The Hunt for Dark Matter in the Universe: New Experiments

[Music] frack noi i'm the emeritus chair of the astronomy department at foothill college and it's a great pleasure for me to welcome everyone to the silicon valley astronomy lectures by remote control what we have done is to shift to virtual mode during the pandemic and so those of you who are returning audience members i'd like to welcome you and those of you who are here for the first time please join us on a regular basis these introductory astronomy lectures are provided thanks to the support of four organizations the foothill college physical sciences division the seti or search for extraterrestrial intelligence institute in mountain view the venerable astronomical society of the pacific and the university of california observatories which includes the lick observatory and i want to just say a word that the recent california fires actually did threaten the lick observatory one of the building's peripheral buildings burned down but thanks to the action of cal fire the main observatory is fine and they are now assessing uh some of the smoke damage and trying to bring things back online so we congratulate them for surviving this very difficult situation in any case we are here to present information about new developments in astronomy um and i should also say that in the last month or so one of our past speakers got the nobel prize in physics so that sets a high bar for all our future speakers um we are going to have questions tonight i should say and there is a notation i hope under me as i speak and there will be a notation uh showing on the screen of a special dedicated email address to which you can send questions and then when our distinguished speaker is done dr jeff matthews the astronomy professor at foothill college will be moderating questions uh and answers so by all means use that email during the talk to ask questions and we'll try to get to as many of those as we can so let me now without further ado introduce our speaker tonight talking about the hunt for dark matter in the universe new experiments this is one of the most exciting aspects of our study of the universe this invisible material that must be out there but whose nature we don't understand and we're privileged to have someone speaking about it who is involved in the experiments that are trying to pin down the nature of this dark matter dr tom schutt is a professor of particle physics and particle astrophysics at the stanford linear accelerator center and part of the coble institute at stanford university he has spent most of his career doing experiments to search for dark matter and has specialized in the development of instruments to do just that he was co-founder of the lux experiment and founding spokesperson of the new lux zeppelin experiment which he's going to be describing for us tonight so ladies and gentlemen it's both a personal pleasure and a professional privilege for me to present to you dr tom thank you very much andy and welcome to all of you i'd like to thank the organizers for giving me the opportunity to do this and um it's kind of a special treat to talk to people who are interested in astronomy uh my background is sort of more from particle physics perspective because i'm looking for dark matter uh assuming it's some sort of particle and so i'm gonna try mix of talking about the astrophysical motivations uh for why we think there's dark matter and the way we're going about looking for um one possible form of it so let me switch over to my um my slides um and uh yeah so i'm going to talk about the hunt for dark matter in the universe and new experiments were really one mostly one new experiment but um uh here we go so uh starting point is uh that we understand gravity really well and by really well that means you know if we go back to the time of newton we understand gravity very well this is uh this wonderful diagram maybe some of you have seen it from newton's principia you imagine is a person standing on a hill so here's a little diagram of the person and they throw a projectile uh a certain amount you know some speed it goes to d they throw it harder it goes to e they throw it harder still it goes to f and then you see down here well if you're on a planet if you're in the earth uh there's an orbit that there's there's a one speed you would throw at where it just perfectly uh circles the earth and that's that's to balance and i apologies uh i know this is a general talk this is the only equation in my talk uh but very basic physics if you balance the so-called centrifugal force uh versus the gravitational force you get this very simple prediction if just uh cancel out the little m there that the velocity goes like one over the square root of r it's prediction for how much the velocity of this a projectile orbiting the planet would be what that velocity would be depending on how far away it is from the planet or the planet it could easily be the sun so over here is what we have for the solar system this is the speed of the planets versus the distance from the sun and there's the dots are the data and the blue line is just a the what this equation predicts a simple one over the square root of our behavior and it works perfectly well um and in fact you know famously of course gravity locally is very weak you know you and your your neighbor or your spouse uh you know have a gravitational force of attraction between you but it's so small it's essentially it's very hard to detect nonetheless people have done precision measurements and in fact um people have confirmed newton's formula for gravity down to the fraction of a millimeter scale um some of these measurements are in stanford but all around the world it's been a cottage industry of making these tiny little pendulum systems uh and and isolated from all forces and seeing what gravity is like and yet if you look at the solar system uh it's also working out to the edge of the solar system which is more than a billion miles so over a huge range of length scales the gravitational relationships that uh newton worked out in the 1670s uh are exactly um that's that's that's the way nature seems to be and um every satellite that's put up is just proof of that because you use these equations to calculate what's going to happen so simple question let's go to a bigger scale we've seen that gravity is newtonian on the scale from a fraction of a millimeter to a billion miles let's go to a galaxy and of course i think everyone at an astronomy talk knows what a galaxy is it's 100 billion or so stars uh in it when it's an elliptical galaxy in this thing that is slowly rotating and you can ask the very same sort of question it's it's an object if i'm out beyond the edge of this galaxy i more or less see the mass of the galaxy in the center and i have some velocity if i'm uh some object that's bound to the galaxy and orbiting around it and it and those measurements can be done they're not completely easy um they really came of they came to fruition in the seventies in fact the woman in vera rubin uh was one of the main people who sort of really pushed this thing so here's here's another photo of a galaxy and this time this galaxy and this data down below line up so this is the same sort of graph i showed you before it's the velocity of an object orbiting the galaxy versus how far out it is from the center of the galaxy and the scale of it is such that the sort of the visible edge where you kind of see the stars petering out is about here and according to the you know newtonian gravity and it doesn't really matter if you think about einsteinian gravity it's the same same same holds here um once you get out beyond the edge you would predict this curve here that uh you're falling off the velocity is is going down just like it was for the solar system the details are a little bit different because the galaxy has some stuff out beyond the real bright edge et cetera et cetera but all that is folded into the calculation of this curve um the data are these points up here and there's this huge discrepancy between these points and those points um some of it is there's a lot of gas this is uh what you predict due to the force from the from the um the gas uh uh um but it still doesn't account what's going on here is it appears there is mass that is holding on and and giving the gravitational force which is allowing objects which are orbiting this galaxy to be orbiting faster than they should and i'm sorry in the last slide i should have made the two slides ago i should have made the point of course you know what's going on here is something familiar to you if you've ever been on a um on a merry-go-round or a little you know when you're a kid on a playground um you know uh um merry-go-round that swing that goes around in a circle real fast right if it spins real fast and you're trying to hold on you need a lot of force and the faster it spins the more force you need that was embedded in that equation i showed you so out here beyond the edge something is allowing objects to orbit the galaxy at really high speeds that just don't seem reasonable given the weight of everything in the galaxy then you can say well how do we know what's the weight of everything in the galaxy and here as a particle physicist i throw my hands up and say well the astronomers have figured this out you know we understand stellar evolution uh we understand you know you can look at the spectra light spectra of all the stars in the galaxy and we have a pretty good idea how much that galaxy weighs in fact a lot of it is gas and you can measure that in different ways radio infrared etc and it just doesn't work and so we think that there is mass present that is not seen and for lack of a more imaginative term it's traditional to call it dark matter it wasn't glowing uh but it's more than it wasn't glowing in fact it in no way can be seen with a telescope people have looked and looked and looked it doesn't absorb light it doesn't emit light it simply doesn't interact with light that much is completely clear and the amount that's there is about a factor of seven-ish more mass overall in this thing than is there in the normal matter that's in the stars and the and the gas and the dust so then we can go to bigger scales still because this this problem didn't pop up in the solar system but now we're you know we go to the whole galaxy and it pops up well what about a bigger scale and the next bigger thing in the universe are galaxy clusters so galaxies tend to be in groups of a few order of a thousand or a few thousand galaxies were in a close cluster here is a famous image of a distant cluster of galaxies the galaxies are seen in yellow and in this image uh was one of the first really striking examples of now what is a almost a mainstream tool in astrophysics and astronomy that is to use gravitational lensing so gravitational lensing is worthy observers here who took this photo the you know of course a nice telescope um the there's a lensing object which is this whole cluster of galaxies it's a huge number of stars in a huge in a thousand galaxies they have a lot of mass and if there's some object back behind them by einsteining um einstein's you know gravity you can calculate that that that that mass will distort and bend the light it essentially will act like a lens so here in this little diagram we've labeled it the lens and then if you think well i'm the observer and i'm looking back this light which was bent appears to be a source here and in fact a single source if if everything is exactly collinear and the lens were completely spherical you'd get a ring of em of image of that source but in fact when everything's a little bit messy and this is not a completely spherical collection of things you get just a set of images and that is all these blue streaks they are um most likely of a quasar a distant quasar it's it's very early and bright and it's bluish just that was its color and all of those arcs are coming from one object back behind this galaxy and then you can take einstein's equations and you can essentially do an inversion mathematically it's kind of like tomography and you can ask what was the mass of the lens required to make the one object create these all these blue smudges and it is this beautiful map here which is sort of famous in the cosmology community um the spikes are galaxies but look be between the galaxies there's just this overall kind of smooth distribution of mass and remarkably the amount of mass there uh the ratio of the total amount of mass in this image to the spikes which the galaxies is again about a factor of seven so um we come to this the statement that sort of there's something called dark matter out there it outweighs normal matter by about a factor of seven and it does it on two very different length scales now um the size of a galaxy it's about 10 kiloparsecs and size of galaxy clusters which is mega parsecs thousand you know 100 times bigger if you go to even bigger scales still what do you see well first off and maybe this is known to people but we've kind of learned that the universe is made of swiss cheese or at least that's the way i like to think of this this is um a set of uh one of now a bunch of images which are taken from really big surveys of galaxies that are come from people you know studying the cosmology of the universe and where you see colors that are those are galaxies and where it's dark that's absence of galaxies and so it kind of looks to me to my mind like i like to think of this kind of looking like swiss cheese where there's holes and then there's kind of stringy or sort of uh sheets uh where there's the cheese okay well so what about dark matter well this this alone doesn't tell me about dark matter although more or less but i wanted to kind of frame things we're looking at now really really big scales and the universe kind of looks like that um and so in fact well i want to take even another jump and think about the big bang so here's just an image in fact this is uh from the wikipedia page on the big bang um and it's kind of a modern view of the way the big bang went there was some starting condition that we just don't understand where all of the matter in the universe was compressed to some tiny spot and there was something that put it in a situation that you know now it exploded effectively expanded and there was an early time where it inflated and then the universe acted kind of normal after that um i want to draw attention to this point right here this is the time at which it was no longer sort of like a plasma where um uh it was matter in fact at this point in time the universe had changed from being weird exotic matter of some sort at incredibly high energies that we cannot attain in our laboratories today to be normal protons but at an early time it was really hot and the protons it was hydrogen gas if you will but it was ionized it was so hot it was ionized so what does ionized mean well ionized is very familiar it's the center of a flame in a flame uh if the flame it's hot enough that electrons are ripped off of atoms and so you have charged electrons and charged protons if you only have hydrogen and and light the the glow of the flame and the light actually doesn't go very far and it hits an electron or it hits a proton and the three of them are all kind of scattering it's just like this little soup and that's to the left of this of this time and then things cooled down enough and uh the the universe became transparent uh the electrons fell onto the protons now they're and it's cool enough it's no longer a flame the flame went out and the light that remained just kept going and the rest of this diagram is that then all sorts of stuff developed and i want to talk about that a little more because it turns out dark matter has everything to do with this picture everything to do with this picture um in fact this is a simple um i played with powerpoint uh view of what that light from the big bang looks it's called the cosmic microwave background it was famously found in the 60s um you know many nobel prizes later we know a lot about it and the one of the remarkable things is it's the same temperature in the same color if you will in all directions i mean it's light it was very uv light it's been redshifted into the radio and it can be measured with radio receivers and it's more or less one temperature in all directions however if you look in detail it looks like this and this is a famous result that was measured about a decade ago after a heroic effort by a lot of people over many years and at about a part and a hundred thousand in some directions the light's a little redder hotter in some directions a little bluer and what that really is telling us is that this plasma of gas of proton of hydrogen and electrons in photons was a little bit lumpy it was very very very smooth it was smooth to a 10 parts per million about a part and a hundred thousand it was a little lumpy and wherever it was more lumpy was a little heavier um those places over time if i just go back to this diagram as we go to the right wherever was a little heavier um gravitationally grabbed material from around it just wherever it's heavier stuff falls onto it material falls onto it wherever it's less dense stuff goes away and so the contrast between what's red and what's blue just grows with time and cosmologists have had a field day calculating this and do we have we measured it well enough can we calculate can we predict can we predict from how we went from this picture which is mostly uniform but in fine detail a little bit lumpy to this today where on big scales the universe looks like swiss cheese and to be clear when we say you know swiss cheese i'm really saying empty space and very solid objects huge difference between the density of earth and the density of space which is you know almost completely empty but has some trace gas in it whereas in the early universe everything was the same density density to about 10 parts per million and the answer is yes we can calculate this in fact i'm going to show you a beautiful movie um i actually have to pop out from um my powerpoint over to uh a youtube video the beginning of this video is this is a simulation and the beginning of it it's essentially the plasma that's at one temperature in fact it's well it's after it was a plasma it's now just neutral gas and you see a little bit of lumpiness and i'm about to hit play and you're going to see the slumpiness change rather quickly but this is very early in the universe there are a lot of people in the world that do simulations like this this isn't happens to be a group that in fact are at stanford and slack that i know very well they did these calculations this involves heavy-duty heavy-duty computer simulations because the gravitational equations are very complex the gravity is all straight forward but it's just uh the equations are complex and non-linear and need to be simulated and it's just a natural consequence of gravity acting in a situation like that where this that's a fluid that's largely all one density but the whatever it's a little bit heavier accumulates wherever it was less dense sort of loses material that the universe grows to something that looks kind of like what we have with these kind of filaments and kind of sheets of material and then these dark these these very bright spots which are going to be i think probably clusters of galaxies at the scale of this simulation and it's just really a remarkable success let me just show you this again of of of our understanding of the universe that this calculation can be done and i just think it's a very cool video to watch and when you do this calculation you put in the known laws of gravity and then the other thing you put into the calculation is what matter was there in the in the universe there's this there's this gas right um it turns out this calculation fully does not work at all if you just put in uh the gas that we know about that was there in the early universe the time of this a cosmic microwave background in fact it completely fails in fact um there's a dirty little secret this calculation i just showed you this video only showed dark matter only show dark matter in fact that's the only thing that's in the calculation we can essentially understand and explain how the universe uh went from all of that simply through the dark matter and and so and and and i and i'll skip kind of the details why but it turns out that in that story not only um is the dark matter something dark we know that it can't even be made out of any of the normal elements it can't be made out of the hydrogen that was there in the early bang or anything later that ever came out of the hydrogen which is essentially everything that we know about you know the hydrogen formed stars the stars finally went supernova that formed all the elements the elements that are in our body the elements that are in the earth everything that we know about was not the dark matter when viewed um from the way the dark matter played the starring role of growing the structures of galaxies uh that we know about in the universe so that's really kind of an interesting thing not only is uh the dark matter is there dark matter oh my video didn't show very sorry um the video was really pretty and i'm gonna stop and uh and i'm gonna show i'm not gonna i'm just gonna do it really quickly but i wanna show the video again so here's the video of the universe starting in a very uniform state and now evolving and uh structures forming and uh so this is going from an early time in cosmology to essentially the present day and i'll let just this run just once and you know the bright spots are either galaxies or clusters of galaxies and it's really just an incredible achievement that people can simulate how we went from the big you know these this measurement of of the gas the fireball from the end of the big bang uh to today and you know our mathematical understanding of the universe it all works um except that we have to stick in um that there was this enormous amount of dark matter so one reason i also talked about the big bang so much is that well the early big bang was unimaginably hot and dense very very high energies if it wasn't hot enough for you go back another few seconds and it was unimaginably hotter and if that's not hot enough go back a little bit earlier closer to the moment of origin and it was incredibly hot still hotter how much hotter still and the result of that is that almost certainly it had to have been that all sorts of high-energy particle phenomena were present in the early universe so particle accelerators of the last 50 years we've created you know we've smashed two protons into each other or an electron into an electron or electron into a anti-electron and you can create this whole zoology of interesting particles uh that are heavy and exotic uh but that live a short time and so most explanations for guesses as to what the dark matter is focus on this idea that hey the early universe had all these weird particles in it the ones we know about that we've studied at accelerators are all short-lived but what if there's some type of new particle that is not common around us that we think that we know about but that was created in the in the big bang um and didn't decay away and is and remains today and that the dark matter could well be one of those particles and um and and so that's what um uh uh i'm looking for so let's just talk about the known particles a little bit i'm not gonna i don't want to talk you through this table but particle physics has resulted in us having something like the uh periodic table the elements these are all of the particles the fundamental particles we know about for instance the up quark and the down quark make neutrons and protons so your body is made of up cork and down quark and electrons but it turns out there's another family of very similar particles they're heavier they live a little while then they decay into those particles and similarly an even heavier sat then there's these weird ghostly particles you may have heard of called neutrinos they're incredibly small they're actually stable and they don't do very much but they they exist and they're created in nuclear reactions and then there's these particles that carry forces like the photon um and these things called w and z which are heavy particles they weigh as much as an atom uh heavy atom um but the z in particular is incredibly tiny so actually if the dark matter were like the z particle uh but lived the z particle lives a tiny tiny amount of time like 10 to the minus 20 seconds um it could be the dark matter if the neutrinos were heavier they could be the dark matter um the reason something like the z particle and the neutrino could be the dark matter and we wouldn't have kind of known that is because they're incredibly small they don't interact much so i wanted to talk a little bit about what do i mean by a particle being small um the modern particle physics view of particles is sort of interesting it's that all particles in fact the fundamental particles the ones on that last chart are points they're point-like we don't quite know what that means it's kind of like not knowing what a black hole singularity is we don't know how tiny they actually are if there's some hard nugget at the center and we haven't ever seen anything like that as far as we can tell they somehow have mass but they essentially are make themselves known just by their force fields and the force fields have different sizes so there's an electric force we all know about the electric force and i've kind of colored it in blue here an electric force field which is about the size of an atom if you give me one fundamental unit of charge from an electron or a proton it has a force field that extends over a range about the size of an atom to get electricity you have to have a lot of electrons or a lot of protons there's a force that holds together nuclei and it's called the strong force and it has a force field that's the size of a nucleus which is a hundred thousand times smaller than the force field than the size of an atom and the weak force which is not very familiar to many people but you may have heard of in a class or something is this other force that mostly manifests itself in radioactivity but also in particle accelerators we can see that it's present and it has a force field that's about a hundred thousand times smaller than the strong force so the size of an electron is really the size of its electric field you'll often hear this funny statement that fundamentally uh at an atomic level matter is mostly empty space because you maybe have a model of an atom that has a nucleus and an empty space out to an electron of course matter is not fundamentally empty space that is absurd if you stub your toe your toe hits the wood you know the door jamb they didn't yield your toe didn't go through the door jamb because at atomic level it's mostly empty space the point is it's force fields and to say how big is something is to say how big is its force field why did i tell you all that well you know the way we know about small subatomic particles is by smashing them into each other it's kind of that's the only way we can know about them is the hard cold fact you can't like grab a proton in your hand and look at it so here on the right i have a just a little cartoon of a proton and it has an electric force field it has a strong force field and it has a weak force field a proton has all three and if i send uh if i send a neutron at it here's a neutron it only has the strong force field and the weak force field most likely it just passes right by it didn't know that there was an electric force field as far as it's concerned this this proton is tiny it's the size of a nucleus an electron would say hey that proton is the size of an atom if i get lucky and i'm a neutron and i just happen to hit the proton then it'll collide off so let me try to run those little those little videos again so here's here's a neutron passing through by a proton if it doesn't hit dead on it doesn't care that the proton has a big electric force field um and here a neutron if it happens to hit the dead center it does so if i have a neutrino that has this tiny force field it sees a proton okay i'll say it it sees it as mostly empty space it totally ignores the electric field it totally ignores the strong field it just sees this this weak field that's somehow associated with the point-like nature of the quarks and the proton and if i get so lucky as it happens to dead on strike one of the quarks whatever that means it's a point like thing and it interacts with that weak force field then it'll collide which almost never ever ever happens so the neutrino is a thing that has mass but is incredibly incredibly small because it carries a force field which is very very small an extent and the final thing i want to tell you is that size and mass are not related and that is really actually one of the real interesting things here um a z boson which is one of the particles we know exists uh it's you know it plays a big role in all of the measurements that are done at cern where they found the higgs blah blah blah it weighs as much as a heavy atom but it has only the weak force which makes it incredibly small the neutrino also only has the weak force but it weighs about a trillionth the mass of a gold atom so two totally different massive particles with the same size if you will this one is stable it's too light to be dark matter this one was perfect to be dark matter but it decays in 10 to the minus 25 seconds so that gives us an idea for what we think the dark matter could be we think the dark matter is uh something that has maybe about the weak force of particle physics which makes it tiny it will also have gravity all particles have gravity and i didn't mention that but that's basic to um in fact einstein figured that out it's embedded in general relativity if a particle has mass it has gravity but uh so that's why it's gravitationally shows up as dark matter but it only has the weak force but it's heavy and we don't really know how heavy and it turns out we don't really know exactly how weak it would be in this in this theory but it's called weakly interactive mass of particle or wimp which is kind of this funny this funny name that arose about 25 30 years ago so how are you going to test this idea here is our dark here's our milky way artist sketch of course there's no photo like this and it's embedded um that the dark matter in every galaxy isn't really kind of in the center of the galaxy it's sort of in a big halo that surrounds the galaxy and we know that from exactly the sorts of measurements i showed you earlier about the velocities of things orbiting the galaxy um okay fine so there's the dark matter whatever it is is in this halo in fact here's another cutaway version um perversely in both these uh images the halo is uh white even though it's dark you know dark so if i'm a particle so if this whole halo of matter out here is made of these subatomic particles they are orbiting the galaxy with about the speed um we don't have to guess at that the the speed at which this thing orbits the orbits here is just given by the gravity in the galaxy and we sort of know what that does it essentially it dictates the speed at which the disc of the of the of the galaxy rotates in fact that's the speed the milky way is busy is busily rotating about the center of the galaxy it's going to take us forever to get there millions and millions of years but it's in fact it's not a small speed it's about a thousandth the speed of light so all of the dark matter here whatever its form is orbiting around and it's whizzing around at about a thousandth the speed of light we also know basically how much dark matter there is everywhere and we know about roughly how much dark matter there is on average around earth and it's about one of these particles per liter which by the way that's a really small density you know another reason why dark matter is not easy to found so far is if it's kind of diffuse it's about as dense as the interstellar medium actually just a gas that's randomly you know between stars um the reason it's seven times more is because it just extends everywhere and there's more of it um but this gives us something to shoot for we this is actually kind of nice i've actually said i think i know how big it is how physically it's got a certain size i've said how heavy i think it is in the wimp idea it's about as heavy as an atom a heavy atom and i know how fast it's moving i can predict what's going to happen occasionally it will bump into a detector on earth occasionally bump into the earth um let me just stop for a second here though an interesting thing if you have a weakly interacting particle let's talk about neutrinos for a second and you wanted to stop them you wanted to put up some material such that the neutrino was guaranteed to run into it i want you to think about going to the dentist and getting x-rays right they put a little lead apron on you uh to stop the x-rays from going into your organs and make it just go into your teeth or whatever right that apron will be maybe i don't know a thin sheet of lead 32nd of an inch of lead something like that right and that will dead stop on all the x-rays um you can think of an x-ray as kind of a high-energy particle right neutrino high-energy particle they are basically born at high energy and weird reactions how much lead do you think it would take to stop to stop uh neutrino and and here normally if i could look at look you all in the eyes i would see anyone nodding or not if they know the answer to this but here on zoom i'll give you a second to think about it so the answer is if you want to stop a neutrino you don't put a 32nd of an inch of lead you don't put a meter of lead you need a light year of lead let me repeat that you need a light year of lead to stop a neutrino that's how small a neutrino is neutrino is so small that all of the atoms and all of the lead it seizes these tiny little things where it sees the weak force on the on the on the on the nuclei and the quarks in the lead and it needs a light year blood to stop it so that's the problem trying to find wimps if the dark matter is these particles if one goes at the earth it's overwhelmingly likely to go right through the earth if one goes at a little detector i have it could maybe bounce off of an atom in the detector but boy is it a rare process so looking for dark matter consists of uh in the form of wimps putting out a detector so here's a particle detector this is uh something that would measure x-rays say if you see a movie where there's radioactivity like the great movie about chernobyl or whatever and people have the geiger counters and they you know when they're met those are particles striking it and they make an electrical signal so my business has been making interesting detectors where the wimp would strike it and you would sense that some energy had hit um the amount of energy deposited in the detector is rather small by particle detector standards it's um it's uh uh at the level of like um um uh of an x-ray in fact in fact like type x-rays you'd go get your teeth done with which is small compared to like uh measurements of the colliders in europe at cern or the collider at stanford slack those were like literally a million times more energy in every particle but you can build detectors that will do that okay fine so that's the idea a wimp will occasionally just strike a detector and you'll say aha i saw a wimp well it's not that easy so first off um the earth is constantly getting bombarded by cosmic rays these are high energy particles from outer space um actually they come from really interesting things they come from like supernova they come from like the supermassive black hole the center of our galaxy there are these high-energy protons and other mostly protons but other things they hit the atmosphere and they can then and they kind of blow apart you know the first the first atom they hit and you get a stream of particles and the only real way to deal with that is to go underground in fact you need about a mile of rock to arrange those uh cosmic rays out okay and in particular it's the muons the heavy the heavy cousin of a of an electron that i showed you in that table of the known fundamental particles the muons that are created by the protons blasting apart nitrogen atoms in the upper atmosphere will penetrate hundreds of meters of rock and so people set up in underground locations and then they've gotten rid of that problem and it's a real problem one of these goes through your hand uh a minute um and then these wimps are only dark matter is only very rarely gonna interact with the detector so then the next problem is that we're surrounded by radioactivity and here i'm not talking about radioactivity like scary radioactivity i'm just talking about there is a little bit of trace radioactivity everywhere everywhere around us and so we're in an underground cavern but there's all this radioactivity coming off of the rock so i put up a shield and i can pick certain materials people know how to make what there are materials that are much better in not having radioactivity it's kind of a happenstance of the chemistry so copper for instance turns out to be very low radioactivity silicon starts out to be really high in radioactivity but people have been very very very motivated to figure out how to purify the heck out of silicon and when you purify silicon enough to work in a semiconductor like in a silicon chip it turns out it's gotten rid of all the radioactivity so we can surround the detector by a shield and it turns out lead is not very high it's low in radioactivity and stop most of the radioactivity from coming around so that's how we're going to hunt for wimps we're going to put out a detector we're going to put it in a shield we're going to put it underground and we're going to be really really really careful about the materials we pick just a couple words about radioactivity i made this little chart um the vertical dose axis is radioactive dose and i want to give you a scale of the problem here this here is the annual human dose on average uh this going up about a factor of a hundred in dose is what the fukushima worker saw and they were fine about a hundred fold more dose is fatal and the poor people who put out the problem at chernobyl got about 10 minutes of chernobyl which was more than fatal uh let me focus here if you eat a banana this is the dose you get bananas are a little radioactive and it's a lot less than the average annual dose going down lower our detector lz the experiment's called lz and we have a detector we have removed through careful shielding all the radioactivity to get the backgrounds down to about here but then it turns out we then are able to just get rid of the backgrounds that have even the radioactive particles it's mostly gamma rays it's from uranium and thorium and the rock we remove the radioactivity that hits the hitsar detector and i'll explain that a little bit more in a second and the problematic backgrounds in our detector are way down here and then if we focus on the energy range where the particles that we're the wimps may be we get down here and i just want to point out what's going on with this scale the the average normal dose you get walking around is i don't know what's this about 100 10 000 times lower than what would kill you if you got it all in a short time but that annual dose is something like 16 or 15 orders of magnitude higher than what we have to get to to be able to see the dark matter we are doing um the proverbial needle in a haystack experiment the hay is the radioactivity from ordinary uranium and thorium and potassium which are the main radioactive elements they're decaying all the time they put out gamma rays they put out beta rays which are x-ray electrons those things kind of look like dark matter and we have to get rid of them so i mean let's talk about our experiment some uh you have to go underground and there's a few locations around the world we happen to be in one in south dakota and the black hills uh in fact it's from the gold mine that originally was the fortune that's launched hearst herst father not the newspaper man his father made his money in this gold strike in south dakota and if you've ever seen the tv show deadwood it was about this mine what became this mine um custer's men discovered the gold in the black hills and and that forced off uh the sioux and the other tribes after they had just been pushed into the black hills and that led to the wars that custer got killed in very interesting place to work uh it's actually beautiful starkly beautiful there was a big gold mine um the equipment's from the 30s mostly although the gold mine dates back to 1880s or so that's in the winter it's interesting place to go and do an experiment uh there's this crazy thing called the sturgis motorcycle rally which is about 15 miles away and unfortunately these guys went and had the motorcycle rally this year and now coronavirus is going nuts in south dakota but culturally it's a lot of fun to to be there i took that photo uh one of the years i was there about 700 000 motorcycles show up in this little town of 3000 people um and just jumping ahead this is what the previous experiment um the lux experiment uh was like we have a detector which is about as size as a person this is one of my students and the wimp is going to strike it and we're inside of a big tank and the tank is about to be filled with water and in this case the water is the basic shield you can purify water very very well and then it serves as a good shield against all the radioactivity that's coming from the rock outside this is a photo from outside the water tank so you know this is just normal well it's weird place for underground et cetera but this is just normal material full of radioactivity water tank water shields the detector and then the detectors inside how does the detector work um the detector is made actually it's kind of interesting of a big vat of liquefied xenon why xenon well all of the noble elements actually does helium neon argon krypton and xenon make wonderful particle detectors in fact the geiger counter has probably argon in it i think that the reason is is that um if you when an incoming particle strikes an atom in the xenon uh it emits a little glow of light in fact probably most materials will do that if a sub if a subatomic particle comes in and smacks an atom and makes it angry often the atom will relax the will by emitting light but in most materials the light's immediately absorbed in the noble elements light gets out also you will knock electrons off of the atoms and in in in the noble elements the noble elements completely inert if you remember your basic chemistry you know those are the ones that have the close shells completely inert an electron can't get trapped on a xenon atom or a helium atom so we have these electric fields um we have sorry we have an electric field created by it's kind of just shown here in a light a light color here um a mesh of wires that are that are at a high voltage and they make an electric field and the electrons that were that are knocked off of atoms when the particle hits the xenon are drifted up to the top of this detector and the lz detector is big it is going to have seven tons of xenon the lux detector i just showed you the photo of in the center was about a um uh um about a half meter it's about a two foot by two foot uh dimension object that had was full of liquid xenon this is going to be a much bigger version and we can sense where the particle interaction happened very nicely because the electrons come up and when they come to the top where the of the liquid we pull them out of the liquid into gas and we have a high electric field there that forces the electrons to fluoresce the glass gas much like what's going on in a neon lamp and much more like what's going on fundamentally in a plasma tv and there are these objects these little circles here are called photomultiplier tubes they are single photon sensitive sensors and they measure the the path the splash of light created by the electrons coming up out of the liquid the initial flash of light from when the first atom was atoms were struck also as measured in these light sensors the time difference gives you the depth of the event and the pattern of light from this this bright flash up top tells you where laterally in xy if you will the event was and so remarkably in a huge tank of liquid xenon we can measure an event we've only put sensors on the top and the bottom the sensors are always a little bit radioactive radioactive so our our challenge in removing radioactivity is reduced to just getting the xenon clean the radioactive sensors around the perimeter and yet we've measured events everywhere in the xenon and it turns out it's very it's relatively easy to get xenon very clean from radioactivity because there's an inert element there's these simple purification techniques that just take out any junk that would be in the xenon and it's in the gas it's easy to purify gases compared to like a solid like a metal or something which you have to melt and do stuff with and gas you can just start working with immediately this is what the data looks like this is a candidate dark matter event in the lux experiment the previous experiment so um this is time here uh it's it's it's 200 millionths of a second there was first two photons measured from this first flash of light and then a bunch of photons measured when this what from the electrons creating the second splash of light the time difference from here gives us the depth the pattern of which pmts were hit which is kind of this number over here uh tells us where it was this could have been a wimp uh but unfortunately this was actually calibration data we can we can we can we can simulate the wimps and this was from the calibration data um but just i think it's kind of an exquisite uh technological thing that in in this case it was 300 kilograms of xenon we could measure this very low interaction very low energy interaction that that created two photons and this is probably about five or ten electrons which created this signal this very very faint ghostly interaction and we and we measure it very well and we know where it was and um and and again the radioactivity is all around the periphery and we look in the center of the detector and that's clean and um and uh we you know we can look for our wimps there um and even a little bit more detail than that um i know i've given you a lot of detail this is kind of the last detail i've got when the a background gamma ray so the gamma rays are these are from radioactivity when they strike an atom what happens is they knock an electron out of the atom at high energy and that electron goes careening around like a i don't know like a it just just weigh a trail of destruction ball in a china shop this is a a a computer simulation a monte carlo simulation of what happens and all the spots in blue are where uh another atom hit hard enough to create lighter charge or scintillation or electrons and this whole thing is in a very small this is microns if that that's a millionth of a meter this is about one percent the width of a human hair is the total range of destruction from this from this event that gave me the charge in light that you saw in the last my last my last slide the dark matter by contrast let me take a little space here that's very small much smaller than my percent of a human hair blow it up the same energy dark matter creates a similar looking kind of path of destruction a little different in detail but the main point is this is an incredibly small scale this is about the width of a hundred atoms all the destruction all the energy is confined to a small space compared to here the wimp events are much denser and that gives us a difference um sorry coming back to this image in the amount of initial light and the number of electrons which give this second flash of light the ratio of electrons to photons is very different simply because the density here is very high and the density here is low so here's our um the the the data from the lux experiment i divide the amount of light a charge by the amount of light number of electrons by divided by the number of photons on an axis here two two different plots in the top we calibrated the detector by a by taking in a deliberately bright source of radioactivity and putting it near the detector or actually in the detector in fact we dissolved tritium into the detector and then took it back out and you see that um we get all these events the black dots each black dot is from a quote-unquote event some of the decay in the detector um and there are between these two blue bands then we calibrate the signal the dark matter signal neutrons are radioactive background which also interact with um like the wimps do what they do is they strike the nucleus of an atom instead of the electrons in an atom i'd forgotten to say that but that was the difference to give a different track um and so the the the dark matter or neutrons striking the nucleus of an atom uh populate they make less charge compared to light which is just a consequence of the density of the whole thing being different and this population is well below this blue line this blue line is the center of where these events are this red line is the center of where those events are the dark matter will fundamentally look different than gamma rays and it turns out we can essentially the neutrons are rare in radioactivity and this is really our problem and not the neutrons so the final data from the lux experiment which is from 2013 had this final data set here are the data all the all the little x's are events of unknown origin it's a smear some of them spilled over into the potential dark matter band but if you just look at this all of them are at the upper half if there were dark matter present it would be centered around the heavy you know the central line and so sadly after a lot of sophisticated statistical looking at this this is incompatible with their having the dark matter in the in the detector we this was the world leading result when it turned on and was a world of leading result for about three years there have been a whole bunch of experiments like lux looking for dark matter over in fact i have been doing this my entire career the first experiments were in the late 1980s um they looked very different than this experiment but basically they did the same thing and here we were we got a crack at the apple we we took our chances but we didn't see dark matter so we're working on a bigger experiment um this is uh the same water tank um much bigger detector i showed you the other kind of cartoon of it this is actual you know computer automated design drawings uh we've added an extra layer of shielding this is a active detector in it of its own right in green it's um a cheaper uh type of detector and it's going to deal with neutrons um and we're still on this water shield but it's a much bigger experiment it's 10 tons um i've spent the last five plus years in fact seven years i would say uh working on this experiment uh i mentioned the fact that you need these uh electrode meshes to create high electric fields so that's something that my group at slack did we um here's a woven mesh of stainless steel it's kind of boring to talk about looks like a a screen door but there's a huge huge effort it's um to do this all these materials are very radioactively pure et cetera et cetera we did a lot of testing so here we are testing a prototype of the detector this this is a long skinny version of the of the full detector and we we basically learned how to uh elements of how to design those grids with this testing and there's the detector so this experiment lz has not turned on it is about to turn on this was last winter just before covid hit we had assembled the detector um i didn't really tell you the walls of the detector which were sort of hidden in my in my images are made of teflon which is highly reflective and so this is a you're looking at a bunch of teflon and on the other side of that is where the liquid xenon will go this is about to get placed in a ultra pure uh titanium uh uh vessel that is cooled down to minus 100 degrees celsius which is what you need to liquefy the xenon um here you can see the array of photomultiplier tubes these are cables uh to read out the photo multiplier tubes there's something like about 700 or 800 photomultiplier tubes in the experiment uh we did a lot of work to make sure there wasn't any dust so it turns out a good way to look for dust is uv light so here's an inspection of the of of the thing we've actually very rarely did we uncover the detector almost the entire life of all these parts they were covered in plastic bags uh um except when we took a publicity photo and when we put the thing in the in the vessel here we are looking for dust in a slightly uncovered area while the rest of it is covered in plastic there's a lot of that sort of activity to keep the radioactivity low and um one other kind of fun thing is and it's a slack one of our groups things is uh removing krypton from xenon so you know xenon is nice and inert and xenon has got no radioactive isotopes it turns out uh krypton which is another noble element has a radioactive isotope krypton 85 it actually comes from more or less from the nuclear industry in france but a little bit from the nuclear industry worldwide and it's in the air it's not really harmful for people but it's way way way hot compared to what we need and when you get xenon which by the way is purified it's distilled out of the air it's a whole interesting story but i won't go into um it'll have krypton in it and we use a technique called chromatography we pass we mix our xenon with helium and we pass the cocktail of xenon and helium plus the unfortunate trace amounts of krypton through a column of charcoal and the xenon comes out slowly this is time and the krypton comes out fast and and this is a generalization of something that's called chromatography which is used in chemical analysis you may have encountered in chemistry lab uh and this can separate the xenon from the krypton and this is a big plant we built uh we have a lot of students in our group this is mostly students this is actually have a partner who's another professor at slack dan acrim he and i run this group and we have this big chemical call it a chemical plant we built that's a charcoal column that's a charcoal column there's some massive pumps around and in fact at this moment while i'm not on the phone oh i'm not online here talking to you all uh uh all of us involved are watching this this plant is busily processing we have 10 10 tons of xenon and we're taking the krypton out of it we're removing it down to something like 50 parts per quadrillion 50 parts per quadrillion that's a less than a part per trillion and at that point the krypton won't be radioactive at a level that matters um this is how we talk about our results um on the vertical axis is the size of the particle we call it a cross section in particle physics essentially if two particles are going to smash into each other in this case the wimp on the zenith xenon atom how how sort of big did they look to each other um a typical nucleus is about 10 to the minus 23 on this scale so we're here about 20 orders of magnitude smaller than a nucleus size the horizontal axis is the mass of the wimp a gev that's particle physics jargon that's the mass of a proton 100 proton masses that's just a an element in the middle of the periodic table and um and this is the kind of range we don't really know any one experiment so lux lux is the blue curve here didn't see anything and excluded this huge space up here if the wimp was effectively bigger it would have scattered more often and we would have seen it we didn't see it and so we're left with knowing that it's less than this the state-of-the-art worldwide right now from a set of experiments which have been neck and neck for some time now and we've been leapfrogging each other is these three lines uh the xenon one time is a little bit out in the lead on this log scale it's only a little bit out in the lead lz and the successor to xenon wonton we're racing to turn on and we're going to get down here in this range uh kind of the mean expectation is we're going to get down here at about to this black line and this green and yellow line is because um we have a lot of computing power nowadays and even while we're building the experiment a lot of smart people spend a huge amount of effort thinking about the statistics of of what we might see and this is kind of the statistical uh fluctuations we might get like when you hear about voter you know voter pull and has a certain uncertainty modern experiments think very carefully about the uh statistical variations we might expect and it turns out to be surprisingly broad but if the dark matter is in this area here we're going to see it this gray area is as kind of um it's a kind of a way to try to say where some theorists not some theorists where kind of consensus opinion in the theoretical community of particle physics uh they're the ones that came up with this wimp idea they're the ones that think it's sensible to think there's a new particle that's weakly interacting and about as heavy as a gold atom uh they they're saying that it might be in this gray area we're including some uh mo some possible models that were now ruled out by lux and xenon anton and his pandex experiment in china but includes all these uh things that we can maybe go find um this orange bit is is is is an interesting thing um about a decade ago even though people have been at this for about 20 years we we stopped and realized that neutrinos from cosmic rays i mentioned the cosmic rays and particles hitting the upper atmosphere though that process creates neutrinos those neutrinos occasionally will interact under detector and technical details but they interact with the nucleus just like the wimps we can't get around the floor of neutrinos and we're approaching it which is to say if dark matter is in for in the form of wimps we may well see it in this next big bite we're getting with the experiments we're just which are just about to turn on if we don't see it and we go a bit further another factor of 10 and still don't see it we will run into an irreducible background that will not allow us to look further it's it's it's not quite irreducible but in practical terms it is and so if it turns out the dark matter isn't is of this story i've told you weakly interacting massive particle and it lies down here sadly we're not going to find out on the other hand it does kind of mean we're not going to be like captain ahab forever and i have been at this for 30 years and we haven't found it so i'm kind of comforted by the fact that we eventually have to stop looking um i do want to just make a point here i am talking i'm talking on behalf of a large number of people the lz collaboration actually has over 200 people now a lot of good friends of mine a lot of young students uh this was a meeting we had it's an international collaboration one of the groups was from portugal this is from coimbra in portugal uh where there's a it's like a 800 year old university wonderful to visit we had a meeting there but this is how science has done big collaborations also i didn't really talk about it but of course there are other ideas for what the dark matter might be other types of particles etc well especially other types of particles and so there's a whole world of people looking for the dark matter if it's a different type of particle but until now for the last 20 years or so people have thought the wimp was the most likely story and the biggest experiments the biggest ongoing experiments have been looking for wimps and this is um so lz is about to turn on it's gonna be exciting time so in summary uh we know there's dark matter we don't know what it is um these weakly interacting particles that would have been built made in the big bang that are analogous to particles we know about from standard you know particle physics are a good guess um if it's wimps liquid xenon detectors have emerged as the way to look for it and we're about it's now exciting time because two major new experiments which effectively have a hundred fold higher sensitivity uh than what's been done before about to turn on and you know two order of magnitude in in this sort of space of how big is the wimp is it's a big deal to get it and advance at that scale and hopefully we're going to find the wimps but if we don't we're soon going to reach this neutrino floor okay and uh with that i think um i i'll finish the talk when you go to questions so why don't you stop sharing the screen and let me in the meantime thank you for this amazing talk showing us the forefront of technology i'm going to encourage people to use the email that you see on your screen to send questions to dr matthews who will in a minute read them but i'm going to take the moderator's privilege to ask two quick questions uh first of all when as reasonably as you can estimate when do you think lz will actually start taking data and the second question is do you have any preparations for success when we search for alien signals with the seti project they always have a chilled bottle of champagne in case uh there's a a moment of success do you guys have any preparations for success yeah when will we turn on uh before the coronavirus hit we thought we would be turning on liter really about now this this fall around now um actually given the nature of the race between the experiments we've kind of gone radio silent on that we are several months delayed because of the coronavirus um but that's all we're saying publicly but it is it is imminent it's you know it's not two years from now let's put it that way uh what will we do if we well yeah we'll go to disneyland and pop the champagne um more seriously if we do see um the particles in it with lz that'll be really fortunate because the next thing you'll want to do it's kind of obvious if you hadn't seen them in the previous experiment given how much bigger lz is best case we'll see on the scale of like 30 or 40. if we were to see more than that in lz then we should have shown up before and that's a little bit marginal that's that's that'd be pretty good but if we see like five or 10 you really would like to see hundreds now what will certainly happen is everyone in the particle in all of physics will get super excited and absolutely we will suddenly have money shower down upon us to build a factor of 10 bigger detector without a doubt and we've all been thinking about how we would do that for a very long time and in fact almost certainly the next experiment will be a combination of our experiment with our our you know currently rival because once experiments get to a certain size you know it makes more sense to work together just cause it's so costly uh so we would immediately start on that big big big detector and fortunately if we do see something in lz with that extra decade before the neutrino floor starts to be annoying we got a pretty good look at it and then of course you'd be trying to look other ways you would certainly redouble the efforts at cern to see evidence for the particle physics that would also be they would go hand in glove with the wimp story because in fact we were hoping to see evidence for something called super symmetry at cern which was one possible reason why wimps would exist and such evidence wasn't seen that doesn't rule out the wimp story but if we saw something it would redouble their efforts because they would expect maybe they're going to see something now i could go more more into why you would think there's that connection but there is that there's that hoped-for connection between what we're doing and what they're doing all right thank you very much and again thank you for the talk i'm now going to introduce dr jeff matthews who's the astronomy professor at foothill college in silicon valley and he's going to moderate the questions from the audience jeff take it away thank you very much andy and thank you dr shutt for coming and speaking with us this evening great talk um i would also like to thank all the people who have sent in questions far more than we will be able to get to this evening um and so thank you for sending your questions to astronomy foothill.edu and so uh let's go ahead and dive into those we can i think we have time for eight or nine questions and so the first question from neil is um is dark matter likely to be many different kinds of particles or just the one no one knows no one knows it's everyone's guess um let me give you two views though there's an occam's razor view which says boy if there's some new form of particle there's probably one of them that's kind of simple-minded but and we could we could fluff it up in fancier language there's another point of view which says if there's another the we're guided by what we've learned in particle physics to date and in particle physics to date particles new types of particles have tended to show up in families of particles now the wimp story in the particle physics world comes from a theory called supersymmetry and in that theory actually there would be a whole bunch of new particles but only one of them would be the dark matter so in a lot of models you would think it's one particle um it's still fashionable for people to say well but maybe it's got several components because you know every much of physics has been kind of messy and complicated so but the truth is we don't know it's it's it's you know it's kind of your your your prejudice and belief we have to do the experiments to find out if we if we're god forbid if we're lucky to find that question out all right and uh another question here from chris we have uh what is the observationally determined limit on how strongly light could interact with dark matter that's a really good question i don't really know the answer directly um [Music] i mean i'll just say this right people have invested enormous amounts of money in telescopes because they want to see stars and they want to see dust and they want to see gas and they've measured in radio they've measured it infrared they've measured a uv you know huge amount of astrophysics in astronomy almost all of it carried out by looking at photons nothing has ever shown a hint of the presence of the dark matter through its interaction with those photons so to put a number on it i don't quite know how to do it but it's a pretty super well established fact certainly no version of normal matter there is no way to take normal atoms and hide them and not be the dark matter um let me be a little careful there if you put normal matter into certain types of really dense planets or like white dwarfs or as as always asked black holes you wouldn't see it via the way it interacts with light and there's other reasons that we don't think the dark matter came from normal matter but certainly you can't put it into sort of normal atoms and sort of hide it in a galaxy that cannot be done i don't i'm not an astronomer i don't have all the details at hand but they assure me that that's absolutely uncontrovertible your answer there anticipated the next question i had queued up which was to ask uh could it be that it's uh black holes so that's really interesting question and especially with the you know the black holes being so abundant as seen by ligo which was a bit of a surprise how quickly ligo saw you know two black holes coalesce and now the huge number of black hole black hole black holes coalescing they've seen since then i don't know if you guys know about the lightweight story but that's a whole cool thing okay there are a lot of black holes in the universe why dark matter um this it turns out i think the statement to be careful is we don't definitively know it's not black holes one thing is certainly true everything we know about stars and the normal way you form black holes is you have a star and the star collapses and has an explosion and what's left behind is a black hole those can't be the dark matter because my story which i know was kind of abstract and i i thank everyone for their patience with my movie i didn't actually show that whole story is is fascinating and complex and tells us that at an early point in the big bang the normal neutron the normal things that became all the neutrons and protons and all the normal matter cannot account for the dark matter no matter what happened next so normal matter in a normal way forming stars forming black holes cannot account for dark matter if black holes are the dark matter they actually had to have been made at some super early time in the universe in some exotic physics that we simply do not know that created black holes at some early moment in the big bang that is that is that is solid so people talk about primordial black holes it also turns out if you try to take the mass of a galaxy and multiply it by seven because that's the amount of dark matter make it black holes they actually aren't very benign just gravitationally they'll be like you know they're orbiting they'll go through the galaxy basically they'll jiggle the stars gravitationally and that's one thing and there's other things they do and in fact that's kind of not my field of expertise there are talks that go on and on and on and they show black holes from being very very small to being very very very large and various techniques have ruled out most versions of a black hole that came from some weird primordial time as being the dark matter but it's actually an active you know one year there'll be a paper saying it can't be black holes in this mass range and the next year someone's like no those guys were wrong and then two years later other people are like no they were right and it's it's not quite settled but it doesn't seem likely and it's not it's it's um people think about it actively but it's it most versions of a black hole aren't allowed and so then there's one more black hole related question which is um could dark matter get pulled into black holes once they do exist and so do the regular black holes end up being kind of mixed matter black holes yeah absolutely no there's certainly black holes pulled in dark matter that's that's the case um the thing we know about dark matter and i didn't really bring this out is that it it doesn't appear to interact in any way and um normal gas and dust in the same way that like coalesce to form stars and planets on the galactic scale a big accumulation of of of stuff that you know i i showed how like that we can simulate how stuff grows you know through gravitational collapse from these huge scales down and when a galaxy is formed the normal matter kind of coalesces down to the spiral disk and a lot of it's in the center what is true is the dark matter because it doesn't interact and can't lose energy kind of stays out more on the perimeter but absolutely and you know there's a supermassive black hole in most galaxies and probably the center of every galaxy and for sure those have sucked up a lot of black a dark matter um and i'm sure theorists would love thinking about that so here's a bit of a technical question so um you you talked a lot about dealing with radioactivity from your containers and instruments and whatnot but what about things like heat from the electronics do those cause problems with vaporizing the uh the liquid medium uh we had to carefully design the way the liquid xenon works in fact that's a little specialty of mine is cryogenics i love cryogenic engineering and there's a lot of thought put into it and we had to pay attention to it it wasn't it wasn't fundamentally that hard we don't have a lot of electronics actually um but we have a bunch of cooling the cooling is provided by ultimately by liquid nitrogen and we actually have a little liquid nitrogen plant underground which you can buy a liquid nitrogen plant if you have a spare 100k you can put one in your backyard and have a lot of liquid nitrogen so here we have a question that's uh sort of zooming back out again a question about from ann about uh the halos of dark matter are those just big spherical structures or are they flattened like the galactic discs really good question um ah what to say so the answer is they're not flattened they're they're probably uh again this is not my real world of expertise there's you know astronomers and astrophysicists think a lot about this the claim is that they're typically football shaped things but there's a very basic reason the fact that they don't interact and it turns out the fact that they can't emit light whereas normal matter can is the fundamental reason why the normal matter which probably at some early time was kind of in a more spherical thing was able to collapse down to a disc by emitting light where and i know that sounded weird and i didn't explain it but there's a whole backstory to it which actually i probably can't explain but i've been told it many many many many many many times uh so the dark matter is a kind of a non-interacting thing it's gravitational collapse kind of sucks it in but realize when things gravitationally collapse they speed up and so now there's they're still moving and that that gives the size of the sphere the normal matter has energy relaxation mechanisms by emitting light and that's why the galaxy the normal matter is kind of collapsed down more and is in a disc so the dark matter is not thought to be in a disc it's a bit model dependent but you also know from these um studies of this of the dynamics of the galaxy in fact that's how you first figured it out that it had to be spherical or football shaped and then from that also people kind of have this explanation about not emitting light and not interacting and therefore they can't form they can't collapse down to a disk all right so we have time for just a couple more questions and so um we will we will take a question from from anon asking um you know when you talk about a particle hitting another particle head-on um you know is there some threshold of their fields that that you're talking about there like is there some clear right i mean it is i'm not quite sure what you're asking it on um sounds like a good question i mean so you know it's these two kind of fuzzy force fields and what it tends to mean is if if here's one particle and here's another particle coming at it they could kind of have a glancing collision or they can have a more head-on collision and that's just it's going to affect how how much how much this particle banged that one as it moves on and you could have a glancing collision where this one just gave this guy a gentle nudge or you could have a more head-on collision where this gave that one a hard uh a hard you know a lot of energy and it also is the case and maybe this is what you're asking there tends to be you know some distance away basically nothing happens um and it's a little different for the different forces the electric force is kind of a little fuzzier the strong force has kind of a very firm distance and if you're a little bit outside that distance nothing happens electric force it's more like what i first described although even for electric force finally if they're far enough apart nothing happens and the weak force is kind of like this strong force it's got a really finite range you either are in that range it's like bbs as opposed to like you know cotton balls if you will all righty and so we'll have our final question here from uh susan you know asking ask me if you could just give a quick summary why is this hunt important for our big picture understanding in astronomy so so so i'm thinking you're like you know how do the different possible results that you find for the nature of the particle end up affecting our astronomical understanding you know that's a really good question and that um okay let me play a cynic for a second from an astronomy point of view maybe you don't care maybe you don't care from a physics point of view where you know i mean maybe it's a little bit the difference in in kind of interests of maybe a pure astronomer and a pure physicist although i don't think people are ever that but you know you know a physics question would be do we understand the nature of matter do our equations understand the nature of matter and the fact that the dominant form of matter is a particle or some type of matter that we haven't understood for physics is this huge crisis it's like this huge hole in our understanding we think we understand so much we understand the four forces we understand the big bang we understand subatomic particles we can you know we understand electricity and magnetism we we understand quantum mechanics we brought you silicon valley right physics did that and yet we don't know what the main form of matter in the universe is so for us it's just disaster that we don't understand this um and interestingly in the early days astronomers were happy with the idea that there was dark matter but very much resisted the idea that it could be a different form of matter because they're not used to particle like particle physicists are always making these weird forms of matter and accelerators at this point everyone understands that the universe is dominated by dark matter and in fact i think you know most astronomers in fact all astronomers think dark matter is a huge huge issue philosophically do you really care that you don't know what it is yeah and a final point i mean i think most people do but you know that that's a taste issue one other issue is though we have been hoping we and this is actually more the astronomers we've been hoping that something about the behavior of galaxies would somehow be able to be modeled where the dark matter wasn't treated purely as a thing that only interacted by gravity i mean i kind of didn't quite say that in my talk but it's really true a lot of things in astrophysics the behavior of galaxies this thing orbiting that thing whatever are need dark matter to explain how the galaxy worked and but the modeling that puts the dark matter in puts it in and says all that dark matter does is interact interact gravitationally and we have all been hoping that in the astrophysical sense something would show up where oh the dark matter you can tell that actually at a very faint level it's colliding with the normal matter or it's colliding with itself that would give us some clue as to what it is so in point of fact trying to understand the dark matter it's kind of a it's actually it's a marriage of astronomy and physics because you you can't even think of physics wouldn't have come up with the problem without astronomy and probably the solution is that it's a new form of matter and that's kind of the domain of physics and the two to come together you know and that's like that's the hope that we can you know the depressing thing is if we never figured this out you know that that might happen right i mean i'm having looked for 30 years and not found it i've been on a set of experiments that all have world-leading searches blah blah we haven't found it so i'm just hoping someone finds it it'd be great if it's lz if someone else finds it because it's a different form i'm still going to be really happy thank you very much dr shot for this wonderful discussion and i will just say as an astronomer that uh just as much as it's a slap in the face of the physicists that you don't know what fundamental parts of matter are made up i think we astronomers consider it a slap in the face that we only know one-seventh of what the universe is made of as you said so we wish you the best of luck with this experiment and thank you again for coming and sharing it with us let me say to the audience uh thank you for participating usually if we were meeting in person at foothill college each of the members of the audience would have received the program with information about the four organizations that support this lecture but in the absence of that i would encourage you to go to the about page of our youtube channel where there is uh there are links to each of the supporting organizations where you can find out more about the good work they're doing and i want to invite everyone back our next lecture in this series will be wednesday november 11th at 7 pm pacific time and our featured speaker will be dr michael brown of caltech on the possibility of finding a major ninth planet in distant reaches of our own solar system so we look forward to seeing you again then and that concludes this lecture in the silicon valley astronomy lecture series

Channel: SVAstronomyLectures

Views: 17,425

Keywords: astronomy, science, physics, dark matter, science news, astrophysics, cosmology, particle physics, Tom Shutt, LUX-ZEPLIN Experiment

Id: cbFQVLLJEE8

Length: 88min 43sec (5323 seconds)

Published: Tue Oct 20 2020