Dark Matter and Dark Energy | Sean Carroll | Talks at Google

Caltech in physics and he's going to speak on dark matter and dark energy he's also a contributor to the blog cosmic variance which if you haven't read it and her interest in physics she should I guess from there I'll let him take it great thanks thanks for inviting me it's great to be here at Google and it's a great to be telling you about this extremely exciting set of things that we've learned about the universe over just the past 10 or 20 years which coincidentally coincides with my career as a working physicist but I don't think that I had much to do with it in fact none of we theorists such as myself have had much to do with it over the last 15 years or so observers and experimentalists have learned an incredible amount about what the universe is made of and so forth and while be telling you about is what we've learned the problem is that the theorists such as myself have not really caught up we do not understand why it is the way it is we know a lot and in fact what we know can be summarized in the form of this pie chart right here this pie chart is the inventory of what the universe is made of we figured it out only in the last few years it's only once in human history that you figure out what the universe is made of so this is a major event the problem is that it makes no sense you guys here at Google could have done a much better job designing the universe than the whoever designed the actual universe did it's kind of messy there's a lot of features that don't really get used that are unexplained that little yellow part of the pie chart there is what we call ordinary matter that means everything that you've ever seen in the universe and by you I mean any human being every particle we've ever detected within the standard model of particle physics anything you've ever made in a laboratory experiment all the atoms and molecules and stars and gas and dust all that is 5% of the universe the tiny little yellow slice of the pie chart the red slice 25% is what we call dark matter so it is matter it's made of some kind of particle but a particle that we've not made yet in the laboratory a new kind of particle that we don't exactly know what it is and the other blue slice of the pie chart' 70% is dark energy not something is not even a particle as a field of energy that we can tell it's there because it influences the expansion and evolution of the universe but cannot be detected in laboratory you cannot even be be composed in individual particles and no one really expected it to be there so this is our job we know what the universe is made of we have an inventory we don't know why it is like that we don't know why there is dark matter or dark energy we don't know what particularly they are we don't know could they have been something very different or is there interesting physics within the sector of dark matter and dark energy can we explain them in terms of things that we do observe here on earth and that's what I'll be talking about here today but to start let's go with the universe and this is at the point at which people begin to use analogies and they start talking about raisin bread or expanding balloons and so forth I don't like any of those analogies they give you the wrong idea because the universe is not embedded in something else but a balloon has an inside and outside raisin bread has a crust in an oven and so forth so to understand the universe but the right tactic to take just to think about the universe think about standing outside on a clear day and imagine you have perfect vision so you could see everything in the universe what would you see well you would see galaxies this is the Hubble Deep Field of galaxies this is a picture of the universe that you would get by taking a powerful telescope and aiming it at an empty part of the sky and letting it sit there for a long time and collect collect a lot of photons what you would see is all of these little patches of light each one of those patches is a galaxy comparable in size to our Milky Way galaxy so of course you see stars when you look at the night sky we live in the Milky Way galaxy a collection of about a hundred billion stars but there's a hundred billion galaxies in the observable universe every single one of those little splotches is a galaxy all by itself so 100 billion stars per galaxy 100 billion galaxies in the universe if you've heard the phrase the Internet's this is what is being referred to with a hundred billion stars per galaxy a hundred billion galaxies and the universe is probably a lot of internets out there on every one of those things and perhaps the president has access to some of them that we don't this was discovered in 1924 a hundred years ago we knew nothing correct about large-scale structure of the universe we didn't know there were other galaxies Hubble came along and finally figured out that these little spa ch's the people had detected in their telescopes were in fact galaxies in their own right it was Hubble again hue the next big discovery a few years later that the universe is getting bigger not only is it big not only are there a hundred billion other galaxies but it's expanding so what does that mean what it means operationally for Hubble is that he figured out how far away the galaxies were and other people had measured their recession velocities galaxies it turns out are moving away from us that was known before Hubble but what people didn't know is whether or not that is meant they were being kicked out of our galaxy or where there was something else what Hubble realized is that if you plotted the distance to a galaxy versus its velocity you got a straight line the farther away a galaxy is the faster it is moving away from us what that means is that we're not at the center of some sort of explosion if you were on one of those other galaxies you would see us moving away from you every galaxy sees the same basic picture around them is a uniform distribution of other galaxies they're all moving away they all move away faster and faster the further they are away so what does that mean what that means is that space itself is expanding the reason why a distant galaxy is moving away from us faster than a nearby galaxy is because every little cubic centimeter in between the galaxies is getting bigger as a function of time so if you have more centimeters in between you and a faraway galaxy you're being pushed apart by the expansion of the universe faster and your recession velocity will be bigger if you trace backwards 14 billion years everything was landing on top of it everything else and we call that the Big Bang so this is the physical effect of the expanding universe it becomes more dilute the number of particles in the universe is roughly constant not precisely but pretty roughly so so if you take an expanding universe in the past things were closer together on the left you have the old universe things were closer together and also you have photons in the universe waves of light that were squeezed closer squeezed to smaller wavelengths remember light has an energy when the energy goes up as the way length is smaller so in the past the photons were hotter they were more energetic we take the universe today and wind it backwards in time it is currently cold and dilute it was formerly hot and dense you could trace it all the way back 14 billion years to the Big Bang we don't know what the Big Bang is we don't know what happened at the moment when everything was purportedly on top of anything else but we notice that the universe started from a hot dense smooth state and we know that because that's an idea that makes predictions if you say ok the universe is expanding now everything was closer together in the past therefore everything was hotter in the past and hot stuff gives off radiation gives off blackbody radiation so the early universe was glowing and you can detect the Relic radiation from the Big Bang it's what we call the Cosmic Microwave Background the hot dense state that was the Big Bang gave off radiation it is cooled as the universe expanded and right now the peak of that radiation is in the microwave range the same as your microwave ovens you can detect that and various Nobel Prizes have been won in studying that cosmic microwave background so what does that mean that's the universe that we see is a very simple picture of the universe I'd like to say that cosmology is the right specialty for young scientists who have short attention spans because cosmology is very simple compared to many other sciences the universe is much simpler than a frog or than a high-temperature superconductor it's the same everywhere the same number of galaxies at different parts in space but it changes with time in this very simple way in the past it was hot and dense and the future will be empty and cooler so once you understand that the next question is what is the universe made of you can see the galaxies but that doesn't mean that what you see is what there is maybe there's parts of the universe that you don't see so to get there I want to first think about what are the things you could possibly have in the universe so instead of trying to just look at the universe and decide what we have let's think about particle physics let's think about the fundamental building blocks that we are made of here in the universe the good news is there's only two fundamental types of particles there are fermions after Fermi bosons named after Bose and they have crucial differences namely that fermions take up space fermions are particles that you cannot stack on top of each other they stick and that's good news because you and I are made of Fermi on so we don't collapse into a little ball the electrons and the protons and neutrons that make us up can't be stacked on top of each other they take up space and therefore we take up space and we can have interesting structures and so forth those are matter particles or fermions the other kinds of particles are bosons they can stack on top of each other as much as you like that is also good news because it means that you can have this out of just one particle in every place you can have a whole bunch of particles in the same place giving rise to a large classical force field such as the force field that is holding us onto the earth by a gravity the gravitational force is mediated by bosonic particles called gravitons also you can see things in the universe because you're getting electromagnetic radiation which is a classical force field constructed from many many boson ik particles called photons photons carry light so the interplay of these two kinds of particles matter particles that are fermions to take up space force particles that are bosons piling on top of each other is what makes the incredible richness of the universe around us possible and we're gonna be thinking about both of these possibilities as we go on in the real world how does this play out well you have atoms we're all made of atoms you know this much atoms again take up space so they're made of fermions it turns out that the lighter a particle is that less mass a particle has the more space it takes up is a little counterintuitive but this is just how quantum mechanics works so in an atom you have light particles called electrons and you have heavy particles called protons and neutrons because the protons and neutrons are heavy and they're sticking together they form a small dense nucleus at the center of the atom because the electrons are light they puff out and most of the space in the atom is taken up by these electrons that are moving around now why are the electrons stuck to the nucleus because of a force the electromagnetic force that is holding those electrons to the neutrons and protons and therefore there are photons that are passing back and forth in between the electrons of the nucleus of the atom if you zoom in on the atomic nucleus if you zoom in past the neutrons and protons you find that they are made up of quarks every proton has two up quarks and one down quark every Neutron has two down quarks and one up quark and they are also held together they are held together by gluons gluons are these strong nuclear force analogs of the photons of the electromagnetic force so within nature you see this pattern played out over and over again there are different levels of structure where you have particles that are fermions taking up space held together by bosons carrying forces well it turns out there are more particles than you need to make up the interesting part of the universe we have in nature a set of fermions that take up space all you need is one set of fermions we have the up quark and the down quark those are what make up protons and neutrons we have the electron that helps make up atoms and the electron has a friend called the neutrino which doesn't really play a big role in our everyday lives because it's so light they just zip away our bodies are being passed through by billions of neutrons every second or neutrinos sorry every second you don't even notice because the neutrinos interact so weakly they have no electric charge but together the up quarks down quark the electron electron neutrino make up a family of particles which makes up everything that you've seen in your daily life everything that you can directly detect in atoms stars galaxies and so forth just those four fermions and different combinations which is remarkable fact all by itself yet nature decides to repeat the pattern another two times there's another family of quarks and leptons leptons by the way or what we call the electron in the neutrino and their friends so you have more quarks the charm in the strange more leptons the muon was just a heavier cousin of the electron and the muon neutrino another cousin of the electron neutrino and it happens again there's a top quark a bottom quark a Tau lepton just like the electron but much heavier and a Tau neutrino so the first one of these to be discovered was the muon that was the first next-generation particle to be discovered and I I Rabi a physicist at the time said who ordered that this is not necessary doesn't fit into anything we understand but there you go there's just three times pattern that exists in nature doesn't serve any useful purpose as far as we know although the people who discovered some of the interactions between these particles didn't win the Nobel Prize this year so that counts for something then you have the bosons the particles that give rise to forces that hold us together you have the photon the single particle that is the most important force in our everyday lives you have the weak nuclear interactions that are carried by three different weak bosons there are two W bosons and a single Z boson you have the gluons there are eight gluons that are more or less the same they're what hold the quarks together in protons and neutrons and then you have the two more mysterious bosons you have the graviton well we certainly know that gravity exists gravity was the first thing we understood as far as elementary forces but we have gravity being so weak that it's impossible for us to make a single elementary graviton in an experiment we see the classical gravitational field that you get by piling up billions and billions of gravitons but individual gravitons remain elusive so part of this is part of the problem that we cannot successfully quantize gravity we have a very successful picture of the standard model of particle physics based on quantum mechanics gravity doesn't fit into that picture not yet anyway so we all think that there is something called the graviton we've never observed an individual graviton yet then you have the Higgs boson this is another particle that is part of the standard model of particle physics and it is easy to quantize it's easy to fit into the system the only problem is we haven't found it yet there's a big ongoing experimental project to actually find the Higgs boson it could be that it's not there it could be that this idea that we have of the Higgs boson needs to be replaced by a better idea we don't have that better idea yet probably there's gonna be something like the Higgs boson or something very similar to it okay so how do we know all this well together all these particles make up the standard model of particle physics it's a story that was put together at multiple particle accelerators you use a equals MC squared energy is mass times the speed of light squared you collide particles at high energies and you can make new particles of higher and higher masses so this picture that we have of the standard model three generations of firming all these different kinds of bosons fits every experiment we have ever done at a particle accelerator or any other laboratory experiment here on earth we have strong evidence against it from outer space but here on earth the standard model suffice it to do everything that we've been able to do so far of course we want to do better that's why we're trying to build the Large Hadron Collider in Geneva and so forth we would like to be surprised to go beyond the standard model but but for the past 30 years the standard model has just been confirmed to higher and higher accuracy by every new experiment that we've built so that is what we call ordinary matter the matter that makes up you and me the matter that we've been able see in experiments so you might ask how much ordinary matter there is in the universe well one thing to do is just look the ordinary matter has the nice feature that it glows it couples two photons it gives off light so you can in principle find it but you would always worried that you were missing something you would always worry there was some form of invisible transparent not radiating matter in the universe after all there's air in this room but we can't see it we can feel it by waving our hands but the light just goes right through it we would worry that there's stuff in the universe that it it really is ordinary matter but that we just can't find it well the nice news is that there is a way to figure out exactly how much ordinary matter there is in the universe that doesn't rely on looking at it and that way is called Big Bang nucleosynthesis what you know from what you know in experiments about the standard model of particle physics is that heavy particles decay in two light ones so we have not only the electron but we have the heavier cousin of the electron the muon the even heavier cousin called the Tau lepton why don't you see muons and Tau's all over the place that's because they're heavier and they just decay if there's a muon out there in empty space which is decaying to an electron into neutrinos you wouldn't see it so the real question is what are the lighter particles in the universe the particles that can't decay because they're carrying some charge that cannot disappear the electron cannot disappear it's the lightest particle that carries electric charge and the proton can't disappear it's the lightest particle that is what we call a baryon it's a separate kind of charge that can't go away so the real ordinary matter constituents of the universe are the things that make up atoms neutrons protons and electrons how many of them are there we can tell through Big Bang nucleosynthesis the early universe was a nuclear reactor as a fusion reactor at very very early times the temperature was so high that you couldn't have an atomic nucleus you couldn't stick together protons and neutrons they would smash into other nuclei and they would break apart but the universe is expanding and cooling so what you do is you start at one second after the Big Bang with free protons and neutrons then you see there are fewer neutrons the yellow guys then protons because the neutrons are heavier and they're decaying if you waited long enough all the neutrons would have decayed away but instead the universe cooled down to the point where some of those neutrons could undergo nuclear fusion with the protons and then the atomic nuclei become stable even though the individual neutrons are not so you turn both protons and neutrons into helium nuclei that's two neutrons and two protons and you have a whole bunch of free protons that didn't get captured there are more protons around so given what we know about nuclear physics the rate of nuclear fusion reactions and given what we know about cosmology the rate of expansion in the cooling down of the universe you can make predictions for how much helium you should get and also how much lithium and deuterium other light elements that are created in Big Bang nucleosynthesis there's only one free parameter that goes into that calculation that free parameter is how many protons and neutrons are there which is the same question as how much ordinary matter is there in the universe so you make predictions for the abundance of helium lithium and deuterium and you compare against what you see and the answer is that you predict 24 percent of the universe in baryons and protons and neutrons is in the form of helium you get trace amounts of lithium and deuterium you compare with the data and it works but it works for one particular value of the number of baryons in the universe that value is only 5% of the total amount of stuff you need for the universe so it doesn't matter it's not that we have not found the ordinary matter in the universe we know how much ordinary matter there is and we know it's not enough to be most of the universe well how would we know what is the universe the other trick you can use besides looking for things is the fact that Einstein here understood how gravity works and he came up with a very brilliant idea called the general theory of relativity this by the way is a picture of Einstein you see pictures of Einstein when he's you know in the 1950s he's 70 years old the hair has gone and he's dressed in a rumpled sweater and so forth but it's important to recognize that back when he was being Einstein back when he was inventing these theories this is 1912 he was a sharp-dressed young man with well combed hair I don't know what happened after that but he looked smarter than everyone else cuz he is he was Einstein you know you'd let it go a little bit after that the idea that he had in general relativity was a very simple one gravity is the curvature of space-time we can think of space and time as a place through which we move in the universe but it has its own geometry and the gravitational field is created by everything in the universe that has energy there's nothing in the universe that exists but does not create a gravitational field when we orbit around the Sun the reason in general relativity why the earth moves around the Sun is because it's moving in a curved geometry the earth is trying its best to move in a straight line but there are no straight lines because the gravitational field of the Sun warps the geometry around it we end up moving in an ellipse around the Sun but that's good news because it means that since everything feels the geometry of the universe everything causes fluctuations or perturbations in the geometry of the universe everything that exists gives rise to a gravitational field so if we want to know what exists in the universe all we have to do is find out what are the gravitational fields in the universe and then we will locate the stuff that is causing them well how do you do that there's many ways of doing that one nice way is through the phenomenon of gravitational lensing if gravity is the curvature of space-time and everything moves through space time gravity affects everything in the universe including the photons that are moving to us the beams of light to travel from distant galaxies so if you have a very distant quasar or set of galaxies and there's something in between you and the quasar the lights in that quasar will be deflected how much will it be deflected well that just depends on how massive the thing doing the deflecting is the more matter the more stuff there is in the intervening galaxies the bigger the gravitational lensing effect will be so you can actually use this to weigh the galaxies you can figure out how much stuff there is in there by looking at how strong the gravitational lensing is it's completely independent of your ability to see the stuff inside the galaxy you won't miss anything in the galaxies by using this method so here is not the quantitatively best with the prettiest example of this and you may have seen this picture before I think was on the poster for the talk this is the bullet cluster this is an example it just came out a couple years ago where you have actually two clusters of galaxies you can sort of see that on the left-hand side of this picture there's clearly a cluster of galaxies there in the middle of the left-hand side if you look on the right-hand side there is a smaller group which is also a cluster of galaxies the astronomers are more careful than that they actually would measure how far away all these things are and see if they are associated with each other there really are two bunches of galaxies here but it turns out that even in a cluster of galaxies these are galaxies that are bound by their mutual gravitational attraction most of the ordinary matter most of the hydrogen and helium and so forth is not in the galaxies it's in hot intergalactic gas it's in gas and dust that is in between the galaxies not in the galaxies themselves and you know this because you can see it because this intergalactic gas heats up and gives off radiation it gives off x-rays so once you have reached the point technology when you can build an x-ray telescope which is relatively recently you get pictures like this this is taken by the Chandra x-ray telescope a NASA satellite that is in orbit that pink stuff there is a hot x-ray gas that is emitting from these clusters of galaxies but unlike a typical cluster the shape of this gas is a little weird you would expect to see the hot intergalactic gas in the same place as you see the galaxies they're all bound to each other gravitationally but what you see is that the galaxies are a little bit to the left of the gas on the left-hand side and a little bit to the right to the gas on the right-hand side what is going on there what is going on is that these two clusters of galaxies collided in the recent cosmological past they went right through each other the galaxies are actually kind of tiny the galaxies just went right through without even interacting very much but the hot gas in between the galaxies smacked into the hot gas of the other cluster and that's why you have this shock wave on the right that's why it's called the bullet cluster so basically big because these two galaxies went through each other the hot gas got stuck in between separating the galaxies from the gas in between so if this were a system that had nothing in it but ordinary matter nothing but gas and dust and stars and so forth if you measured the gravitational field of the bullet cluster you would find it centered on the pink stuff that is where most of the matter is most of the ordinary matter most of the atoms most of the protons and neutrons are visible here as the pink not as the galaxies so you can do that you can look for where the matter is by looking at the galaxies behind the cluster and seeing how they have been lens there will be slight distortions in the shapes of the galaxies behind this cluster because the light from them came to us through a gravitational field you can reconstruct where the gravity is in the bullet cluster and where the gravity is is here this is a reconstructed map of what the mass density looks like in a bullet cluster based on lensing measurements of galaxies behind the clusters so what you see is that where the mass is is centered on where the galaxies are but where the ordinary matter is it's not where the galaxies are so this is the ordinary matter this is the total matter ordinary matter total matter are in different places how can that be true the way that it can be true is if most of the matter in the bullet cluster is not ordinary matter it's some other form of matter that does not smack into the matter of the other cluster when they collide it's what we call dark matter you can weigh how there is you find there's much more dark matter than can possibly be accounted for by ordinary baryons giving the limits given the limits that we have from Big Bang nucleosynthesis so we have some sort of stuff that is in the clusters of there it's clumped together under its gravitational pole it's giving rise to gravitational lensing but it's not ordinary matter doesn't interact in the same way as ordinary matter does it doesn't participate in Big Bang nucleosynthesis in the same way the ordinary matter does we call it dark matter it's an old idea dark matter goes back to the 1930s Fritz Zwicky at Caltech astronomer first proposed the idea Mira Rubin here in the 1970s found that there needs to be dark matter to explain the dynamics of individual galaxies and these days we find dark matter again and again in clusters of galaxies and large-scale structure of galaxies but we've only known for sure in the past 20 years or so is that the dark matter is not just hidden ordinary matter it's something new some new kind of particle something is heavy stable and doesn't decay so we want to ask ourselves what could that be however before we get there there's another question we want to ask ourselves we've been looking under the lamppost we're looking in galaxies and in clusters of galaxies and asking how much stuff is there what if there's stuff in between the galaxies and clusters of galaxies how would we know we want to make absolutely sure we've found everything in the universe well there is a way to weigh the entire universe which is to keep track of how fast it's expanding under the ordinary way of thinking about things as the universe expands the galaxies are pulling on all the other galaxies in the universe and what you'd expect that the expansion the universe slows down because of the mutual gravitational pull of all the stuff in the universe so what you can do is you can try to weigh the whole universe by not just measuring the expansion rate today but comparing it to what the expansion rate of the universe was in the past by basically doing what Hubble did measure velocity versus distance but do it to much much larger distances than Hubble was able to do so the way to do this is to make take advantage of supernovae what you want is something that is very very bright so you can see it very very far away but also something that is reliable something that when you see it you know how bright it is you can see galaxies very far away but you don't know how bright they are intrinsically some galaxies are big some galaxies are small etc but supernovae are special there's a certain kind of supernova called a type 1a supernova that is always more or less the same brightness and that's because it comes about from a picture like you see in the upper left here you have a white dwarf star that's a star that is given up on fusing light elements in its core it no longer generates energy this is a star that has died and collapse so the white dwarf star is just sitting there all the electrons are squeezed it's close by to each other the only reason it doesn't collapse is because the electrons take up space their Fermi on so they can't be squeezed any more but this white dwarf as a friend has another star next to it which is leaking more and more matter on to the white dwarf so the white dwarf is getting heavier and heavier it can't get smaller because the electrons are taking up space but the gravitational field is becoming larger and larger so at some point there's a crisis at a point called the Chandrasekhar limit where the gravitational field is so strong the white dwarf will collapse whether it likes it or not and what happens is the electrons disappear the electrons get eaten up by protons and turning to neutrons and the white dwarf collapses to a neutron star neutrons remember are heavier than electrons therefore they're smaller therefore they take up less space and you have a more compact object a white dwarf might be the size of the earth a neutron star will be the size of Los Angeles they're much much more compact than white dwarfs so this event where the white dwarf reaches the Chandra Sekhar limit is so heavy that it has to collapse and form a neutron star of course it's not perfectly efficient the outer layers of the white dwarf are just blown off in this explosion and because the Chandra Sekhar limit is the same for every white dwarf everywhere in the universe that explosion is more or less the same for every white dwarf everywhere in the universe that's what makes a type 1a supernova and you know how bright it's going to be because you know what it happened it happened when the white dwarf reached the Chandra Sekhar limit so this is a picture of supernova 1994 d they're not always that nice but you can see this is supernovae in a distant dusty galaxy and you can see it the supernova is very very bright it's comparable in brightness to the hundred billion other stars that make up the total rest of that galaxy so they're easy to see far away and you can calibrate how bright they are the problem is they're rare and the galaxies this size the supernova will happen only once every century so you have two options either you go to the telescope and you wait for a century for the supernovae to happen or you go to a telescope you take a big picture of many many galaxies at once so there were two collaborations in the 1990s that set out to do this they took large pictures of the sky and they would take them at new moon and the sky was dark they would come back four weeks later and take another picture the same patch of sky then they would hand them to their least favorite graduate student who would compare them with a little magnifying glass or they can put them in a computer either way they want to compare the two pictures and then what they're looking for is in this picture you see on the left three weeks before on the right supernovae discovery that little tiny blob in the upper right became a tiny little bit brighter that's a supernova that's a real supernova I mean we show you pictures like this but the data really looks like or like that that little blob a little bit brighter and then you call up your friends at NASA and you say we found a new supernova we need better data that we can get can we use the Space Telescope and they say sure because they're your friends so they take a picture in the top right and you see indeed there's a little spot there in that galaxy because the Hubble Space Telescope can take much more precise pictures than you can that's a supernova you can not only see how bright it is but you can get its spectrum to make sure that it's the right kind of supernova so what you can do is you have standard candles you know how bright these events are you can figure out how far away they are you know how fast they're receding you can do what Hubble did you can measure the total amount of matter in the universe by seeing the deceleration of the universe the expansion rate used to be very very large in the early universe the mutual pull of the matter slows things down as the universe expands so you can figure out how much matter there is it was therefore a little bit surprising when the results came out and the universe is accelerating it is not slowing down this was a huge surprise in 1990 what we realized is if you're looking at an individual galaxy that has a certain velocity away from us you wait a billion years and come back and look at it again you'll be moving away from us faster that makes no sense this is if you threw a ball up in the air and after slowing down for a while it began to speed up and just went away how could that possibly happen but that is what is happening we have the data here the bottom curve on the right is what we expected what should have happened if we had a lot of matter in the universe and was flowing the universe down the top curve is what we got the university is actually accelerating the galaxies moving away from us faster and faster this is the original 1998 data it's gotten a lot better since then so this is a little sketchy but you can be sure that the result has held up the distant supernovae are getting further and further away from us faster and faster and this is a true order that question again not something that we anticipated to be there in the universe but we are not short of ideas for what it could be there's basically one idea that is working and that's called dark energy the idea that there's not some new not only some new particle in the universe called dark matter that clumps into galaxies and so forth because that we would see through gravitational lensing and we didn't instead the dark energy is some form of energy that is smoothly distributed through space so it's not more of it in a cluster or in a galaxy than in between there's the same amount everywhere in the universe the same number of urge the same amount of energy per cubic centimeter of the universe the second thing we know is that it's a constant density in time and this is the dramatic and crucial feature of dark energy so you have a cubic centimeter of space it has a certain energy density if that energy density came from particles as universe expanded it would get more dilute and the energy would go down but the dark energy doesn't go down the amount of energy in every centimeter stays more or less constant as the universe expands so the total amount of energy in the universe is going up is increasing as the universe gets bigger and it's that more and more energy that is providing the impetus to make the universe accelerate that's the explanation and finally of course we can't see it it's invisible it's dark that's what dark energy is it's something that is not a particle not something it can move around it's almost like a feature of space itself it's a certain number of eggs per cubic centimeter in fact it's ten to the minus eight eggs per cubic centimeter that's not a lot if you took all the dark energy within the earth today and converted it to electricity it would power one average American for one day so we're not going to remove our dependence on foreign oil sources by applying the dark energy but there's a lot of cubic centimeters in the universe so even though it's not a lot of dark energy in this room there's a little bit but not that much in the universe it's dominating in the universe we added up 70% of the total energy in the universe is dark energy and if you think about it that's a number that changes with time because the dark energy per cubic centimeter stays the same but the ordinary matter in ordinary radiation and dark radiation dark matter and so forth all grow in the past the more larger in the past when things were closer together so in the future ordinary stuff will dilute away and the dark energy will take over but in the past the dark energy was negligible because everything else was more dense and more important so that's nice we have a complete inventory of what the universe is made of the problem is like I said before it doesn't make sense this is a universe that has issues and it's now our job as theorists to take this wonderful picture they've been put together by observers and try to make sense of it so let me give you a very brief overview of what some of the issues are this is you know there's two choices when you give a talk like this I could have picked an issue and done it carefully so you'd understand something or I could just cover a whole million things you could be inspired to go learn something else so this is more an inspirational talk than an educational one here's the problem with dark energy there's not enough of it the problem is not that there exists energy in empty space that's very easy the problem is that if there exists energy in empty space there should be a lot more than there is once you're given the idea that empty space can have energy you can sit down and say well should I estimate how much energy there could be yeah and you can actually do a back-of-the-envelope calculation of how much energy density there should be in empty space it turns out to come from many different contributions there are many different things that give rise to dark energy one of them is the set of virtual particles that exist in empty space this is not the only one but an important source of dark energy is the fact that an empty space according to quantum mechanics empty space is not boring empty space is alive with particles popping in and out of existence we know these virtual particles exist because we can detect their effects they're not imaginary or purely theoretical constructs they give rise to measurable effects in atomic physics and they're very very well-established but they should carry energy and you can say how much energy should they carry you should add it all up and the answer is infinity so that's too much you say I already made a mistake so I will fix that mistake by saying well the reason why I got infinity is because all these virtual particles popping it out of existence carry different amounts of energy and I was including arbitrarily large amounts of energy in the dark energy particles even larger than the Planck scale where quantum gravity becomes important where space and time should really not have any meaning so let me not include those let me only include the Low Energy virtual particles and add them all up what do I get the answer is I get a theoretical prediction for the density of dark energy that is 10 to the 120 times bigger than the number I have observed this is famously the largest discrepancy between theory and observation in all of physics I've always felt that was a challenge to come up with a bigger discrepancy but this is big enough that's not a number this is not experimental error 10 to the 120 1 followed by 120 zeroes is the experimental prediction divided by the observational reality so that's a problem so that's just one example of the problems that we face with this picture so I want to emphasize both things number one is there is a lot that we know it's not like anything goes and we're confused we know that general relativity is right we know that standard model of particle physics does a really very good job of fitting the data as far as particle physics is concerned and we have an inventory of the universe where the standard model exists along with dark matter and dark energy but we're stuck with these puzzles that we don't know yet and of course that's always the fun place to be as a scientist you want puzzles to which we haven't yet given the answer how do you reconcile gravity with quantum mechanics what is the dark matter what is the dark energy why do they have the amounts they have so what we're looking for our big ideas that could solve more than one of these problems at once I'm going to just run through some big ideas and the common thread I'm going to use to do that it's the idea of extra dimensions of space this is a big idea it's a hypothetical idea may or may not be true but we live in a world post Einstein where we think of space-time as dynamical we live in a world that has three dimensions of space right we have height width and length if you like there if you if someone gives you three meter sticks and says tie them together so that everyone is perpendicular to every other one you can do that you can make three meter sticks perpendicular if someone gives you four meter sticks and says tie them all together so they're all perpendicular to each other you can't do that experimental proof that there are three dimensions of space but there could be more than three if they were hidden from us somehow so I'm going to talk about two different ways two different versions of the idea of extra dimensions one is what you could call ordinary extra dimensions if an extra dimension can be ordinary just another dimension of space where I could actually tie a meter stick together and find another direction to move in except that they are curled up but they don't actually extend as far as a meter they're much smaller than that so you can't see it it's like seeing a piece of string very far away it looks one-dimensional if you look at it very very close you see AHS or three-dimensional thing like anything else but if you look at it from a large distance the other dimensions disappear and appears one-dimensional maybe our universe appears three dimensional just because other dimensions are curled up the other idea is an even weirder one called super dimensions this idea that there are dimensions of zero length that are completely hidden from view and I will explain to you what I mean by that let me back up a little bit first part of the motivation for exploring these ideas comes from something called string theory remember one of the outstanding problems is that we have the graviton we have this particle of gravity that gives rise to a classical gravitational field but we can't yet fit the graviton into a framework that also has the standard model of particle physics on a nice quantum mechanical footing we want to be able to quantize gravity so right now the leading candidate for quantizing gravity is something called string theory and string theory is a very very simple statement to begin with it says remember the proton the proton which we said was made of quarks and gluons and the proton has a size a characteristic size of about 10 to the minus 14 centimeters string theory says that if we had a really really good microscope and we zoomed in on any one of those particles to look at it at ultra-high resolution the particle would not be a point it would be a little one-dimensional line segment curled up into a loop and we call that loop a string if you ask what is the string made of you're not allowed to ask that question string is made of string stuff that's the stuff out of which the universe is made instead of particles it's made of strings while we're saying is that the elementary building blocks of the universe are not zero dimensional points they're one-dimensional loops you have to zoom in a lot though if the proton is 10 to the minus 14 centimeters of cross a typical string is the Planck length 10 to the minus 33 centimeters across so our best microscopes the equivalent to a microscope is really a particle accelerator the best we can do these days is something like 10 to the minus 17 centimeters we are not down to a resolution where you can see 10 to the minus 33 centimeters nor will we be within my lifetime but it's still an idea worth thinking about because it comes with predictions for what the universe should be like so far those predictions are not really concrete they're too fuzzy to actually compare with the data or rule anything out but they are inspiring us to ideas that we're trying to compare with data one of the features of string theory is that it can't live in four dimensions of space-time it's just not enough almost every theory of nature you can pick the number of dimensions of space-time but with string theory it's not true certain numbers work and certain numbers don't the answer for string theory is you need seven ordinary dimensions of space 32 super dimensions 32 of these zero length super symmetric dimensions so let me tell you what I mean a little bit about that ordinary dimensions of space are just cornea general relativity spaces dynamical things can curl up space is not a rigid absolute framework in which we move space respond to the stuff in it so we can imagine that indeed the total number of dimensions of space could be as large as 10 you can actually imagine whatever we want but string theory says think of nine or ten dimensions of space but think that in the early universe when all the dimensions were small seven of those dimensions remained small and the three that we exist and perceive grew bigger if that were true then at every point in space that you and I think about today every point that we think of is just a point there's actually a little ball a little curled up region of extra dimensions of space that are so small that our current particle accelerators or microscopes cannot perceive them it's like a piece of paper you cannot proceed a thickness of unless you are able to resolve that tiny little amount that's the old-school way of having extra dimensions just everything is curled up so tiny you can't see them this goes back to the years immediately after general relativity was invented by Einstein it's actually a very old idea due to Kaluza and Klein there's a new idea which is again about ten years old can come from the 1990s which says maybe there are extra dimensions that are big maybe this actually mentioned that is infinitely thing that is not compact at all but the reason why you don't notice it is because you can't get there so it really is like a piece of paper this is a two-dimensional piece of paper embedded in this three-dimensional world imagine that there were little beings that could only exist on the piece of paper that cannot turn to the direction perpendicular to the paper they would think the world is two-dimensional they can't get out there the problem with this idea the reason why this idea wasn't popular long ago is because there is one force that can always get outside and that's the force due to gravity gravity is the curvature of space-time gravity goes everywhere and if you have space outside then gravity can leak out there and we have evidence that that's not true so the thing that was done in the 1990s is people figured out how to hide gravity from going into the extra dimensions so in fact you could have large dimensions we just can't get there and we are confined to a three dimensional subspace which scientists call a brain they come from the word membrane which was generalized to three brain for a three dimensional surface embedded in some higher dimensional space well how would we know this is something that is a hot topic among experimenters trying to figure out whether or not ideas like this are right if there are extra dimensions but that are curled up imagine the extra dimensions are let's say one millimeter across a slides where you could see without any microscope at all you can perceive a millimeter no problem with your eyeballs but we're imagining now that we are stuck to a membrane that is much much smaller than a millimeter and we could not get out there so we don't know that millimeter is there the reason why we choose a millimeter is because that's the size at which we have experimental evidence that we understand how gravity works if you take two objects and bring them close together Isaac Newton tells you the gravitational field between them goes as one over the distance squared the inverse square law due to gravity if there was an extra dimension of space it wouldn't be an inverse square law being inverse cubed law remembers to the fourth law depending on how many extra dimensions you had so if there were extra dimensions of space that were a millimeter across gravity on scales smaller than one millimeter would go as one over the distance cubed or to the fourth or a higher power and that is very plausible idea people are now doing experiments trying to bit bring heavy things together and measure the gravitational field between them it's easy to do that the hard part is removing all the other forces all the wind and the noise and the electrostatic forces between the objects and in fact they have ruled out the idea that an extra dimension is as big as a millimeter across but 1/10 of a millimeter is still perfectly open so the game is to decrease that distance to put an upper limit on the size of an extra dimension into which gravity could go the other thing to do is that if you actually smash particles together in a theory where extra dimensions can be large a nice spin off of that idea is that gravity can become strong you don't need to wait to get to the Planck scale these super high energies before gravity becomes important in quantum mechanics and a theory with large dimensions gravity can become important at scales accessible to particle accelerators so when you smash protons and antiprotons together at Fermilab or protons and protons together at CERN in Geneva you will not only make top quarks and Higgs bosons you'll make gravitons how would you know well the answer is that you throw things together at high energies and if you make gravitons they just zoom out of your detector and you don't find them so what you're looking for is actually missing energy you see in this picture there's a simulated event we've collided two things together and towards the lower part of the diagram you see a spray of particles to the upper part of diagram you see nothing that's strange because momentum is supposed to be conserved so when two particles came together if everything that came out went down something had to go up but you didn't see it that's called missing energy and could be the gravitons were going up there sadly this is a simulation this is not yet data but this is the kind of thing we're going to try to be doing when the Large Hadron Collider turns on next year the other idea is that you could get dark matter out of extra dimensions so if you have extra dimensions we said that they're small and you can't get there but it could be that some particles are actually moving in the extra dimensions because of the miracle of quantum mechanics you can only move with certain quantized value of momentum you're either not moving or you're moving or you're moving twice as fast or three times as fast you don't get to choose any possible velocity there's only certain velocities you can have and the velocity that you have in the extra dimension makes you heavier from the point of view of us here in three dimensions if we took a photon which has zero mass and we gave it momentum in the extra dimension it would suddenly have a nonzero mass and that begins to sound like a new massive stable particle in other words a good candidate for dark matter so one idea is what is called Colusa Cline dark matter dead either the dark matter is just ordinary matter but with more mass because it's moving in the extra dimensions so the dark matter could be photons could be the massless particles that make up light but they're given mass because they're actually moving in the extra dimensions as well as in our ordinary dimensions this is an idea which would become testable at the next generation of particle accelerators so that's the dark matter question what about dark energy can the dark energy get any information from extra dimensions well the idea is maybe again a controversial idea that we're discussing right now the crucial fact is that if you really do have six or seven extra dimensions of space that you want to curl there's more than one way to curl them up you could imagine spheres or Tauruses or higher dimensional surfaces of some sort so some people set themselves the question of given that there's brains and magnetic fields and all these crazy things going on the extra dimensions how many ways are there to curl up the extra dimensions the answer is a lot the best guesses are that the number is at least 10 to the 500 different ways to curl up the extra dimensions arguably an infinite number of different ways now why is that interesting it's because once you curl up all those extra dimensions the particular geometry and topology of the extra dimensions shows up in our world as affecting the constants of nature it affects the mass of the electron it affects the coupling constants the charge of the electron the strength of the nuclear forces the electromagnetic force the strength of the gravitational force and also the amount of dark energy so dark energy is much smaller than it should be but what this is saying is that they're actually 10 to the 500 different possible values of the dark energy and we just live in one why is that relevant it's because if the dark energy were as big as it could be and should be we wouldn't be here talking about it remember dark energy makes the universe accelerate the dark energy right now has a large enough value that has begun to take over our universe is in fact accelerating but the dark energy is nowhere near as big as it could be what if it were as big as it could be the universe would be accelerating at an incredibly fast rate be accelerating so fast you could not make an atom much less a human being or an Internet or anything like that if the dark energy had anywhere near the value it should have life would be impossible so people have gone to the obvious conclusion I say look maybe we live in a pocket of the universe that is not typical maybe there's a selection effect where in fact there's a huge multiverse out there that's not really separate connected disconnected parts of the universe but maybe there's different parts of space and time each one of which in that local region there's a different compactification of the extra dimensions different ways to curl up the extra dimensions and get different constants nature well in most of those regions if that were true life would be impossible you wouldn't have the right conditions to make a complex long-lasting metabolic organism the only place it would be possible is where places where the dark energy were small because of the dark energies big life just can't exist so there's a very heavy selection effect in a multiverse in the region in a configuration where there are 10 in the 500 different places with different constants of nature physicists would only arise and those very very small subsets of places where the dark energy was much much smaller than it should be this is the anthropic principle and it's if you believe that there could be many many different regions of the universe where conditions are different that's practically a tautology obviously things will only arise in the parts of the universe where things can arise including lights the difficult parts of this are number one is it true that there really are ten to five hundred different places like that that's very far from clear string theory gives you a way to make it happen by having all these extra dimensions curled up that doesn't mean it does happen that's a much harder problem to solve and number two is the dark energy really the right amount that it should have if the scenario is behind what's going on does it really need to be as small as it is or could it be a little bit bigger these are the kinds of questions people are trying to figure out right now the sad news is that this idea that the dark energy is not a concept of nature but something it takes on different values and all these ten to 500 different places of the universe and we only live in the place where it's hospitable to us living is the best current explanation for the value of the dark energy it's not very good we have to invoke 10 to 500 different universes to explain one number people begin to worry that you're not being parsimonious but the point is you didn't invent 10 to 500 different universes because that's what you found interesting it's a prediction of a theory if you have one theory that follows from one simple idea particles are not point particles there are actually little loops of string and therefore you derive that there are 10 to the 500 different places of the universe with different laws of physics that is actually relatively parsimonious that doesn't mean that it's true we're not anywhere near showing this is true yet but it's kind of is like I said the best idea we have right now if someone else tomorrow comes up with a better idea we will all pretend that we were never talking about this but right now it's the best thing we have on the market okay what about this super dimensions stuff I said this is a dimension of zero size so what is that supposed to mean the idea is that there is something that is mathematically looks like an extra dimension of space but with the property that if you move in it you turn bosons into fermions and fermions into bosons so there's a symmetry in ordinary dimensions of space it doesn't matter if I'm out in empty space in the vacuum between the stars whether I'm pointing right or left or up or down all the directions are created equal if there were a soup if there were a symmetry between ordinary dimensions and super dimensions that would mean there was a symmetry between bosons and fermions for every boson there would be a matching Fermi on the same mass in charge and vice-versa that's not true in the real world this is an idea called supersymmetry today there's a symmetry matching up bosons and fermions there is no particle with the same mass in charge of the electron but that is a boson instead of a Fermi on however that doesn't stop physicists just because something is not obviously there in the real world you can have a symmetry that is hidden so the very popular idea among physicists is that there is something called supersymmetry relating bosons and fermions but it's hidden in the real world so it's not manifest it's there but it's just not immediately clear it's not there in the standard model of particle physics there's no match up between the fermions and the bosons so what you do instead is you invent twice as many particles as you have you imagine that for every particle in the standard model down there at the bottom of the graph there's a heavier superpartner with more mass and for every Fermi on there's a boson for every boson there's a Fermi on and the rule is to start with the Fermi on and make it a boson you add the letter S so leptons becomes leptons quarks becomes quarks electrons becomes electrons for every boson to make it a Fermi on you add the suffix Ino so photons become photino x' w's become we knows gravitons become gravity knows etc why don't you see these particles because they're heavy that's the idea they're not the the supersymmetry is not manifest in the real world they are a broken symmetry that gives mass to all the particles so this is obviously a bonanza for experimental particle physicists they can look for twice as many particles as we found then that they are looking for them here they are looking for them Fermilab an accelerator currently working outside Chicago the Large Hadron Collider which was scheduled to turn on this year briefly did and then broke it will turn on next year for real and they will be looking for all these super partners so do they help us with dark matter and dark energy it absolutely helps with dark matter the leading candidate for what the dark matter is is the lightest supersymmetric particle it could be the part the partner of the photon or the partner of the Higgs boson we're not sure what yet is the lightest supersymmetric particle but it has all the right properties to exactly be the dark matter it actually fits together remarkably well this is one of the reasons why people are very excited about the prospects for supersymmetry experimentally so we're looking for it not only in particle accelerators but also in the universe we're building detectors deep underground shielded from radiation that could detect when the Dark Matter particle comes in and bounces into an ordinary matter nucleus we've also NASA and the Department of Energy have recently recently launched a satellite called the Fermi gamma ray satellite that is measuring gamma rays high-energy radiation from all across the sky and here's the first map that they made and you see if you look at that map you see the Milky Way across the middle there's a spot and right in the middle of the Milky Way that's a tiny bit brighter than everywhere else it is possible that a center Milky Way galaxy where there's more dark matter than in other parts of the Milky Way galaxy the dark matter is made of particles and antiparticles these particles interact very weakly but when they're dense when they're packed close together like at the center of the galaxy they can come together and annihilate giving rise to gamma rays it is possible that right there you are seeing in a very strange color scheme right now you're seeing the signature of dark matter particles annihilating into gamma rings we don't know yet we haven't collected enough data but it's possible the next five years it is extremely plausible that you will see front-page news that we finally have detected the dark matter directly one way or the other so the final question I'm almost done can supersymmetry help with dark energy we said the super somebody could help with dark matter it gives natural candidates for what the dark matter could be could it put it up with dark energy the answer is again maybe you see that we're not nearly as good at dark energy as we are at dark matter with dark matter we can have concrete predictions with dark energy we're still in the maybe I don't know kind of speculating phase so remember the problem with dark energy was there's too much of it you added up all the contributions from these virtual particles and you got this huge answer well it turns out that bosons and fermions give you opposite answers the bosons give you a positive contribution to the dark energy the fermions give you a negative contribution to the dark energy so that is completely irrelevant if there's no such thing as supersymmetry because one will win it just depends on the details of the particles is it more bosons or more fermions you're gonna get a large number one way or the other but if they exactly matched if for every boson there was a Fermi on and vice-versa you could cancel precisely the contributions to the dark energy sadly you know they don't exactly match in the real world you don't have a solectron with the same mass the supersymmetry particles if they exist are much heavier so what that does is it brings the prediction down from ten to the hundred and twenty times as big as the observation to 10 to the 16 times as big as the observation so on the one hand you've just saved yourself 60 orders of magnitude that's awesome on the other hand you're still nowhere close to the final answer again we're a little bit stuck as far as this problem is concerned so to end the lesson I want to leave you with is that we have this really great set of models discussing that fits all of the data in the real world it's a model where you have ordinary matter stars gas dust planets you-and-me atoms electrons protons neutrons photons and that ordinary matter interacts with itself through the standard model of particle physics then you have dark matter and dark energy which interacts only through gravity as far as we have so far detected yet but our goal is to understand is there physics in the dark sector there's all sorts of ways that you could invent interactions between the dark in the ordinary matter the dark matter in the dark energy the dark energy in the ordinary matter and every sector within itself if 95% of the universe it could very well be that it's not boring that we're just beginning to study it right now we may find there's just as much interesting phenomenology in the dark sector as there is an ordinary sector and we're doing this in two ways surveillance and interrogation you look at the universe out there and you bring the universe home and you interrogate it and you get different sorts of information by interrogating you can ask direct questions but sometimes the suspect claims up by surveillance by eavesdropping in the universe you can't ask questions but sometimes the suspect will say things that are incriminating so we have a complimentary approach to learning new things about the universe it's brought us a tremendous amount of knowledge now we want to go from knowing what the universe is doing to knowing why it is doing it thank you so questions yes thanks for dimensions yeah right so the question is that the dark matter is moving in extra dimensions will they be weakly interacting so there's two different definitions of the phrase weakly interacting in particle physics one is not interacting very much the other is interacting through the weak interactions of particle physics okay so it turns out that to get the right abundance of dark matter it works well if the dark matter particles actually interact through the honest-to-goodness weak interactions of particle physics that's the right strength of interaction that you want however it doesn't need to be precisely that interaction it could you know that interaction which is carried by the W and Z bosons if you have another interaction which is a similar strength but a different interaction that would work just as well so if the photon for example has a partner that is moving in the extra dimensions to make a collusive klein photon a heavier version of the photon it turns out that that does not have the weak interactions the photon does not interact through the W and Z bosons but the strength of its electromagnetic interactions for such a massive particle are the right amounts to make the right dark matter so it need not be weakly interacting in this strict technical sense of interacting with W and Z bosons it would be weakly interacting in the sense that it would have a new kind of interaction with that strength yes mm-hmm so the question is what do leading theoretical physicist thing about string theory it's mixed I think that with overwhelming numbers theoretical physicists who are not string theorists have great respect for the theoretical physicists who are string theorists these are smart people they are not going around wasting our time trying to chase things that aren't going to work on the other hand people have made some people including very well known ones have stood up and said look it's been you know you've been spending a lot of resources thinking about this since the mid-1980s and you haven't brought us the mass of the electron yet that's what you were promising us all those years ago so people are beginning to become impatient I think that you know it is certainly still true that string theory is the most promising candidate for quantum gravity on the other hand it might be that it's just too hard to make immediate progress on and therefore instead of spending a tremendous amount of intellectual resources working on string theory we should balance that effort more towards some string theory research but other research on more down-to-earth phenomenological particle physics so there's within that there's a spectrum of opinions from we should be doing string theory because the theory of everything - it's a mathematical waste of time but everyone respects the effort we're not quite sure how best to balance that effort against other things yes right right that's a very good question so that experimental evidence from looking at the universe is that the Dark Matter doesn't interact with itself very much you see here in the bullet cluster you see the Dark Matter just seem to go right through without interacting and that you can use to put an upper limit on the strength of the interactions between different dark matter particles but the truth is that upper limit is not great and in fact it's not different in any quantitative way from other upper limits we derive from less pretty methods of doing it so we have we know that if the dark that well I should back up a little bit there's two possibilities there are long-range interactions like gravity and electromagnetism there's short-range interactions like a weak and strong nuclear forces if the Dark Matter interacted with the strong nuclear force strength that would not be compatible with this data that's ruled out if it interacts only with weak nuclear force strength that's perfectly compatible with this data so there's a large range in between there if you have long-range forces the situation is much more complicated and honestly we don't know the answer you can put some limits on gravity like new forces but you could also have electromagnetism like new forces I just recently wrote a paper just a couple weeks ago on what we called dark radiation dark electromagnetism that there was a new electromagnetic force that only coupled to electromagnetism and you can ask how strong could it be the answer is kind of you know not too weak it's a little bit weaker than ordinary electromagnetism but not too much so as far as we currently know so the interactions between dark matter particles are certainly weaker than ordinary matter interactions but they it is not true that they need to be so weak that they are uninteresting right right yes so that's a very good question how clumpy do we know the dark matter is just from data well you shouldn't believe the details of this picture too much cuz it's not that's right it's it's gravitational lensing you know it is true that there are literally thousands of galaxies in the background that are used for this but nevertheless there's modeling that goes in and it's an imprecise science right now this is cutting-edge stuff right it's never going to be perfect when it just starts so the prediction is there should be clumps of dark matter on small scales because there should be clumps in the early universe it's grew a little bit not because they stick together just goes through gravity exactly galaxies grew and so forth um the Dark Matter doesn't stick together but it does have the gravitational force that pulls it together so we would love to know what is the mass distribution of dark matter look like on small scales we don't know that experimentally yet there are different theories that make different predictions the problem is that on those small scales that's exactly the scales on which ordinary matter begins to influence the dark matter so well the scale the cluster of galaxies you can ignore the ordinary matter it's not even relevant you can just follow the dark matter and get a pretty good idea what's going on when you get to down the scale of an individual galaxies or smaller then the ordinary matter begins to become important in the ordinary matter is a lot more complicated than the dark matter so it's an open question right now both what the theoretical prediction is and what the experimental situation is but this is exactly the kind of question that we can guarantee tremendous progress will be made on in the next 10 20 years well I don't think neither one of those are assumptions these are ideas so inflation is a hypothesis about what happened in the early universe this hypothesis that has made predictions which have so far come out correct so a lot of people are very optimistic about the idea that inflation happened in the early universe and and created some of the initial conditions for what we see is the Big Bang but then of course it ended like you said you know inflation was a period where there was a temporary period of domination by dark energy like stuff that made the universe accelerate and then it stopped that dark energy like stuff decayed into ordinary matter and radiation so it is absolutely possible that the same thing will happen right now the early universe was dominated by dark energy in the inflationary era for a relatively long period of time so we're just beginning right now so it's certainly no reason to think that soon the dark energy that we see now will decay away but it could happen I think it's an open question it turns out that you can tell the difference during inflation the energy density of the dark energy like stuff was not absolutely constant it was slow slowly varying and we can ask is that happening today that's a major experimental program to look for variations in the density of the dark energy so far we haven't seen any right there's a good question does do particles explain along with space the answer is no because if they did you wouldn't know right if everything expands nothing happens what the theory says and what the experiments and observations seem to verify is that space expands but objects which are held together by some force in space do not expand so atoms do not expand because they're held together by the electromagnetic force the size of a hydrogen atom is not set by the expanding universe set by the mass of the electron and that and the charge of the electron the size of a galaxy is not expanding because they're all held together by the gravitational force within the galaxy there's there is a distance past which things begin moving away from each other but if they're close enough so they can be held together by their mutual gravitational forces they will not expand away well it just depends on what you mean if you if you if you put a meter stick in space it would not expand because it's held together by the atoms in the meter stick okay any questions from far away all right well then thank you again

Channel: Talks at Google

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Keywords: talks at google, ted talks, inspirational talks, educational talks, Dark Matter and Dark Energy, Dark Matter, Dark Energy, dark matter dark energy difference, antimatter, sean carroll

Id: cxFfUsDgnaU

Length: 73min 42sec (4422 seconds)

Published: Tue Feb 10 2009