Dark Matter in the Universe

Oh welcome to the science for the public lecture series science for the public is an organization committed to bringing science information and issues to the general public visit our website for our program listings and blog tonight it’s dark matter and as many of you know most of the matter in the universe is invisible and it’s elusive and it’s impossible to understand and that’s why you have come out here tonight to be you know enlightenment it’s one of the greatest mysteries in science certainly one of the most enticing and it’s probably for that reason that it engages many of the most brilliant scientists and the most sophisticated research labs in the world it’s a very great honor to have as our guide tonight dr. Peter Fisher he’s a veteran in this pursuit of dark matter dr. Fischer is a professor of physics in the physics department excuse me at MIT and he heads the division of particle and nuclear experimental physics there he also holds appointments at MIT laboratory for nuclear science and the Kavli Institute for Astrophysics and Space Research dr. Fischer’s major research interests are dark matter and high-energy interactions he’s published extensively on neutrinos and cosmic rays he and his team have been involved in a major experiment at CERN and if this is designed to make high precision measurements of cosmic rays for all historians and despite a very hectic schedule dr. Fischer is very committed to an adept at translating for us non specialists including non physicists the very abstract phenomena that make up the structure of the universe for an example that I think is wonderful see his answers about the a Collider on the Nova now that it which you’ll find on the link on our web page we’re of Peter Fisher tonight dr. Fisher will do three things first he’ll tell us all the basics about Dark Matter that’s stuff that holds the galaxies together he’ll also describe how scientists are trying to locate and identify dark matter and finally he’ll explain the unique dark matter experiment in which he or his colleagues engage well thank you that that’s a lot to live up to so I just wanted to tell you that I’m really happy to to come here because I speak to the public quite frequently the public being you know MIT alumni and you know there are there’s kind of an agenda and at about this time everybody’s kind of sitting there knowing that they’re gonna be asked for something later on so there’s none of that I’m just gonna tell you about it and it’s really I think it’s great that people come out you know a night like this to learn about things like dark matter it’s actually kind of upsetting and I’ll tell you why it’s upsetting in sort of a metaphysical way I chose this title because it’s the word to a song that was written by an astronomer named David Weinberg called the Dark Matter rap and it’s actually a tremendous it’s a rap piece and it’s a tremendous piece because he goes through the entire history of dark matter in a historically correct way with footnotes and I used to play it for you know parts of it for the alumni and finally my friend Dean Kastner told me to knock it off it nobody liked it it’s really bad rap but the the the chorus is dark-matter do we need it where is it how much where is it how much and through the course of the song it gets its gets progressively more and more frantic and I think what I want to tell you about now is is it’s kind of the the franticness has reached a criss

crescendo and I think in the next five or 10 years you know we’re either gonna find out what Dark Matter really is and I’ll tell you what that means to find out what Dark Matter really is or we’re gonna find out that we’re probably never going to know what it is and we’re just gonna have to live with not knowing which is not something that anybody ever really talks about but it could happen okay so I mainly show this to irritate colleagues science magazine is kind of the the big place to publish results particularly if they’re wrong and for their hundred and twenty-fifth anniversary they they had the hundred and twenty five most important questions facing science the one hundred and twenty five things we don’t know and and number one is what’s the universe made of and you know they’re after this there’s all kinds of you know gene and biology stuff but but you know this this says something about you know what what we as a race or as a as a species thing you know if you if you put yourself in the place of cavemen and cavewomen and cave children you’re sitting around the cave and you’ve just killed something and eaten it and there’s nobody actively trying to kill you and you’re not planning to kill them anytime soon and everything’s kind of okay you begin to kind of take an inventory about what’s around you and there’s you know rocks and dirt and other cave people and some bones and sinew and you look up in the sky and there are some points in the sky and you can’t touch them and they move around in this kind of complicated way and it’s if if you’ve anyone’s been out in the desert recently and looked up at the sky there’s really just huge amount of stuff there and we’re really just drawn to it I think I have a good friend who’s an astronomer Paul Schechter he just couldn’t have been anything else you know that that when you look through a telescope and you see the rings around Saturn and you see the Horsehead Nebula you see all these things you it’s just astounding and so there’s been this kind of psychotic group of people drawn to look up and and try to understand you know what’s there so the the problem of dark matter is really caused by the intersection of two fields and I’m going to give you a very short history of particle physics and a very short history of astronomy starting with 1905 and ending up now and to show you how they kind of really collided and and created the present circumstance which you know is is all together sort of unsatisfactory so in 1905 Einstein published five papers any one of which would have made a magnificent career one of them was the special theory of relativity and I’m gonna start there in both cases and then work up to now and just tell you what’s what’s happened a little bit later on 1912 he published his paper on the general theory of relativity the special theory of relativity kind of undid the separation between space and time and and showed that space and time are really related in a way that was not at all obvious to two people like Isaac Newton and then in in 1912 he published his general theory which showed how gravity is operates in a way that’s really different than other forces it actually acts on space time and so when when things are deflected by a gravitational field what they’re actually doing is traveling in a space in a straight line in a bent region of space-time and that came out in in nineteen 1912 but the special theory was added to the the first ideas of quantum mechanics the mechanics that describes very small distances inside the atom by Dirac in in 1927 and there was this really kind of amazing thing that came out of this theory it was the idea that for every particle there is an antiparticle and this this there’s actually just one equation with like four symbols in it that’s the Dirac equation that just says everything you know the really great things are simple so for every particle there’s an anti particle and this very simple

observation kind of launched the modern study of particle physics so in 1936 this prediction was concerned confirmed by Carl Anderson who looked in cosmic rays and saw something that looked like an anti-electron so if you remember high school chemistry and atom has a positively charged nucleus it’s kind of in the middle and then orbiting it like a little planet is an electron electrons negatively charged because Ben Franklin decided electrons would be negatively charged that’s true so there was a particle that is exactly the same as the electron but oppositely charged and so this guy anderson saw it in in cosmic rays and that got him a nobel prize and an on-campus parking space at Caltech which i think was probably more valuable in this time during the war and immediately afterwards a full theory of electricity and magnetism was was developed and so this explained everything about how atoms work how they interact so how charged particles scatter off of each other and so it this theory emerged in 1948 and is kind of the prototype for other theories of other interactions in particle physics and I’ll say a little bit more about that in a moment so the anti-electron was 1936 1954 the antiproton was discovered so the simplest atom there is is a hydrogen atom with a proton positively charged it’s a big heavy thing and a little electron kind of scooting around and so even though the proton is a very complicated object it’s got three quarks inside of it and all kinds of other stuff this theory said it should have an anti particle and so at a custom-built accelerator at Berkeley they actually made the first anti protons and observe them so these ideas really kind of built up you know very very quickly over this this period of time oh and chamberlain and emilio sub gray both got nobel prizes and on-campus parking at berkeley which is probably more valuable than Caltech so in 1976 building on a lot of work that had been done here first quarks were discovered by Kendall Friedman and Taylor at MIT physicists working at Stanford so Stanford paid for this big laboratory in this big accelerator and Kendall and Friedman went and used it got Nobel Prizes and on-campus parking at MIT and what they showed was that really inside protons and neutrons and all of this stuff there are actually little constituents little point-like particles called quarks and they are real little things they’re all stuck very tightly together and they’re they’re whizzing around and they showed that there were three different kinds and then ting and Richter ting at MIT Richter at Stanford showed that there was actually a fourth quark and at this point all of this theoretical work that had been done through this kind of collapsed together into one description of the way particles interact at the some atomic level and so the idea is that there are three different kinds of interactions there’s the electromagnetic interaction which you know about from chemistry there’s a strong interaction that holds the quarks inside of the nucleus and there is a weak interaction that’s makes radioactive decay and they all kind of follow the same rules from this original quantum theory of electricity and magnetism there’s a fancy name for it which I’m not going to tell you because I don’t doesn’t make any sense to me but it’s it’s like it’s not exactly carbon copies but variations the one thing that doesn’t fit into any of this is gravity so gravity is a completely separate thing it bends space-time it works over cosmological distances but we arrived here in 1983 with a discovery at CERN made by a guy from Harvard so people in Massachusetts are like really good at going somewhere else and using somebody else’s stuff to do really good science this discovery of the heavy light particle which was predicted by this theory was kind of the next-to-last missing piece and the last missing piece which is called the Higgs boson is the

object of the efforts at CERN now so CERN is this huge laboratory in Geneva actually it’s not in Geneva it’s not in any country it’s international territory but it’s between France and Switzerland and they have a big accelerator and they’re gonna bang them together and try to find the Higgs boson but nobody really doubts that it’s there because in this period this theory was tested and this is where I got into the game by people like me at the better than a percent level so what does that mean it means you can use this theory to calculate that if you smash an electron into an anti-electron and you make one of these heavy photon particles and that decay in a certain way how often does it decay how many particles does it decay into what kind of particles does it decay into you can predict all of that as accurately as you want and the actual accuracy is a fraction of a percent largely limited by theoreticians ability to calculate things there flighty people they lose interest quickly but you know I mean who who here keeps track of their checking account that the you know 1/10 of 1% level so we arrive here with kind of one little unfinished piece of business called the Higgs boson but a perfect description of everything else so everything in this room and the air in us the fundamental interactions you can calculate as accurately as you want now what actually happens how molecules and things like that form our vast complexes of those interactions that you can’t calculate easily but that’s a complexity problem we know all of the basic rules okay it’s like if you know all the rules to football and then you know you can study all the rules and then you watch a play you’re not going to understand what’s going on because there are 22 people all doing their thing um okay so you know where we arrive here at the year 2000 with thinking that we just know everything okay so over here again 1905 we have the theory of relativity 1912 the description of the mathematical description of gravity so Einstein writes down the mathematical description of gravity it’s you know it’s like 18 characters or something it’s it’s not very complicated but nobody can solve it for you know about towards the end of the Second World War a guy named Schwarzschild who was serving in the trenches actually worked out a solution you know in between charges here in the early 1920s it was kind of the start of modern astronomy there was an event called the great debate I think it took place at the Smithsonian and the question was you look up in the sky and you can see lots of stars and then you see these things that don’t look like snorri’s they look like smudges and the question was what are they okay so this is astronomers starting to take an inventory of things is that smudge a big gas cloud that’s in the Milky Way or is it like another Milky Way that’s very far away so that was actually the debate between Harlow Shapley and Curtis Heber and it kind of became clear that there were other milky ways hundreds of millions of light years away called galaxies based on a series of measurements by Hubble who is an astronomer and in 1929 Hubble made an even more remarkable observation which is he just looked at galaxies and by measuring the reddening of light as the galaxy moves away he could measure the velocity and then through is kind of a complicated series of measurements he could determine the distance of the galaxies and he found out number one the most startling part is that every galaxy is moving away from the Milky Way which means one of two things either the Milky Way is in a very very special place in the universe or that the universe is expanding in some way he also found out that the velocity with which the galaxy was moving away was proportional to its distance so the further away a galaxy is the faster it’s moving so somehow space itself is just kind of inflating and this was really very very challenging there were people I mean respectable astrophysicists well into the 1970s who just could not accept you know this idea that space was expanding worked fine in

Einstein’s theory no problem during the Second World War there was a real explosion in the understanding of nuclear physics and immediately after the first idea of the origins of the universe began to take shape so the obvious thing is you go in and you measure all of these galaxies they’re all moving away you know how far it is you can extrapolate backwards and and so they all came from you know sort of one starting point and you can figure out that that was about 14 billion years in the past and this was the beginning of the Big Bang Theory and Alpha Hermann and gamma were able to calculate how many protons how many helium’s how many electrons would get made early in the Big Bang so this is really kind of an amazing things how many protons are there in the universe well they calculate it’s about 10 to the 80th power so that’s one with 80 zeroes which sounds like a lot but you know it’s a finite number the other thing that came out of this is that early in the Big Bang is space expanded there are these protons or these electrons and and things were moving apart and about 400,000 years after the Big Bang the the electrons stuck onto the protons to make what’s called primordial hydrogen the original stuff of the universe and when that happens some light was emitted so every time you stick an electron onto something some light comes out you know these are atomic transitions you might have learned about in chemistry so in that these things there are atoms with the electron getting kicked off by some radio frequency field and the tube and then the electrons sticks on some light comes out and we can see this light the big Big Bang was actually a derogatory term for this theory you know I forgot what the original name of the theory but but somebody said oh it’s just a big bang and it’s not 1963 the light from the Big Bang was actually observed by by Penzias and Wilson this was a huge thing because this light couldn’t be anything else and it’s really a signal from the beginning the universe there were so during this time there’s a lot of work done on how this expansion early in the universe started and there was the inflationary cosmology that my colleague Alan Guth developed that explained how the universe expanded very very quickly and a lot of measurements looking at supernovae that I’ll tell you in a minute but the bottom eye is about the beginning of this orange line these guys these astronomers come to the particle physicists and say well we’ve measured the astronomical measurements how much stuff there is in the universe and we’ve weighed it and we’ve kind of taken a census and we found out that your stuff you know the stuff that you have this great theory to describe is 4% of the total matter in the universe and the other 96% we don’t know what it is so the question is is this success I mean so we can be forgiven for this because we live on the earth that’s dominated by the stuff we know about which is why we know about it but in the limitless voids of the cosmos there is all this other stuff and we really don’t have much good idea of what it is so from the measurements they are able to to kind of categorize it into two parts one is called dark energy and one is called dark matter and I will tell you very briefly what dark energy is I don’t work on it so I don’t want to emphasize it too much but this is the the statement I mean nobody is this is really true and in terms of fitting with our theory of fundamental interactions there is no way to put in something like dark energy that’s at all credible I mean there are people who try but let me just tell you a little bit about how one kind of measurement is this is a galaxy and that’s a supernova and you can see that the supernova is as bright as you know the black hole and billions of stars in this galaxy so when a supernova goes off you can really see it across the universe you know so the universe is well you can you can see it

halfway across the University and a certain kind of supernova called a type 1a has a very distinct sequence of how it emits light it gets brighter and then it dims in a certain way and by measuring that brightening and dimming you can calculate pretty accurately what its total energy output is and if you know that you can use the fact that light spreads out as one over radius squared to measure its distance so you can measure how far away this is by how bright it got okay it’s just like you know if if I took a light bulb and I looked at it through a telescope and it was you know 10 yards away and then I moved it to 20 yards away it would look 1/4 as bright because it spreads out like radius squared if I knew how bright the light bulb was I could measure how much light hits my line my little telescope divided and figure out how far away it is so you measure the distance that way the other way is you can use Hubble’s trick by measuring the the reddening of the light and you can use Hubble’s law and measure how far away it is in a different way so you measure one way by how fast it’s moving plus Hubble’s law you measure the other way by how how bright it is and you compare I just said all this so this is kind of a so this is a typical astronomer plot here is is the measurement of how far away it is by its velocity from the reddening that’s called redshift and then here is how bright it is these are units of dimness because astronomers are kind of perverse so the larger this number the dimmer it is okay and the the big thing that you can see on this plot is that when you get very far away this is halfway across the universe when you get you know a good fraction of the way across the universe the supernovas are a little bit dimmer than they should be for their distance and the conclusion from this is that there have been accelerated from Hubble’s law it’s not just a simple linear relation but they’re getting away from us faster than they should be and that’s because there’s something called dark energy that’s pushing them faster than you would expect actually this this is a map of the sky of the photons from the Big Bang so this is actually a picture of what the sky looked like 400,000 years after the Big Bang which was 14 13.7 billion years ago and by analyzing these bright and dark patches you can also make a measurement of the expansion of the universe oh that’s it’s it’s kind of complicated to explain but using that you know the light from the Big Bang and the supernovas it turns out that 73% of the total matter has to be this dark energy 23% is dark matter and 4% is ordinary stuff and it doesn’t all just come from the supernovae measurement there’s been a lot of cross-checking there are now five or six different ways of getting at this ratio and of course you can combine them all together so the uncertainties on these numbers are a few percent so this picture isn’t going away this isn’t some you know fluky thing that somebody made up in the middle of the night okay so why is dark energy so weird it’s pushing the universe apart faster than it should be and what that means you can think of in terms of a relationship between density and pressure so I like names for things Boyle’s law is the relationship between density in a bottle of gas and pressure and a bottle of gas so if you take a bottle and you stuff twice as much gas and into it the pressure on the surface of the bottle doubles and you put in twice as much again and it doubles again it’s just called the ideal gas law you know children in the audience a fun thing to do is is take a bottle and you put some vinegar and baking soda in it and then screw the cap on really tight and leave it on under some furniture or something and the pressure will build up and it will explode because there’s more gas going in that’s making the pressure

go up dark energy works in exactly the opposite way the pressure is in and the more dark energy you put in a bottle if you could put it in a bottle the more it would want to implode so from an energy point of view putting creating dark energy in space doesn’t cost you anything so the universe wants to expand so that it can have more dark energy and there just ain’t particles that that can do that so that’s kind of my extent of knowledge of dark energy there’s lots more to say about it but it’s it’s more details what are we gonna do about it well the only thing we really know how to do is study more supernovae study the way galaxies are scattered around in space and for that you need new telescopes this is the hubble space telescope which has just been an amazing piece of equipment I remember I was at Johns Hopkins when it launched and it went through that horrible debacle of the optics but it’s it’s been going just like crazy ever since it’ll it’ll die soon and they’re replacing it with this the James Webb Space Telescope that’s probably up five years away and then this is a based telescope card called the large synoptic survey telescope that’s really a supernova hunter so this thing works by photographing a fair fraction of the sky every night and then you can compare with the night before and the differences are are always interesting usually you can see a lot of supernovas going on and so then you can you can put more points on this plot and you can see if you know the plot is the same for the supernovae over there and the supernovae over there you can do all kinds of things so these are things you’re going to be hearing a lot about okay now dark matter I mean if you’re just confused and unhappy with dark energy you know join the club deal with it it’s just a mess dark matter is a lot more tractable and it starts with with cranky guy named Fritz Zwicky who was really the first one to well he came up with the name and he was the first one to make the the key observation this is Fritz in in later years he was astronomer at Caltech and he was just notoriously obnoxious he was brilliant but he’d come up with these ideas like if you’re an astronomer a big thing is the atmosphere and the atmosphere kind of jiggles around and it blurs your image he did some calculations and found out that a shock wave from say a passing supersonic jet would smooth out the atmosphere for about a minute so you could make observations so you know he immediately called the Navy and demanded a jet flying back and forth you know he just thought like that he did amazing things and he’s he’s kind of unsung there hasn’t been the great fat book written by you know David McCullough about him he makes this gesture because anyone who didn’t agree with him was was bastard that was his term and and this was particularly disagreeable people were spherical bastards so this kind of physics humor a sphere looks the same no matter how you look at it so if you were as if you were a spherical you know you just had no redeeming qualities and the last thing I had to say about him was I my he was a Swiss and my PhD adviser Felix Bohm was also a Swiss so when I got the Caltech in 83 I asked if Ricky was still around and and Felix is a very my advice very formal man but he immediately said oh no he’s dead thank God so he’s a real character and if anyone has a yen to write a great scientific biography I think he’s he’s ripe for it Wiki yeah there’s a there’s a famous yarn company in Switzerland name of Zwicky yarn what he did was really incredible he made a survey one of the first surveys of a cluster of galaxies using this telescope which you notice is not particularly big but has a very large field of view and this is important because when you take a survey you want to see a lot of stars at the same time so that you can measure their brightness in a consistent way now a mile or so from this is you know the hundred inch Mount Wilson telescope and everybody’s fighting over who gets to

use that but you can’t do surveys with these with you know really big telescope because you look at a very small patch of sky so this was a very clever thing this is an 18-inch Schmidt refractor which was designed by another nutter named Schmidt he’s also quite a character but what Ricky looked at was this cluster of galaxies so this is really an amazing picture if you look at it that’s a star that’s probably a star there’s a star all these yellow things are galaxies you know all of these yellow things every you know that that one right there that little tiny one that’s probably a billion stars with a big black hole you know probably the size of the Milky Way and if you look that’s all you see there are thousand galaxies in this cluster really it’s really astounding that’s a star their stars are pretty boring so these are you know this distance from here is probably a hundred million light-years this this whole mess I mean probably 50 million light years this whole mess is about 300 million light years away so it’s and it’s one big gravitationally bound system so they’re they’re not orbiting in circular orbits but they’re all kind of moving and swirling around each other bound by gravitational attraction and what’s Wicky did was he really used just this picture and you know he reduced it to data so you can see there’s there’s kind of a cluster of galaxies in the middle and then it gets fewer and fewer on the outside he used the positions to determine the relative potential energy and that’s the energy that you have from being in a gravitational field like if I’m up here I have more potential energy than I do down here because I’ve moved myself up I’ve stored energy in the gravitational field so the fact that these guys aren’t sitting on top of each other means they’ve stored some energy by being apart kinetic energy is that you know when I hit the floor I’m moving with some velocity why am i moving with some velocity because I jumped down and accelerated in the gravitational field so these are swirling around he’s able to again using the reddening of light for this redshift it’s able to measure the velocity and what he was able to do statistically by measuring the velocity of several hundred of these and using their position was to determine the hole the mass of the whole system okay so it’s really pretty simple you know you this was 19 32 or 33 you’ve just decided that these are not gas clouds inside the Milky Way but huge Milky Way’s somewhere else so what’s one of the questions well how much does all that stuff way right so he was trying to weigh it and he weighed it by relating their motion and their position using the virial theorem so the most boring class you take when you’re in physics is the class I teach which is mechanics and kind of the most boring part of part of mechanics is the virial theorem so he paid attention during that part of the mechanics and use this theorem to determine the mass of the whole thing and then on average each galaxy is one 1000th of that mass you measure the mass of the whole thing they’re a thousand galaxies divided by a thousand that’s the average mass okay fine who cares the other thing he did again you can figure out how much light’s coming out of say that one by measuring how much it activated the photographic film he was using and this was a pretty standard procedure just how many light particles hit the film and how many she actually count the grains of the film that are that have gone dark and you can determine how bright that collection of a billion stars it okay you can look at a star near ours there’s a star I think it’s three light years awake Kapteyn star you know how far away it is you can measure its brightness you can measure its mass and you can make a ratio of how much brightness there is to how much mass there is and you can compare that ratio to the same ratio for that galaxy and what he found out is that that ratio is different by a factor of 10 for that galaxy than it is for a star so the picture that the mass of this galaxy is

all in the stars is not correct there’s some other mass in that galaxy that’s not related to the light output of the galaxy and that’s why it’s called dark matter it’s dark it doesn’t make light it’s very simple and that was really all there was I mean that is a very simple clear argument absolutely I’ll come to that so in the rap song after rapping about this the line goes and then for 40 years the dark matter problem sits because after all it’s only crazy fred’s and that’s sort of true at the time this is 1936 it was not known you know what particles there were how galaxies operated so nobody worried too much about the Dark Matter problem in the 1970s there was an interesting measurement made by Vera Rubin of m33 which is a galaxy not too far away it’s the Triangulum galaxy and so here you can see the galaxy there are the stars and out here there aren’t any stars there’s kind of some hydrogen but again by measuring the velocity in slices as a function of distance which is shown here by the the yellow data points what she was able to do is measure the mass because if you have if you think about the Sun the Earth’s in orbit around the Sun the the the velocity of the Earth’s orbit is related to how much mass there is inside the Earth’s orbit and so if you can measure the velocity systematically and take differences between the velocity here and the velocity here you can figure out how much mass is in between so the data looks like this now if you just again took the light and assigned a certain mass to each mount amount of light based on what you saw from the Sun what you would expect was was this and so there’s mass a lot of mass way out here where you can’t see any stars and so this is telling you that there’s a whole lot of mass as much mass in here out here actually a lot more this is my favorite about the same time some guys Ostreicher in people’s at Princeton got hold of a PEP 11 does anyone remember pdp-11 yeah pdp-11 which made here in Massachusetts was the first computer that a university could afford to buy and operate I mean it was Digital Equipment Corporation I think one of the saddest days of my life was when I heard they had been bought by compact but the pdp-11 was the first first computer where you could actually have in your office and do stuff this is in the early 1970s so what what they did was they took a collection of mass points you know like stars in a galaxy and they started them off with the velocities that you would have if everything were rotating around the way the galaxy rotates and then they just solve Newton’s force laws and watched what happened just numerically brute force solved the equations that describe the motion and down here is time as measured by the orbit of one of the outermost things so this is time equals zero time equals 0.2 time equals 0.6 time equals 0.94 and what you see is it very quickly the whole thing kind of collapses into this messy grungy little bar in the middle and that’s not what galaxies look like you know what galaxies look like I showed you a picture there of these big sweeping structures with certainly a concentration of matter in the middle but but spiral arms and this beautiful structure and no matter what they did it would collapse into this unless they put a halo a spherical distribution of dark matter around the whole thing and then they would get something that looks like our galaxy so numerically they they said look to get our galaxy to look the way it is you need you need all this dark matter okay this is actually very recent thing but this is this is extremely interesting so all of these yellow points here are our galaxies and what’s actually happening is there are two clusters of galaxies that are colliding with each other kind of passing through each other and overlaid on the picture of the stars are

two sets of data this pink is the intensity of x-ray emission so when these two galaxy clusters of galaxies pass through each other there’s a lot of gas that bumps into other gas and the gas heats up and emits x-rays and so the pink is tracing out where most of the matter the normal matter in the galaxies are the blue is showing a region where the dark matter is and that’s been determined in a very complicated way called gravitational lensing based on looking at the shape the distortion of the shape of stars and galaxies here as the light is bent by the Dark Matter what you see then is this is one cluster of galaxies and the dark matter is a head of the normal matter going this way this is going that way dark matter normal matter and so what’s happened is in the collision the normal matter has been dragged because it’s hitting the other normal matter the dark matter which isn’t affected by the presence of normal matter or other dark matter has run ahead so you can actually see there’s there’s a stripping of the normal matter this was about three years ago and it’s kind of now the iconic picture of of dark matter so I’m really summarizing all incredible amount of work just just to give you sort of a flavor there are about 300 galaxies that have been measured like this is called a rotation curve so 300 rotation curves they all look like this and they’re then many many other numericals these observational studies they’re all saying the same thing which is that there’s about 10 times as much dark matter in a galaxy as normal matter so kind of the the dumbest picture you can make now of the situation is that this is the part of the galaxy that you’re used to seeing there’s the center with the big black hole and there are the spiral arms and Earth’s about halfway out you know 45 light years and then around the outside roughly to scale is this big fuzzy ball of dark matter and this big fuzzy wall of ball of dark matter is making the gravity in here just right so that the galaxy looks the way it does and from our numerical simulations baryons is the fancy name from for normal matter now this is kind of the picture that everybody agrees is the simplest and everybody agrees is wrong there’s a huge amount of complication to this but nobody agrees what it is there could be clumps of denser or lesser dark matter there are you know could be little satellite galaxies in fact our galaxy has two satellite galaxies the large and small Magellanic Clouds and there are a couple other galaxies even further away but kind of the simplest picture to think about how to look for dark matter where it might be is is this okay so you have if you’re gonna look for dark matter you’ve got to make a gas and the best guess is dark matter are particles that weigh more than a hundred proton masses and that comes about because using accelerators we’ve kind of found every particle with a mass less than 100 proton masses okay it’s like the drunk looking for his keys you know under the lamp because that’s where it’s light it certainly gravitationally interacts it may or may not weakly interact with ordinary matter that’s a hope so dark matter to summarize this point all of the evidence is from astronomy and from the way dark matter gravitationally interacts the hope is it weakly interacts with ordinary matter because if it doesn’t it’s gonna be very hard to observe using the simple picture I showed you before in in a court volume in this room there are three dark matter particles and they’re moving with about a thousandth of the speed of light now that sounds like a lot you know how could you not have found dark matter if there are three of them in a court but you know so kourt’s maybe twice this in in one cubic centimeter there are you know 10 to the 25 atoms or one with 25 zeros after it so in this room on this earth there’s just an incredible amount of stuff that’s going on that with the

normal matter so that the three particles that don’t interact much that are kind of wandering around are very hard to find unless you do something special to find them so let me tell you about it so we know dark matter feels gravity may be dark matter interacts with the weak force with ordinary matter if it does it can bump into an ordinary atom and give it some energy so the first thing we tried to look at and I did this one as a graduate student is you know here’s a nucleus sitting somewhere and a Dark Matter particle is going to come and it’s going to bounce off of it exactly like a pool ball so what you need to look for is a nucleus that has a little bit of energy about the same amount of energy as as one x-ray when you get a chest x-ray which isn’t much but something you can do alright so there’s this dark matter three perk or moving with a thousand the speed of light it’s going to go like that and my job is to find that so the question is you know how often does that happen so if you take two pounds a kilogram current best gasses that’ll happen once a year okay now if you think of a kilogram of stuff even a kilogram of stuff just sitting here very quietly there cosmic rays going through it there are trace radio if contaminants you know there’s probably some living cells on it because the Earth’s are pretty filthy place but there’s a lot of other stuff going on so the challenge is really signal-to-noise how do you its create an environment where only the Dark Matter does stuff and you know that’s what we spend our time doing so that’s a dark matter particle that’s hitting a nucleus that’s attached to a molecule and the nucleus is recoiling and as it moves along it it hits other atoms and knocks off electrons and this is the way that particle physics is done you you count these electrons using an electronic circuit so this was one of the very first Dark Matter experiments and it was my PhD project it’s eight diodes diode is about the simplest piece of electrical circuitry but it’s eight diodes of germanium and it’s all set up so that it can collect those electrons from the nucleus after being hit by by dark matter now I’ve told you that there’s a lot of stuff going on so what we had to do is we had to make these diodes out of very pure germanium so that there weren’t any radioactive contaminants we had to surround it with copper to keep any ambient gamma rays or radioactive stuff from hitting the germanium and then we had to surround the copper with lead that was an economic thing the copper is pretty expensive the lead is pretty is cheaper but the lead is a little bit more radioactive than the copper so it’s an optimization problem you have this is about twenty thirty centimeters of copper and twenty centimeters of lead so there’s about twelve tons of lead and about three tons of copper that I stacked up by hand and I didn’t get to stack it up by hand in some nice lab at Caltech I was in this tunnel which is a 17 mile long tunnel through the Swiss Alps because their cosmic rays now the cosmic rays penetrate hundreds of yards into the earth so a nice help that’s about a mile high it’s what you need to stop those so this is going in and this is what the tunnel look like and there are actually two tunnels this is the one for cars this is the escape tunnel and and so every kilometer so there’s a tunnel connecting the two and that’s where we put this experiment so I had to this is this is in southern Switzerland so I had to drive down there a couple times a week and this is a Swiss Alps pretty nice place actually but had to go down there to work on the experiment it’s pretty miserable place now we operated this experiment for a time and we were able to show that dark matter wasn’t a heavy neutrino which was kind of the idea neutrino is kind of this weird particle that that doesn’t interact much either and there was this kind of idea that might Dark Matter was in neutrinos and that was pretty low hanging fruit so with this thing which was actually built to do something else we were able to retrofit it and show it wasn’t neutrinos but even with all this lead and all this copper and the Swiss

Alps and blah blah blah there were still things that look like fake signals and there was nothing we could do to get rid of them so it really needed a new technology now doing this experiment and delayed me writing my thesis for about six months so when I was done with this I was just like so disgusted with dark matter that I went and did something else for twenty years at the same time so we heard about this possibility of using this kind of detector to look for dark matter at a meeting in a ski resort in France and so at that meeting was where my advisor told me you go do this at the same meeting there were three guys David Caldwell Bernard satyr leh and Blas Cabrera who said okay this thing Fischer has is probably a okay to start but we want to really build something designed to detect dark matter and their idea was actually very smart this measured the energy of the recoiling nucleus only one way what they decided to do was measure it two ways and by measuring to two ways you could obtain much better understanding of what the particle was that got hit so let me explain that this is also a piece of germanium and down here is a little electric circuit that calc that collects electrons up here are a whole bunch of really really really sensitive thermometers because what they were doing is making use of the fact that when the nucleus moves along it bangs into other nuclei and generates a little bit of heat and only a dark matter particle can hit a nucleus to make it do that other things hit electrons or interact themselves and the signal is all electronic for the dark matter induced events the signal is partly Heat partly electrons so they very intelligently decided to measure both Tali Figueroa at MIT is is working on this so this started at the moreand electroweak meeting in 1988 and these guys just got their first result so it’s a tough game oh by the way because you’re looking for this little tiny amount of heat not only do you have to survive surround this thing with copper not only do you have to surround it with lead not only do you have to put it in the bottom of a mine in Upper Minnesota you also have to operate the whole thing close to absolute zero so here you can see it it’s there’s a guy it’s glove this is in a helium dilution refrigerator which is you know one of the most god-awful complicated things ever made no don’t I’ll show you so this is just the principle that I just described this is the total energy and then this is the fraction of energy that’s electronic so everything that isn’t dark matter shows up up here everything that is dark matter would show up there so this is their result twenty years of development and three months of operations so this is the same total energy fraction and so this is just the bottom part of the plot and these points here are interactions that they were able to explain using timing as not very interesting the red star and blue are things that look like dark matter and you know there’s one and this is the lines that they figured out from the way their things work there’s one that looks like it’s dark matter unfortunately they calculate and simulate everything very carefully and they do expect statistically one that looks like that so they can’t really say that they’ve discovered dark matter but what’s interesting is after twenty years this thing is a factor of a billion times more sensitive than my thing it’s pretty impressive okay so this is just that dark matters and neutrinos and then these guys there’s another experiment there now a factor of a billion more sensitive so the way you get more sensitive is you build a better detector and you build a bigger detector but some work I dated with Jocelyn Monroe showed that the next generation of dark matter detectors which will weigh about a ton we’ll be ultimately limited by neutrinos from the Sun which interact just like

Dark Matter so when I told you you know we’ll either get it in the next 10 years or not or you know this is this is the problem we’re we’ve come up to a place where there’s an irreducible background plexy these annihilations could be taking place and protons anti protons positrons electrons all kinds of stuff could come flying out of these different annihilations and so you could look for those particles to see if dark matter were annihilating in our galaxy so in 1994 a couple of people and I started thinking about how to do this one of them is the my colleague Sam ting so we came up with this idea that you’d look for them in space by just measuring them by just building a detector that can tell a proton from an antiproton a positron from an electron and we just look at stuff from Dark Matter annihilation so the way this works is this is a big magnet and particles bend in a magnet so if they’re positively charged they’d been that way if they’re negatively charged they’ve been that way and then this blue stuff is ways of measuring their trajectory so you measure their trajectory as they go through the magnet you can tell how much energy they have and whether they’re positive or negative and then here green labeled scintillators here and here are timers so when the particle goes through here a clock starts and when a particle comes out here the clock stops now the time to go from here to here for a cosmic ray is about a billionth of a second or 3 billions of a second and these clocks operate with the precision of a hundred trillions of a second so you can measure their velocity this is a known distance and when you measure a time to go over a known distance you measure the velocity and here you measure what kind of particle it is now there’s a rub this is not a very big thing this thing is about is it’s you know that’s about a meter here and this is this kind of thing particle physicists have been building for 40 years this is called a magnetic spectrometer there’s some other detectors here that that kind of help out but the main thing is this curvature measurement the problem is all of these particles from Dark Matter annihilation don’t make it to the Earth’s surface because we have this nice thick atmosphere that absorbs them if the atmosphere didn’t absorb them we’d be dead okay from radiation sickness also is fixie ation but but we’re focusing on the cosmic so the problem is you have to get this up out of the atmosphere and there are kind of three ways to do that there was a Japanese group about five years before as that that built a much smaller one of these about this big and they didn’t have enough money for a balloon they had nowhere near enough money for a satellite so they bought a seat on a 747 on a jl 747 that flew from Tokyo to Sydney and you know it get up to altitude and thing just had its seat it’s a good thing you know you can do that when you have a nationalized airline we actually had to put it on the space shuttle so that’s it right there and this is discovery just after it undocked from mir for the last time so we had our are the first version of this experiment taking data on this Space Shuttle I got to go and sit in Mission Control and listen to all the stuff going on and be a part of it so it was it was really just tremendously exciting and the only bad part is that we didn’t find any dark matter this isn’t 1998 so this is this is my last students PhD thesis this is this is energy of particle this is what you expect this is what we measured and this is the largest amount of dark matter that could be accomodated and it’s it’s nowhere near a signal so we we kind of didn’t succeed yet again but undaunted we have built a much much bigger one and in February it will go on the last space shuttle the last space shuttle to the space station and the Space Shuttle is going to dock here and then this robot arm is gonna grab it and put it there and it’s called the Alpha Magnetic Spectrometer and it’s gonna stay there forever we’ve built it so that it will last for 20 years and there’s no way to get it back

actually because the the Space Shuttle is the only thing that can carry it the Space Shuttle is an amazing piece of hardware it’s it’s it gets dumped on a lot for for good reasons but it its capabilities are astounding so if you can go to this launch the space shuttle launches is one of the most amazing things I’ve ever seen so one last remark the other thing you can do to look for dark matter is you can try to make them in the lab now to make something with a hundred proton masses you need to bang particles together with a huge amount of energy and as you’ve probably heard this big accelerator the Large Hadron Collider is just starting to operate in Geneva Switzerland and so I can suggest several excellent people to come and talk to you about it one lives here in Belmont and it could be that in the next weeks or months you read about the production of particles that could be Dark Matter at the Large Hadron Collider so we’re all hoping but you know the bottom line is you know we’re kind of at the end of the line as far as being able to do experiments to look for both dark matter and dark energy this Large Hadron Collider it took 25 years to build its many billions of dollars this a and x6 AMS experiment well my colleague Sam is the guy who’s in charge of numbers and he tends to flee he says it costs two billion dollars I think it probably only cost about 900 million but you know that’s that’s real money for for looking for something like dark matter and these other experiments that look for the nucleus banging when I casually said oh well there’ll be a ton that’s 200 million dollars so it’s kind of to the point where we either find it or not because the next step I don’t think you know anybody really is too interested in paying for but we can hope so I just like to leave you with a few things if you’re if you’re interested in the Big Bang this is the first three minutes by Steven Weinberg who is a theoretical physicist and really the guy who put together particle physics as we know it now he’s a brilliant brilliant man and this this books probably 20 years old it’s the story of the Big Bang but it’s it’s fabulously well written my colleague Alan Guth explains a lot about cosmology in his book inflation this is not about economics this this is a tremendous book this book is about dark energy and it’s by Robert Kirchner and it’s it’s very good about the science it’s very good about the sociology of astronomy I’m also writing a book for Princeton University Press so it’ll be out in a year well I’m finished the third graft so maybe sooner but it’ll go into a lot more detail and then this stuff so this is the big time for dark matter and in addition to these books I really recommend looking at science magazine they have a podcast they have lots of online stuff as you know they science is a technical journal but they have a nice section in the beginning called editor’s choice which is kind of a a layperson summary of what’s in the technical articles and you know I can elite in a given issue of science I can only read two or three of the articles because I don’t know what the rest are about because they’re all about biology but these these summaries are really pretty good ok so I think that’s oh yeah the people who paid for all of this so you know this is really it’s really hard to do this because unlike a lot of other scientific enterprises there’s no like big accelerator or big physics things the thing I do are kind of one-off things so we get a support from a lot of a lot of places at MIT and then of course the Department of Energy and National Science Foundation and then there’s a whole other story the Dark Matter detector we were building actually turns out to be good for looking for stuff you’d make atomic bombs out of so I work with the Institute for soldier nanotechnology at MIT and then this is well somebody can ask what whip is it’s it’s a whole different story anyway I’ll just leave

you with my cat whose name is dark matter that’s the one on the one on the right is is dark matter that’s my daughter Olympia and you know I I hope you know I hope dark matter is going to be in our headlights pretty soon so thanks a lot