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5 Things About The Search for Dark Matter

5 Things AboutThe Search for Dark Matter Today we're going to learn about dark matter and it's a wonderfully I literally elusive sub...

5 Things AboutThe Search for Dark Matter
5 Things AboutThe Search for Dark Matter


Today we're going to learn about dark matter and it's a wonderfully I literally elusive subject to talk about because when we look out into space as soon as you get outside of our own solar system everything we know about space we know because of the way we analyze the light that comes our way the way it's emitted or its reflected or its absorbed and blocked or refracted by all the cosmic objects out there and one of the most surprising things we have learnt about the cosmos is that the amount of matter that actually interacts with light in any of those ways is only a tiny fraction of what's out there there's far more to the universe than meets the eye or you know even the telescope so today's talk is about part of that invisible universe I'm going to be talking about dark matter and let's just be absolutely clear this is something different from dark energy I've talked about dark energy before that's related to the way the universe is expanding its associated with the space between the galaxies today I'm going to be concentrating on dark matter which is something else completely dark matter is associated with mass is associated with structure in the universe so if you like is the antithesis of the dark energy and we've known about dark matter for 80 years it was first you know first evidence for its existence was mooted in 1930s and since then we've come a long way in

establishing the evidence for the fact it's out there looking at this distribution what kind of objects it's around how its distributed maybe more about its nature but we still haven't quite got to determining what this elusive substance is made of and so today I'm going to be talking about some of the physics behind it I'll be reviewing the observational evidence and support of it II will go through some of the candidates why we think some things are valid why things are not valid so what we think it could be and what it definitely isn't and then I'll end up with talking about many of the ongoing searches to understand the nature of dark matter and the thing that's great about this topic is it brings together


disparate parts of physics you'll hear about the structure in the universe on the largest scales and how it could be influenced by what we understand about physics on quantum scales on subatomic scales and so this is the world of astroparticle physics again where astronomy and particle physics are both working the same problem from completely different viewpoints so let's step back let's just do a little bit about forces this is just your physics 101 five minutes physics 101 before you get to the astronomy so we think there are four fundamental forces that rule the whole of the cosmos and the one that's really important for graph and for astronomy of course is gravity gravity operates over the longest length scales in the universe it's comparatively weak compared to the other four forces and even that strength and diminishes following the inverse square law so if you go twice as far away it drops to a quarter of its strength the amount of gravity that's present depends on the amount of mass that's present but the things that well and cane astronomy of course always dealing with the very biggest masses of all the things that make gravity so important for astronomy



 first of all it's always an attractive force okay it acts on all matter whether that matters electrically charged or neutral whatever size it is that's not true about all forces and the other thing is it's always ads so you keep piling matter together gravity just grows bigger and so the bigger the object the more gravity you have contrast that with the second force I'm going to mention electro magnetic okay and just to say gravity is moderated by the laws of general relativity contrast gravity with electromagnetism now this is a much shorter range force compared to the other two it's it's comparatively long you know but you're talking the distance of atoms nothing like what you see in the universe electromagnetism is a again something that follows the inverse-square law it drops off with distance quite sharply but it doesn't act on all the matter out there it'll only act on the matter that has an electric charge with that's positive or negative it will not create motion in neutral particles the other thing it can be an attractive or repulsive force so if you par lots of matter together you don't just keep continuing adding to the force some may be attractive some may be repulsive you can cancel out that force so you can have a lone large body which has lots of gravity but absolutely no electromagnetic force and so it's it doesn't influence all matter and it doesn't just keep growing as you add matter therefore it only influences a certain subsection of the matter out there in the universe and it's very important for the production of light of course light all comes in at all colors in the electromagnetic spectrum and that's sort of highlighting the relation between that and this force and then there are two further forces that operate on the subatomic scales you've got the weak force which is involved with the decay of the nucleus of atoms where they kind of spit out particles and your atom will transmit from one kind of atom to another different elements and then you've got the strong forces watch what is which is what binds quarks together to form things like protons and neutrons and it also binds together very strong you know charges like protons which would otherwise be repulsed because they've both like positive charges it binds them together to form the nucleus within the atom but both of those are very small range forces and just operate on those suburb Tonique scales and in fact all of these three forces the electromagnetism be the weak and the strong force they inhabit the world of quantum physics which is very different from that of relativity and indeed when particle physicists analyze the quantum world they tend to use something called the standard model and this is often referred to in terms of supersymmetry or Susy and the idea is that this brings together understanding of those three forces but is still an incomplete description of the world of physics and in the interest of elegance and simplicity a lot of physicists believe that there should be one set of rules so instead of having general relativity for stuff on the bigger scales and to talk about gravity and quantum physics to define the subatomic world really there should be one set of rules and sometimes these are sought under that sort of Susy umbrella extending that standard model of particle physics to encompass the relativity relativistic world and as yet this hasn't been achieved but just to bear this in mind it's going to be relevant for some of the particles we're talking about later that could possibly be associated with dark matter there's also the idea that these four forces in the current world then all disparate but very early on in a much more energetic universe so it conditions just after the Big Bang these are all unified in one force so imagine very energetic conditions just after the Big Bang before the universe starts expanding way in relation you've just got one very simple force that moderates what's going on in the universe but as the universe grows and cools these forces decouple and separate from each other the first to go its own ways gravity strongly after up towards quickly after that is followed by the strong force and then the last sturdy couple are the electromagnetism and the weak force which up to then have been one force together which is the electroweak force they separate out and you get the four forces of the current universe and so again just this idea there's something yet we haven't quite got right in physics we think there's something that we're still searching for we're looking for that unification model and the way that it can bring all these four forces under one set of rules of behavior that apply right in the early universe okay let's get to the real universe and I said that dark matter was first mooted in the 1930s and indeed it was this chap fritz Vicki he was a great discover and cataloger of clusters of galaxies like the one you see behind and indeed he first did analysis on this cluster this is the Coma Cluster it's the densest the richest nearby cluster where you've got thousands of galaxies all tied together by gravity within a volume about a few millions through tens of millions of light years across and he analyzed the motions of the galaxies within that cluster they're not stationary within this image and we don't see the move because it's so far away we don't see them move on our human timescales nonetheless all those galaxies are kind of swarming and on orbits through the cluster and they're moving at phenomenal speed speeds of up to a thousand kilometers per second and Vicky immediately saw this was a problem the galaxies are moving too fast they shouldn't be part of this coherent cluster they should have just zoomed off into outer space and the whole cluster should have dispersed many you know eons ago if you're going to keep all these galaxies together in this volume of space you bind it by gravity and the problem was if you can to pull the Stars and all the galaxies in this year the humongous cluster there isn't enough mass there to produce enough gravity to keep them all detached you need more mass and therefore more gravity than you see with your telescope and he first raised this issue at first it was called missing mass and he published the first results in this in 1933 and by four years later he was actually referring to and Dark Matter Dunkel matter E in his papers though arguably you might do better by calling transparent matter because it kind of gives you slightly better feeling for what we're dealing with here now this result was puzzling it had since been confirmed by observations of many other clusters of the galaxies is not peculiar to the Coma Cluster every cluster of galaxies we study the motions the galaxies are swarming around too fast now some of you might say well of course the galaxies are not all that's there in the cluster especially if you've been to one of my previous talks on x-ray astrophysics you know something one of the things I study it's not the galaxies in the cluster but the hot x-ray atmosphere that lies between the galaxies and the cluster so this is a rich cluster every one of those little yellow fuzzy blobs is a large galaxy in between them if you look with x-ray eyes you see a hot atmosphere millions of degrees so hot it only shines in x-rays here it's color coded purple you can see it fills the space between the galaxies and it's probably about seven two times more seven to ten times more mass within that hot atmosphere than there is in the galaxies but that is still nowhere near enough to account for the gravity you need if you add up all the light in all the x-rays and all the stars it's still only about 15% of what you need okay so maybe we don't understand the galaxy motions in the cluster of course you come with a result to say you know that 85% of your cluster galaxies is missing you need to check that result in great detail you can use x-ray observations to do this this gas that's colored purple here is emitting x-rays because it's very hot and the way it works is you get more x-rays according to how dense the gases and the temperature so for example if I've got my cluster of x-ray Syria x-ray emission from a cluster that is hot because it is being squeezed and what you can do is you imagine this as a big ball of gas you go gravity pulling in the outer layers and they're squeezing the inner layers now the amount of an x-ray emission you get depends on the density and temperature so you can analyze the x-ray light you can work out the density and temperature within this gas you can work out the pressure that it's been squeezed to so effectively you've got inlet filter gravity squeezing the gas until that outwards pressure starts to resist it so by looking at the x-ray emission look at the properties of the x-ray image and you work out the pressure at different layers in the gas you work out the gravity that's squeezing that gas and again it fits exactly with what you get from the motions of the galaxies there is much more mass there there's much more gravity there than you see from the observable matter and then this match is pretty large if you're looking at the gravitational mass so that's you know all the the gravity that is needed to keep either the x-ray gas in place at that pressure or the galaxies in the cluster compare it to everything you see all through the all through the cluster there's a mismatch of at least about an order of 100 and it's not just on the scale of clusters of galaxies we see this also within individual galaxies including our own Milky Way now imagina got a spiral galaxy that's the flat disk type galaxy it's rotating all the time here's one seen face on you know we live in a galaxy like this we are rotating around the center once every about 220 million years we've only gone round the center of the galaxy about 19 times since the Sun was born and we travelling the speed of 220 kilometers per second and that gives you an idea of how big this galaxy is now what you can do is you can map out that rotational motion because in the same way that the the speed at which the Earth rotates around the Sun depends on how far out the Earth is from the Sun and the mass of the Sun therefore the gravity that's pulling it around in its orbit you can look at the rotational speed of these stars they're responding to the gravity of all the mass that's within their orbit or you know around and imagine you've got your galaxy that's kind of more easily sidon like here you can use the Doppler effect to measure the speed of stars and the discs that are coming away fruit and coming towards you and coming away from you can map out that rotational motion now this was first attempted again in the 1930s by her as babcock who looked at our nearby spiral Andromeda galaxies again he what he found was that the velocities didn't behave as you'd expect they were too high and this kind of was under question but really was affirmed again not until the more 1970s when Vera Rubin and Ford and their collaborators looked at lots of external spiral galaxies and they found the same result and the problem is this what you expect from a system you know even think of the planets in our solar system planets that are further out from the Sun travel much slower in their orbit around the Sun and the ones whizzing around the center you'd expect the same behavior for the stars in the disk of the galaxy this is what you predict okay that it would drop as you move out from the center of the galaxy the problem is what Babcock saw in Andromeda and since been seen in almost every spiral galaxy looked at is that instead of dropping like this now this is a plot of the velocity so this rotational speed with radius out from the center of the galaxy what you observe is that instead of dropping it continues flat this is true even for our own Sun we're about here 26,000 light-years out from the center we're travelling about 70 kilometers per second too fast that means the galaxy this stars right out here at the edge of the disk are whizzing around far too fast again they should have been flung off into space unless there's more gravity than we expect tying them anchoring them to the galaxies and you can try and construct what is causing this velocity curve you can add in all the gas in the disk that contributes a bit but what you need is an extra component of mass and this is the Dark Matter halo this sort of is what you'd expect if you have a large spherical world kind of Fira call halo way outside our galaxy so that is what's needed to keep those stars bound to the edge of our galaxy so if you've got our galaxy probably 100,000 light years across then that is embedded within a much large invisible halo of matter made of this dark matter and extends out like six hundred thousand light years pretty about ten times the size of our galaxy altogether and it's invisible and it's in every direction and it's not just for our galaxies you can do the same okay again just numbers if you work out the total gravitating mass that you need it's about an order of a hundred more than if you add up all the light in all the stars and the gas and gas clouds within the disk of the galaxy you can play the same game with elliptical galaxies okay they don't have a nice steady rotational motion makes the maths easy all the stars are swarming about in orbits in any random direction but they're still responding to the gravity of the galaxy and again what you find is they're moving too fast you need more mass within the elliptical galaxy than you get just from adding up the Stars and it's not just the stars moving in response to that gravity elliptical galaxies also have x-ray halos and if this is an optical image of this is Messier 60 a big elliptical galaxy I'll just show you an x-ray image again has an x-ray halo x-ray gas is hot gas the particles in it are moving very fast again they have no reason to be bound to remain bound within this halo they should fling off into outer space but they're being anchored by much larger mass than you expect and when you infer the shape of these Dark Matter halos from the galaxies okay they're much more extensive than the galaxies they retain the same basic shape so if you have a kind of rugby ball shaped galaxy then the Dark Matter halo follows and tracks that but it's just on a much more extended much more distributed basis and if we go back to the clusters of galaxies we know the dark matters they're from the velocities the galaxies we know it's there from the properties of the x-ray emission but you can do something also that's quite neat you can use gravitational lensing now gravitational lensing comes from what Einstein tells us about where gravity is generated you plunk a mass in the middle of space it distorts the shape of space around it any light rays traveling through that distorted space are bent and curved and they can fool our eyes so we think they come from a different origin so if you put matter in space it distorts the shape of space now put a lot of matter like a cluster of galaxies it distorts the shape of space a lot and if you've got background galaxies then you they're if they're light paths from that background galaxy passes through the foreground cluster you get weird distortions of their shapes you can see some of them here within this galaxy in fact once you start looking for them you see everywhere within a rich cluster of galaxies here are some of them blown up but remember that distortion depends on the amount of bassmaster it doesn't care whether there's a light-emitting mass or dark mass it's measuring the total gravitating mass there and you can use and map out all these different lens damages these mirages that this distortion creates you can work out the mass that's needed to do this distortion what's more you can all because clusters of galaxies are distributed across the sky you can map out how that dark matter variation tracks the galaxies within the cluster so for example going back to this cluster again very rich example lots of lensing going on if you infer the mass distribution from the lensing again it's distributed and extends further than all the galaxies and here you have again three different ways of measuring the mass in a cluster galaxies that all agree there's far more mass there then you'd expect so those are the observational things that anchor it in place there's also evidence from theory in support of a vast percentage of our universe being dark matter ok one more thing keep throwing extra slide things I want to show pictures here you have a very distant cluster of galaxies in fact it's two clusters of galaxies that have undergone a collision there's one about here and one about here is a long way away so it's kind of difficult to see the individual galaxies within the two clusters now that's the distribution of the galaxies these galaxies sorry within these clusters these clusters have had a head-on smash so they smashed and gone through each other now but if you have a head-on collision between two clusters it doesn't affect the galaxies much because they're separated by huge distances compared to their size galaxies themselves don't collide with each other that inter cluster medium though it fills the space in the galaxies that kind of collides and squeezes and gets slowed down so the galaxies carry on through and if you look in x-rays you see the inch custom medium of the two galaxies has got slowed and fills the space between the two clusters so remember there's much more matter in the x-ray gas and there is in the galaxies now if that were the dominating gravity gravitational mass in the cluster when you do your lensing mass you'd expect most of the lensing to happen between the two clusters what you find instead and here the lensing is shown in blue it's followed the galaxies it hasn't interacted with it with itself or with the other galaxies except through gravity it's just gone through and if the fact that it's not in the same place as the x-ray grass shows you that there's the dominating mass within the cluster is not associated with the x-ray gas and it tracks the behave of the galaxies.

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Sowhat: 5 Things About The Search for Dark Matter
5 Things About The Search for Dark Matter
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