The Dark Universe - with Adam Riess

thank you very much for having me here I'm going to tell you a little bit about the universe so the universe behaves like it's been kicked we call it the Big Bang and now space expands is literally stretched you could think of the galaxies like raisins in a rising loaf of raisin bread whichever galaxies are raising you sit on you look at the ones around you and they all appear to be rushing away from you the further away they are the faster they appear to move because there's more space so there's more dough between you and distant objects so how do we actually know this about the universe this is not something we can theoretically derive this is something we actually have to observe about the universe so if you look at this animation that you see here the key aspects are that you have to measure how far away objects galaxies are around us and you have to measure how fast they appear to be moving away from us so how do we figure out how far away objects are one of the favorite ways we have is known as the method of the standard candle it's a little bit like a lighthouse and the way in a ship you use a lighthouse to gauge how far away the shore is by how bright it appears in this case our lighthouse is a single star which explodes in a galaxy of a hundred billion stars so here on the right you see is a galaxy with a hundred billion stars and then one of those stars may explode and we can gauge how far away it is from how bright that explosion is the inverse square law makes this very quantitative that if a supernova is twice as far away it'll be four times as faint if it's three times as far away it is nine times as faint so we go and we look with our telescopes for these exploding stars in nearby and distant galaxies and we gauge their distances from the brightness of their life now the other aspect that you saw in that animation of the expansion was that we see the galaxies apparently rushing away from us how do we see this well the light is emitted by any object including these standard candles at certain known wavelengths wavelengths of light we can determine in the laboratory but as that light travels to us like you see in this animation the wavelengths of light are stretched not it's a little like the Doppler shift you may be familiar with but the the cause is different it's not actually the motion of the object it's actually the expansion of space itself we call this the redshift and it allows us to measure this motion away from us so we go out in the universe with our telescopes we measure the distances we measure the redshifts and now I can go back to that animation I showed you except now this becomes a quantitative science we look at galaxies around us we measure how far away they are we measure how fast they appear to be moving away from us and as I said the further away they are the faster they are moving because there's more space and so this plot this little graph that you see in sort of cartoon form there is the way we measure the expansion rate of the universe this is known as Hubble's diagram after the astronomer Edwin Hubble who showed that the universe was expanding in the late 1920s and by measuring the slope of that line we can tell just how fast the universe is expanding around us today now there's a catch I sort of lied to you a little bit that I said we can tell how fast the universe is expanding around us today and that's because we forgot to account for the fact that it takes light a very long time to reach us from distant objects in many cases billions of years so we can never actually really measure how fast the universe is expanding right now this minute we have to look at delayed information and infer therefore how fast the universe was expanding in the past now this is actually very powerful for us because we can look at a distant Stan piano a supernova like the one you see on the top row that's more nearby and it might be telling us how fast the universe was expanding a billion years ago you go to the next row find a more distant object it's telling us how fast the universe was expanding two billion year ago and three billion and so on so with this trick that we can look back in time by looking out we can not only measure the expansion rate of the universe but we can measure the expansion history of the universe we can see how it's been changing just like when a geologist takes a core sample of the earth the deepest layers tell them about the past we have the same ability in observational cosmology so this is good because in the mid-1990s cosmologists wanted to know how the expansion of the universe was changing in particular the expectation was that the expansion was slowing down I said there was the Big Bang but then there's all the matter in the universe and the attractive gravity of the matter would pull back and slow the expansion and the big question was whether we lived in a very heavyweight universe like the one you see on your left on on your right my left is that would imply that the universe would expand but there would be enough gravity to slow it down it would be decelerating and eventually it would stop expanding and start contracting and the universe would end in something like a Big Crunch the opposite of the Big Bang or the universe could have been the one on the right a very lightweight universe very little matter very little deceleration and like a rocket with escape velocity the universe would have escape velocity from itself it would continue to expand forever and it seemed like a straightforward question measure the amount of slowing of the expansion way the universe and figure out the fate of the universe seem pretty straightforward for a graduate student project now not just any kind of supernova will do there's a special class of supernovae first explained by the Indian astrophysicist Chandra Sekhar who won the Nobel Prize for this work explained how stars of a certain type called a white dwarf star can only hold themselves up against the crushing force of gravity up to a certain mass now known as the Chandrasekhar limit when a star like our Sun becomes more massive than 1.4 times the mass of our Sun you will get a runaway thermonuclear explosion it's like nature's own standard bombs or standard candles they all blow up at just about this same mass and so they have the same luminosity so we can use these to track distances across the universe when one of these explodes we could see it halfway across the universe or 2/3 of the way across the visible universe they are as bright as about four billion suns so when we did the measurement ha it was neither the the heavyweight universe nor the lightweight universe the universe was doing something completely different it wasn't decelerating at all it was accelerating again that's like you fire the rocket from the earth to see whether it will fall back or not and instead it is able to just keep going and going this was acting opposite to attractive gravity and so we realized that the universe really is accelerating this was the breakthrough discovery of the year in science magazine in 1998 it's so exciting to us because it implies that about 70% of the universe is made of some other stuff this stuff we called dark energy it's important property is that it would have repulsive gravity now that sounds like a pretty strange concept but it is something that Einstein had first explained could exist in his theory of gravity I'll say more about that in a minute so we have a prescription now for the universe which is that it's about 0.05 percent in planets about half a percent in stars about 4% in gas so if you add up all the material in the universe that's made out of the parts you see in the periodic table of elements that's less than 5% of the universe 23% is in a form of matter that is exotic we think a particle we have yet to find definitively and about 73% in the form of this weird dark energy so I'll end with just telling you our best ideas about dark energy the first idea and still the one we sort of hew closest to is that it starts with a theory in physics called quantum theory that tells us that the vacuum of space is not really an but it's chock-full of particles that appear and disappear all the time and that there's an energy associated with those particles and there's a gravity associated with that energy which strangely enough is repulsive and would cause the universe to sort of self accelerate that's an interesting idea it looks like what's happening the problem is when we try to do calculations based on that idea we get an answer that's about 120 orders of magnitude off from what we actually see so it's just a word level idea at this point because we don't really understand why that is but as crazy as it sounds if there's energy and empty space we've begun to see this in other areas so if you've heard of the Higgs boson or the Higgs field this is an energy and empty space that we have now seen directly in the laboratory it's not it is some dark energy it's not our dark energy causing this but dark energy is not just a made-up concept there is another possibility that it's a transient form of dark energy that is it's related to a field like the electric field or the magnetic field we think there was a field like this early in the history of the universe called inflation that caused an accelerated period of expansion so it may be that these arise from time to time or the last possibility and I think while maybe it's least likely as most intriguing is that we don't really have the right theory of gravity that Newton thought he had the right theory of gravity until there were certain anomalies which finally Einstein came along with a deeper understanding of gravity which resolved those anomalies now we have these funny parts maybe we will find them directly but we have to keep our eyes open for the possibility that this is not the right theory of gravity so I will end there and right angle pick up thank you it's lovely to be here I was just saying earlier that I've only ever seen this room from a television so it's lovely to be here so Adam introduced us not only to the fantastically exciting concept of dark energy but also showed us graphs of the history of the universe and we're going to look at one of these again and the measurements that we're making of supernovae are really made today from from Earth and we're looking at objects you know a few billion years away but I want to take us back to that time of the Big Bang back to the really early times and and figure out what that time can actually tell us about other dark components so if you're ready this is what the early universe looked like no really so we believe the universe started in a very hot dense phase and this is an artists representation of what it would have looked like the whole universe was in what what what I call my favorite state of matter which is actually plasmas so it's incredibly hot and we have protons and electrons really in this soup this hot fiery plasma and they're interacting together and what we do as cosmologists to measure the Cosmic Microwave Background is we want to use information about this time to tell us what the universe is made of so when the universe is hot as plasma is actually oscillating so we have dark matter in the universe which is pulling like gravity pulling all of the parts of the universe together but because the electrons and the protons are free is actually radiation that's bouncing off the electrons in the early universe and that's why we can't see through it so the only universities are Paik for the same reason the Sun thing is opaque light is bouncing around we say that the mean free path is quite small and so because this light is bouncing around there's additional pressure so the photons are moving and they exert a pressure and so the early universe is really ringing like a bell we call these acoustic oscillations the key thing is as the universe expands it cools down and just as you take a bunch of excited five-year-olds and then they slowly get tired they stop running around the universe is similar so as the universe cools down so eventually the protons and electrons can combine and form neutral atoms when that happens the photons now can't interact with the electrons anymore and the photons can propagate towards us and those are the photons that we measure from the early time so the light that I study has been coming towards us for over thirteen and a half billion years okay so what do I do I actually measure the temperature of this radiation on the sky but what does that tell me about the early universe the key thing is that the temperature of this Cosmic Microwave Background is related to the density in that early times because you imagine if there was a little bit of a nova density it would pull a little bit more because it would have gravity works gravity sucks as the adage goes so the more there is the more it attracts and so areas that are dense act in in different ways to areas that are under dense in the early universe and they change the temperature and so we use the temperature of this radiation to really tell us about the density in the early universe so what do we do actually well we want to measure this radiation on the sky and the way we do that is we make a projection we basically project longitude and latitude into the heavens and so you'll see in this animation we have the sphere around our own earth and then we open it up we typically open it so that it's centered on the graph on the galaxy so if you see plots like that there's a lot of very interesting stuff happening along the middle of this egg and this is a standard map that we use and we really want to measure what is the temperature like in this part of the sky compared to this part of the sky because that tells us that the conditions in various parts of the sky parts of the universe were different but in order to do that we need to make incredibly precise measurements of the differences in the temperature so in fact we know that the differences from place to place in the universe in temperature are one part in a hundred thousand so that's roughly one drop of water in a gallon so only very small differences in temperature and that's kind interesting and very exciting because it's like saying imagine if I polled everyone in this room what your favorite song was I know you're all going to say thriller but but that would be strange because you don't necessarily know each other at the universe at very early times there are very small differences everything is basically the same temperature and so to measure these differences we really need to make very precise measurements of the Cosmic Microwave Background so we measure it in the microwave the radiation has been traveling towards us for billions of years and so because of the redshift that Adam mentioned it stretches out and becomes very cold in fact when we measure this average temperature on the sky today it's only about three degrees above absolute zero so about three Kelvin and so we need to really build incredible instruments to measure the contrast between different parts of the sky and so that's what you saw in the movie I'll play it again just so we can see so we basically increase the precision we take away the average and we keep building finer and finer instruments to measure the difference from parts apart in the sky now of course you'll see the big band and that's our galaxy that we need to remove because it's telling us not about the early time but about physics now so I said the temperature was linked to density and the way that we see that is because areas that were slightly over dense in the beginning of the universe act a bit like a sink and so they'll trap photons inside them and so photons at the early time if Aran in an over density are a little bit colder areas that are a little bit less dense will be a little bit hotter and so we can actually link the fluctuations in temperature that we see to the fluctuations in them in density and this is a movie that was made by NASA and the W map team which is a space satellite that measured the Cosmic Microwave Background and you can see in this animation they're evolving time and those fluctuations in temperature grow and become galaxies that we see today and reefs of course is going to tell us a lot more about exactly how that happens so we have these incredible measurements of a cosmic microwave background but what does it tell us about the universe that we live in both now and it's past well we're really lucky we actually have an incredibly simple model of the universe atom introduced it a little bit already but we actually have just six numbers that really tell us almost everything we need to know now if you haven't already read Martin Reese's book it's fantastic I recommend it but in the important parameters the dark ones I've highlighted in the orange circle so the three on the on the Left tell you about what stuff is in the universe so baryons are things like you and me gas and that makes up a very small fraction of the universe called dark matter that Omega and the CH squared that's telling us how much of the dark matter is in this early time and then Omega lambda is telling us about this vacuum energy or the stock energy and then as I've said we believe that that dominates most of the energy density of the universe there are two other parameters that tell us a little bit about those density fluctuations and they tell us basically what are the sizes of the Peet's so imagine I come into this room and I say roughly what is the average size of the person in the room I know that would be human sized right because I see that most of the people most of the structure in this room is people I'd also see that there's some structure that is the size of a step and so if I was to measure just as a function of physical size how much of the room is made of various things that would tell me something about the power spectrum of fluctuations in this room and we do something similar in the universe we say roughly how big other things in the universe and we measure that than using a power law which is just a mathematical formula and that has an overall amplitude and it has a slope and those are two parameters we also learn about how the universe stars begin to form and that's something I want to talk to you at all about now so I mentioned this a little bit before that we have these acoustic oscillations happening in the early universe and it's really incredible because as this plasma is vibrating so we have pressure from the photons pushing outwards gravity from acting on the matter pulling inwards and so the universe really is oscillating and the cool thing about that is if you change up the makeup of the you change how that's happening so if there's more dark matter you make the wells a little bit deeper or the oscillations change in their structure and we really can use these oscillations which get transmitted in the radiation that we see to tell us something about what how much matter they is in the universe for example here is some of the most recent results so there's there are many experiments on the ground in the South Pole in the Atacama Desert and in space that make measurements of the Cosmic Microwave Background and they have all been working together to try and figure this figure this model out this is from a recent release by the Planck satellite which was in space measuring the radiation and you can see how the date of points on that graph from three experiments match our theoretical model beautifully and that's no mean feat because there are only six parameters that make up that Wiggly curve and that's wonderful we have this perfectly simple fantastic model unfortunately it has a lot of dark stuff in it that we don't understand but it fits great and we use understanding how these equations work in the early universe and codes to try and predict different models and we use the data to really fit and see which one of those models make sense for example if I change the amount of matter in the universe you can see those peaks and troughs change a lot and because we understand theoretically how they will happen we can match the curves to the data and that's really what I spend a lot of my time doing is this kind of sleuthing next mixing data and theory but something else happens we don't only learn about the early universe the Cosmic Microwave Background also tells us something about what the universe is like between us and the CMB and that's because I told you that the light has had to travel to us from you know roughly 13 and 1/2 billion years from from today and so as it's the photons are travelling towards us they're interacting with everything else in the universe kind of like in the beginning before everyone came inside if I wanted to go to the ladies I would have had to walk through crowd of people so my path would be a little bit deflected and that's exactly what happens to the photons from the Cosmic Microwave Background they want to get to my eye but you're in the way and so these small deflections tell us something about what the distribution is of matter and the geometry of the universe between us and the CMD in this schematic you see a little snippet of the CME at the back and we've drawn with an artist has drawn the photons coming towards us and they're moving through this beautiful purple cosmic web and the structure the big galaxies and clusters are going to deflect those photons and they'll get to my eye and a slightly different path so how does this structure change what we see I'm going to show you two simulations of what lensing does to the Cosmic Microwave Background blink and you'll miss it luckily I put on Linden lanes just in case so these effects are tiny and one of the things that makes it so exciting is that we weren't able to even see this effect until a few years ago we just didn't have the resolution so if we had smooth to these maps over you would never see the effects these small fluctuations from a structure but because we now have exquisite measurements of this radiation we can start to see these deflections and we can actually start to figure out what was doing that so what was the universe made of how much dark energy is there how is that changing the structure how much matter is there what does it look like how is it distributed along the line of sight we learn a lot from these observations about the content of the universe at the Big Bang the structure of the universe today and because we have models that can take us forward in time we really also learn about the death of the universe I don't have time to show you the movie but I worked with the Ted team to make a very short movie animated movie about the death of the universe I think it's fun not depressing it animated so it can't be depressing but just to close I want to really reiterate the fact that with exquisite measurements and with actually pretty simple models were able to learn so much about the universe and what it contains and how it's going to end thanks so I'm gonna tell you about what happened after this first moment of the universe actually about 400,000 years after the Big Bang that you heard about from Rene and I'm gonna focus on how we get from that early time to the universe that we see today many of you might be familiar with this beautiful deepest picture ever taken of the universe so far from the Hubble Space Telescope so how did we get this beautiful set of galaxies in all their variety all their structure on the sky from that sort of hot dense picture that we started with very early on we not only have these galaxies but we also have these beautiful planets so how do those galaxies form how are they distributed in space and what can that tell us actually about the physics of the universe and the part of the universe this dark universe that we've heard about that we can't see so as we heard already the universe at early times was very smooth but there were these small tiny little fluctuations in the temperature that were mapping out fluctuations in the density okay so those were really tiny in the beginning and what we see today in the universe is that those fluctuations are very large the universe is very clumpy not only do we have auditoriums full of people watching learning about new planets but but we also have galaxies that are actually not randomly distributed they're quite clustered in the universe this is a map of the nearby universe the galaxies this is the same projection you saw earlier and what you're seeing here in Milky Way in the middle but all of those other little every single point on this red on the rest of this plot is a galaxy that's been mapped out and this is actually just the very nearby galaxies in terms of in the terms that I think about since I am really interested in the galaxies spanning the entire history of the universe so much much denser than the early times that we saw that the early universe was about one part in a hundred thousand the late universe is much much more dense at sort of the very outskirts of a galaxy it's about 200 times denser than the average and Pluto is about 10 to the 33 times as dense as the average part of the universe so how does this happen how does the universe go from you know that baby picture to actually it might not be in its advanced years yet we think we have a long way to go so how the universe evolves actually depends on what it's made of it depends on what are those six numbers that you heard about how much dark matter do we have how much normal matter do we have how much dark energy do we have so we actually have pretty good theoretical models for how this happens based on what those six numbers are and the reason this is actually a fairly simple physics problem is that on large scales gravity is dominating gravity really rules the show on large scales you know on small scales within this room there's lots of other physical processes that matter but on large scales gravity is really the important thing and that means this is actually a really beautiful problem because we have the initial conditions of the universe those initial conditions set by the Cosmic Microwave Background tell us about what the constituents of the universe are how much matter we have how much dark energy how much normal matter how much of it is dark and we can then add gravity to this picture and it turns out that it's it's a it's sort of a simple problem conceptually but it's a very difficult problem computationally so we also have to add a really big supercomputer we actually use the largest supercomputers in the world to do these kinds of calculations we ran we ran a simulation just last year that took about 30 million CPU hours so but we can do these calculations quite precisely here's an example of one of those this is again starting from the very early universe when it was very smooth and evolving it forward and this is actually just looking at the dark matter in the universe and you can see as I told you that it's very structured it's very clumpy and those bright spots that you see there are where we think we get galaxies so what happens is that we have this cosmological expansion we heard that the universe is not only expanding but it's also even now accelerating but gravity is pulling things together in small regions and in those small regions where gravity pulls things together we get something which is no longer expanding with the universe and the gas in those in those dense regions can actually start to cool and then can start to form stars and then we can get a galaxy we call those collapsed regions Dark Matter halos it's basically just a clump of dark matter that is surrounding what ends up forming a galaxy for example like our own galaxy the Milky Way so this is a complicated process and here's just showing you a zooming in on one of those movies basically to see it in a little more detail you can see in this movie this is the formation of a galaxy cluster where you have lots of galaxies coming together you can see the universe is very active that structure that we see exists essentially on all scales in the universe and measuring that structure really allows us to determine something about again about what the universe is made of how it evolves and what is the basic physics that's driving it so we heard that we have these parameters but we don't know for sure that those parameters actually describe you know all of the universe maybe they just describe what's happening you know at early times so by by doing these kinds of simulations and then comparing them to measurements of the universe we can see whether this same model is actually able to describe what the galaxies look like and how they evolve and how they cluster in the universe so this is just an example of doing this simulation now with two different sets of those numbers and what I've done in these two movies is both of them are actually tuned to match the parameters of the Cosmic Microwave Background but one of them has larger fluctuations today one of them has larger fluctuations in the early universe and you can see that they actually evolved differently and one of them ends up being a lot more structured than the other one okay and so we can do this with all of the cosmological parameters we can sort of run these movies forward and we can actually test them against data so let's take one example we start again with those initial conditions that we measure from the Cosmic Microwave Background and we start again with with this really simple idea that galaxies form at these density Peaks and that that is actually what sets the structure in the universe so we can then use that theoretical model and make a prediction for how galaxies should cluster in the universe and we can then actually go out with a telescope and look at how galaxies cluster in the universe so this is an example of doing that kind of comparison in this in this animation every point on in this movie is a galaxy that has actually been mapped out by a telescope called the Sloan Digital Sky Survey that survey got redshifts for about a million galaxies it mapped out their 3d positions in space but actually I'm going to show you that movie one more time and I played a trick on you because it turns out that only half of this image is the real universe and the other half is a universe that I invented in my computer so I what I did was I started with those parameters I put it in a supercomputer I made a prediction for where all the matter should be and then I used this idea that the galaxies should trace that matter distribution actually in a fairly simple way and you can see by eye that it looks very similar but we can also do a lot more detailed statistical tests to decide whether this simulation actually agrees with what we observe and it's it's quite amazing that we have this model which is very puzzling that involves most of the matter is not made of the same stuff as us most of the stuff in the universe is not even matter but it describes supernovae it describes the very early universe and it describes the evolution of galaxies over several billion years with essentially just these six numbers so it's pretty incredible but we really do want to test this idea a little bit further we want to for example Adam mentioned three possibilities for what could be doing this acceleration maybe it's just a cosmological constant maybe it's kind of like a cosmological constant but it changes with time at some field that that you know that that is somewhat different in the early universe than it is today or maybe it's actually a modification of gravity and in order to test that one of the best ways to do that is to really expand these surveys of galaxies as I said this a Sloan Digital Sky Survey took spectra of about a million galaxies and the next generation of galaxies surveys will allow us to test these models with much much higher precision and I'm just going to very briefly tell you about two examples one example is an imaging survey called the dark energy survey this survey has already started and just a few months ago we published this beautiful map of the universe this is actually just about 1/3 hundredths of the sky so it's still a small piece of the sky this survey when it's done is going to map out about 1/8 of the sky and this map is actually made using the same kind of technique that Renee told you about gravitational lensing but here now we're not looking at the CMB photons were getting at the distortions of background galaxies that actually allow us quite amazingly because mass along the line of sight distorts those photons to map out where the mass is in the universe and to really trace not only where the galaxies are but where the dark matter is so so far this is this survey is quite preliminary but it's quite exciting because this survey actually is going to find a whole bunch of new supernovae it's going to be able to do gravitational lensing it's going to be able to find other objects that allow us to test the dark energy and dark matter in different ways to really decide whether these various ideas match the data that we have and then we heard about one more survey already this is the dark energy spectroscopic instrument this is a different kind of survey instead of just pictures we actually take spectra of the galaxies and that allows us to make a three-dimensional map it's quite amazing this survey will do thirty-five million galaxies currently there's about not quite I think three million galaxies and quasars that have been taken by any instrument ever so far so this will increase that by an order of magnitude and that will really allow us to make this very precise 3d map of the universe to help distinguish between these various ideas for what is accelerating it what is the dark matter and how did the universe begin so we have lots to look forward to I'll make sure that the measurements of the Cosmic Microwave Background don't contain any discrepancies from galaxies or stars that emit light how do you how do you ensure that that radiation is definitely from the beginning of the

Channel: The Royal Institution

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Keywords: Ri, Royal Institution, Lecture (Type Of Public Presentation), Dark matter, dark universe, physics, talk, discussion, lucie green, adam reiss, nobel prize, Adam Riess, dark energy, Renée Hlozek, renee hlozek, Risa Wechsler

Id: rgdKlabAn1c

Length: 37min 39sec (2259 seconds)

Published: Wed Oct 14 2015