WSU: The Accelerating Universe with Adam Riess

it really is a pleasure to be here so I'm gonna tell you the story of how observations of exploding stars called supernovae revealed to us that we live in a universe that is not just expanding we knew that already but is actually accelerating that is expanding faster and faster propelled by a mysterious new component of the universe that we call dark energy but about which we know very little before I can get to the exciting new stuff let me just take you back a little bit and tell you how we viewed the universe before this discovery this is a deep image of the universe this is not just any deep image this is actually the deepest image ever taken of the universe we picked a blank spot in the sky and we use the Hubble Space Telescope and its advanced camera just to stare with the shutter open for the equivalent of a number of weeks and so what you get to do is collect the light of very distant objects there's tens of thousands of galaxies in this image some of which the faintest of which are about a trillion times fainter than anything you could see with your eye and this is just a tiny patch of the sky this is a patch so small if you took a single grain of sand and held it out at arm's length you could cover a patch like this so obviously the universe is just chock full with billions of galaxies so when you look at a picture like this it looks like the universe is static you don't see anything moving I said we kept the shutter open for a long time but nothing is blurry so we don't get a ready impression of the universe as in motion but it really is the universe access oh it received a big kick we call the Big Bang and now the separation between galaxies grows you could think of this like a giant loaf of raisin bread in the oven that it's rising in the oven and galaxies are like the raisins whichever raisin you're sitting on you look at all the other raisins around you and they are all moving away it doesn't matter where you sit everything appears to be moving away you also notice especially in the animation that the further away raisin or a galaxy is from us the faster it appears to recede from us because there's more dough there's more space between us and the other objects so how do we really know this about our universe that it's expanding and the answer of course is we checked and but you wonder you know how would you check this and looking back at that animation you would actually want to witness the motions and the positions of galaxies around us so in order to do that we have to tackle the first major problem in cosmology which is to figure out how far away things are before we can think about figuring out how far away things are in space it's useful to talk about how do we figure out how far away things are let's say here on the earth so on the earth we have lots of different tricks first I'll mention is called parallax that you make a virtual triangle in space you see surveyors do this all the time they plant their equipment they cite some distant tree that they want to measure the distance to and they complete the triangle by then moving their equipment to another location and they measure the angle through which that tree appears to move when they move from one location or the other as you remember from geometry you have a triangle you have a length you have an angle now you've solved the whole triangle you can figure out how far away that tree is this doesn't work so well in space I'll say more about this later space is very huge and so it is difficult for us to set up triangles in this way another method is the method of lighthouses so ship captain at night wants to make sure that they have sufficient distance from the rocky shore and so they look out at a lighthouse and they understand that a lighthouse is intrinsically very luminous so the fakeness of the lighthouse is what reassures them that they are still far away from the rocky shore now you could be fooled if it were let's say a foggy night that a lighthouse even it was up very close to you would still look faint fool you into thinking that you're still far from the rocky sure but we have other tools foghorns this is the same principle that you have something intrinsically very powerful that is diluted or attenuated by having to cover a larger and larger volume as it moves out so when the foghorn sounds quiet again you know that you're very far away and finally there are what we call objects of known size so all around us we see faraway things that we recognize what their intrinsic size is we've been up close to them before you look at this squadron of airplanes and you immediately recognize these there are the same kinds of airplanes just the ones that appear small are very distant airplanes my four-year-old son might not know this he might think that those are baby airplanes and that they're up close with their families but we recognize that these are actually different the same kind of object in different locations so these are all great but these are all human-made objects so we don't have access to anything exactly like this an experiment that we can set up precisely in space so we have to use telescopes and whatever nature provides and it turns out nature provides an object which is very similar to that lighthouse something that cosmologists call a standard candle because we understand its luminosity so well we can treat it just like this lighthouse so a galaxy like the one you see there will typically contain some hundred billion stars and about once every hundred years a single star may explode in that galaxy and this is known as a supernova here goes one right now and when we see one of these supernovae they act like these lighthouses they're extremely luminous they're as bright as billions of times the luminosity of our Sun and from the brightness of the supernova we can gauge how far away it is we can do this in a very quantitative way because we know that light and distance will obey the inverse square law that if the object is twice as far away it'll be four times his fame three times as far away it'll be nine times as faint you could imagine this as though the light has to paint the surface of a larger and larger sphere centered on us which is why the light declines as one over the distance squared so that's great we look for the super novae and we can measure distances the other thing from that animation we needed to be able to measure is the apparent motion of the galaxies away from us and we do this in a different way light is emitted by objects in the universe like a supernova at generally known wavelengths wavelengths of light that we can determine in the laboratory where nothing is moving and then we notice that the pattern of wavelengths of light is shifted it shifted in a particular way by the expansion of the universe here's an animation of that so the light is emitted at the known wavelength but as the light propagates to us space stretches and we receive a redshift that or longer wavelength version of that light we can measure this effect and this might remind you of the Doppler shift that you've heard of but I just want to be clear the interpretation is different and this is critical it's not that the distant object is really moving away from us it's that space itself is actually expanding and as I said that's a key distinction if it were just an object moving away you would have no reason to believe that any other object nearby would have any predictable motion and a predictable blue shift or redshift if it's space that's expanding then it creates this entire pattern I'll give you what I think is a useful analogy if you've ever been to an airport and you see a bunch of people standing together moving along at some constant speed you can't see them below the knees you might think they've all decided to walk together at exactly the same speed some conspiracy or conversation collusion involved but you also might recognize there are these things called people movers these conveyor belts that bring people along and that is a deeper understanding you would understand the actual phenomenon and you would be able to explain the whole pattern you'd have expectations as well so in our case we understand that this is actually due to the expansion of space so when we see one of these standard candles we can measure immediately how far away it is and how fast it appears to be moving or how much space has stretched over that interval so now we can go back to the animation that I showed you before you see on the left except we can become quantitative about this we can pick out individual galaxies that have the supernova in them and we can measure their distance and we can measure their apparent motion away from us and this linear relationship between the two is the signature of an expanding universe its famously called Hubble's law and we can use a measurement like this to tell us how fast the universe is expanding so if the universe were expanding faster it would be a steeper line if the universe were expanding more slowly it would be a shallow line if the universe were contracting instead of expanding it would actually be a negative slope we would actually have blue shifts individually now this is not just a thought experiment this was first witnessed by the famous American astronomer Edwin Hubble for whom we named the telescope for he had combined measurements of redshifts of galaxies around us by the astronomer Vesto Slipher with his own measurements of the distances to those galaxies and he made this very iconic I think this is one of the most important simple plots that's been made in the twentieth century I say I don't have a tattoo but if I got a tattoo I would probably get it of this that's that's how meaningful it is to me but you can see you could just tell this linear relationship between these that indicates that the universe is expanding now if that didn't convince you in my thesis work in 1996 I used the supernovae to extend these measurements out further so Hubble's original diagram would fit in this tiny little red square in the lower left-hand corner there of the diagram and then we continue to make measurements farther and farther out so now there really is no doubt you could see a very clean linear relationship hence the universe is expanding so this is exciting you've already learned one thing that the universe is expanding and how it is that we measure that and we can immediately address a deep philosophical profound question when did the universe begin this is something that I got into this field to study because I thought this is a kind of question that you can only ask you know your rabbi or your philosopher or something like that I'm amazed to find that this is a quantitative science and we can act we addressed this in a in a very physics based system so Hubble's law of the expansion of the universe tells us how fast the universe is expanding this is great because we can treat this like a movie that we are watching forward we could imagine rewinding the movie and so instead of the universe expanding at this rate we've carefully measured we can imagine the universe contracting at that rate we keep running this movie backwards that is using Hubble's constant the present expansion rate of the universe to run the movie back until everything goes further and further closer together until everything is on top of everything else and that is a measure of the age of the universe it's like when you turn on the marathon in New York City Marathon and the runners are at Mile 20 and you measure the speed at which they're running you can guesstimate how long ago the marathon race started that is assuming that they haven't been speeding up or slowing down the whole time it's what we call a good first-order guess and so the inverse of the Hubble constant gives you that first order guess now interestingly when Hubble himself measured this number he was way off and he got a silly answer for the age of the universe the answer he got was that the universe is about two billion years old now even in 1929 when he did this work we knew that the universe we knew that the earth was older than that I should say from dating rocks and different isotopes we knew the universe the earth excuse me it was at least a few billion years old but things changed over the next 80 years we learned to measure the expansion of the universe better this is actually one of the hardest measurements to make and it requires us to understand a great deal about the astrophysics of the objects in the universe that we are using as tools so this was a nice article in the New York Times a few years ago in the science times on our quest to measure the Hubble constant back from Hubble's initial measurements to where we are today and I show here the inverse of the Hubble constant giving you approximately the age of the universe so we learned some very important lesson along the way we learned that there's been more than one generation of stars stars go through their whole lifecycle they may explode they may just burn out but either way another generation comes along with heavy elements produced by the first generation and those stars have different luminosities than the first generation so it would be like confusing one light house model for another and so we had to learn that that gave us a great correction the this became a much more quantitative science when we replaced photographic plates in telescopes with the same charge-coupled devices that are in your smartphone's that you used to take digital photography we launched the Hubble Space Telescope which was really designed to help us make this measurement much more precisely than before we learned how to measure distances to supernovae and if you look just in the last decade or so there's been a tremendous tightening in these measurements my colleagues and I made the most precise measurement of the Hubble constant the local expansion rate just a few years ago we get a number of 73 kilometers per second per megaparsec the inverse of that is about thirteen point five billion years so that is what we get as determined locally as the age of the universe and later on in the talk I'll get to how I think we could improve this measurement even more but let me jump ahead to the next topic you might wonder okay the universe is expanding but what happens next right well like anything in motion you wonder what happens next this reminds me very much of when Isaac Newton thought about launching a cannonball from the surface of the earth and he wondered what would be the fate of this cannonball he recognized he could give it greater and greater velocity and it will make it further out if he gave it enough velocity it would have a special value of velocity we call the escape velocity enough velocity to overcome the gravitational pull from the earth and its way out to infinity even greater velocity would go way beyond that of course it matters what the mass is of the planet you're launching it from as well so the value of the escape velocity actually depends on the mass of the planet you launch it from if you're trying to launch the Cannonball from the moon which has much less mass this is much easier to do so our expectation was that the universe itself was a little like this cannonball in motion that sure there was this initial kick from the Big Bang and everything was moving but now the attractive gravity from all the stuff in the universe was pulling back on it and the really profound an exciting question they asked was does the universe have escape velocity from itself that is if you take the combination of the velocity we measure this expansion and the mass or mass density of the universe how do those equate will the universe expand forever like the cannonball escaping or will it fall back and start collapsing in the future like the cannonball that doesn't make it out now Einstein had a completely different idea and he was at a big disadvantage in thinking about this you know just a hundred years ago for a more or less this year Einstein had developed a new theory of gravity called general relativity which imagined gravity in a very different way and he decided one of the first things to do with this is to throw it at the whole universe to see what general relativity had to say about the dynamics of the universe now at the time astronomers were telling him that the universe was static that neither it was neither expanding or contracting and the astronomers at the time were themselves confused because what they called the universe was really just the Milky Way galaxy so they were unaware of the general expansion of space so they gave they gave in Stein some bum information and he tried to work with it and so he recognized if the universe was static and then whoever set it up that way let it go then this attractive gravity would tend to make things fall together again so there must be some way to counteract this attractive gravity and he made an amazing discovery he discovered essentially that while the gravity of matter in the universe is attractive the gravity of empty space itself could be repulsive something he called the cosmological constant today we would more generally call of dark energy and he thought the that this repulsive gravity of empty space and attractive gravity of stuff were in perfect balance that's what was going on now I don't know if Einstein thought very deeply about this he might have recognized that even if this were true this would be what we call an unstable equilibrium that means that I can balance a marble on top of a basketball if I'm exceedingly careful and I get it just at the top but if I am off by any little amount rather than restoring itself to the top the marble will run away it's an unstable equilibrium the same thing would be true of the universe set up the way Einstein imagined it if the universe got a little more dense than attractive gravity actually wins starts to get stronger because it depends on the separation of objects but more importantly Einstein learned later about ten years later that the universe was not static that the universe was actually expanding there's a famous trip he took in 1931 to Mount Wilson Observatory where he visited Edwin Hubble and his colleagues there even took a look through the telescope just to make sure Hubble was getting this right this is funny this must be a PR shot because you know at the time Hubble was using photographic plates to add up like for an hour you couldn't actually see any of the things that Hubble was actually seeing so I don't know what Einstein you just must be seeing darkness but you know he's a theorist so he probably didn't know he shouldn't say anything he looked stupid but anyway so famously he retracts this idea and it sort of falls back into the the wastebasket of physics okay so then can we actually measure if the universe is slowing down in its expansion like that cannonball and when you measure the deceleration of the cannonball to the earth you are in essence weighing the earth and taking that information and the current velocity of the cannonball you can make a prediction will it will it escape the gravitational pull of the earth so we can do the same thing if we can measure the deceleration of the expansion we will in essence determine what the mass is of the universe that combined with our measurement of the current expansion rate will tell if we're on a trajectory to rika laps or if we have escape velocity so how do we do this how do we measure the deceleration of the expansion of the universe well if the universe were like the economy right you can just wait until the next quarter and check new performance versus past performance but these things change very slowly and so this would not be a useful way to go so instead we use what I think it was kind of a trick which is the realization that the universe does not instant-message when we look out and measure the expansion rate of the universe as I told you in the first part of this talk I was sort of a little dishonest there we cannot actually measure how fast the universe is expanding right now the information we get from these distant supernovae has a built-in delay often millions or billions of years so we are learning about the past expansion rate of the universe now we can turn this around to our advantage and reach much further back so if we let's say find a distant standard candle we might be measuring how fast the universe was expanding a billion years ago we find a more distant one this might be telling us about two billion years ago more distant maybe three billion years ago this is a powerful way to actually observe the past changes in the expansion rate of the universe so we can't and rather than measuring in the future whether the universe is decelerating we just measure in the past how it was decelerating now in the mid-1990s when I got involved in this work the expectation was that the universe either resembled the model on the left or the model on the right the model on the left would be the heavyweight universe this would be the cannon ball launched from the surface of an extremely heavy planet which therefore would not have escape velocity it would expand and at some point would start to contract and end in kind of a Big Crunch sort of the opposite of the Big Bang alternatively we could have been in a very lightweight universe very little matter like launching that cannon ball off the surface of the moon very easy to do and so with the velocity that we currently have the universe would expand forever and another possibility was that we lived on the knife edge just between these two that at exactly escape velocity for the mass of our earth and so as I said we had measured already how fast the universe is expanding we wanted to weigh the universe by measuring the path deceleration of the universe now it turns out not any kind of supernova will provide this standard candle for us there's a special class of supernovae called type 1a supernovae this was first really explained by the famous Indian astrophysicist Chandra Sekhar in the 1930s that a star actually the core of a star like our Sun called a white dwarf star in a particular configuration can only hold back the crush of gravity up to a certain critical mass known as the Chandrasekhar mass so that's all well and good that white dwarf star can be happily holding up the crushing force of gravity by a special kind of it's called electron degeneracy pressure but if another star let's say a companion star starts spilling material over onto that star little by little the the original star the white dwarf can grow in mass until it crosses Chandra state car's limit and then you will get a runaway thermonuclear explosion you have the inability to hold back gravity so the star crushes and fusion occurs throughout the star immediately this is a great formula for a standard bomb right you have in this case 1.4 times the mass of the Sun runaway thermonuclear explosion leads to a very uniform luminosity about four billion times the luminosity of the Sun at peak what's also so great about these objects is we can see them very far away you want a standard candle that's very luminous so we can see these halfway across the visible universe maybe three quarters of the way with the Hubble Space Telescope so in in my graduate work I started looking for and measuring the supernovae here are four of the ones that I studied in my thesis from 1995 you might wonder how do we find these supernovae well you can see it's quite easy you just look for the arrows and the pictures and there's usually a dot at the end of the air now we add the we add the errors in Photoshop later no but how do we Eddie really find a supernova well it's a needle in a haystack problem or it's a you know trying to win the lottery it's incredibly rare so there will only be one as I said in a galaxy like ours about every hundred years so the way you win the lottery is you buy all the lottery tickets right so in our case what that means is we recognize that we can turn the odds in our favor if we have enough lottery tickets so if there's one in a galaxy like ours every 100 years that means there is one supernova in a hundred galaxies in a year so you monitor 100 galaxies you'll probably find a supernova in a year but we want many more than that so then you start monitoring thousands or tens of thousands of galaxies at the same time you saw that picture I showed you in the beginning we can easily take images that have tens or hundreds of thousands of galaxies all in them and then you use computers computers are very good at doing this very monotonous process of taking two sets of images separated by some period of time typically a month or a week and digitally matching them and subtracting them an alerting a human observer that there's a new point of light that has appeared between the two exposures and this often turns out to be a type 1a supernova I could say more about that later if people are interested ok so in the mid 1990s my colleagues and I had been good at measuring nearby supernovae to measure how fast the universe was expanding so we began using large telescopes to measure the very distant supernovae that would tell us about the past expansion rate that we could compare to today and so we formed a team and just to show you how strongly convinced we were we were measuring the deceleration of the universe you have to come up with a credo or a tagline and so ours was to measure the cosmic deceleration of the universe with type 1a supernovae except we got a great surprise when we did this and I was the sort of tip of the spear for seeing this surprise by about 1997 we had collected our first inset of the super novae and it was time to see what the answer was and so as I just to remind you again I was using the super novae to measure the deceleration of the universe that would equate to some amount of mass in the universe that would tell us whether we were we had escape velocity and sorry I was using a very simple equation that the deceleration would equate to the mass of the universe and I wrote a little computer program to tell me okay fit the data and tell me what the masses of the universe I want it to jump right to the answer don't even tell me the deceleration just lets go right to the right-hand side of that equation and this is the key page from my lab notebook so the mass should have been either a small number like 0.3 or a big number like 1 and instead it was a negative number now this made no sense there's no such thing as negative mass but computers don't know physics they just know you know what you tell them and what they can fit so when I gave such a simple equation and force the computer to find a match to the data it wasn't able to say hey dummy you should notice that the universe is not decelerating that I actually I need to change the sign on the left here but you're not letting me do that you're only letting me play with math so I'm gonna change that sign so I realize pretty quickly that that is not physical that you need something else something like Einsteins cosmological constant so I've added here now another term that as you see can act the opposite way you could explain the universe accelerating only if you have something very much like Einsteins cosmological constant not just have it but it actually has to be dominant it has to be more important more larger in size than the mass in the universe we equate mass and energy frequently in physics so you understand that I'm saying that the the energy of this has to be greater than the energy of that so a few days later I started thinking about the cosmological constant could this be what was going on and here's another page from my lab notebook where I was calculating the likelihood that this really was in the data and it was quite high it was high enough that I had to admit that it was time to talk to other people about this that this was probably not just what we say a fluke or bad luck I did a lot of cross-checking to make sure I hadn't made a simple mistake and then I contacted my colleagues I was collaborating with about 17 astronomers spread all over the planet we spread all over the planet because well it's always nighttime somewhere on the planet and we're observing the supernova all the time so it's very useful to have colleagues in South America Hawaii Europe Australia and so over the course of about 24 hours we began a discussion of do we really believe these results and I'm gonna show you some emails back and forth from that discussion because it's very interesting to see how scientists react to something unexpected scientists tend to be very conservative because most new things turn out to be wrong so my colleague in Berkeley California Alex Filippenko wrote to the team Adam showed me fantastic plots before he left for his wedding our data imply a nonzero cosmological constant who knows this might be the right answer Brunel Ivan good from Germany who is a supernova expert responded concerning a cosmological constant I'd like to ask Adam where anybody else in the group if they feel prepared enough to defend the answer there's no point in writing an article if we are not very sure we are getting the right answer Brian Schmidt my colleague who had helped find the supernova wrote from Australia I agree our data imply a cosmological constant but how confident are we in this result I find it very perplexing my thesis adviser Bob Kirchner at Harvard but on sabbatical in Santa Barbara row I am worried in your heart you know the cosmological constant is wrong though your head tells you that you don't care and you're just reporting the observations it would be silly to say we must have a nonzero cosmological constant only to retract it next year John Mark Phillips in Chile my colleague there a serious and responsible scientist huh we all know that it is far too early to be reaching firm conclusions about the value of the cosmological constant John tonray in Hawaii who remembers the detection of the magnetic monopole and they're gas on the other hand we should not be shy about getting our results out he's referring to there of course there are many many famous Mis discoveries in physics and you know we didn't want to be the next people to discover cold fusion or magnetic monopoles or any one of these things but it's funny in physics you're sort of sometimes you're just you have to play the hand that you're dealt and this is this is the hand that we had Alex Filippenko if we are wrong in the end and so be it at least we ran in the race there was another team located I was at the time working at UC Berkeley down on campus and there was a competing team at the Lawrence Berkeley National Lab which was a Department of Energy Lab doing the same experiment so there was a feeling of competition and an interest in working well but quickly so then I responded the results are very surprising shocking even I have avoided telling anyone about them because I wanted to do some cross-checks I have and I wanted to get further into writing the results up the data require a nonzero cosmological constant approach these results not with your heart or head but with your eyes we are observers after all Alejandro Cloe Hadi from Chile wrote if Einstein made a mistake with the Cosmo chill constant why couldn't we I never quite understood if that was supposed to be reassuring or not but you know I guess if the grand old man had made this mistake what we'd be safe - Nick sunset from Chile I really encourage you Adam to work your butt off on this we need to be careful if you are really sure that the cosmological constant is not zero my god get it out I mean this seriously you probably never will have another scientific result that is more exciting come your way in your lifetime of course Nick was right so in 1998 we published this paper observational evidence from supernovae for an accelerating universe and a cosmological constant in other words it looked like the universe was about 70% you'll keep hearing this number 70% in the form of dark energy something like Einsteins cosmological constant and the other competing team came to the same conclusion about the same time so this was this rapidly became part of the the story the fact it became the breakthrough of the for science magazine in 1998 apparently Einstein himself would have been pretty amazed but this this feature what some people might have thought was a bug but really a feature in general relativity was actually being exercised or actually being witnessed in the universe so why do we actually think the universe is accelerating now well in detail we really don't know that is we have a kind of general story hand-wavy word salad but we don't really understand the physics or the nature of this dark energy so sometimes we refer to it as the vacuum energy so there's an idea that comes to us well a feature of quantum mechanics quantum theory the physics of very small objects that the vacuum is a very rich place where particles appear and disappear that we cannot have total certainty that it's empty as a result of that we actually have certainty that it is very active place with lots of energy of this type but when we try to calculate all the energy states that are possible we get an answer that's about 120 orders of magnitude off from what would allow our universe to exist so as I said that's an idea and it sounds it smells like what we're seeing but there's a giant disconnect between calculation and actual observation maybe we could ask Brian more about that at the end nevertheless it is still a very powerful idea we have seen other I would say dark energy in the universe if you've heard about the Higgs field and the Higgs boson this is a built-in energy and a built-in particle in space that would act like dark energy it's not the dark energy or all our dark energy but it is a place where we've been able to touch and see that empty space has actual activity in it has actual important physics that we just don't understand that well it could be we have a dynamical dark energy so this would be like a time variable or changing form of this it would be a vacuum energy that changes from place to place in time to time it's associated with a field you could think of the electric field of the magnetic field that has a value in all places but it will change with time this sounds like a weird idea but we actually believed that some like this an episode we call inflation occurred shortly after the Big Bang when the earth was actually dominated by a temporary vacuum energy that caused the universe to accelerate greatly tremendously exponentially much greater than what we see now that period ended perhaps we are entering a new sort of gentler version of that or it still could be that we don't understand gravity that we still have the wrong theory of physics and dark energy and dark matter and these extra parts are just the you know the ether and the epicycles and the you know the extra stuff when you don't have the right theory of physics but I I would say these in order are probably our best guesses now back in 2000 this was such a crazy set of possibilities we worried about something just much more basic what if we were just wrong what if when we saw a distant supernova and thought that distant meant faint like the light ship captain that a faint lighthouse meant far away what if we were being fooled by something in which case the universe wouldn't be accelerating at all how could we be fooled well there could be a kind of I talked about fog a kind of cosmic fog what we would call dust grey dust if it lived in between galaxies it would make distant objects look faint now nobody had ever seen this gray fog before but then nobody had ever seen dark energy before so you couldn't use a Occam's razor and say one is crazier than the other we actually had to go out and prove it another possibility could have been that there was an evolution that we were looking at supernovae that were born when the universe was much younger and actually had less chemical enrichment maybe those supernovae were born fainter so the idea here is when we looked out at something the faint that meant that it was far away and hence there was dark energy but what if instead faint meant dusty not so far away and there was no dark energy so we realized that if we could look out even further to more distant supernovae this story where we're looking through some some kind of fog would just continue to add up we would keep seeing things look ever fainter or if there was this evolu younger super novae and the age in the universe were fainter that would continue as well whereas if we really had this new cosmological model where there's dark matter and there's dark energy the universe would have decelerated in the beginning and it only would have accelerated more recently and we should be able to witness this change that would break the degeneracy I would say with these other stories now in order to do that we needed to use a more powerful telescope more powerful than what we had mostly used before here's a short history of the improving power of telescopes this is compared to let's say here's Galileo's telescope got about almost a factor of hundred improvement in sensitivity over your eye and then telescopes got bigger we moved them to better sights we changed the detectors from our eyes which are not very efficient detectors KITT integrate light for very long to photographic plates and then to electronic detectors and then sometimes we're really lucky when astronauts go and put a telescope up into space the Hubble Space Telescope it sits above the blurring effects of the atmosphere if you've ever sat in the bottom of a pool and looked up at your friend outside the pool everything looks all swimmy and wavy well that's what happens to the lights that we see from stars so when we put that telescope above the atmosphere we can get very sharp images so here is the image of a supernova and its host galaxies as seen from the ground same one as seen with the Hubble Space Telescope and if your goal is to be able to measure the brightness of the supernova without the light from the host galaxies you could see that's an easier job here on the right and then we've been particularly lucky the astronauts sometimes return to the telescope and put state-of-the-art technology newer detectors in there and that improves our ability to observe the very distant objects so in early 2000 2002 the astronauts put the advanced camera in and we used it to find about 25 of these type 1a supernovae all from this earlier era of the universe when it was still decelerating when it was still compact before it gave way to dark energy and we were able to rule out those alternative stories as well as the original cosmological models to get a convincing case that we live in a universe that has both dark matter and dark energy that was decelerating in the beginning when everything was very compact but as it got larger at some point there was a transition we believe about five or six billion years ago and the universe is now expanding about 20% faster due to this dark energy now it's not just dark energy asari it is not just acceleration as witnessed by supernovae that convince us that the universe is filled with dark energy we as you saw in the email scientists are very conservative and we like a lot of redundancy and cross-checking and so now there's about five or six independent lines of investigation which give us this same picture super novae were the first in 1998 in 2003 we saw evidence of what's called the integrated Sachs Wolf effect so that means that there are photons of light from the Cosmic Microwave Background which are on their way to us but along the way they run into these kind of potholes that are the gravitational potentials of clusters of galaxies and they kind of fall into this pothole they gain energy that way and they kind of climb back out they lose energy but dark energy acts to pry apart or overcome the gravitational attraction in the cluster and so the amount of energy that they lose when they climb out is less than when they fell in that allows us to measure the universe is 70% of dark energy same answer that we got over here another way we can tell is we look at the global geometry of space there's a there's a relationship between the geometry you see in space and the amount of mass or energy in that space this has been measured exquisitely with observations from the Cosmic Microwave Background that tell us that the total energy or mass in the universe is in our kind of funny units one whereas when we actually measure how much just mass there is including the dark stuff that's only about thirty percent so this extra piece seventy percent is obviously there that's as I said independent line of investigation that tells us this we've seen the action directly of dark energy the largest clusters of galaxies are always pulling in new members new new galaxies because they have a lot of attractive gravity but on the largest scales they've become less effective at doing that we have seen the you know arrested development of the largest cluster of galaxies that allows us to measure the amount of dark energy again about 70% and something else I've been working on recently is is to book in the universe to take the initial conditions of the universe as measured from the Cosmic Microwave Background and use them to predict how fast the universe is expanding today and by comparing those two we again get that the universe is about 70% dark energy so we have lots of independent evidence but the way we really know that this is true is because we won the Nobel Prize in 2011 and they don't give you the Nobel Prize unless they're pretty sure but now seriously we learned to our great excitement and amazement in 2011 that the accelerating universe one was awarded the Nobel Prize this is our team of 18 members who all came with us and we had a great time in Sweden you know partying and dancing with the king and queen and all that good stuff that you hear but there's kind of a dark side to this which was displayed in this recent episode of The Big Bang Theory show [Music] - am i doing up Nobel Prize acceptance ceremony streaming live from Stockholm I want to see what all the scientists are wearing this year look at these men they've managed to win the top science prize in the world with no more understanding of the quantum underpinnings of the expansion of the early universe and God gave a goose you should they have good writers because that is exactly true that we we do not really understand the quantum underpinnings of this we don't have a quantum theory of gravity yet - and that's what's so fascinating about dark energy is it really cries out for it I mean it's something that we can't really fully understand without that so if you're an optimist you see the glasses half-full Wow we've made tremendous progress in the last decade or so we finally have the recipe of the universe we know what it's made of this is not something that we knew 10 15 20 years ago it's a surprising recipe about 0.05 percent in planets about half a percent in stars about 4% in gas so less than 5% in all of these these are the things that are built out of the materials in the periodic table of elements that you learn in chemistry class this is what we call baryonic material or normal matter the unfamiliar stuff 25% in dark matter and about 70% in dark energy so we really have our work cut out for us because still about 95% in of the universe is in a form that we don't really understand fortunately we keep getting new instruments to work on to help us understand this in particular the astronauts returned to the Hubble Space Telescope in 2009 for the last time unfortunately they put a new camera in in particularly new wide-field infrared camera much more powerful than the old camera let me just show you what it means when you get a more powerful camera here's a distant picture of space with the old near-infrared camera this is an exposure taken 48 hours of keeping the shutter open so it's very deep here's what the new camera in just 10 hours so I'll hop back and forth but pick out some very faint object and notice how much better you could see it how much more light there is from it and now bear in mind that's in 150 exposure time oh and also the field of view is a hundred times greater in the new camera so if you want to look for a supernova and you want to monitor lots of galaxies this is really a great advantage and we've been using this over the last few years we found some of the most distant supernovae I'm gonna tell you about a fun project I've been doing just in the last couple of years with this new camera I told you about parallax just plain old geometry parallax this is very important to measure distances with parallax because it forms what we call the first rung on a distance ladder our ability to calibrate successive ever more luminous lighthouses further away you want somebody to walk up to that first lighthouse with a measuring stick and actually figure out how far away that one is to know how luminous it is then if you see that kind of lighthouse next to another lighthouse you could calibrate the other one and and so on but that first step is very important but very difficult the reason is because stars are very far away these are our first lighthouses are the Stars and so you wait six months for the earth to move around the Sun and you try to detect this tiny motion of a star your target relative to even more distant stars the problem is that stars are so far away we want to be able to measure their distances we have to be able to detect the motion of this star over six months that motion being as small as 1% of a single pixel on the hubble space telescope so this is an incredibly fine measurement that has to be made so what an image actually looks like I've shown here is this kind of noisy thing where these spikes that you see are that's the image of an individual star and you are trying to figure out the location of the star here compared to its distant friend over here you're trying to measure that separation and then you take this picture again six months later Yury measure that separation and try to tell that it is shifted by 1/100 of a single pixel that is tough but a technique that we came up with a few years ago to improve this was what if you spatially skinned or drag the telescope along one direction while you're taking the observation instead of getting two points here you will get two parallel lines of light and your ability to measure the separation between two lines is much greater than two points because there are many more pixels involved in the measurement you could think of this as though you took a thousand sets of these images every set gave you a chance to measure this 1/100 of a pixel so with an image like this we can actually measure down to one one thousandth of a pixel so we can measure this parallax effect ten times farther or ten times more precise just if you're having trouble picturing what I'm talking about we literally drag the telescope across the sky and so the Stars turn into lines of charge and then we are measuring the separations of those lines and so this is working pretty well so far this is my first pilot program trying to do this so what you see is a measurement every six months that's what a dot is and what you see is for a number of different stars in the field you see what looks like double use these double use are is this back-and-forth motion due to parallax and after you see a few double use you're convinced that you're measuring this properly the amplitude of the W the size of that change the inverse of it is actually what the distance is so for example here is a star that I would say is relatively speaking very close to us it's 250 parsecs away which let's see converted into Hollywood movie units that would be about a thousand light years I say Hollywood movie units because we don't really measure distances in lightyears nobody is there to spark the light and then catch the light at the end this is how we actually measure distances is by these deflections so one parsec means a deflection of one arc second so we are down here to measuring just milli arc seconds of deflection and so the inverse of this tells you that this is a very nearby star relatively speaking we measure that with good precision here's a very distant star this is eight kiloparsecs away this is actually the edge of the Milky Way galaxy that we can measure with this technique and then here is a special kind of pulsating star called a Cepheid variable these are used to build a distance ladder to super novae and I say more about that maybe in QA meanwhile we're building new telescopes to go out and measure dark energy we only get a small fraction of the time on the Hubble Space Telescope maybe a couple percent a year of its time think how much progress we'd be able to make with a dedicated Hubble Space Telescope so the Europeans and NASA are both planning to build these telescopes over in the next decade and the goal of these telescopes is to measure whether the properties of dark energy are static in which case the first option that its vacuum energy is most likely that they are changing in which case it's this dynamical energy like occurred in the early universe or whether Einstein's theory of general relativity actually fits all of the data whether it works on what I would say are small scales and large scales do we need a new theory of gravity so I just want to end with reminding you why is it that we're so excited about studying this dark energy well first of all it's most of the universe so it's hard to say we really understand the universe when the biggest part of it is still such a mystery to us it will determine the fate of the universe we need to understand whether dark energy is changing or not if it doesn't change that's one prediction but if it changes it's still possible for the universe to rika lapse it depends on the way in which it changes but I think we're most excited about it because we have these two great understandings of physics from quantum theory physics of small to general relativity physics of large these two theories are not compatible and generally we don't have to use them at the same time so we you know as much as we can we happily use one theory or the other in the right situation dark energy sort of lives at the Nexus of these we have to understand how at the microscopic level empty space how it is defined what is going on in it and then we have to add that up over vast distances in the universe and describe how it gravitates what gravity it actually has show we feel like if we follow dark energy it will give us a clue as to how to do physics at that interfere until now and actually right at the moment it's quite embarrassing that our understanding of how to unite those is so off that as I said we get 120 orders of magnitude disagreement between what we see and what we imagine so thank you very much for listening [Applause]

Channel: World Science U

Views: 4,554

Rating: 4.6666665 out of 5

Keywords: Adam Riess, cosmology, dark energy, Cosmic Deceleration, Searching for Supernovae, Supernovae detection, Nobel Laureate for Physics, Nobel Laureate, Brian Greene, World Science U, University, science unplugged, New York City, NYC, Physics, Stephen Hawking, Albert Einstein, Quantum Mechanics, General Relativity, black hole, WSU

Id: 76XKCfCJUdY

Length: 53min 55sec (3235 seconds)

Published: Wed Jul 22 2020