Cosmology for Feynman: What We Know & What We Don’t Know - Michael Turner - 5/12/2018

okay our next speaker is Michael Turner Michael is rauner Distinguished Service professor and director of the Kavli Institute for cosmological physics at the University of Chicago he helped establish the interdisciplinary field that combines together cosmology and elementary particle physics to understand the origin and evolution of the universe today he will tell us about the state of that field and some of the big questions were facing please join me in welcoming Michael it's really a special privilege to be here today I learned physics at Caltech and how to think about physics at Caltech from Richard Feynman and his colleagues and my introduction to Caltech was this auditorium so I was a high school student in Los Angeles and my high school physics teacher brought me to the Monday night lectures I think they're called the Watson lectures now no permission slips no seat belts I came here and I heard about things that I had never heard about before and I knew this is the place I wanted to be and so the Beckman auditorium holds a very special place for me so let's see I think I said this I was a undergraduate here here are the years 67 to 71 we learned physics from the Fineman lectures Feynman was a very big influence on the campus as would be shown by the yearbook here so this was one of the yearbooks when I was here so the big tea lectures on on Caltech and these are some pictures from the yearbook let's see this who's that oh that's me and here's Richard Feynman from that year and this is Robby vote one of the other physics professors here who really influenced me and the reason I show this picture of Robby is that Robby arranged for a tutorial between me and Fineman once a week for a quarter and it was one of the greatest adventures of my life and I learned a lot and so the theme of my lecture here is kind of a little bit of payback because cosmology was not as you'll see was not an area that Fineman was that interested in so this is cosmology for dr. Fineman this is my telling him about where we are and seeking his help it's always important to tell people what you're about to do is hard so this is challenging because he wrote not one paper on cosmology the closest were his lectures on gravitation he was interested in fundamental issues that were ripe to solve and ripe to solve means he wanted data and that was not cosmology at his time so cosmology was the province of astronomers and it was not ripe for solving there was not very much data however he once attended a colloquium I gave at Caltech on cosmology and asked me a question I'm still trying to answer he has Parton's which I think we heard a little bit about last night really was one of the first steps in pushing cosmology to be closer to fundamental physics and so I'll talk about that and what we've heard in countless talks is he was always ready to look at something and if he looked at it he was going to look at it in a new way and so we could use his help in that regard so let me start out slowly on the universe so here are the basics a hundred billion galaxies in the universe lit up with the light each of about a hundred billion stars carried away from each other by expanding space from a Big Bang beginning 14 billion years ago all of this got started in Pasadena when Fineman was a teen in New York so that was interesting for me to think about I had never really thought about it that lit that way and Hubble this is the sky I don't know if you could see this from Far Rockaway maybe it's far enough from New York City that you could but a big puzzle when when Fineman was young was these fuzzy little nebulae so that's the Andromeda one that's the one you can see with the naked eye what are they and Hubble solved that riddle with the hundred inch telescope on Mount Wilson and showed that they were galaxies like I own they were Island universes and so he enlarged the universe by a factor of a hundred billion we'll come back to that number a couple of times and so here's the kind of picture we have of the universe today with the with the space telescope named after Hubble this is one ten-millionth of the sky and i'm not going to show you this slide long enough to count but they're about 10,000 galaxies there and if you multiply those two numbers together in the right way you get a hundred billion of course Hubble was also famous a few years later for discovering the expansion of the universe and the Big Bang so you use that same telescope to find that those galaxies are moving away from us in a pattern that we call the Hubble flow the ones that are farther away are moving faster and if you look at that picture and extrapolate backwards you find out that 14 billion years ago they were all in the same place and a Big Bang and here's a picture I think this is a picture of Einstein at Caltech and with some help because when you're at the frontiers things are really confusing Einstein had help on this the Big Bang is not an explosion of galaxies into space but it's an explosion of space with galaxies being carried along so when I was an undergraduate here the big questions in cosmology were summarized by Hubble's student Allan Sandage who worked down the block from here and he said it's all about two numbers H naught and Q naught I'll tell you what those are some of many people probably don't know what they are and we're going to solve it with this 200-inch telescope that was the mirror was polished here it's now called the Hale telescope at Mount Palomar and so this is from a famous article that he wrote in physics today cosmology is is just those two numbers and so this is the size of the universe I'm going to illustrate these two numbers so it's expanding and the issue is how fast is it is it expanding because if we know that we can figure out how long ago was the Big Bang and so that's H naught the expansion rate the slope of this line and it must be slowing down due to gravity and so that's the droopiness of this line how much is it slowing down and so that Q naught quantifies that and that tells you the destiny and and cosmology really was the province of astronomers and there weren't that many of them working on it maybe I don't know 30 however change was in the air at about that time a mysterious microwave signal was discovered by Penzias and Wilson in 1964 radiation left over from the Big Bang we call it the microwave echo of the Big Bang and the import of this for which they were given the Nobel Prize in 1978 we saw some stamps last night so here's a another stamp was that the Big Bang was hot and that starts to change the way you think about the universe in a fundamental way and so you heat something up you reduce it to its basics that's starting to sound a little more interesting define 'men and so the universe is hot and simple it's expanding and cooling and I'm gonna skip the quarks here for a second but it does start with quarks ooh neutrons and protons when the universe was second sold it was a nuclear reactor and then the it got cool enough for atoms to form and when the atoms formed the universe became transparent to light and so this is the microwave background that Lisa was talking about and we'll talk about some more so we're starting to change the way we think about the universe at about the same time Fineman was thinking about what are neutrons and protons made of are their seeds inside and there were some experiments going on at Stanford at the linear accelerator Center and Fineman was starting to interpret those experiments with his Parton's and so part on was his name for a stand-in that we now realize is the quarks and so here he is giving a seminar at SLAC writing some equations down about these constituents of the neutron and the proton that would make a life a lot simpler and more interesting for cosmologists and david gross this morning talked about the standard model of particle physics where this was all put together the part tons are actually the quarks that murray gell-mann and others talked about and here's the standard model and so in the beginning the universe was just a soup of elementary particles and it was a lot simpler than it had any right to be for cosmologists to study okay so in nineteen in the 1980s the standard model of particle physics allowed one to make big speculations that David talked about this morning about the unification of the forces and that these ideas might even be relevant to cosmology and so the cosmologists started exploring the bold ideas in particle physics we heard about grand unification and supersymmetry and super strings and we supported ourselves with t-shirts so here we have cosmology takes guts that's grand unified theories let's see here's another t-shirt that helped pay salaries of postdocs in short this was a really really great some people call this the go-go junk bond days of early universe cosmology it was a very exciting time to be alive and I just want to pause here and somewhere around 1980 or the mid 1980s the conversation and the vocabulary about cosmology were changed it was no longer Q naught and H naught it was a different vocabulary a big idea changed the conversation and that big idea was the deep connections between quarks and the cosmos this early quark phase and the universe that we see today and the connection is very deep it's not just that the universe began as quarks ooh it's things happened in the quark soup that shaped and determined the way the universe would be today so that's the big idea and so you know I showed you Q naught and H naught and so this is today's vocabulary much of which you saw in earlier talks today dark matter I'll talk about inflation dark energy baryogenesis I won't be able to talk about but Lisa talked a lot about where the dark matter came from what about the atoms so we we have a theory of where the atoms came from but it's a little more complicated and I won't get to it we heard about wimps so this is the new vocabulary so the language of cosmology was changed and it also brought in some outsiders that's the wonderful thing about astronomy as they welcome outsiders they I think Lisa gave you a flavor of it at first they they don't like them and they say we don't need you and then they became full members of the community and oh yeah we embrace you from the very beginning you're really an astronomer kind of remind you of immigration in the United States and so dark matter that's the first piece I don't need to say very much about dark matter so the evidence for dark matter Lisa mentioned the flat rotation curve stars moving around the Centers of galaxies much faster than they should be if they were held only by the gravity of the Stars and this is Vera Rubin whose careful work demonstrated this in the 1970s and the idea here is dark matter is a new elementary particle left over from the quark soup and I think Lisa gave you a really great flavor of okay so that's the big that's the you know that's the paintbrush version of it but could we have a few more details and the few more details were struggling with right now inflation so let me talk about inflation so inflation is probably the most important idea since gammas idea of the Big Bang the most important idea in cosmology since gammas idea of the Big Bang and it involves an early burst of tremendous expansion and I'll tell you what it's caused by in a second and so why would we be interested in that well that bursts of expansion could explain the fact that when we look across the sky and I'll show you some images across the sky it's we live in a very smooth universe smoother than it ought to be oh the echo of the Big Bang the heat of the Big Bang where did that heat come from that's another big puzzle and then there was a problem called the monopole problem that's no longer a problem that john prescott worked on and it also would explain why we're not overrun with monopoles and it made some predictions about the universe and this what causes it well our best understanding which best does not mean correct is that it has something to do with a field like the the field that gives rise to the higgs boson so when Higgs boson was discovered we were very excited because that at least there's one scalar field in nature and we describe all of inflation in terms of another scalar field let me talk about the most amazing thing that inflation does which shows this deep connection between the very small and the very big so inflation is a tremendous stretching of things so Lisa mentioned that we had a lumpy universe at one time small variations I'll show you a picture of it in the density of matter and gravity got ahold of those amplified them and turn them into the galaxies and all the structure we see today so where where did that initial lumpiness come from and inflation explains that in an amazing way so it says guess what the lumpiness came from quantum fluctuations so they have to be there and you might say well yeah I then don't really buy that because you know they're on scales I can't even see and you're talking about scales that I can't even imagine how does that work so these fluctuations on very small scales get stretched due to inflation to astronomical scales and then there's a few details I haven't put in here and so galaxies were seeded by quantum fluctuations so that gives you a sense of the connections between the very small and the very big the details of inflation were worked out at a workshop that Stephen Hawking held in Cambridge England and Caltech and fineman's influence was on this workshop well of course Stephen Hawking was a frequent visitor to Caltech Paul Steinhardt is one of the fathers of inflation he was an undergraduate here I don't know who that is there's John Prescott and this is Jim Bardeen who's one of the few graduate students who did their PhD work under Richard Feynman okay the third element that comes into the current cosmological paradigm is called dark energy and this burst on the scene in 1998 with the discovery that the universe is not slowing down but it's actually speeding up and it's doing that because repulsive gravity is a feature of Einstein's theory and I don't mean that studying his theory is repulsive but that some stuff rather than having attractive gravity which that's kind of the defining principle of gravity some stuff can have repulsive gravity and this is very very weird stuff and we call it dark energy and if you're puzzled you've caught up with us ok and our best example of course in Southern California having Zen in your talk is good so this is a Zen part of the talk the best example of this is the energy of nothing and of course nothing isn't nothing according to quantum mechanics it's filled with particles living on borrowed time and borrowed energy and I'm sure you're going to trust me on this it's easy to show that the if if quantum nothingness has energy it has repulsive gravity you'll probably trust me on that one and theorists are very very proud because when we figure out how much how repulsive the gravity of the quantum vacuum is we get more or less the right answer well to within fifty five orders of magnitude and well actually it's not even that good but so this is I'm gonna bring Fineman in on this I'm going to remind Fineman that the he knows this but I'm gonna remind him anyway to show him that I know that he knows that so we heard about his tricks to get rid of infinities and this is an infinity that his tricks didn't work on so this this this is a really hard one okay so we call this model that we have sometimes we call it the concordance cosmology sometimes we call it lambda CDM because this quantum vacuum energy is mathematically equivalent to Einsteins cosmological constant lambda and lambda is shorter than quantum vacuum energy so let me just tell you or sorry let me tell dr. Fineman how great this model is so a jiffy after the beginning the definition of a jiffy is down there there was this tremendous burst of expansion that's the inflation it's smooth out space-time created hot quark souped and termed subatomic quantum fluctuations into the seeds for galaxies until the universe was a my a hundred ten to the minus five seconds old we had quark soup and during that time ordinary matter arose and dark matter arose so you're starting to see that early earliest moments that the universe are really really important from ten to the minus five seconds to three hundred seconds this is when the neutrons and protons formed and then some of them came together to make nuclei and we actually have these elements left over from the Big Bang and that's one of the big pieces of evidence from a hundred thousand years to five billion years the gravity of dark matter that's what Lisa was talking about builds cosmic structures from these quantum seeds and from five billion years on the dark energy takes over in the universe is speeding up so that's our story here it is here's this very early quark soup phase that really didn't come into being and tell about the time of fineman's death where so many interesting things were going on here origin of dark matter and ordinary matter and inflation and oh let me I'll show this next slide quickly so gravity is building structure with the dark matter and this is what Lisa was talking about is we have lumpy dark matter and so this theory predicts that you make small things first like galaxies and they have the visible part of the galaxy embedded in a much larger halo and then clusters form and then the universe starts speeding up and the formation of structure ends giant computer simulations this is very predictive it says galaxies most galaxies should be forming around redshifts of two to five if you don't know what redshift is you'll see it in the next slide so I'll bring in another Fineman quote I mean it doesn't matter how beautiful your theory is this is a pretty beautiful theory it doesn't matter how smart you are well I don't know how smart I am if it doesn't agree with experiment it's wrong and so what's wonderful about this story is that it's backed up with a wealth of experimental data so I'll just show you a little bit of that oh I've been talking about theory because Feynman was a theorist and because I'm a theorist but we had some help from some powerful instruments I believe those are the two Keck telescopes and there's some really good telescopes in Chile too and there's some really nice telescopes at the South Pole maybe we'll come back to those later and we have telescopes in space so powerful instruments helped this story as well so this is one of my favorite astrophysics slides and it's it's from the Old Testament so you have to read it from right to left so here's redshift so the dark McCole Dark Matter this dark matter theory said that most of the galaxies should be built right around here and that's what you see here's time if you're interested in regular time and what's shown here is the star formation rate galaxies are made out of stars and so this is the kind of data that we have that test this theory okay let me come to the microwave background because this this is a rosetta stone this is really important it allows us to see the acai blown up the last bit of my complete history of the universe we look out into space we look back to time in time and we the microwave background allows us to see the universe before stars before galaxies when it was really simple and when theorists can understand it and so the Kobe Sal satellite gave us our first good glimpse of it and its resolution was not so good not because it was an ounce of failure but because it was you know that's that was the first try w map had better resolution and the best map we have from spaces from the planck satellite so here is what the universe looked like when it was 380,000 years old and for those of you trying to decide whether or not you want to go into cosmology if you stare at this and your heart goes pitter-patter you want to be a cosmologists if you look at it and say that looks like noise well actually it is Gaussian random noise except for Stephen Hawking's initials there Sh so if we do a magical mathematical trick to this called the spherical harmonic transform and plot the temperature difference between different points on the sky look what jumps out so we have these acoustic speed peaks and a curve that I hand drew through here no I didn't hand draw it through here this is what the cold dark matter theory predicts so that's pretty good Oh Allan Sandage the Hubble constant is 70 I'm sorry about that I know you had your heart set on 40 and equally sorry that Q naught is negative so Allan defined that with a minus sign so that if the universe was slowing down it would come out positive and so we have a 14 billion ol year-old accelerating universe but Intel and he thought Q naught would solve the destiny but until we understand the dark energy we don't know the destiny oh my I had to say I already said this once that the biggest things in the universe so back to Fineman so I've told him this story and he would probably say remind me the first principle is that you must not fool yourself and you are the easiest person to fool and from firsthand experience I know that's true so let talk a little bit about the chinks here so here is this beautiful description of the universe connecting the very big to the very small and it's built upon these strong pillars I wish I had an animation where the pillars start to shake and of dark matter let's see Lisa explained very clearly that we don't know what it is dark energy I think I explained very clearly that we don't know what it is and well trust me we don't know what inflation is yet either and so three shaky pillars and I think Feynman would say you know okay that's one way to look at it is from David's talk the optimistic talk this is new physics or maybe it's stand-ins maybe their proxies or I hate to use the word epicycles but maybe they're they're just describing in in the poor language we have today something much richer so the big questions what is the dark matter particle or is that even the right question what is the nature of dark energy or cosmic destiny and why does nothing weigh so little a question that we tried to ignore when did inflation take place and what about the multiverse a limited show you one thing about the multiverse how did orden ordinary matter originate I didn't talk much about that what happened before the Big Bang so that's a great question and I already said are the three pillars just stand-ins so I have to talk just ever so briefly about the multiverse so in inflation it's one of those things that's the mouse in the cookie if inflation took place once it probably took place a lot of times and whenever a physicist says a lot of times they really mean infinity and so here's the cosmic river of time here is the inflation that led to us so everything I just told you is this but there were a lot of other ones and so the universe is a lot bigger than we thought and so this is the multiverse and the pieces are disconnected so they can't communicate with one another they might look very differently the multiverse makes you know Hubble's discovery that the universe is a hundred billion times bigger than we thought makes him look like a piker but oh it might even solve the problem of what the dark energy is and why nothing weighs so little is the multiverse you have a landscape the cosmic landscape of string theory that indeed in most places the vacuum energy is incredibly big but in one of the 10 to the 500 universes out there it's small enough I'm gonna borrow a phrase of someone in the audience that seems like a very extravagant explanation for one fact and I'll bring Fineman back in the multiverse dilemma in my mind is this could be the most important idea since Copernicus but if it's not testable is it science and so that gives me a headache and so here's the Fineman quote I'll bring in I would rather have questions that can't be answered than answers that can't be questioned and I'm not saying the multiverse is an answer that can't be questioned but I haven't seen a good way to question it so far and I guess I have one minute left err yeah okay so they see ten minutes left let me just skip to the well I wanted to talk about testing inflation because I can bring Caltech in again so inflation also predicts gravity waves and they leave a polarization signature on the microwave background it's called the curl or the bee mode it's a really critical test and the this experiment that I showed you area early or the bicep experiment and the South Pole telescope that's the Chicago experiment detected these B modes look at that's that's what they detected that's the swirl that's amazing unfortunately the first thing you learn about astronomy is dust is the bane of astronomers and so you know is it inflationary BMO's or not and so far we know some of it is dust but I like to think positively there's room for something else and gravity waves and so this is one of those the chase continues this is one of these big things so let me just finish I hope that dr. Fineman would be pleased with this story especially that it's not over especially that it is ripe for progress because there is data and now because it involves fundamental physics and the last thing I want to say I added this this morning because I realize Fineman is one of the few scientists this is one of the quotes I haven't seen anyone put out he was not afraid to say I don't know and I I think I heard in every lecture somebody saying and Fineman said I don't know and that is so important in science and I don't know but certainly his spirit was and let's find out I don't know and let's find out and so that's the cosmology adventure we're on thank you any questions right here hi thank you for the very inspiring lecture I saw a figure that I didn't hear a full explanation of I was wondering if you could explain the moose diagram the moose diagram well Lisa explained that oh and we can't see it anymore so the moose diagram oh boy I get headaches in cosmology because I'm getting old maybe I'm just getting headaches because I'm getting old but if we if we go back to the WIMP and I've got a trademark on that Lisa violated and if we go back to the WIMP we thought dark matter was really simple we thought it was one extra particle and that moose diagram if you're a theorist even if you say you have one answer you really give three and so one was the wimp or the or the neutralino the lightest supersymmetric particle another answer was the acción which comes out of QCD and we thought life was really simple that we were just missing one piece of the puzzle it wasn't going to open the world to a whole new Dark Sector and it wasn't gonna make life more complicated we were very close to the end and that might actually be the case but when I hear Lisa give a talk I go boy I don't think I want to bet on we're close to the end I think there there might be a more twists and turns and so the moose diagram is is sort of a relic of early simplified thinking thank you previous talk there was a list of seven or eight concrete experimental sort of reasons why dark matter should be there they seem to be pretty clear cuz dark matter can go through each other itself when things collide etc I take it inflation is sort of the reason dark energy should be there but is there sort of similar lists for for experience with experimental evidence for dark energy yeah so the the number one so right now we don't know if dark energy and inflation are related they're both accelerated expansion but we we don't know if they're related in fact I don't think there's any really good theory where they're related so let's do dark energy so the evidence for dark energy the primary evidence is the universe is speeding up and that evidence came in 1998 and the evidence was a little bit shaky and the evidence has gotten incredibly stronger and all kinds of leases list we could make a same list for dark energy where there's ancillary evidence the microwave background clusters of galaxies tell us about the expansion of the universe so the evidence is really really good that the universe is speeding up but we don't know what's causing it and likewise for inflation I probably my poster child for inflation was the was the curve with the acoustic peaks so for both of them we have really good at the universe is speeding up but we don't know why on the topic of big questions what you rather fight one fireman sized duck or 100 duck sized diamonds oh you're gonna have to repeat that again I on the topic of big questions would you rather fight one Fineman sized duck or 100 duck sized fineman's in white that's dilemma I think I'll take see you had a C right you talked about the river of time kind of expanding into many multiverses which yes presumably with different initial conditions and parameters are there any thoughts on like why it suddenly triggers an expansion into a inflation into a universe and kind of what the nature of that is yeah so the thoughts there are really do - Alex Ville Alex Vilenkin and Andrei Linde a that the same here's the dilemma about the multiverse so we like those quantum fluctuations that seeded galaxies we love them and we see evidence for them in the microwave background and what Ville in Caen and Lynn day and others have shown is that those same quantum fluctuations seed other universes and so that's the strongest argument for the existence of the multiverse which is if you want those quantum fluctuations for galaxies you got to take the other ones as well now that it turns out that Hawking's last paper just came out and it was about the multiverse and the wonderful thing about Stephen is he was in in the fray until the very end and his paper hoped to give I don't think he liked the multiverse that much hope to give the multiverse a big haircut and made it a lot smaller but the multiverse still gives me a headache does it give you a headache good good then you're really smart Hey we have a question over here it's two parts um I was in the supermarket and I picked up a prestigious looking magazine which I'd never seen before American scientist May June issue as opposed to New Scientist which I've seen all the time and in it there was an article by an american-born cosmologists or physicists who is a tingling or Dutch based University which states that the the latest compilation of all survey material says there's two trillion galaxies down 100 billion that's question one the more interesting question is in the your diagram of the river of time spawning an innumerable number of universes so you know that's a Monet that's that's your that's a abstract art you're not supposed to interpret lit literally but continue with your bugs sorry I thought it was Kandinsky I love Wassily anyway um what is if if we're going to have something as simple as the river of time what is the origin of the river of time what came before and what are its characteristics so let me answer the second one first not answering it and then I'll answer the first one so the that river of time is one of the answers to what happened before the Big Bang and when we asked nature really really hard questions we say is that a or B is it a or b I know it's got to be a or b it's D right it's C and so when we asked what what happened before the Big Bang the one of the interesting things about the multiverse is there was no beginning there were infinity of beginnings and that should go send you back to your desk to think new thoughts so that so that takes you out of that there was I'm not saying I understand it but one of the one of the nice V another anyway that's one of the nice features of that the the article that you read is fascinating I'm teaching undergraduates and I had them count the ten thousand galaxies in the Hubble Deep Field and then I asked them on a pop quiz how many are there and so one of my students and please Chicago students are really smart I don't know if Caltech or I'm staring at your in here now Caltech students are brighter and so the students said I know I counted a hundred billion galaxies in the Hubble Deep Field but I googled it on the internet and it's two trillion so I thought oh geez so I went and googled on the internet and found that article and it's a very interesting article because those if I understand it correctly and we probably have some real astronomers in the audience those two trillion galaxies have not been seen but in in this paper that was it was European astronomers from Holland where's our yeah from Holland there Robert and said that you know if you look at the Hubble Deep fields one of the amazing things in it are the train wrecks that you're seeing the Assembly of galaxies so it was an extrapolation back that if you look at the hundred or two hundred billion that are here today the pieces that came together there must have been two trillion pieces is the way I read that article but they didn't really see them and so I don't think that counts I think that the correct answer on my pop quiz is still a hundred billion so if the river of time has if it has no origin because that question has no meaning because it's infinite why is there only one color in the Monet how many other colors should be used well I'll let you know artists never explain their work so yeah yesterday one of the speakers mentioned that there was that some people postulated that there was only one electron and they use that to house plants wonderful and I think it's even true and then yeah and then they also in another one said that we really don't understand time so that's also true my question is if there's only one electron if that happens to be true and we don't understand time is the understanding it's it possible that there is only one photon in our universe and in other words if you are standing here and say you have a black hole in the middle and you have a star on the other side that photon goes around the black hole it's the the star galaxy on the other end comes back and then goes back and then comes around so we actually see two distant galaxies but the time sequence that we have from our perspective is it's the same time and and that would that could maybe explain something that's going on there and as far as dark matter is it possible that there is a another photon of a different frequency that we cannot detect simply because we don't have the physical ability to detect that photon this is a complicated question I encourage you to okay to Michael now the session is over we're a little bit over time so please come on up and finish your question in person so it's it's time to go to lunch I want to remind everyone to take their belongings with them so please don't leave anything in the auditorium and there are lunch opportunities listed on the program so we'll see you back at 2 p.m. Thanks you you

Channel: caltech

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Length: 43min 29sec (2609 seconds)

Published: Tue May 22 2018