What is dark matter? – with Peter Fisher

yes hello thank you Lisa and um our topic for tonight is you can um as you heard is what is uh dark matter I'll give you kind of the the main bullet points which are we know Dark Matter exists because the way things are shaped in our universe but we don't know what Dark Matter actually is so the rest of the talk in my my book which I'll show you here um will uh flesh out and elucidate uh those two ideas um so you should be seeing a slide that's a pie chart um and it shows how the energy and matter in our universe is apportioned uh according to the best measurements we've been able to make both in particle physics and astronomy and there's some kind of an odd history to this slide over the last hundred years particle physics uh which started just about just about 120 years ago um evolved so that uh by the early 1990s we had a very good idea of all the different kinds of particles there there were in the universe and about how many of them there were and that started with quantum mechanics in the 1920s and then Nuclear Physics began in the 1930s and then after the second world war a lot of new technology became available to make microwaves and accelerators and there was an explosion and understanding of particle physics that lasted until uh well until the present day but really reached it Zenith uh in the 1990s and so coming into the early 1990s physicists thought that we really understood the totality of matter or energy in the universe and we talk about matter energy interchangeably because E equals m c squared as Einstein folder told us and um so we can just talk about one or the other along the same time astronomers were developing bigger and better telescopes and more advanced observing techniques and so in the 1920s uh astronomers discerned that the fuzzy star-like objects that they called nebula which means fuzzy or foggy were actually distant galaxies like the Milky Way with hundreds of thousands hundreds of millions or trillions of stars I believe I'm sorry trillions of stars in them gravitationally bound and very shortly after that uh there were a series of observations that indicated those stars were moving too fast to account for the matter in the in the galaxy and we'll talk about I'll talk about that more more later as observational techniques improved the acceleration of the universe became clear and the acceleration of the universe is actually space time getting bigger which means that those galaxies are seem to be moving apart with a velocity that's proportional to the distance between them this was quite surprising uh and um measurements of these acceleration which is also called the Hubble parameter uh improved through the 50s 60s uh and 70s by the mid-1980s astronomers had developed techniques using exploding Stars called Supernova to be able to look really halfway across the Galaxy or halfway back in time and remember light travels at a fixed velocity so when an astronomer sees light from a distant star uh in in their telescope what they're actually looking at is light that was emitted by that star uh uh hundreds of millions of or billions of years uh before and so when an astronomer looks at a distant Supernova they're actually looking back in time at How the Universe was for example if it's halfway across the Galaxy half the uh halfway across the universe when the universe is half as old as it was now and so being able to look halfway across the universe these supernovas showed that Not only was the uh Universe uh accelerating uh expanded but that expansion was accelerating and that gave rise to dark Matter's cousin dark energy so the two discoveries of dark energy and dark matter one of which I'll talk about tonight told physicists and astronomers really um and then we can have the pie chart uh that the stuff we know about which I've labeled stuff we know about which are the atoms and molecules and photons and all the particles we made in accelerators is really only four percent of the matter energy in the in the universe 23 is dark matter and uh about uh 73 uh is is dark energy so it's a little ironic that two Fields closely related but distinct pursued roughly parallel courses for 100 years and one informed the other that it had only done four percent of the uh of the work uh can we stop sharing for a second so tonight what I want to tell you about is a little bit about Dark Energy because it's a cousin of of dark matter and I want to tell you a little bit about it so that it's clear that it's it's really a a different phenomenon and a whole different story a very interesting one but not one we'll talk about tonight then I want to show you two pictures from two very different telescopes and talk about how dark matter makes those pictures look the way they are because a major theme for this talk is how dark matter makes the universe appear the way it is now I want to give then uh three or four examples of different phenomenon that have been observed by uh astronomers uh showing very different mechanisms by which the dark matter has has shaped our universe and then move on to the pick the question of what is our current picture of the universe what particles do we know about and why are none of them the Dark Matter particle and then I want to give you three examples the three most popular examples of what Dark Matter could be and say a little bit about how we search for them so this is something I've been involved with for about 35 years I worked on one of the very first Stark matter experiments uh as as a graduate student and um had a really marvelous adventure worked with many uh wonderful people and done a lot of interesting things I'll tell you a little bit about them um but I have not found uh dark matter and I'll give a perspective on the the future of this endeavor um in a few minutes oh what's shown here is the universe evolving from left to right you can see big bang labeled there in the in the uh White glow of the start of the universe and then as time passes and the Big Bang was 13.7 billion years ago as indicated on the bottom uh the universe expanded and that's indicated by the the way the disc that represents the the universe gets larger so there was an initial very rapid expansion labeled inflation that's a theory by my colleague here at MIT Allen Guth and um that's an entirely different and no less fascinating story and then one of the first thing that happens in the universe is the cosmic background Afterglow and I'll tell you what that is in a in a couple of minutes but after that happened you can see the universe gets bigger only very slowly going from left to right and that's indicated sort of by the diameter of that trumpet shaped uh figure and you can see where the first Stars start stars and galaxies develop and then you can see a little bit more than halfway down that the uh radius of of the universe starts getting bigger faster it's starting to to curve a little bit which means that it's accelerating and that's really the action of of dark energy so dark energy is somewhat mysterious there's no quantum mechanical theory of dark dark energy that's been shown to be correct um but it fits very naturally into Einstein's theory of gravity which is not a quantum mechanical theory in fact it's just one number that appears in Einstein's theory you put that in you measure that number and you can explain how the universe is expanding and that expansion is accelerating and as I mentioned before that acceleration was discerned by looking at very distant Supernova whose energy output you could calculate because there are special kind of supernova and that tells you how bright they are you can measure their their red shift which is a technique astronomers have for measuring how how fast something is moving away away from you and um compare that velocity with the distance and get the uh acceleration of the expanding universe what this says is that every Square centimeter of our universe is filled with something called dark energy that is a substance that has negative pressure and that negative pressure somewhat contradictory uh makes the universe expand it drives an ever increasing positive feedback in the expansion of the Universe um I uh well I know a few at least a few of you are in finance and uh so you can think of um dark energy as compound interest on space time and you all know that compound interest um you know it starts out small but you give it enough time and it gets big that's how University endowments work in the United States because universities can be around forever um it's the same thing uh the the universe right now gets about seven percent larger uh objects space time expands by about seven percent per billion years and so for that whole first you know eight billion years you didn't notice the universe doing anything except sort of expanding linearly with time and it's only now uh in the last five billion years that the the the quadratic part or the Kirby part of the compound interest on the space time of our universe is really starting to make the universe expand so dark energy in in a certain way shapes our universe as well but um it's it's a little bit boring it's just really the same everywhere and um aside from this effect uh you don't really see uh what what Dark Energy uh does okay so uh you should be seeing a a colored oval labeled plank this is the first of two pictures that I want to show you that tells us what the universe uh really uh looks like uh as formed by by dark matter so this oval is a map of the sky and for example if you think about the center of the oval that's looking straight up and then the sky is unrolled to the left and right and top and bottom so that all the coordinates in the sky are are represented uh on this oval and it's a temperature map of a specific frequency that corresponds to light that was emitted uh about 400 000 years after the big bang uh when neutral Hydro hydrogen formed hydrogen is the simplest atom and before hydrogen formed there were protons floating around and electrons floating around in an equal number and it was so hot that the electrons and could not stick on the protons to make a hydrogen atom as the universe cooled at 400 000 years the electrons and protons bound together and form neutral hydrogen and allowed the photons that were also uh in the universe from The Big Bang to stream off because before the photons were banging around knocking off of the charged particles because charged particles scatter light once neutral hydrogen formed the light streamed away and by measuring these photons which was first done in 1963 you can really see what was happening during the big bang and actually make a map of where matter was uh at the time of the Big Bang so the red uh points which indicate the places where the temperature is three millionths of a degree uh I'm sorry 300 millionths of a degree above the average temperature which is about three degrees are the denser parts and the blue areas are the the less dense parts so what does that mean density is a number a measurement of the number of particles in a little piece of space so the more particles the higher density and what you can see is there's kind of a random looking pattern but you can see that there's kind of a characteristic size actually several characteristic size one of them if you look at there's a big patch kind of over to the right uh right at the Meridian uh you know that is about the same size as the big patch kind of in the middle just below the meridian and and so on you can see the blue regions the biggest blue regions sort of have the same size and so on there's actually a hierarchy of different sizes and that's because early in the Big Bang everything was mixed together all the protons and neutrons and electrons and dark matter and quantum mechanics dominated and quantum mechanics introduces uncertainty into all of the physical parameters including the density so there were places where the density was a little bit higher and places where the density was a little bit lower because of the quantum mechanical uncertainty of dark matter and as the universe expanded matter was attracted to these places from high of higher uh Dark Matter density and away from places with lower Dark Matter density and that's what you're seeing in this in this image and those regions of higher density went on to form clusters of galaxies uh that we see today now what we're looking at is 13.7 billion minus 400 000 years ago so we're looking at a distant part of the universe but we believe that the Galaxy clusters that we observe today are because of the quantum mechanical uncertainties or fluctuations in the in the dark matter uh density this is a second picture uh so just going back to that to finish up so this is the first example of how dark matter through quantum mechanics makes the way the universe look the way it does we look out we see clusters and super clusters of galaxies those were seated by Dark Matter quantum mechanical fluctuations the second picture looks for more familiar as a picture from a telescope and that's because it's from the Hubble Space Telescope which was the precursor to uh James Webb and I'll just say that the Hubble Space Telescope has got to be the most productive and exciting scientific instrument of of all time I remember when it was launched in 1992 and the terrible problem they had with the mirror that was fixed in in that famous shuttle mission and four shuttle missions uh sense making repairs uh and the thing is is still an amazing scientific instrument and this picture to me is the most amazing picture from that scientific instrument this is called the Hubble extremely deep field or xdf so a field is a word that astronomers use for for an image of of a stellar uh astronomical image was part of one of the things that Hubble was built to do before Hubble launched uh a team worked very hard to find the darkest place in the galaxy the place where there's the fewest stars and the least light in Toto this is in the fornax constellation and during the course of its lifetime when it wasn't doing something else Hubble stared at this darkest spot of the the of the universe and just slowly accumulated the image that you see deep means that this image is of objects that are very very far away objects that are far away that emit light are dim and so all of you who are photographers know that if you want to take a image of a dim object you have to either open the f-stop on your camera increase the aperture or lengthen the exposure the Hubble's face telescope is as wide open as it gets so the way they had of increasing the exposure was by just looking at the same place in the sky over and over this is probably a total of about 10 million seconds um of observing time something like a hundred days and I know astronomers who will scratch each other's eye out for uh you know a few thousand seconds of Hubble observing time so this represents a significant amount of time on this this very precious uh instrument and what do you see in this image what you see is almost all galaxies everything you see there is a Galaxy I think there's one or two stars in in the entire image and if you look closely they'll there's a spectacular spectrum of differences you can see little um Blobby red ones you can see beautiful blue spiral ones that look kind of like the Milky Way you can see some that are very irregular you can see um uh some kind of collected near each other and some if you look really closely are just a single Pixel and that's some Galaxy that's halfway or three quarters Across the Universe with a trillion stars and we're just seeing a little trickle out of it and one of the things I'll tell you about in the next part of the talk is that every one of these galaxies looks the way it does because in addition to the stuff that makes the light the protons and electrons and things that we know about there's a surrounding Halo of dark matter that holds the Galaxy together and makes it pancake or makes it irregular it makes it do what uh what galaxies what that galaxies do as you see in in this picture this is a great screensaver you can just Google xdf and and you'll get it and when you're not doing anything else you can just kind of contemplate uh what it means for all of the all of these galaxies okay so those were two very different pictures that's a radio telescope this was a normal optical telescope so what I'm going to tell you about next are uh three different examples of the way dark matter has made galaxies and clusters of galaxies look the way they do and I'm going to tell you a little bit about astronomy uh and and the the the the the people involved but I just want to be clear that uh there are probably dozens of of different examples of the way Dark Matter shapes our galaxy and I'm just choosing uh a few of the the most most dramatic um one of the the most interesting and it's in my book is that uh Dark Matter around a Galaxy can actually actually act like a lens and focus the light from something behind that Galaxy and make a an image writer uh make an image of an object behind brighter so that that it can be seen so this is a picture of a cluster of galaxies called the the coma cluster uh the galaxies that are in the cluster are uh actually all the yellow objects uh you you can see and uh you can see they're kind of grouped together um in the upper left or upper right I'm sorry you can see uh a star and and so you can see how a star looks very white and very round in contrast to the the galaxies and the coma cluster they're about 2 000 galaxies uh in this cluster and uh if you look up on the sky with a telescope they're about two degrees across so they're kind of spread out you wouldn't readily recognize this if you uh didn't have a good telescope and and where to where to look what makes the these galaxies a cluster is that they're gravitationally bound now a Galaxy itself is an ensemble of gravitationally bound uh objects and you can think of gravitationally bound as as meaning things that are orbiting around some central body the way the solar system orbits around the Sun so we have solar systems orbiting around Suns we have Suns or orbiting around the center of a galaxy and one of the interesting things we've learned in the past 30 years is that every Galaxy com contains a a million solar mass black hole so everything in the galaxies orbiting around that black hole and then this coma cluster all of these galaxies are orbiting around each other and uh this image uh the the coma cluster was first studied um maybe five or ten years after the first understanding that galaxies were distant assemblages of of order a trillion Stars uh that that work was done um in like 1924 and the study I'm going to tell you about was in the early mid 1930s now in order to understand something about how all of this worked astronomers of the day were just trying to answer some really simple questions how big how big is the cluster how many galaxies are in a cluster maybe how many stars are in a galaxy and they could make some relatively sophisticated measurements they could make images like this from from large telescopes like the one on Mount Palomar they could also measure the velocity of of things along the line of sight using redshift which is just the stretching out of light as as an object recedes um technology played a role in this too because in order to get an image with good resolution all on one plate one needed a a telescope with a wide field of view which at the time was technologically very difficult to make but this shows an early version this is an 18 inch uh Schmidt telescope and the astronomer is one of the great characters of astronomy Fritz Wiki and you can see hit him uh there this is the way that you you know you worked if you're astronomer uh you can see this picture must have actually been taken during the day because you can see Shadows and Light sources and things so normally when he was working he it would be very dark now this is a small telescope and he was only a few hundred meters away from 100 inch telescope yet he preferred working on this telescope because it had a large field of view and in one photographic plate he could measure the uh entire cluster coma cluster uh through an image and then he could measure the red shifts of individual constituents so through this work and uh a brilliant statistical analysis that he did in a paper in 1936 in German and uh subsequent paper in 1937 in in English was he found that the objects in this coma cluster were moving too fast to account for the mass that's holding the cluster together so remember that the force of gravity is proportional to the mass of the gravitating body and how fast an object goes around uh in an orbit is proportional to the force So the faster you see an object moving that's gravitationally bound the greater mass that must be present in the system that's exerting the force on the gravitationally bound object now you could look at the amount of light coming out of the uh an individual Galaxy and how far away it was and uh from there you could form a mass to light ratio and compare that to a mass to light ratio of nearby stars that you uh knew very well so you could estimate the total amount of mass associated with light emission in these 2000 galaxies and compare that to the amount of mass associated with the motion of the galaxies inside of the cluster and those two different masses turned out to be wildly different they were different by something like a factor of a hundred with very large uncertainties but it was clear that there was some other form at least clear to Fritz Wiki that there was some other form of of matter that was creating gravity to make those stars move through the gravitationally gravitational potential that was holding them together um there was much more of that mass than there was mass uh generating light um I'd be a little remiss not not to mention that um zwicki did this work in in 1937. um he was really uh a brilliant man but he was also uh let's say somewhat acerbic uh and and rude and uh he was he was really quite a character he was at Caltech um and when I was uh at Caltech as a graduate student in the 1980s uh there were still people who were uh you know quite afraid of him even though he had had left and passed away a number of years but before and I always thought that the slow progress in in Dark Matter uh might have been the sort of the founder uh chasing people away because he was just a little bit uh too crazy for for anyone um to to Really tolerate um so by let's say the night late 1930s there was this idea around that there was a lot of unaccounted for mass in large astronomical struct structures large meaning uh clusters of galaxies perhaps even even galaxies uh them themselves and people thought about it astronomers worked on this there was some discussion um but things really didn't get going until um the 1970s when uh Vera Rubin and and Ken Ford made an important uh measurement of a nearby Galaxy called Andromeda um here's a picture of your Reuben and next tour is a new piece of technology which is a high-speed image intensifier that was I believe was on the Mount Wilson uh telescope and allowed uh them to collect light uh much more efficiently than uh usual on a on a photographic plate so what they were able to do is actually make a density map of this uh Galaxy so this is a well-known Galaxy it's kind of a classic idea Galaxy it's called Andromeda uh it's the nearest galaxy to Earth it is I think two million light years away so it's it's closed by astronomical standards and it's structured a lot like the Milky Way in the middle you can see a glowing concentration of stars and then as you move outwards you can see uh spiral arms which are really kind of density waves that move through the the galaxies and those are all stars um and then as you look further out from from the center uh you can see that there's really kind of a sharp edge where the the Galaxy ends and that's called the Schmidt boundary so our classical notion of a galaxy is that it's it's really this spiral object um and it has a really distinct Edge uh you can also see uh to the lower right um a little satellite Galaxy that's uh maybe a few hundred thousand stars uh perhaps orbiting perhaps falling in uh to to Andromeda so what Reuben and Ford wanted to do was really kind of make a density map of Andromeda to again measure its mass because astronomers always start with with really simple things and the way they did this was by measuring uh the brightness and how fast the stars were moving as a function of distance from from the center so in in the center uh you you expect the Stars to be moving rather quickly because uh remember Gravity the force of gravity goes with the inverse square of how far you are away from massive object and as you move out one would expect the Stars to move slower and slower and slower and so they were able to use this marvelous marvelous image intensifier to make the the precise measurements of the velocity of the Stars uh at different points moving out radially and then others were Robertson was able to use radio uh waves to measure even further past this uh edge of the of the Milky Way so this plot summarizes uh their their measurement um this is a tipped version of the plot that of the picture I just showed you so that uh Andromeda is is sitting horizontally and then the data points in the curve show the velocity and you can see over uh the scale on the the right side is velocity in kilometers per second so a number to remember is that things move at hundreds of kilometers per second uh inside of a of a galaxy um and that's okay because uh you know the Galaxy's pretty huge uh Andromeda is probably 50 000 light years uh across um you know from edge to edge and remember I told you that as you moved out you'd expect stars to move more and more slowly because they were further and further from the gravity gravity gravitating Center and what in fact you see in this curve which is both radio and Optical measurements is that the velocity is more or less constant at about 220 kilometers per second now what that's telling you is that there's some other form of mass that is making the gravity strong enough so that the velocity remains constant and as you move out one's orbiting more and more of the mass um inside inside the Galaxy so this was was very striking and uh these results really caught people's attention and within about 10 years the same measurement had before been performed on something like 40 or 50 uh galaxies that were were close enough to really resolve the the Luminous desk the bright part which is necessary for measuring the star's velocity and every single one of them had a plot that looked like this this was called a flat rotation curve flat because it's it's constant it's not decreasing and rotation curve because it's measuring how fast the stars are rotating around the center uh of of the Galaxy so this really by the 1980s put a dark matter um solidly on the map so that even particle physicists like me started really paying uh attention um and in 1983 in particle physics there was really a lot of work being done to consolidate the the three different kinds of interactions that atoms and and uh nuclei go through into a single unified theory called now called the standard model which is really a rather uninspiring name and using the theory of the big bang and the particles in the standard bottle theoretical physicists and cosmologists working together uh closely we're able to predict how much matter there should be in the universe and it simply wasn't enough to account for how much dark matter there should be based on the observations I've just just described uh to you so there was really the start of a of a problem also in 1983 was the beginning of a new theory of particle physics called supersymmetry that you might have heard about that predicted all kinds of heavier particles that we wouldn't have observed in accelerators because they weren't powerful enough so in the 1980s in the dark matter picture there was a lot of different ideas a lot of confusion most physicists and astronomers believed there was some new substance uh in the universe some didn't but um it was uh actually a very exciting time um in in physics so uh my story is I started graduate school in in 1983 as as all of this was going on and I was not working on any of this stuff I was working on uh trying to detect a very rare process with a very simple detector that had to be deep underground to keep away from cosmic rays and uh oh actually I'm getting ahead of myself uh I'm sorry um I just want to show you can we share a screen again the picture that came out of the early 1980s was that um a Galaxy actually looked like this so in the middle you can see a thing with little spiral arms that's the visible part of of a galaxy like the Milky Way and I've put a little blue star labeled solar system uh that's about where the Earth is so the Earth is uh and and Sun are about halfway out to the edge of the Milky Way kind of at the trailing edge of of one of the Spiral arms and that visible part is about um 50 uh 50 000 light years uh uh from Center to Edge so about a hundred thousand light years across to account for the observations that astronomers had made many at this point what needed to be added was a large Halo a spherical ball of dark matter that extended Way Beyond the visible Edge about uh you know 150 000 light years and 300 000 light years across that you couldn't see because for whatever reason it didn't produce light but what you could see is the motion of uh the disc in the center that told you that the gravitational field from the Dark Matter had to be very strong because the constituents of the Galaxy the stars and solar systems were moving very quickly so resuming my story in the in the 1980s this is a picture of of all the particles uh that we now know make up this this model that we have um shown in green are the quarks those are particles that bind together to make the nucleus shown in in blue are um are purple are lighter particles called uh leptons and uh you'll probably recognize from chemistry the electron and the electron has two cousins the MU and the Tau um and then at the bottom are neutrinos which we'll come back to in a minute so the purple and green particles those uh 12 particles are all matter particles and everything in the universe is made up of those particles really only the up and down quarks which make protons and neutrons and the electrons um all of the other particles either don't interact which are the neutrinos or Decay rather quickly to the up and down quarks or the electrons so that those are the matter particles and then the particles labeled in light blue are the force carriers the glue on the photon the Z and the w and so for example the photon is the thing that makes electric and magnetic fields and the w and z particles make different fields that make particles Decay and the gluons glue up and down Parks together to make neutrons and protons and there was a whole Theory checked out in the third decimal place that shows how all of this works so the way these particles talk to each other uh their masses everything has been experimentally measured um very very well and there's no gaps in the theory and there's no particle here that could really be a dark matter particle off to the right is the one remaining miscreant which was the Higgs boson uh the w and z were were the last ones uh what were discovered in 1983 the top Quark was 1994 or something and then uh the Higgs took about another 20 years and was discovered at CERN in um 2012. these are all the particles that are in this model they've all been observed they've all been measured and we know all the rules about how they work so that's why in the 1990s people like me thought we really understood everything about the universe uh about the matter in the universe uh when in fact the totality of of these uh particles uh 17 particles comprise about four percent of the mass energy of of the universe okay so that's kind of the situation uh when I showed up um so I started in 1983 uh this was largely the the standard model was largely assembled by then and in um nineteen eighty seven and 88 I was working on my my thesis experiment here's a picture of it this is actually from my PhD thesis and this was a wonderful experiment because this thing was so simple it was an experiment to look for a very rare kind of Decay called double beta Decay that took place in the four little boxes with x's in the very center uh up on the upper left side of the left hand diagram labeled GE detectors so these were detectors made of germanium um and uh if one of the germanium nuclei decayed in a certain way there was a very simple electronic circuit that would detect that decay and carry the the information out uh so this was a wonderful project as a graduate student I worked on it with just two other people um it was what I did all the time and and they were kind of part-timers most of it was uh most of the experiment was actually lead in Copper and you can see the the source which were the GE detectors are surrounded by um about 60 centimeters of copper and uh about 30 centimeters of lead which which we had to Stack Up by hand so that whole apparatus is about two meters tall and um we had it in the Saint gottard Road tunnel to protect it against cosmic rays that could make that could fake germanium decays and so I was just happily analyzing the data each week from the experiment um and and working on my thesis then I went to a talk at a conference in France and there um a guy named David spurgle who was then at Princeton and and now uh works for the Simons Foundation he's president of the Simons Foundation gave a talk about dark matter which I had been hearing about and how dark matter could be like a really heavy neutrino now in the previous graph I showed you there were three neutrinos those have all been detected and they're very very light they weigh much less than an electron his idea was if there was another neutrino or something like another neutrino that was very very heavy it wouldn't interact very much because neutrinos don't interact very much in fact you could take a neutrino and you could shoot it through 200 Earths you line up 200 Earths side by side by side that neutrino would go through all 200 Earths and probably wouldn't wouldn't interact so that's why you'd never detect it and it's just floating around in space and all it's doing is making the gravitational field we need to explain dark matter well what he said was that every once in a while one of those neutrinos could bump in to a germanium atom and leave a little bit of energy that would be recorded uh in my experiment so my PhD advisor Felix bones suggested you know which I wanted like a hole in the head remember I'm writing my thesis to get out of graduate school that we make a small change to the apparatus uh and look for dark matter which we did we didn't find dark matter but uh we did show that dark matter couldn't be a heavy neutrino which was uh an important result because it had been a heavy neutrino we'd have seen it and that's what the plot to the right is is showing now at that same conference there were a couple of guys who were really working Bernard sadelay uh and David Caldwell and uh there was somebody else uh they were trying to understand exactly how the energy of the struck germanium got out of the detector and they found out that if a dark matter particle hit it a third of the energy would be electronic and two-thirds of the energy would be Heat whereas if anything else hit it all of the energy would be electronic so they created this marvelous thing which is a detector with thermometers all over it that could tell whether the deposited energy was Heat or electronic or or both this was called cdms there have been a series of of experiments since mine in in 1988 um they've gotten bigger they have this marvelous new technology the um radio purity of of the things which is important for fake signals got a lot better to the point where now these experiments are 10 billion times more sensitive than the one I did in in 1988. um that is an amazing achievement and what it tells you is that a dark matter particle if it works like a neutrino could go through 10 trillion Earths without hitting anything so these things are very very hard to detect and have not been yet been detected so that's one way of looking for Dark Matter particles and this is a specific version of dark matter called the weekly interacting massive particle picture or Wimp And this is a very active area of research today my experiment weighed a couple of kilograms the current experiments weigh many tons uh this is just in order to get more things for the dark matter to hit this is a picture of the accelerator at CERN uh CERN took the approach of colliding particles together which happens in this ring down below and trying to make dark matter particles that would be detected in in the uh detectors that are labeled CMS Alice Atlas and lhcp uh they haven't observed dark matter and for many years I worked on this experiment which was supposed to look for dark matter in cosmic rays this is called the alpha magnetic spectrometer and it's sitting on the International Space Station this was kind of an amazing ride I had this experiment started in in a decrepit conference room here at MIT with five people in in 1994 and uh now is sitting on the International Space Station measuring uh cosmic rays we've learned a lot about cosmic rays but we haven't detected dark matter so that's one kind of dark matter the weekly interacting massive particle there's another kind called Axion Axion uh can we stop sharing screen for a minute Axion is an Italian uh laundry detergent and uh the creator of the Axion Theory Frank wilcheck uh called the part of axions because they clean up a problem a theoretical problem in one of the interactions oh and by the way axions could also explain uh dark matter and if we can share screen again this is a picture of an Axion detector that a colleague of mine Les Rosenberg at University of Washington built where the Axion labeled a in the little diagram to the left kiss a magnetic field Photon and turns it into a microwave that's collected with very high Precision in this cavity and would indicate the presence of of axions uh that too has not revealed uh Dark Matter so no wimps no axions now if we go back to 1972 there is a man who's thinking about what the very beginning of the universe would look like and remember I talked in the very beginning of the universe there's all this Mass around and it's very dense and it's in in a regime where quantum mechanics is important so the density is is rattling around and fluctuating and changing and he thought could it be that the density would be high enough to form a black hole and he did some rough calculations and found out that yes they could be a black hole they collapsed into a black hole and in fact there'd be a lot of black holes and there must be some means of getting rid of all those black holes because we we don't see them that man was Stephen Hawking and he thought about this problem for a year and in the following year invented a mechanism by which black holes radiate light at a very low level and this is called Hawking radiation and this is a remarkable notion because this is the only connection that we have right now between gravity and quantum mechanics and he developed this Theory to get rid of most of the black holes but not all of them and it could be that there are black holes left over from The Big Bang that would weigh about as much as a comet and be the size of a proton those are called primordial black holes and there are various ways of looking for them um but but no result yet and here if we can just share screen one more time this is a a picture that a colleague took the Event Horizon telescope of uh the surface of of a black hole and you probably saw this in the newspaper so weekly interacting massive particles axions black holes some people have their preferences the story some people think the story is closed on one or the other but these are the three active areas um the weekly interaction acting massive particles are certainly the most popular that's where a lot of the money is going but uh I think all three are are viable and we have two groups here at MIT working very hard on on on axions so that brings us back to what is dark matter the best I can say right now is more what Dark Matter isn't it isn't any particle we know about and that's we're very sure about that we have 17 known fundamental particles and none of them account for for dark matter the properties are that dark matter should not have electrical charge or very small electrical charge should not interact very much if at all with um known particles and need to be able to form in galaxies and create the gravitational potentials uh that we see so an enormous amount of work has been done I would say a great deal has been learned but we still don't know what what dark matter is and uh it may be sometime before we we really understand it's it's a great mystery uh when people complain I remind them it was about 300 years between Newton and Einstein and it might take that long or longer for for dark matter but it's a it's a great way to spend your time uh I learned a lot I worked with some really terrific terrific people and um I did some really exciting stuff and that's my talk and I'm I'm happy to answer people's questions foreign [Applause]

Channel: The Royal Institution

Views: 248,216

Keywords: Ri, Royal Institution, royal institute, dark matter, what is dark matter, what is dark matter and antimatter, what is dark matter in universe

Id: DDMOHLZtYLM

Length: 56min 47sec (3407 seconds)

Published: Thu Jan 05 2023