Well. There's a science. Publisher and we're very happy to talk a little from. STANFORD UNIVERSITY I'm going to talk for a long time. Let's just start three. Products were just on this for you so you got a speech the other Maxwellian university really dirty and they went on to post docs and gamers versus their own real bridge and park. And Harvard University at the center of it and then it was a system and it was a professor at the State University and he went to Stanford University shortly after that and where he is now is associate professor at Stanford. Stanford lyrics already said. And also he's already one of the I guess I want to say the only person or you're one of the first Professors. Which is the holiest cities for particle at this coffee and I'm very happy here because I was one of those Grad students way back when I went it's a pleasure so that's a lot of time. I've Thank so much well it's a great pleasure to be here we actually had a mini symposium earlier this week and so I got to meet all of the people here at the center and then turning up tremendously. The idea for today. We have about an hour so I'm going to try to not speak too long so that we have also some time for questions of talk about the things that you might be interested in and I want to talk about dark matter and raise over the whole history of the universe in some sense and think about. It give you a flavor of our current best understand. But the universe is made off and. The whole way of how we look at the universe now where we have a paradigm that is within which we can calculate how things evolved from shortly after the Big Bang all the way to the present day and sort of just some of the founding principles of this whole picture and of course you can imagine it's thirteen point seven billion years of history and evolution in the universe so we'll have to leave out a few things. If you want to finish early enough. I'm just going to get out of the first of thinking a little bit about our place in the universe where do we actually live then talk about some of the evidence we have for dark matter and this really mysterious stuff that seems to be just so much more abundant than any then all of this regular stuff that we're familiar with and so walk you through some of the evidence in most of P. Tauriel way nobody questions today. And just give you a sense that we have many avenues of getting at this and give you a flavor of why so many of us are absolutely convinced it's there. And then I'll walk you through some of the implications that actually has. Wires and what that does to the universe and at the end of time to think a little bit about why we should take the universe itself of the personal Ok so here we are that's out of the the night sky. Sorry wrong button. One more time on. The night sky. Winter constellation that you might recognize Arayan that's a Ryan's belt he has got a sword hanging from it so he's hunting I think some bear or something over here. That's his head over here that's an elbow where he sort of shooting with his arrow. That of course is the band of the moon. The way you see all these dark splotches is just dust clouds blocking out from light that comes WAY behind it and you think that's that's really lovely if we can take a nice picture of the night sky we get some sense of the universe we live in but much more fun about this is that over the years we had satellite missions that measured the exact positions Luminato they saw how bright stars are their colors of just. All the nearby two hundred thousand stars OK sounds a lot is really tiny there's a tiny little region in our own Milky Way galaxy which has about one hundred billion or two hundred billion depending how many of the small little stars you count. But for us now having three D. positions that we can actually go and fly out in the universe and so we fly a straight for this interesting region that's in the sort of variety one thing you notice right away is all the stars that made up the nice constellation just dissolved constellations are pure chance projections on the sky we're already one thousand five hundred light years from Earth. So we're going much much faster than the speed of light Horsehead Nebula here these are just massive stars that shine out a lot of radiation lighting up the gas around them. That gives rise to all this nice red color coming from hydrogen which is the dominant form of regular matter in the universe OK that's the result nebula there's about ten thousand stars that form in that region and some of them are massive some tens of them are very massive stars that are you know as much as a million times brighter as our own sun lighting up that whole nebula. Now we come by another little guy who looks very similar you think at first that's actually However just the nebula made by one single star it was an explosion. About a thousand years ago that made this. Supernova remnant we call it and the little blinking you saw in there was a neutron star that actually is as big as Atlanta more or less in spins around in its axis thirty times a second. All right here we are flying out of the Milky Way. That. One part of the movie now it's completely made up OK We have no idea what the Milky Way looks like from the top right so right now. For good well most a million light years away depends on what kind of camera we would be using of what the distance would be first but it's about a million light years away from the Milky Way And so we had to take another galaxy it was with paste that in this thing is just another galaxy that has some theaters that are the same as the Milky Way in the sense that it's a spiral galaxy that has multiple spiral arms there's a very small component of a bar so we picked one that's very close to what we know about the Milky Way but the image itself is not the right one. However all the images now of all the other little galaxies out here is again the correct ones they have the correct positions is the correct images of these galaxies you see all this is the Large Magellanic Cloud and the. Small Magellanic which we could all see if we were to move to the southern hemisphere. But you see them on the night sky these are two galaxies that are essentially already orbiting within our own galaxy. And I'll talk about what we mean. The known galaxy so there's lots of little galaxies we call them toward galaxies they tend to hang around big galaxies. And in fact of the past few years we've been discovering many many such small galaxies around our own Milky Way and also around our sister galaxy and drama. Appear to a little bit larger than the Milky Way but also a spiral galaxy and you see it already has a few small ones around it too. This is the third largest spiral in our galaxy with a very strong star forming regions here where new stars are constantly being formed but again it looks like a field of stars in this view but it really every single dot here now is a galaxy so we're really out hundreds of millions of light years now and you see the galaxies themselves are not distributed randomly they are distributed long filaments There's regions where there's hardly any galaxies they have all sorts of shapes the typical spiral ones again here but there's also much rounder ones in the one that we call elliptical galaxies is typically older they don't make as many new stars and you see that this wild glamorization of about a two thousand galaxies or so that's the Virgo Cluster we're actually falling towards that at about six hundred kilometers a second. It's so far away we'll actually never get there. And that's just a fact that the universe started accelerating getting faster and faster and faster again so even though we're falling there the universe the space between us and that object was exhilarating even faster so we're not going to make it there. As a pity we'll make it out there and I'll talk about that that's the center of this very cluster that maybe seven is one of the very largest galaxies we have around. It has a big black hole in the center that a little thing sticking out there relativistic jet of material where there's a black hole is a billion times the mass of our own sun and it's throwing our things at very close to the speed of light. It's nice. Great for us to get the study such awesome phenomena. All right now we would be just flying back but do get a sense. Too because mom. Just most of these things here are very lovely if we are connected to it but it's only boring this is super close we like to study things much much further away so the things we talk about here was only a sort of a distance of two hundred million light years so in the scale for the whole universe that's really not much at all so you're diverse of course is thirteen point seven billion years old so we have light that has been traveling to us from galaxies for Already some sort of twelve point eight billion years where the light started at those galaxies has been traveling to us twelve point eight billion years in the meantime of course the universe itself was expanding further so the region where the Galaxy was that emitted that light they came to us is already much much further away by now. It's pretty big. I'm not telling you anything new in this part OK here's a quick view of the Milky Way galaxy is in the wonderful thing of course with us using telescopes in detectors is that we have a lot more than just our eyes and we can use wave bands in the radio using radio waves X. ray gamma ray an enormous spectrum probing very different phenomena. Around us and that you know this is beautiful thing of astronomy that we can sit on earth we build our telescopes we point them at the sky or we launch a satellite so we don't have to deal with it pesky atmosphere and we get to see radiation just informing us what the physical conditions are there and just studying that light we can infer some quite remarkable things what the sun is made of for example you know you can fly there and take a little sample put in the lab and. They studying all the different wavelengths of the light we can gather an enormous amount of information. Legally right from our armchairs so to say and so in the. In this waveband here we can sort of see the emission of atomic hydrogen is just neutral hydrogen just a proton and electron around it and the proton in the middle it can flip. Because the proton itself has a spin and so there is actually a spike they're friends of whether the spin of the electron and the protons are aligned or whether they misaligned and there's a tiny little difference in energy negatives of very little energy and that's why the radio waves radio waves are not very energetic. And so what's wonderful about that light though is that it can travel unimpeded through all these dust clouds that we saw that sort of block out light here in the optical we see we can see we only get to see the nearby stars because all these dust the dust bunnies are around in block all the light from behind not so in atomic hydrogen so this way we can study the far reaches of our galaxy and sort of get a sense of how fast material is moving even on the other side of the galaxy. Super useful tool use infrared wavelengths are actually also good in peering through the dust you see there's much less light blocked out here and this near infrared bat and there's a little bit of the blockage there you can see in the optical it's the worst in the X. ray again. Less of the like it's absorbed they so energetic they also make it through. Into media things. OK And now we're even starting in the in the gamma rays some very energetic particles. OK but there's a whole picture of you know we're sort of in astronomy think about this is in the galaxy in this cartoon way is that all these optical pictures over just the regions are of the stars is just a tiny part of the story that they are really is just a very small fraction of the overall volume that makes up the galaxy. Because there's a whole region around it that we always just call dark halo now it's just made of dark matter. Because it's big. Not very dense it doesn't come up very much it's not very dense at all but it has most of the mass in the galaxy and I'll show you all sorts of evidence why we came to believe that even though we can see there actually that stuff it doesn't absorb light it doesn't emit light it doesn't scatter light. And so it's not nice to the astronomers. But it's lots of it of the place we've got a whole bunch of globular clusters just interesting star clusters sort of orbiting around the Milky Way they are some of the oldest things in the universe we've got these Magellanic clouds here and actually by now we've discovered another fifty such companion little galaxies with the smallest one of them only having a few hundred stars in them. You know we still call it galaxies OK so they're a million times million times a few stars in the Milky Way itself we still call in galaxies. That keep orbiting around the Milky Way in fact and that's just the fifty we found and we didn't look everywhere yet we were always in we always have trouble actually because we're right in the Milky Way so right in the center when we look out you already saw it's hard to see stuff because you've got all this inside that disk so there's whole regions where we can't look you know. Any right so now we have a little bit of a sense of how we go where we are in the universe what the Milky Way is a very roughly. Now just walking very briefly through one of the earliest evidences we have that there is such a. Component of dark matter and I'm not going to do this is hardly correct or anything I just want to get you the physics flavor of how you actually go about that determining where stuff is in the universe and how much mass it has in so there's a very simple approach when we sort of look out and you see a pretty spiral galaxy out there you see a nice disk in the middle and again there is some sort of just stuff here but all this. Brighter line here it is because there's a whole bunch more stars that just orbit. In this disk and so from the top it would look like this year some of the spiral arms perhaps but all the stars themselves they just go in circles for a bit. So the sun takes two hundred fifty million years to go around the center of the Milky Way. That's a while so you know the last time we were at this position there were no dinosaurs here so way to think about it and so we've gone around what twenty times thousands the earth has formed. So you think that that's that seems like a long time but at the same time we're going to hundred kilometers a second. Which is actually quite fast so I have to make it back to California and so at that speed if I were a twenty seconds or something. You notice I could start an airliner with that technology anyhow so the but the fantastic thing is that we can actually measure directly how fast these stars are going in that's again because atoms give all of light in very specific wavelengths and so you have an emission line that comes from a particular atoms so say for hydrogen you are taking a line that gets emitted but if now things are moving towards you really fast that line will appear more energetic and if it's giving away from you that light will appear less energetic and so with this red shift or blue shift of the light we can tell exactly how fast it's going. Which is great because now what we do is we just go along this line we just measure how fast of a star is going and then you say OK how fast are they going as a function of radius and that's always of plotting up here the distance from the center of the galaxy and just the velocity that we measure and since we see how much light there is and we can see there's a lot more light in the middle than there is on the outskirts you also have an expectation if you know how the heaviest. Stars and how brightest stars and you say OK Given how much light they see I know how much mass there is in stars. And so from that mass you know how much gravitational attraction there is to the center and so if you want to stay on a circular orbit that mass that contains the star in this orbit has to match exactly the speed that the star is going on right and so that simple balance if you just look at the light what you would expect is that you get a curve like this because there's less and less light as you go out and you know gravity falls off like one over just and squared so that in fact the velocity is as you go far away you should get less and less and less doesn't take much to go around. But the really. Crazy thing is no matter which galaxy we look at what always happens is that it actually goes like this. And the speed just doesn't drop at all it was just sort of seems more or less constant just flat so stars that are out here or out here or even the rare ones that are out here they're all going to the same speed. And that is one key. Element to tell us wait a minute there's going to be more mass than we can see just from the light OK. Already one big evidence here and we've known that the actually since the middle of the last century there was earlier evidence that al that also go over now the. OK great so very easy in a telescope spectrograph measure how fast the stars are growing. And then just compare it to. Use your understanding of Newtonian gravity and. Now here's an example of actually something that is going to happen in a few billion years from now where the undrawn the galaxy the Milky Way galaxy are going to become one galaxy. As an astronomer it would be the best. I'm to be around you know have two hundred billion stars right in front of your nose it's fantastic. But you know I wouldn't want to wait five billion years now either. You see that all these features that sort of happen as these galaxies collide this of course now was a numerical calculation where we just took what we observe of how the stars are aligned you also measure how much dark matter they have just from how it just explained and then in the numerical model just calculate the forward what's going to happen in a few billion years time. But it's this type of experiment that we can do on a on a computer where we can also check our ideas did we get this wrong with this dark matter stuff that we have too little too much. Into the sea of whether that's consistent with the structures that we observe so very useful to in that sense. Perhaps just because I'm having fun and how we've got two galaxies like when you throw me at each other like how many stars would hit your guess. There we've got the experts here yeah there is actually there's only a one in a trillion chance that two stars would actually hit each other. And essentially what that tells you is that galaxies are just crazy empty I mean in terms of stars that you know we always see these pictures and all the starlight sort of makes it all hazy that's purely because we don't have good enough cameras in our cameras one pixel so big that there's always like a million stars in one pixel if we had better cameras and would resolve this even further we would actually find that there's only space in the stars themselves or just a very very aware things. OK. Now we can have sort of think about dark matter in another sense and I can walk you through this but I. To give you a good evidence that dark matter really exist. Just from one simple picture we see here is a cluster of galaxies very much like the various cluster we've looked at the other one somewhat higher Richard which all these yellowish galaxies they're all at the same distance they're all part of this cluster of galaxies and these galaxies just keep orbiting each other. In this very large ball of matter but there are these funny streaks in here all these very long streaks that connect up so this well you see another one here you see one there that's connected all the way down here this is the light from individual galaxies that are way behind this galaxy cluster where that light was going off into space and thought I'm going to go over here but what happened is the gravity of the cluster and the light you know spacetime was distorted around this and actually the light instead of going in a straight line. Was bent and it was bent towards us. Hitting our detector and so the the mass that was bending it is all the mass that was interior to the particular arc that you see here. And so what I like so much about looking at these is if there were only stars if there were no dark matter that sort of more uniformly still clump but more uniformly distributed in the stars then I would have expected that all these arcs would not be so nice and round I would have expected them to be all have a curvature just like the galaxies and if the light is where the masses then you would have expected all these little arcs to have more or less the curvature just like this galaxy has but instead it has a very broad curvature and in fact it turns out it's only a few percent of all the mass. In this entire region is in. Back in stars the rest is in hot gas that's between the galaxies but by far the dominant part is again this dark matter component that leads to this bending of the light what we call a strong gravitational lensing you can sort of by I already sort of feeling well is going to be some stuff there that's more fuzzy and we usually do this a little bit more accurately but OK so here's a theoretical calculation what that would look like if we could sort of fly around and for a given mass distribution just see the light behind it all these the shapes of these are exploding keep changing right it just depends exactly where you are of how light we've been towards you. OK so now that's a very very obvious example it doesn't happen that often only happens for the most massive clusters and things have to line up just right you have to have a nice background galaxy that just comes around and if this galaxy cluster is too close to us we don't see much lens and if it's too far away we don't see much lensing but where we can do with most of these galaxies clusters is a sort of slightly simpler thing but that's a more statistical method where we look at the shapes of galaxies. That are in the background so we look again at one of those clusters of galaxies for example and then we look at all the shapes of these thousands and thousands of galaxies we can detect when we have a deep exposure from a telescope and they're all randomly oriented right and so if we measure the ellipse to cities or of the orientations of all these galaxies on our images. We expect them to be randomly oriented However if one like gets bent because of the mass between us and those galaxies all of a sudden we see that the galaxies again start lining up a little bit like those big arcs but it's a much more subtle effect and so we do a statistical which is sort of measure OK How different is it from a random distribution here. You know how the angles are those galaxies in the sky in from that we can infer how much mass there is the super powerful tool that was only in you know in vision about fifteen years ago and really came to fruition very rapidly in the field and we can do something really some very fantastic things with this and here's one particular example it was quite popular a few years ago of a so-called bullet cluster is quite a remarkable object so these you have now three things overlaid the one is sort of the regular optical image taken from the Hubble space telescope and you see all these galaxies here. Typically all galaxies more ellipticals tend to hang out in clusters of galaxies there's a little one over here that's a bit smaller roughly about a tenth the mass of that whole assembly of galaxies on this side and. The blue contours here is where this blue light is what we infer that how much mass there must be given how the light of all the galaxies in the background here has been distorted but how they have been shifted in position from that we infer how much mass there has to be and that's what sort of shown in this blue here and so more stuff here and more stuff here and this red one is actually now an overlay of what the gas does in that there's a hot X. ray emitting gas that's between the galaxies in these galaxy clusters and the key thing here is that the shapes are very different what we have for the dark matter is two blue balls if you want you know one much smaller than the other one but for the gases you see a big interaction of a strong sort of shockwave here and what we learned from this is that dark matter is very different than regular matter so when you take two things you throw me to each other gas will just get stuck gases. No what happens is you've got the atoms they all bounce off each other and we just like the air in this room you have a nice distribution mix where in this beauty of how the particles was around and that's all it is because the particles bounce off each other dark matter it doesn't bounce off each other and that's the direct proof here because dark matter here went straight through this little clump actually came from this side and went all the way through. From that we get the constraint that dark matter already behaves quite different than regular matter. In these very colleagues of ours actually post I'll get our institute that some of the analysis for this and. And I put together in the day an animation trying to show what actually happens and you see is the little ball running through the big ball in red was the gas and the blue is the dark matter so this is a theoretical calculation where we just took the parameters that they observed and you see exactly the shape here that we saw in the hot X. Ray Milland gas that we can reproduce directly in the calculations even with much effort and that's only possible because we had such a good idea ready expectation of what dark matter or how dark matter should behave and so it made it easy to compare this and here just for fun reason figure out let's figure out what happens in the next few billion years. Just kept going. For a while there OK so now I convince you that we've got a. We can do this just measuring the velocities of stars we can look at how like it's bent in strong reputation lenses we can use the how the light slight perturbations on many galaxies what we called weak gravitational lensing can use that to infer how much dark matter there is there is actually quite a bit more to the story yet. I'll give you just a little bit of a favor of this. And for their ideas just bring us back just to this very basic point that of course any observations we do it's really like a time machine for us right there's so much light that we can observe now that really started at a time where the universe was much much younger so in a way we do get to see in the past but we don't get to see what the Milky Way looked like in the past but we get to see what other galaxies look like in the past and so any of our observations that go far out you can really think of a time machine I always loved his image because that's actually an astronaut here and you can see the scale of the Hubble Space Telescope is just enormous compared to these guys. But one of the best probes we have there is we can look back to radiation that that started its journey towards our detectors at a time when the universe was only four hundred thousand years old. And so that was really. Long ago and just is the information that tells us the best of what the very early universe looked like and I understand in the same series he will have John Mathur. Talk about the cost Mike with background he's a wonderful man and. Absolutely should have the Nobel Prize for sure and encourage you to go listeners talk. In that sort of a story about what's called the Cosmic Microwave Background Radiation is really just radiation that comes. Really at this time in the universe was only four hundred thousand years old and when you first would look at it it actually is the same in every single direction on the sky ideas there's a small distortion here that just comes from our own motion how we are flying towards the Virgo Cluster we actually see in some regions. That the light is more blue shifted as it's more redshifted just because our own single motion but when we take that out what do you see it's actually completely uniform in all directions and. That wouldn't have gotten you a whole Nobel Prize and so if you look much more character and that's what. Matters team did. They managed to actually observe this so well they could discover a very slight fluctuation So the fluctuations in of part one in one hundred thousand so that some regions of the sky are brighter by one part in one hundred thousand and what's so fascinating for us Stephen Hawking call it the biggest discovery of humankind he was very excited obviously. He somehow lucked out because he has his initials in here. I don't know how they did that. This was very mean of me to do because you could look at this thing now or ever in the future and you will always your eye will always pick that out again if it didn't mention any have changed your brain. Sorry about that and. This one what is so fascinating about this is an extraordinary technological achievement to to make these very careful measurements to see these tiny fluctuations and the reason why Stephen was so excited is it really closes the whole loop for us to understand of how the universe came to be the way it is today if we start from a completely uniform universe there is no way we would understand the galaxies come about at all that any structure would form if you're absolutely uniform where would the structure come from what this shows us directly is although there are some regions in space that are slightly denser even though it's just a tiny little bit the universe has time and spends its time EVERY this Larry little region that just has slightly more mass you can grow just gravitationally and the reason is that the material inside that region feels just a little bit more gravity from the material inside and from all the material outside. As a consequence it accelerates less or expands less for the universe and through that becomes relatively smaller over time OK compared to the overall expansion less and less and eventually it feels it's own gravity so much that it actually collapses to form objects and it's this picture of gravitational instability of some preexisting then to be fluctuations that give us the whole framework that allows us to understand why we have this whole film entry network of how galaxies line up in the galaxies themselves come about in the universe. It's really cool i think i might just sort of skip this the the fun part really is just for the experimenter the bills the telescope is that those photons really went in straight lines until they hit your detector and they've been going for thirteen point seven billion years just in straight lines until you finally catch them and you can still use all that information to picture together of what the universe is made of and then today I'm not talking much about this dark energy. There's a lot more of dark matter than there are atoms in the universe and we also learn from this that. Just a bit about the age of the universe and by studying exactly on which scales in the sciences of these blobs there are in these maps we can learn about what the universe was made up early and this is a little fancy graph but it really is just a variance for for very small objects so small distances on this map. Do I have a lot of variations of this signal on small scales and the answer is no. On small scales there's a lot of variation on a particular scale here this is really just like an angle. What plodded here to the particular angle on the sky is actually quite a bit of correlation. But very little on small scale but what that tells us is that what we see where all these photos come from is all how the photons interacted with the electrons that existed early on so they bounce around with those electrons and eventually they escape and come to us in a straight line because it's the electrons that matter so that would probe were all the regular matter is all the photons in this cosmic with background were interacting with regular matter not with dark matter we already talk about that one actually doesn't. Scatter light of light so what that means is. That we know where all the regular matter how that was distributed even just four hundred thousand years after the bang and what we learned is that there is no it's just not distributed on scales they correspond to sort of the sizes of the mass that will make a whole galaxy. So without dark matter we could not make galaxies is what we learn directly from these observations. So now we had a whole bunch of sort of indirect evidence that we measure there's a lot more mass than we can see that gives us an indication for dark matter then we saw there is all these fluctuation that we can see how the very largest structures in the universe could come from that then right away also tells us that there is this dark matter component has fluctuations on scales that we cannot see in the maps in they are responsible to make galaxies that's one of the reasons why I keep. Inspiring me for the talk of there without dark matter we really would not be alive. OK So one more line of evidence really quick we have in way too much fun with this already so is this. There is a dot for every single galaxy that's observed that's from one of the largest galaxies surveys ever carried out the Sloan Digital Sky Survey. And we're flying enormous distances part of the entire observable universe that there's regions that are dark here is only our own fault because we didn't observe there OK they don't. Just in the regions where we did look we find this enormous web of galaxies and there's a sort of a journey back to where we lived and so obviously we look much more closely in our surroundings but studying this web in how all these. You know hundreds of billions of galaxies that we can observe in the observable universe are arranged also again tells us about how much dark matter we have around. OK now a bit more about that and so now I'm now let me take all this evidence that we have in instead of just work a little bit forward in tires of explore a little bit the model the rhythm built on top of this that tries to explain all these different structures in the universe and so the remarkable thing is for our best hypothesis of what the dark matter actually might be namely some unknown. Particle mentary particle. A good guess at the moment is that we have many experiments going after whether that hypothesis is correct is a particle that would be about one hundred times the mass of the proton it would have been made extraordinarily early on and the universe history wave within the. Split second of the age of the universe. And that particle and there would obviously be an enormous number of them so in our own galaxy we would have about a tenth of the sixty seven of them so one hundred sixty seven zeros quite a whole bunch. And you know what's intriguing about this hypothesis is that we are testing is that the very first thing that would form in the universe would be tiny little balls of dark matter. They all sort of fuzzy things and you can throw two at each other and they pass right through each other right to be don't think. Or them as clumps or rocks or that sort of thing that they are these hazy dark matter halos that are fluffy and they don't they collapse somewhat but eventually what happens the particles have enough speed that they speed counterbalances their own gravity so they're just in a nice equilibrium in a way so they can sit there for a long time these dark matter halos because the Dharma part is orbit just to balance out the gravity and if they wouldn't have enough velocity all they would do is contract a little further then they would fear more gravity and until they find an equilibrium but the in this our best current guess we actually expect that the very first thing to form in the universe are tiny little such halos perhaps with only the mass of Earth. And that's all what happens out of one light year scales and supercomputer calculations we can that of look at what this might look like and that part looks then very simple because really the very first thing to form is the regions where you have some fluctuations of some region were gravity is a slightly different than the mean and you see these objects come together you would call that a halo that's a film interest. But it's this funny sort of fluid in a way so it's a fluid that falls together but it doesn't interact so you go through each other in that sentence like light you know you can have two flashlights and point at each other and absolutely nothing happens in the two beams of light meet in the darkness very much like the same way the only thing is it doesn't go in straight lines it goes in much more complicated orbits in the gravity in this gravitational potential well that itself is making and so it makes for some very pretty things that have cost six and very intricate structures to it and there's all sorts of when I call caustics that sort of lead to the formation of these early objects. And I'm going to do so. Think crazy so this is light years scale so super super tiny scales in the very early universe and now I'm just going to go into the entire history of the universe but go to a scale that's about one hundred million light years across and notice in this all these animations we take the overall expansion of the universe out so this is really like our eyes are also growing at the rate of the universe that in reality all this stuff would even be rushing away from you if you wouldn't go in this co-moving reference frame but this allows you to see a lot more of the dynamic and so here you get the sense why we actually think of this large structure we often call this just the cosmic web that has this web like features where we get all these dark matter halos they are arranged along filaments and sheets and evolve along these trajectories or to make in this case there is a galaxy cluster that forms here just a modest one only has a few hundred galaxies. But has the whole evolution of a cause me time. Yeah right thirteen point seven billion years in less than a minute but crazy. But now you can imagine. We made a virtual universe. That would tell us where all the locations of the galaxy should be right and so now we can test that with these enormous galaxy surveys where we had telescopes looking for years measuring exactly where huge number of these galaxies are here's an example of that where on the left we have the virtual universe and on the right we have the real universe and their color is in code the galaxy luminosities sort of how bright the individual galaxies are and fortunately I wrote it there because I usually can never tell the difference anymore and so that just is a really quick visual. Way of telling us that we're getting to the point where when we do these calculations of worth all the halo. Those are in our numerical simulations and we have to do some work to translate that into what we expect what type of galaxies would form and when we compare that then with the real observed I mean it works fantastically well and so we're now using this much more and even already as a tool to measure exactly how much start matter it is you know so that we get to sort of a small error bars if you want you know how many percent is it exactly and so on it becomes a powerful probe at the same time this whole picture of how we do those calculations now afford us to do completely new things and really ask questions that. Would have been very tricky otherwise So for example one that I have worked on for fifteen years was to try to understand what might the very first luminous thing be that forms in the universe and with this whole picture now of being able to have a good heart is what dark matter is and how to do those calculations we can try to predict the past doing these type of calculations and so. In the way it's just becomes this big question mark how do we connect these observations from. When the universe was really an infant to all the interesting thing it does afterwards and we just show you that all the animation here of we're sort of a scale of some tens of thousands of light years across we just look at how the very first objects come together in here so these different colors just mean denser and denser gas here not showing the dark that I'm showing actually the Geishas component or the hydrogen helium gas that was made in the Big Bang and here all comes together to make the first objects in the universe you have a whole bunch of turbulence like with gets created because here when gas clouds run into each other they really interact they don't just pass through like the dark matter did and that's what creates all this worry in this here. Right now we're flying into this object because what we found was that there's a knob to about a million times the mass of our sun that's the dark matter component deep inside we have a whole bunch of gas there now makes the mistake of giving off lots of radiation which lowers its temperature if it has less temperature it has less pressure to resist its gravitational pull as a consequence it will fall more and more and that's how gas regular gas can get much much denser than dark matter in fact our sun is ten of the thirty times denser than the average density of the universe. So that tells you that if it actually contracted a factor of ten billion in each direction from the mean then three of the universe just to make our sun. Get back to the insect we used to be huge. Anyhow so what we learn from these doing these calculations is that we have very massive stars that are the first thing to form in the universe they then will go supernova and already start injecting the first elements carbon oxygen things they made during their lifetime. And release that through supernovae into what we call the intergalactic medium and of sending the seeds the first chance at all to do something interesting you know make planets create life that's our thing make some silicon so we can build computers. OK. And so in the interest of time perhaps I'll just skip that one quick. That this whole process that the we understand to be hierarchical So we have little things that form first it might make an individual star that goes off make a supernova there's another region that does that in the region that becomes our Milky Way does that one hundred thousand times making all these little objects that have individual stars in our own Milky Way Comes from. You know one hundred thousand different lumps that all had in. Interesting history and the individual also fall together in ever more bigger structures and make up. The galaxies. Whether it goes and so here is just an animation. Doing a little bit more of this total cosmic history this is just the first three billion years of the universe or so but here we focused on really making the images as if we could be there with the Hubble space telescope making observations in three different ways bands we look at the special colors that we have depend on the age of the stars and their metallicity of or the content of heavy elements of hydrogen helium and you see the dust lanes in these spiral galaxies one once in a while you see red regions that's the edge to regions that much is very much like the RIAA nebula where we saw red gas emitting. But these are all direct calculations that a supercomputer that's my favorite one here pointed out. Unfortunately that doesn't live very long. The billion years it's merging in there but you know is it really her rocket core and. You know working on doing ever larger calculations on the computer could follow that down to where the smallest details where we try to build these galaxies one single star sort of at a time and understand the history of every one of them. OK There's a few more really cool things going on at the moment that our experiment friends are doing here you can go deep underground to shield all sorts of cosmic ray background you can build detectors that are so crazy center that if in the off chance that one dark matter particle tensor had one nuclear in one of the atoms in your detector then you'll actually be able to measure it and so there's a few generations. Experiments are going to happen and. Currently is just starting to be the really exciting time it's the last few years we saw for the first time that those experiments are actually ruling out. Popular models popular guesses of what dark matter might be and we're now technologically ready to sort of to push that to the next limit where we are actually ruling out OK does this have hours of work at all or not. And it's the technology behind it is very fascinating. Shielding yourself from background that you're convinced when you see a little bit of energy again in your detector that that must have been from a collision of the dark matter particle. That's one exciting way we call that direct detection experiments there's another exciting way actually in our best hypothesis that already hinted at the whim of particles weakly interacting massive particles making up that are matter. In that whole theoretical picture it's actually dark matter dark matter and I lay ssion that that sets how much dark matter is still around today but it's a built in the theory that dark matter hitting dark matter in a rare chance can actually annihilate an islet in all sorts of products which depends on what that was made of but many of these models eventually even get some photons out that means that some of the gallery telescopes that we now have flying part of it actually were built to our laboratory slag national exam or the Accelerator Laboratory that we might even get so lucky very good observer get to observe radiation that originates from dark matter in our living environment OK if you know given that that's a possibility of course we've got to look very carefully and see the work the hard part there is you can get confusing because other things also make gamma ray radiation and how do you convince yourself OK this was really dark matter that's why it's not just a trivial job. To do that now and of course another really cool way would be to just make some dark matter. You know just make it in a particle accelerator. And this might actually happen the only problem there is that it's also sort of tricky to figure out. For you to be sure that it was actually dark matter that you made and so what happens is you shoot some particles at each other at some crazy high speeds and it makes a complete mess it's a total car wreck and there might be thousands of new things coming out of this because you put it in such high energy you can create lots of other junk along the way and so what you have to do is you have to do to detect all that junk and add up all the energies and say wait a minute there is this much energy missing because you already know that in the did tech there is that you have around your particle accelerator you can actually find the dark matter that would just escape directly from the detector so you have to look for missing energy. All right so let me just wrap it up a little bit of where you want to take home as you know there is no doubt in the minds of astronomers and physicist that dark matter exists. You know no matter what you read in the newspaper. And the evidence really is everywhere you see this from the cosmic microwave background surveys where we measure millions of millions of galaxies galaxy clusters themselves and they do this by for a strong lancing weakly and saying and of course directly measuring how fast cars are going around in galaxies you have some galaxies where we know there is a thousand times more dark matter than start is just you know it's not a subtle thing it's not like. Not doing your math right or you know you're off our factory of two or stars a little heavier than you think and it's not subtle it was like wow you know every time there's just so much more undetected mass. To explain how. Stars moving galaxies OK Now of course the exciting bit is that you know. With the with a bit of luck we may in fact very soon find out what we're made of and so let me just. Do a couple of things because we did talk briefly about the house structure very early form. And it was just a couple of personal things to tell over. Ways are a funny thing in a way right because you're throwing a rock in water and there is this information that travels away from wherever the location was where you put the rock in but the water molecules all they doing is they going up and down and there's actually no element moving along so there's a little flawed philosophical thing. That I always find so fascinating about is that it's just with the elements in our body like we exist but the elements they move through us in the sense of over your lifetime you keep changing out all the atoms that. You know make you so all replaced by food I don't have any atoms in me anymore that I had when I was a baby. And it sort of just moves through but it's all these atoms have these incredibly interesting histories of where they were made were made essentially all of them of all the ones that are not hydrogen in your body. Are older than five billion years the hydrogen itself was already made right at the beginning of the Big Bang thirteen point seven billion years old and so I usually tell my students a much older than they think. Right at the fascinating thing is to me was when the first rays are a thing about it is there's probably about one hundred billion stars the contribution in making all the elements that are inside of us so it's not just some rare little thing that is like one star went off and made to carbon oxygen and that later allowed the sun and earth to form it's really sort of a collective thing. You know. Un-American idea but anyhow so there is a one hundred million. Things work together. And we just heard about it so that then get mixed over this time as all these building blocks get put together to become our Milky Way. When that finally makes the earth you know all our bodies contains such a huge number of atoms that we actually sample all these trajectories all these individuals are they came came on us. So there was. You know it's just integrated how much the atoms that we have are actually connected to the whole whole body but that has this bizarre consequence that if you think about where they all came from in space. Is the sun million light years or so so you know so I'm going to be slow in my diet because they used to be really huge and yeah thanks so much.