's something that looks a little bit more like this although bigger so this is the prototype for the Dark Matter radio experiment which I flashed very early on this logo here when I was discussing other Dark Matter candidates it's actually looking for accion's but what we realize is that actually it could be used as the detector for those deflections of those Milli charged dark matter particles that I was telling you about so if we now look at the experimental prospects we see that we have this complicated looking plot where here along the bottom axis I'm showing you the mass of our Dark Matter as a function of the mass of the proton so at the top here is one-tenth of the mass of the proton and at the bottom here is one in a billion times the mass of the proton so these dark matter particles would be very very light and here we see the charge compared with the charge of an electron and we're starting at roughly one in a billion and going down to one in a billion squared so one in a billion billion times the charge of a an electron so it's effectively not charged in terms of observing it which is why it remains dark as we try to observe galaxies and clusters of galaxies but it would still interact with those photons the gray regions are regions that are already ruled out by other experiments or observations and this sort of bluish region is a region that's particularly interesting theoretically just for reference if we were to take an electron and put its mass on this scale it would sit roughly here and this is what you could do if you took the dark matter radio experiment and just put it in the wake of that deflecting electric field that doesn't look to remember Slee great but that's because the Dark Matter ray do is actually optimized for looking for magnetic fields if instead we build a new dark matter radio to look for electric fields now we're talking now we can actually probe a large chunk of the parameter space of this Dark Matter candidate and indeed a lot of the region that is theoretically very interesting but also these other regions where people haven't really thought too hard about what dark matter could be living there so this is something that hopefully you should stay tuned will appear at SLAC in the future so that brings me unfortunately to the conclusions which are where are we with these fundamental questions that we discussed right at the very beginning so I would like to argue that the question of what are we has been fairly conclusively answered and indeed many of the answers came through experiments here at SLAC like the discoveries of the Sun quark and the Tau lepton I will also argue that we know at least in the Cosmo jabal sense how we got here so we know that all of the forces that we discussed in the early part of the talk played an important role we also know that these dark matter clumps in these gravitational wells also played an important role in where we've got here in this galaxy in this cluster of galaxies I think we obtained a partial answer to the question why are we here and hopefully you've all come away with the conclusion that dark matter certainly played a crucial role in explaining why we got to this point here today although of course the the grander question of why we are here I'm not going to touch with a ten-foot barge pole where are we going well that one gets sort of a red tick because there's an idea of where to go but we haven't really started so we have this idea for detecting this new Dark Matter candidate which would have a very very small electric charge but the next steps are going to be difficult ones we have to make the experiment and then we have to take a lot of data and get a lot of sensitivity but hopefully this is something that will be coming to a slack near you in the near future so that concludes everything that I had to say thank you very much for your attention [Applause] [Music] great all right we have time for a few questions so uh what we will do is notice that there are microphones at the tables if you're recognized please push the button little bar on the microphone and concerns speaking so maybe we can start with the gentleman and a gray half go ahead we're on the electromagnetic spectrum are these photons are they dark photons or regular purposes so they're regular photons and indeed the the signal would be at this frequency so it would be at 100 kilohertz so that's a little bit below radio in the spectrum so so dark matter radio is a radio in some sense but ours is even more of a radio very sensitive radio go ahead can you please talk more about the experimentation that's to come you know was a time frame how difficult isn't is it going to be yes the time frame is that we really just put out this paper in August and it usually takes sort of order of years to actually go from a theoretical paper or an idea of how to detect dark matter to an implementation the next steps are to find an experimental group that are willing to spend five years of their lives doing the experiment and in that regard you know we've already made some approaches both here at SLAC and also at other labs and then of course there's the eternal funding question but we hope that all of these can be resolved and we can get moving as soon as possible so hopefully and if you ask me that question again in 10 years we will either have made a discovery or we will have placed a new constraint on that part that I showed you right go ahead I've ever been to any like recent like sci-fi movie that you dark matter as a subject or like plot of the you know film that is somewhat accurate so there's a lot of movies that have dark matter in them probably most of them are very inaccurate I haven't watched all of them so I specifically which ones would be more or less accurate but my guess is that most of them are very inaccurate all right lady in a blue shirt please can you turn yourself please thank you I wanted to know you showed the prototype of the detector is it in process of being built and I had a second question about the military dark matter are those little particles theoretical and you're looking for them or do have you seen that those Miller charged particles yet so we have not seen Millie charged particles yet so I'm gonna answer your questions back to front the prototype which of course I skipped back which I will show again here is one the the Dark Matter radio experimentalists have already built so that's why there's a photo of it and actually I fail to mention so thank you for drawing our attention back to this this dark matter radio is an experiment being done here at SLAC so this was a nice way of trying to repurpose something that was already ongoing at SLAC and and use it for a different purpose so this is their prototype and they've actually received funding now to to do R&D so research for a one meter cubed guy so that would be sort of a little bit smaller than this podium for detecting accion's and you could also use that for our purposes but as I showed you the constraint or the detection prospects for that are not be great but actually the the sort of stronger line that I showed you would also be in an experimental apparatus about the size of this podium so it doesn't require these big underground mines or the LHC that's huge right it was ten thousand times bigger than us in length to detect to possibly detect dark matter you could do it in sort of this size that's here as eros and Costas well right many fewer zeros in cost yes right okay gentlemen I'd behind the lady yeah go ahead you indeed so your description of the experimental approach the Year proposal calls to mind old experiments trying to discover the ether and I wonder if if this is one of a whole series experience people are considering that are like the experiments that were done then where perhaps you might fire some streams of matter in different directions looking to see how they are deflected by the rush of dark matter as we're passing through it yes there's actually a lot of different ways you can leverage our flow through dark matter to try and discover it you can also leverage the fact that we flow through many things to discover it's in fact gravitational waves were discovered using a very similar apparatus to the one that was attempted to do attempting to discover the ether right so this principle of just using the deflection of regular matter through some unobserved otherwise unobserved matter is one that it's it's fairly basic right you try and move something if it moves you observe that movement then you've detected something right so yes in that sense it's it's sort of akin to the ether but it's just a basic principle that we use in detection strategies all the time there are other experiments similar to actually gravitational wave experiments that look for other dark matter candidates so using these interfere metric devices as then so if you want to ask me more questions about that later I can certainly tell you more all right maybe we can go to the jump line here and then so go ahead hi I'm you you mentioned the primordial black holes as a possible contender is that still the case or it really depends on who you ask so I don't want to bash primordial black holes and I also don't necessarily want to promote them the reason why I showed them was to show just a wide range of masses that could be that could have Dark Matter candidates hiding in there my personal feeling is that primordial black holes are probably not looking so good right now things can always change and indeed in the recent past constraints on primordial black holes have been revisited and have gotten less tight than they were previously in certain regions of the mass range that they could exist in so things do tend to move although more often than not they move in the worse off not better off direction I think there was one more question there okay go ahead in the back um what happens if one of these particles falls into a black hole or gets can they be captured by planetary bodies is there like additional mass associated with astronomical objects from captured dark matter so I think there's two parts to your question one is can these things be captured by big astronomical objects and the answer is of course yes right so if dark matter interacts with nothing except gravity it would already accumulate right as we discussed it sort of accumulates in these regions of under dense space-time and so you would actually expect certainly that dark matter would accumulate in the Centers of stars or even in the center of the earth and there are lots of people including some of my colleagues here at slack in the theory group and probably also at Cape I could have considered the signatures of accumulation of dark matter at the center of celestial bodies and you can also produce them in celestial bodies so in fact one of the the constraints here this big region here that is grayed out comes from the non observation of the production of these particles in stars so stars would just be able to produce them regardless of whether they were dark matter or not they would just be emitted and stars would lose energy and so the fact that there are stars that are still extant now that could have been emitting these stars and these particles and therefore losing energy places this constraint here so yeah they certainly do get absorbed in there's certainly lots and lots of signatures associated with that absorption and emission in astronomical objects I have a question right here observations from the 19th century that couldn't be explained by Newtonian cosmology were eventually explained with relativity is there any prospect for there being an update to theory that solves the unexplained observations but without dark matter or dark energy for that matter is that possible so part of the reason why in my motivation for dark matter I spent so much time on the Cosmic Microwave Background and on structure formation is because unlike the rotation curves these cannot be explained using modified gravity so modified gravity supporters always point to rotation curves and sometimes they can also explain the bullet cluster they say oh look if we just modify gravity on certain length scales you can explain all of this very nicely but they cannot explain the Cosmic Microwave Background nor can they explain the structure formation that we observe simultaneously with everything else so I would say with almost 100% certainty that modified Newtonian dynamics is not going to be the solution to this unfortunately and in fact many people who work on modified Newtonian dynamics today include some amount of dark matter to try and explain the Cosmic Microwave Background so you're almost back where you started all right oh yeah go ahead experimentally how did you come up with 100 kilohertz as a sweet spot it seems like a noisy place to be trying to see things very good so the size of the signal grew with frequency so we wanted to go to higher frequencies as much as possible but there's actually a cut-off at about a megahertz so if you oscillated the electric field at a megahertz the dark matter wouldn't pass through the detector before you flip the sign and see we'd actually start to reduce the size of the signal so that was the the upper limit and then just to be on the safe side we pick a hundred kilohertz but we'll try and push that as close to the the upper limit as possible all right I see one more question there go ahead you know if you think about the sphere of dark matter around our galaxy and this collapsed into this sphere seems like by conservation of angular momentum it would have be rotating or very likely be rotating and shouldn't it form more of a dish shape than a sphere because of that so the disc shape actually occurs because there's additional forces that allow for dissipation of energy before you dissipate the angular momentum so so in that analogy of the the pizza although making his pizza and spinning around the dough there there's just one force at play right it's just the fact that it just gets stretched out because of the rotation here there's not just gravity but there's also electromagnetism in particular and electromagnetism allows regular matter to dissipate a lot of that energy before it dissipates the angular momentum so it's left with most of that energy in the form of angular momentum and just spins out and flattens out dark matter because it only interacts gravitationally or at least if it has interactions they're extremely small right then it's not able to lose that energy so efficiently early on and so it doesn't lose it loses most of its energy just through gravitational dissipation which tends not to flatten things out actually it just sort of if you think about a superposition of discs if you add enough of them together they're just gonna form a sphere all right I don't see any more hands-on work is the Milli charged dark matter your idea no okay how long ago did it what's it proposed ah that's a good question so there are certainly papers from the early 90s where people discuss two million our dark matter but there it was actually not Millie charged it was charges of order one but it was just much much heavier and so then that's sort of morphed continuously over time into this region of mass and charge space that I showed you here most ideas and don't come out of vacuum right they sort of start as a different idea that is maybe roughly the same but somewhat continuously deformable into a new idea and that's generally how things happen all right maybe at this point we should finish our formal questions and there will be probably a bit of time for us to chat outside I think we have to vacate this room pretty soon so that's one of the reasons so let's thank Sebastian again for a very exciting job thank you [Music] you

Channel: SLAC National Accelerator Laboratory

Views: 8,367

Rating: 4.7714286 out of 5

Keywords: SLAC National Accelerator Laboratory, SLAC, Stanford University, Science, Dark Matter, Physics, Astrophysics, Particle Physics, Gravity

Id: -2OvHQ8qVOo

Length: 66min 35sec (3995 seconds)

Published: Wed Dec 04 2019