The Physics of Dark Matter - Lecture 1

all right good morning so the focus for today is going to be on physics beyond the standard model and in particular I'm going to take the angle of focusing on dark matter because arguably that's the most important piece of evidence we have that the standard model isn't complete so what we're going to do over the series of today's three lectures is to start off by kind of reviewing the evidence that we have for dark matter then we'll move into discussing what types of guesses we have for the masses of dark matter and what types of particles it can be and then in the the third part we're going to discuss how we actually search for dark matter and kind of what the current status is of these experiments the lectures today are going to be hopefully somewhat interactive so make sure that you're kind of sitting next to I think everybody's kind of clump together but if you're finding yourself kind of far off on the side you might want to move closer towards the center or not on your preference it's also going to be a blackboard talk so you might want to make sure you have a good view of the blackboard okay so first thing is how we know that Dark Matter exists most of the evidence that we have is coming from Astrophysical observations so one thing one theme that you'll kind of end up seeing over the course of today's lectures is we're going to veer back and forth between particle physics and astrophysics because when it comes to dark matter and thinking about what it can be and how we can observe it the two become really fundamentally linked together so when we we look out at the Milky Way at our own galaxy we need to ask ourselves what are the you know where can we look to to find evidence for dark matter so to start off let's just kind of make a map of what there is and what we know exists so obviously none of this is going to be drawn to scale but all very schematic so we have the Galactic disk which for scale is on the order of ten kiloparsecs in radius can you see this or should I be drawing bigger it's fine okay good and height is roughly half a kilo parsec so most of the disk is comprised of stars and in particular there is about 10 to the 11 stars in this in the disk with the total mass of roughly 5 times 10 to the 10 solar masses so the unit m with a circle and a dot in it I'm going to denotes one well one solar mass so this is essentially five times ten to the ten Suns so this is about the number of stars we observe in the disk at the center there's a black hole so and that's roughly 10 to the 6 solar masses and what else do we have we have gas the gas is spread out along the disk consists primarily of molecular and gaseous hydrogen and it's about in terms of total mass 10% of the mass that's in stars so this is everything that's observable in our galaxy and if we look at all of these components in a square you know what can we use to search for dark matter it turns out that stars provide the best clue and the reason that stars provide the best clue is that stars are essentially two first approximation non collisional which means that their interactions are pretty much governed solely by gravity and that's you know about what weeks I mean that's essentially when we expect dark matter to to be as well you know with its interactions governed solely by gravity so to kind of kick things off and to show this explicitly I want you guys to do just a brief couple minute exercise to estimate essentially what the time is in years between collisions of stars in the Galactic disk so for this exercise you can assume you know that we have everything I've written down here but also that just let see that every star has a radius that's equivalent to the sun's radius and that every star has a random velocity of 50 kilometers per second so take a couple minutes feel free to chat amongst yourselves or not as you wish and using what I've written up on the board just estimate the amount of time between collisions of stars in the Milky Way's disk it'll maybe take another minute or so all right so um what we're going to want in order to be able to do this is essentially we just need the mean free path for the Stars which the first approximation is just one over the number density of the stars times the cross-section for their interaction and then the collision time or the time between collisions is then roughly just going to be the mean free path divided by the velocity of the stars so I'll just write this out so we're very clear about this but the the number density is going to be the 10 to the 11 stars in the disk times the volume of the disk and this comes out to be roughly 0.6 inverse cubic parsecs the cross-section is roughly pi times the radius of the stars squared which is approximately 10 to the minus 15 parsecs squared and then the time between collisions is going to be lambda over the which if you substitute in the numbers you get 10 to the 21 years so the time between collisions of stars in the Galactic disk is way longer than even just the age of the universe right so so from this simple exercise we can see that stars in the disk are pretty much governed solely by their gravitational interactions not by the collisions between them and as a result they end up serving as very good tracers for dark matter because when we talk about dark matter we're talking about matter that does not collide very much with itself that its dynamics or governed primarily by gravity so this means that we can look to the motion of stars in galaxies and from that try to infer what's going on with the dark matter that's there and from exercises such as this we've gotten the strongest pieces evidence for dark matter now in particular rotation curves of stars so this is what I want to discuss next the evidence for rotation curves or the best measurements for rotation curves came in the 1970s and then have continued to improve sense but it was really the 1970s that the measurements became robust enough that people were able to trust the inferences that you can make from them and so what we're doing with rotation curves is looking at this motion of stars about the center of a galaxy so if I put a star here I'm looking at its rotational velocity around the center of the galaxy and then I do this as I move further and further away so I can determine what the circular velocity is as a function of radius from the center of the galaxy now once I get out beyond the disk the radius of the disk then we know just from Gauss's law that the the circular velocity should be dominated by all the mass that's inside which if we're just looking at the visible matter should just be the the total mass of the that we can observe with stars in the disk and so from Newton we see we just know that circular velocity is the gravitational constant times the mass that's enclosed divided by the radius so once I'm outside once I'm like sort of sampling the motion of stars beyond the edge of the the disc then I would expect something like like that I'm sorry where I'm just being dominated by the mass that's enclosed which is just the mass of the disc if that's if that was all the mass that existed so this means that if I make a plot of the circular velocity as a function of radius if Newtonian you know if if the mass in the disc was what was dominating then Newtonian gravity tells me that the circular velocity should be falling off as 1 over R square root of R but that's not actually what's observed what's observed is flattening of this velocity so the first measurements were done with M 31 or the Andromeda galaxy it's our nearest neighbor and the measurements by Rubin and Ford which came in the nineteen seventies and then were followed up by measurements by Whitehurst were essentially sampling this rotation curve in M 31 out in this region here where you know they could see with pretty good accuracy that the data showed that the velocity were flattening out and we're not actually falling as one over square root of R so this flattening of the rotation curve tells us a few things we can interpret it one way is that we have some additional mass that's there so that even as we move beyond the visible matter outside of the Galactic disk where we're actually we're being dominated by invisible matter that's there that we can't observe that's actually just governing the dynamics of the stars out at these really large radii so one interpretation is that there's this new matter that's there it's invisible to us so we'll call it dark matter another interpretation is that maybe we shouldn't be using Newtonian gravity and maybe this is what needs to be modified now to date these are so these types of models are called modified Newtonian gravity or mom'd to date these types of models haven't really been mostly phenomenological and and haven't been sort of absorbed into a full cosmological picture and so the counting the dominant hypothesis is that what's there is a a new matter particle and that this matter particle has to be neutral otherwise we would observed it because it would have emitted photons and the only possible neutral particle in the standard model is neutrinos but neutrinos actually don't give us enough of the density so from that we infer that there needs to be some new neutral stable particle beyond what's predicted in the standard model sorry oh good yeah so I should note that there's so there's lecture notes that are all online so everything that I drop on the board that super schematic you can find detailed version of in these notes but yeah so let's see this is roughly 220 kilometers a second this is for the measurements in m31 and the data is going out beyond roughly and kiloparsec but they have the data only well in these first measurements the data was going out to roughly 30 kiloparsec so we observed the phat the flattening in a particular range of radius but not beyond which is important because actually we we expect that something different should be happening beyond that point all right so we're gonna take the first interpretation and run with it in the course of these lectures and what I want to do now for the first part is see how just how much we can actually infer about dark matter from this simple picture of the rotation curve and it turns out we could actually do quite a bit at the level of like told the level approximation so that's what we're going to do now we're going to get a sense of just how much dark matter there is and how its distributed relative to the Galactic disk okay so the fact that we observed that the rotation curve flattens means that the mass that's enclosed in that energy range where the the velocity is flat this has to scale as R right so that I get a cancellation between the R and the numerator and the radius in the denominator so this cancellation would then be able to explain the flattening if I take that the mass scales as radius then I can write down how the mass density should scale so the mass density is just going to be the mass of the Dark Matter divided by volume the mass I'm going to say scales as R and the volume is going to scale as our cute there's an important assumption that's being made right in that step there so I want to just make sure to delineate it which is that colored chalk there we go which is right here what I've assumed there is that the dark matter is distributed in a big sphere and not in a plane that is making the assumption that the dark matter is surrounding the disc and hasn't actually collapsed to form a disc the reason why I can make that assumption is the following so I've made you the fact that we can't observe dark matter means that it's not interacting very strongly with the matter that we do see as a result we don't think it should dissipate energy very much that's in contrast to the baryon that end up forming the disc barians interact strongly with each other so they can dissipate energy and in that process they can collapse to form a disc but because the dark matter is non dissipative it's not going to collapse to form a disc and so it's just going to sort of live in this big what's called a halo around our galaxy and the reason we can say it's in this halo is because it's not dissipating energy in the same way that the very arms are so if I make that assumption here and then that tells me that the mass density scales as 1 over R squared for the dark matter so just how big is this halo them relative to our disk let's get a sense of the size scales here so the total mass of the halo and again this is super schematic but it'll give us the sense of order of magnitude is going to be the integral of the mass density over the volume and we're integrating from zero to roughly the edge of this dark matter halo we know some of these numbers from experiments so we know roughly from looking at measuring the dynamical motions of stars again all of this information is coming from stars that the mass of the halo is approximately 10 to the 12 solar masses this is the mass of our halo so the milky way's halo we also know that the local density of dark matter so by local I mean the dark matter that's very close to where the earth is the local density of dark matter is roughly 0.3 kV per centimeter cubed now given these two inputs we can solve this and get an estimate for what the radius of the halo is and if we do that we find that the Dark Matter halo in our galaxy has a radius that's roughly 100 kiloparsec so compare that to for a sense of scale to the disk so what I've drawn here is the halo is way not in proportion right this is 10 kilo parsec from our very simple estimate we see that the dark made of dark matter halo extends way out much much further than that right almost 10 times the radius of what's in the disk so what we're seeing is really only a really tiny fraction tiny tiny fraction of what's there oh I forgot to draw on here where we are so we live about eight kiloparsecs from the center so we're close to the outer edge of the disk oh yeah that's neat so the the way we know that is by measuring the motions of the stars near us so if this is the Sun here there's measurements that are done looking at the relative velocities of stars moving in and out of the Galactic disk near us and based on that we can infer how much mass density there is near us so that's how we get estimates on what the local Dark Matter density is I should mention that there is a big uncertainty on this number you know it's roughly a factor of two uncertainty on this as the measurements continue to improve that number is going to you know get better but yeah it can go up as high as so there's some estimates that that pin it as high as 0.6 so this gives us an estimate of the halo mass let's get an estimate for how fast the Dark Matter is moving in the halo so to estimate the average velocity we're going to use just the virial theorem so that tells us that the average velocity is G times the mass of the halo divided by the radius of the halo and we got these numbers now so mass of the halo roughly 10 to the 12 solar masses and then approximate order of magnitude hundred kiloparsec radius if you plug in these numbers you find that the average velocity of dark matter in the Milky Way is 200 kilometers a second so these particles are moving really slowly in in the Milky Way I'm not we're talking non relativistic speeds and the fact that this isn't that the velocities are nonrelativistic is actually going to play a really important part later on when we come back to how we can actually look for dark matter it it sort of changes the phenomenology and in some cases make certain calculations manageable versus not being manageable if the dark matter was moving at relativistic speeds as opposed to non relativistic speeds yes sorry I say to say that one more time oh why this oh good okay yeah so I should be clear about what's coming from data and what's coming from theorist approximation so this is a number that's inferred from Veda and this is also a number that's inferred from data both coming from measurements of stellar kinematics so again this is why we started off with that approximation about the collisional time between stars because so much is done looking at the motions of stars and galaxies and based on the motions of stars inferring things like what the total masses of the dark matter and also what the local dark matter density is so the star service tracers and based on measurements of their motion we can make these kinds of inferences here Oh for for this yeah yeah no it's it's everything it's so most of the stars are in the disk but we also do end up with stars far away they're just they don't there's not as many as there are on the disk but you do have stars that are also in the halo so measurements of their velocities but then in addition you're right also if we look at orbits of satellite galaxies that knot also affects the determination of that number yeah so satellite galaxies is galaxies that are in orbit around our Milky Way so our Milky Way is some large 10 to the 12 solar mass halo but we can also have small little galaxies that are orbiting around us of where these small galaxies can be you know 10 to the 10 10 to the 9 solar masses so that's what I mean when I say satellite galaxies oh yeah so this is a very naive way of doing the estimate so and it sort of makes this inference that there's kind of a hard cutoff in in the Dark Matter distribution and that's not the case so it's a much more subtle point to actually you know to discuss how you actually define what the radius of a halo is please technically it's just it's all the dark matter particles that are bound to the say the center here of the galaxies so and that can extend out very far out and so there's different measures that people use for for what the actual halo mass is so sometimes it's you know the very old mass so it's all the mass within the virial radius there's there's different yeah so there's different ways of defining the mass and the radius and I thought was something I kind of wanted to avoid because it sort of depends on the application so this is like the simplest possible thing but yeah you're right and you wouldn't want to think of it as sort of a hard cutoff that's because it's just sort of the density falls off as a function of radius and there's no reason for it to be cut off oh that the that the density falls off oh these so the the only thing that we can say is that the density is falling off roughly as 1 over R squared in this region we don't know what's going on outside of what's been measured with rotation curves but we know that it can't be flat forever because if you were to integrate this to get the total mass it would blow up if it were to extend out like this forever so yeah so this really only applies in this region where we've actually been able to do the measurements no no no how far do you have to go for what oh that for so for those kinds of measurements we'd be we'd need to go to we look at large-scale yeah yeah so definitely not on not not on these scale so but this is very local in comparison yeah good so let's see what to cover next so I just want to make a few points before moving on to estimates of the actual masses of the dark matter particles that could be constituting this dark matter so like I've said these estimates that I've done up here are just very tilde level approximations and and just based off of the rotation curve measurements if you want to do something that's more realistic you need to turn to numerical simulation and the reason is that you need to be able to account for the gravitational interactions between all of the different galaxies and all of the Dark Matter halos between those galaxies so what the simulations do is they they start off with a certain set of initial conditions and then they watch as the Dark Matter halos kind of Bro with time and merge together so it's a it's an embodied gravitational problem and as a result we can't actually do that analytically I need to rely on simulations to get the answers when we do you know do these simulations we can look at what comes out for the Dark Matter density distribution and what we find is that our estimates here are actually not too far off but they're also not exactly exact answers so for example so the results from n-body simulations that only include dark matter and this is important for reasons that I will say in a second but the results from these n-body simulations suggests that the dark matter density is given by what's called an NFW distribution so that stands for Navarro Frank white which is actually double following power law where RS is the scale radius but for our purposes you can think of it just as some numbers so simulation suggests that this dark matter density where this is log on a log-log plot looks like this so essentially it has some slope here at small radiuses and then at large radius described by another slope so an inner and outer slope if you were to go through the literature there's a lot of different ways that you know other different kinds of profiles that people get from these simulations the nfw one is the one that's most commonly used in the literature which is why I've written it up on the board but the important thing to note from here is that our one over R squared estimate for what this mass density is only applies I guess for mfw around well I don't want to quote the number right off the top of my head but only applies it essentially one point where you have a slope that's roughly one over R squared but yeah so for for mfw the inner slope is roughly R to the minus 1 and the outer one is roughly R to the minus 3 there's a lot of debate as to whether or not this is the correct football and the reason is that the simulations that have been done that have obtained these kinds of profiles only include dark matter particles they do not include any gas physics and the gas physics is expected to actually change the result because what you end up getting at things like explosions by the black hole or supernova explosions along the disk and these explosions essentially can redistribute the dark matter and the present you know by this sort of redistribution what it can potentially do is flatten the density of the dark matter close to the center of the galaxy where you expect the exclude these explosions to be most relevant and so at the moment there is a lot of effort to improve the simulations to include this gas physics it's a really hard problem because it's just computationally more intensive to include that and so those simulations right now are not at the same level of resolution as the Dark Matter only simulations but the improvements have been happening steadily and very quickly so probably in the next few years is this something that will be resolved in more detail coming from the simulation under the spectrum it's a very interesting problem so as a you know I'm coming from the particle physics end because if the gas physics can explain this core that's one thing but if it turns out that observations tell us a core that a course should be there and the simulations are not finding the gas physics can explain it then that can actually tell you that something interesting is happening with the Dark Matter particle itself so that it must be self interacting or something like that so it can actually potentially end up telling you something about the particle physics properties of the dark matter yes yeah so this this flattening is the evidence forward is a little bit tenuous but people have started trying to measure these density distributions in some satellite galaxies that we know are dominated by dark matter is called dwarf galaxies and some groups find that in those satellite galaxies there is evidence for these cores and if that's the case the cores can either be coming from music you know supernova explosions a GM feedback things like that the baryonic physics or if it can't be explained by that and we'll know as these simulations improve it might be coming from the fact that the Dark Matter interacts with itself and if it interacts with itself it's going to mostly end up affecting what's happening near the center of the galaxies where the density of the dark matter is the largest Yeah right now we don't know there's it's a big debate that's kind of going on I think that'll become much clearer both as the observational evidence improves with the measurements from the satellite galaxies and also as the embody simulations the resolutions improve the the simulations that include the baryons and then but right now this is kind of a big question mark we don't know if there's cores in these oh yeah so the simulations that have been done so far that include baryons seem to indicate that you can easily form cores if you have this kind of these baryonic feedback mechanisms so the the sort of first results coming out from those suggest that you can actually explain these cores almost entirely with these feedback mechanisms but it might be as the again the resolution isn't quite at the level where it is for the dark matter only simulations so it might be that if the resolutions improve the story might change we don't know yet oh [Music] so yeah so with the simulation students so there's several kind of interactions that are that could potentially be happening here one is just gravitationally so essentially if I have a supernova that explodes here what it does is it blows out gas at extremely high velocities that redistributes the gas locally in that region and by redistributing the gas locally in that region it changes the gravitate the local gravitational potential which then ends up affecting the Dark Matter distribution so the kinds of processes that are being simulated are those kinds where you have this you know these explosions or the the the feedback from the black holes that we distribute the the gas locally and then from that change the gravitational potential which changes the dark matter density so that's one way that you can get these scores a separate way is if you just have the dark matter interacting amongst itself so if you have that kind of interaction and you get scattering processes just between the two dark matter - dark matter particles and then that can change the slope here does that answer your question okay yep oh here yeah yeah no so when you do the simplest possible thing here you don't expect that there should be the core the only place so the the reason because that's that's sort of the zeroth order estimate of what can be done the presence of the core is something that we're starting to see with these simulations which are taking an account much more I mean that those are if you want to think about it as a perturbative expansion and what you understand about the dark matter properties that's the zeroth order term and then this is much higher this is including all the higher order Corrections that are accounted for by the simulation so and also from observation because there's some observations that suggest that this might be the case so if it is chord then our simple approximation there just didn't didn't quite capture all of the physics and the reason for that is because these cores are our most important very close to the center part of the galaxy so like here and the rotation curve measurements if you notice the scale right the flattening is on 10 kiloparsec 30 kiloparsecs so rotation curve measurements are kind of probing what's happening out here and the core would be in here so we can't actually really rely on the rotation curve measurement to be telling us what's going on here okay so what I want to do next is given what we've done here in terms of just setting up the scales of the problem and kind of mapping out where the Dark Matter is relative to where the baryonic matter is in the galaxy what I want to do now is use this as a way of making our first set of guesses about what the masses the dark matter particles can be so what we're gonna do now is going to essentially be the most generic statement as you can possibly make about dark matter in the sense that it makes essentially zero assumptions about the particle other than its distribution in the galaxies and whether or not it's a Fermi on or a boson okay so what we're gonna need for these estimates are one the fact that dark matter forms halos which I hope I've convinced you of given our previous discussion and the second thing is whether or not the dark matter is a boson or a fermium that's it so let's start off with the bosonic case first if the dark matter is a boson then imagine so we've got our dark matter halo and what we want to do now is essentially pack it as much as we can full of dark matter particles and when we do that we're essentially putting in a particle in a given cell a phase space and if it's a boson then then just the you know Fermi boson statistics tells us that we can continue packing as many bosons as we want in any volume of phase space here and there's nothing preventing us from doing that right this the spin statistics doesn't care if it's opposed what it doesn't care I can have as many particles as I want in each volume fate in each phase space volume so because I can do this it means essentially that I can treat the dark matter field as being coherent I've now packed so many of these particles in a single volume of phase space that it essentially acts like a coherent field and if it's acting as a coherent field then there's actually our arguments for the stability of the halo are going to come down - all right well these arguments are actually to look really similar the kinds of arguments you make for the stability of the hydrogen atom so for example these so the stability of the halo is going to be set by the uncertainty principle in the same way that the stability of the hydrogen atom is set by the uncertainty principle for the electron that's orbiting around so if this is my halo here and I say that it's radius is our halo then Delta X is roughly twice the radius of the halo and Delta P is roughly the Dark Matter mount's times its velocity if I substitute this in here and so I get twice Dark Matter mass the our halo about 1 and I solve for the dark matter mass I'd get that and if I make the assumption that V is on the order of 200 kilometers a second non relativistic right this is matches what we said before using the virial theorem and I put in the radii of the smallest Dark Matter halos that we can observe which is coming from dwarf galaxies if I put in these numbers what I find is that the Dark Matter mass or if it's a booze on cannot be any smaller than roughly 10 to the minus 22 Phoebe so dark matter that's at the bottom of the scale so roughly 10 to the minus 22 easy is sometimes called fuzzy dark matter so the super ultra light these are ultra light scalars but this is getting it somewhere in the sunset now we know that you know there is a lower bound on the mass of bosonic dark matter it's you know it's very small but we can't actually make things any smaller than that because if we did then the halos would not be stable and we would not be able to form any we just wouldn't be able to form any halos we can repeat this whole set of argument now but doing the case for fermions and the results going to be a lot more constraining and the reason is because of Pauli exclusion so when we have fermions we can't just pack them continuously into a single solid phase space Fermi statistics tells us that you know fermions are picky they don't want to be living very close to their neighbors and so you know you can only pack a matte you know maximum of one Fermi on in each unit of a phase space and that restriction is enough to loosen to tighten this bound considerably so for fermionic Dark Matter Pauli exclusion becomes important yes oh um okay so our estimate here is coming from this is the average velocity right but you can have higher velocities than this but you're still you can't have arbitrarily high velocities because what'll end up happening is after a certain point your Dark Matter particle can just escape the halo there's an escape velocity and so you can look at you actually just estimate you can calculate the escape velocity for a halo of that mass with that radius and you would find that it's consistent with the escape velocity like especially the fastest stars that we can measure in our galaxy and that's roughly on the order of 550 kilometers per second so anything you know anything faster wouldn't actually be bound it would just be passing through and that happens I mean we have we you know not everything is bound to our halo you can have stuff that's passing through but it's not going to be the dominant it won't be the dominant thing that's that's in the halo yeah well yeah yeah that's what I'm gonna do next after I do this like I said this is the most generic estimate that you can make in the sense it's the most model independent one the only thing I'm requiring is that I form a Dark Matter halo what we're going to do next when we finish with this is consider what happens when we make the additional assumption that the Dark Matters thermal in the early universe then that the mass range is going to be more constrained but it's coming with an additional assumption so yeah so it's always important to remember that you know what the most generic a possibility is yeah so what I'm using here is for the disorder of the smallest ones we've seen so these dwarf galaxies it's just because that gives you the tightest bounce so you could use you could put in the radius of the Milky Way and you'd get a bound it would be weaker than this but we know we've seen smaller halos so we can use that number instead and that would be the halo associated with these dwarf galaxies any other questions okay good so let's finish up with the fermionic example and then I think we'll be ready for the break and then when we come back we'll we'll consider a thermal dark matter in more detail so for the fermionic case I can take the mass say that the mass of the halo is roughly the mass of each individual fermionic dark matter particle and multiply that by the velocity of the velocity the volume of the halo times the phase space density F of P D cubed P for fermions we know that this can't be this is the maximum value this could take is one so that gives me that constraint here times D cubed P this step here I was making the assumption that F P max is one and I can estimate this as being 4/3 PI R cubed and then D Q P as being Fermi on mass velocity cubed so putting in again if we put in estimates for the mass of the halo and the radius of the halo and then infer from the O and the velocity which we also know then we can solve for what the the limit is on the mass of the fermionic Dark Matter particle and what we find is that this is larger than order and evey and you know what I wrote up on the board sort of the simplest possible way that you can do this but you can refine these phase based arguments and if you do not you can make this even tighter the current limit that is in the literature is 0.7 ke V so a fermionic Dark Matter particle has to have a mass greater than 0.7 ke V in order to be able to form the halos that we observe today notice that this constraint is much larger than what we got for the bosonic case and that coming entirely from the fact of just Pauli exclusion and how you can pack these particles into interface base to form to form the halos so these are the lower limits what's the upper limit the upper limit on the Dark Matter Mouse is really high so it is 10 to the 59 Evy and where that's coming from is measurements with called macho's so massive compact halo objects so you can look for these things using gravitational lensing because if they happen to be coming in front of you know something that's emitting light you can look for the the lens signal and based on that we know that you can't have dark matter that's larger than 10 to the 59 evey in mass so this is what we have to work with so we now know that we've well over the last hour we've used information gained from the motions of stars so looking at these rotation curves to make inferences about the distribution of dark matter in the Milky Way we've gotten estimates for for the total mass of a halo the size scale we've seen that it's much luck send out much further than the Galactic disk we've seen that the velocities of the dark matter in these halos are nonrelativistic and we've used the fact that these halos can form to then tell us something about what the masses of the individual dark matter particles have to be and what we get is something that falls in a humbling ly large mass range right so if it's bosonic it could be anywhere from 10 to the minus 22 DV all the way up to 10 to the 59 if it's fermionic anywhere from 0.7 ke v up to 10 to the 59 evey so this large range that's we know is possible really means that we need a very diverse set of experiments to try to probe this we can also then begin to make some additional assumptions about the properties of the dark matter to kind of limit this map mass range a bit further so when we come back after the break that's what we're going to do we're going to talk about dark matter that's there in the early universe and discuss what that does to the allowed mass range and how we can actually probe for it so I guess let's break now and then come back I'm not sure 1 11:30 ok so we're back at 11:30 [Applause] [Music] you

Channel: ICTP High Energy, Cosmology and Astroparticle Physics

Views: 25,686

Keywords: ICTP, Abdus Salam International Centre for Theoretical Physics, Theory, String theory, Black Holes, Holography, Particle Phenomenology, Large energy colliders, Cosmology and Nongalactic Astrophysics, Inflation, Dark Matter, Experiment, High Energy Astrophysical Phenomena, Astrophysics of Galaxies, Earth and Planetary Astrophysics, Instrumentation and Methods for Astrophysics, Solar and Stellar Astrophysics, Lattice

Id: J5vcAZoxhAc

Length: 58min 48sec (3528 seconds)

Published: Mon Feb 26 2018