Brian Schmidt (2016) - State of the Universe

Thank you. Excellent. So we are going to talk about the state of the universe. And, ladies and gentlemen, the state of the universe is good in 2016. The universe is expanding, that turns out to be useful. The universe is 13.8bn years old, not 13.9, not 13.7. The universe is very close to being geometrically flat and the universe is composed of dark energy, dark matter, atoms, neutrinos, photons and perhaps some other things. So, what I want to talk about is how we know the state of the universe today. So, let’s start with history. Cosmology really got started in about 1915, both Einstein and the first observations happened in that year. But Hubble was the one who sort of put us on our current track when in 1929 he announced his discovery that the universe was expanding. And the way he did that was he took data, he looked at how bright stars appeared in galaxies and judged their distance by how bright the stars were and therefore inferred their distance. And he, or Vesto Slipher really, measured redshift which they thought was Doppler shift. That is, they looked at the spectrum of galaxies which have nice little narrow lines and then you can go through and judge very accurately the redshift of a galaxy which we now know is the stretching of space between us and those objects. That is something we can actually measure to about one part and a million right now, for objects across the galaxy or potentially across the universe and that’s what’s going to allow us eventually to hopefully find planets that are habitable. And probably eventually planets that may harbour life we do not know. But that’s coming and that’s a different talk. So from this diagram, there was a relationship that the further away the more the redshift, that is what Hubble proclaimed in 1929 and meant that the universe was expanding. And so I'm going to expand the universe just to get this idea in your head, so I’ve expanded the universe there for you. And let’s look what it seems when we look at that expansion. I overlay before and after and what do I see? I see nearby objects. Well, they’ve moved a little bit so their redshift is going to be small. The distant objects – well, they’ve moved a lot in that expansion; their redshift will be large. In a universe that is expanding, you expect to see what Hubble saw, the further the more the redshift. Now, Hubble didn’t really have his head around how this interacted with general relativity. And so we’ll talk about that in a second. But I also want to just take the logical conclusion that if the universe is expanding then we can run it in reverse. When you run it in reverse you sort of come to almost an inevitable conclusion that there will be something like the Big Bang, where everything in the universe is piled up on top of everything else. So that is the thing that we thought happened 13.8 billion years ago. What is the Big Bang? That’s one of the most common questions asked of me in talks. I have no idea. I only know what happened after the Big Bang. We think the Big Bang, the time right after the Big Bang - and I'm not even going to use that but let’s use that for a second. Right after the Big Bang there was this period of inflation where things at the quantum scales were magnified to the universal scales. Quantum fluctuations were expanded to the scale of the universe. And those are the seeds of what we see in the cosmic microwave background. The sound waves which I’ll talk a bit later and eventually have led to galaxies, led to the Earth and everything that we know. Now that magnification of the universe from the subatomic to macroscales seems kind of crazy but it keeps on kind of predicting things that we see in the universe. From my perspective that period when the universe was exponentially expanding really is the Big Bang. There may be something before it, I don’t know, that may be an ill-formed question. But if we were to understand that period then I think we would have a much better chance of understanding what the Big Bang really was. And as I said, it may well be what the Big Bang is from our perspective. Alright, now the first thing that Hubble could do is he could do a very simple experiment and he could say how fast is the universe expanding now. That’s distance divided by redshift and I could run the universe back in reverse and therefore you get the age. You run the universe back in reverse and you figure out how old the universe is. So we call that the Hubble constant and that’s exactly what I worked on for my PhD thesis which I finished in 1993. And so here I am as a much younger person, I'm afraid, doing my first big experiment, showing my PhD supervisor, Bob Kirshner, the age of the universe. And the answer I got, which turned out to be about right, I wouldn't say I get credit for telling everyone this was the answer; but the age of the universe is about 14 billion years when you linearly extrapolate. Alright, general relativity. We haven’t talked too much about that. But Einstein, of course, in 1907 came up with the idea of equivalence that no matter where you are in the universe you can’t separate out the idea of whether or not being in a rocket being accelerated, or being on Earth accelerated by gravity. Those things are equivalent and he had that revelation while watching someone fall off a roof and thinking, That is what makes Einstein different from the rest of us. Because the rest of us would be thinking something different when we see someone fall off the roof! (Laughter). But he thought about that for eight and a half years. Everyone told him he was crazy, he was never going to get there. He collaborated widely with the best mathematicians of the time. And he struggled. But in the end he ended up in 1915 with the theory of general relativity. His last person he was working with was Hilbert. Hilbert actually got there four days before he did - a little fact most of you probably won’t realise. But I think Einstein did most of the hard work. But Hilbert was the better mathematician than was Einstein. So this is the thing that put Einstein on the world map. He was, when it was shown to be correct via eclipses in 1920, all over the front pages of the newspapers. And he became a celebrity in the form that we know him now. He was famous amongst physicists before this, this is what made him famous amongst the average people. Even though that’s not necessarily what the average person remembers him for today. We always think of E=mc^2 but the thing that made him famous back in the day was this. Alright, astronomers, of course, are useful v,ery occasionally, for physicists in that we can sometimes make experiments that can show things in physics to be correct. This is something that has become more true recently, but occurred back in 1919, when by looking at eclipses, we could go through and verify that the deviation of light around a body like the sun was indeed predicted by Einstein’s equation. And not by something, for example, a hacked-up version of Newtonian physics. Soon after that was shown, general relativity became a very popular subject and people worked on it the world over, most notably Alexander Friedmann who was in St. Petersburg. He was able to think very quickly about the idea of what happens if I have a universe that is full of some sort of material, what’s the solution to general relativity. And he was going to make a major approximation, which is the universe is homogenous and isotropic, but it’s the same everywhere. That turns out, as I’ll show you in just a second, is a major and very important approximation which seems to be valid in our universe. The person up here, George Lemaitre, a Belgian monk and graduate of MIT, in 1927 had a thesis which was essentially: Here’s the expanding universe from Friedmann’s equation, which he derived himself with data from Hubble and Slipher, which showed that the universe was expanding. He showed that in the Solvay Conference in 1927 to Einstein and Einstein – this was too far of a leap for Einstein – he told poor old Lemaitre, "Your mathematics is fine, your physics is abominable." And so Lemaitre, who discovered and derived the expanding universe in 1927, is not as famous as Hubble, even though he got there first. So Lemaitre went on to figure out that there should be something like a primeval atom, the Big Bang and many other things. I think he was a bit of a crank in the days and people didn’t perhaps take him as seriously as they should have, but we should take him seriously. So Friedmann is what we call the standard model, Friedmann equation from 1923. And essentially you can take a couple of equations that are general relativity. And if you assume that the universe is isotropic and homogenous you can break it down to a single differential equation which is quite easy to solve, even as an astronomer. And so it has a few pieces that may be unfamiliar, a scale factor, that’s a ruler at any given time of the universe. And since it’s kind of hard to define that in absolute terms we define it in relative terms. We call it A right now, or A at any given time t, over A naught, the time, the scale right now of the universe. That turns out we can measure extraordinarily accurately through the redshift. It is essentially directly related to the redshift and so, as I said, we can measure that to one part in a million over time. The other thing we have in here is geometry. Of course, general relativity says space is warped and we need to worry about that. And so there are actually 3 families of solutions which I’ll talk about in just a second, the +1, 0 and the -1 solutions which are closed, flat and open respectively. And finally you have Rho, which I will talk about in just a second, which is the density of what’s in the universe. So within that you get a series of solutions but there are things that are important in these solutions. One is the Hubble constant, so that’s how fast the universe is expanding now. And of course that does change over time, but it’s the fractional derivative of the scale factor. So, in other words, it just tells you in a unit of time how much the scale factor is changing. So that’s why one over that gives you the age of the universe as a linear extrapolation. We think that number in astronomer’s units is around 70 kilometres per second per megaparsec or a fractional change such that 1 over 13 or 1 over 14 billion years that is the relative value of that. We have something known as the 'critical density'. That is the density where the universe switches from being closed to open geometrically, flat. So, a knife-edge universe in practice cannot exactly be flat, although it can be very, very close. And this has a value, when you put in that value of the Hubble constant of a very low density indeed, about 9x10^-27 kilograms per metre cube. Now you might wonder: We live here on Earth where the density is 5500 kilograms per meter cube, so clearly the universe is closed. Well, no, we live in a very special part of the universe. The Earth is nothing like the rest of space. Space is very empty and so as we’ll see the density of the universe is very close to the critical density. Finally we keep track of how much stuff there is in the universe by this parameter Omega, which is the density of the universe divided by that critical density. So, for example, I’ll show you that the density of matter that gravitates is about 30%, so Omega is .3. But you can have anything, you can have energy in the form of a cosmological constant, you can have atoms. You can have something we’ll talk about called 'dark matter' – photons, neutrinos, anything that has energy is expressed as part of that. Alright, so the solutions if you just have matter which is, of course, a sensible universe that has normal gravitating matter, then you can have an empty universe, the universe is just coast and gets bigger and bigger. You could have a universe that has some stuff in it, so it slows down but keeps on getting bigger over time. You could have a universe that’s heavy, has greater than the critical density and is finite in volume. And it’s finite in time. And so all universes start with this Big Bang but only that one ends with the gnaB giB, the Big Bang backwards. Related through geometry, if you have a universe that can have these different shapes. And if you think of the form of a triangle they have, you know these hyperball geometries, the light universe K=-1. Circle geometry K=+1 and then just normal Euclidean geometry and the K=0. All right, so how are we going to go out and do this as a measurement? We are going to go out and measure the universe’s past, and see how it changes. And, of course, the universe is very big. And so if I look further and further away, I'm looking farther and farther in the universe’s past, because light takes many years, billions of years to reach us if we look far enough away. So if I do an experiment to measure the Hubble constant back in time effectively then I can see if the universe is coasting or whether or not it’s on the other side of this critical line where the universe is on the knife edge. So, one side of that gravity wins, the other side of that gravity loses. That was what we were planning to do in 1994 when we started our experiment to measure the past history of the universe. So, to do that we needed something really bright we could see on the other side of the universe. And nature gave us something in the form of a type Ia super nova. So, imagine you have 2 stars a little bit bigger than our own sun. Stars start running out of nuclear energy and their cores they puff up. If they are a close binary the bigger star will donate material to the smaller star. Eventually it will lose most of its material as it evolves. You'll get a white dwarf, which is an electron-degenerate ball of gas that weighs roughly .6 to 1 times the mass of our sun, all compacted in something about the size of the Earth. You get a planetary nebula, we think, which you can go out to see - it’s a very pretty object in a telescope. Meanwhile, inside the bigger star now, it has got all this extra materials, heated up and burning its material quickly. It will start to expand and it can donate its material under certain circumstances to the white dwarf and make it grow in mass. And as Chandrasekhar showed that when you reach 1.383 times the mass of our sun you become unstable. It turns out in practice just before that the centre gets dense enough that carbon starts burning and over about a 1000 years runs away and then: Bam! You get an exponential burn and the whole shooting match goes up in a period of about a second. Turning that 1.4 solar mass ball of gas into about .6 solar masses of nickel 56 and a mixture of other stuff. And that nickel 56 is highly radioactive and so over 21 days or so, or 17 days, its radioactive energy leaks out and you get this incredibly bright ball of gas five billion times brighter than the sun. That you can conveniently see up to probably 10 or 11 or even 12 billion years in the past. A group in Chile that I worked with for my PhD thesis, well, they figured out how to measure distances with these very accurately. And so that was the key ingredient - that happened in the early 1990s, And so all you have to do is go out and find them. Now part of going out and making a discovery is starting a team. And as I was telling people earlier today, when I was 27 I worked with a bunch of people, my mentors, to start on this project. I had no power, I had no money, I had enthusiasm. That turns out to be enough to start a team in science. That’s why science is so great. So, we started our team and the key was to find supernovae. Now in 1994, we would take 1000 2000 x 4000 pixel images - that was a lot of data in 1994. And in a period of 24 hours we had to find the needles in the haystack. There was like 1 object in every 250 frames. And so let’s see if we can find one. Here is an object - the reason we know it is we compare a before-and-after image. And so by comparing that you can see what has come out. In that we had to deal with essentially big data of 1994, and we used rudimentary artificial intelligence algorithms back then which I can tell you weren’t very good. They are much, much better now. But, it was enough, well enough with a lot of help. In practice this is how we found them. We went off to a telescope in Chile. As the data came down from the sky we had to be extremely well organised. So there’s Nick Suntzeff who helped to form this team with me in 1994. And you only get six nights a year. And you need two pairs of observations. So, you don’t want to screw anything up because if you screw up once they are not going to give you any more telescope time. And so as the data comes in we are checking it, checking it, checking it, the software is going through and trying to find the candidates but it finds lots of drunks - not drunks, finds lots of duds. It finds cosmic rays, it finds asteroids, and occasionally finds supernovae. We have literally a team of people there pouring over the data in real time trying to find the things that are most interesting. We get together and then we have to send the data to Hawaii where the ten metre telescope has just been built – Keck – because it’s the only telescope in the world big enough to take a spectrum. So there we have this amazing telescope. That’s Adam Riess and Alexei Filippenko. They are going to take a spectrum so we can get a redshift and make sure that what we are seeing really is one of these type Ia supernovae. So, after three and a half years of doing this you get to see what the data looks like - this is what our data looked like. You have redshift versus essentially distance where I’ve made it obvious what is going on here. Each supernova is a point on there, and you compare nearby with the distant objects and what do we see? Well, we see the distant objects are not in the bottom part of the diagram where the universe is slowing down over time, rather they are on the top part of the diagram where things are speeding up. So, that was a surprise and you don’t go: Eureka! You say: Oh dear, what have we done for 3 and a half years that we are getting a wrong answer that no one is ever going to believe. So you spend a lot of time checking your work and trying to understand it. Then something interesting happened. There’s a team that you are competing with, I should say Steve Chu was the boss of this team, so I have to make sure that we announce it well or he’ll give me a hard time later on. But you’re in serious strong competition with this group and you suddenly realise, both of us are getting the same crazy answer at the same time. That’s highly motivating for publishing quickly (laughter) but in the end there were 2 papers that came out. And we both got this crazy result. That doesn’t mean it’s correct, we could have made the same mistake, we were using the same basic technique. But at least it provides some assurance. And so it is on the basis of that paper that we were given the Nobel Prize, here we are, our team. Now you will note there are 19 men on that group. That reflects my discipline in 1993. The good news is right now half my group are women, so we are changing but it takes time. Of course, the other team, that is our competition. They came from physics and they actually did have some women which was great for them (laughter). I mean that seriously, we should be ashamed of ourselves they were better than us on that. So, what causes this? Of course, Einstein put the cosmological constant in his equations back in 1917, probably for a poor reason but he did put it there. And that energy which is part of space which is how you can think of the cosmological constant causes gravity. It has negative pressure and it causes gravity to push rather than pull. So in practice, the stuff we now call dark energy, well, we needed the universe to be about 70% of that stuff to explain our answer, the 30% is just normal stuff that we were used to. So, if you think about it, what we are looking at is a model that looks like this, a universe which spent its first 13.8 billion years slowing down and has only recently started speeding up and we expect it to exponentially expand. That’s what the Friedmann equation with the cosmological constant says the universe is going to do. It’s a complicated trajectory that the universe should not be doing that. We live in a very specific place where things are changing. It’s a funny place to be. It has very conveniently, it gives almost exactly the same age it’s just the simple extrapolation, within 3%. All right, so let’s say we are going to go out and make another measurement. Astronomers can go out and look at galaxies, there are probes of gravity. And, of course, within a computer you can simulate different universes to see if I start with quantum fluctuations expanded out I can go through. And I can see what the result would look like in terms of large scale structure. And so this is one such simulation done here in Germany and you can see that galaxies start to form as gravity collapses the structures and then what you do is you say: Let’s make our own universe and see what things look like. For example here’s the data that we took in Australia. There’s a similar larger survey done a couple of years later in the United States. And you can say model 1, 2, 3 or 4 - which one looks right. Now, of course, we use power spectrum ways of statistically quantifying the data. But the statistics show very clearly it’s model 3 which, interestingly enough, essentially had a matter content 30% of flatness, so Omega =.3. So, that’s what they can do, completely insensitive to the cosmological constant these measurements, just like it doesn’t exist for this type of measurement. So, they get this measurement of Omega matter = .3, there’s a problem of course because there’s about 6 times more matter than there is, that we can account for from atoms. And so that brings up the age old problem of dark matter. Wherever we look in the universe, whether it be galaxies or in clusters of galaxies, there’s always more gravity than there are atoms to explain that. So, dark matter, how would we find that out? Well, the cosmic microwave background is really the thing that allows us to do precision experiments in the universe today. So, these, as I said, are essentially sound waves of matter splashing around the universe after the Big Bang. And so we can go through and we can learn from them because if you think of the universe as a pond if you throw a rock in a pond you get wave action. It depends if the pond is made out of water or treacle, you get different waves. And so you can actually use the wave action to figure out what the universe looks like because the Big Bang, those quantum fluctuations are like throwing gravel in the universe. And then you let the waves go around for 380,000 years when suddenly the universe turns transparent, we get a snap shot and you can look through and say, what does the universe look like? Well, in detail you can go through and see this is the wave action for different amounts of atoms and different amounts of total matter including dark matter. And you can deconstruct that because the data is so insanely good and there’s a model, fits through the data beautifully. That is a very complicated model so it’s very precisely able to tell you what’s the universe made out of, 6.5 parts dark matter – dark matter is stuff that only interacts by gravity, that’s as simple as it gets - to one part atoms. The other thing you can do is you can use the fact that the universe’s geometry magnifies or demagnifies the universe. And so depending on which universe you are in these things are going to appear smaller or larger. And so if you look at the curves you can see that the geometry makes a big difference of magnifying shifting things this way. Dark energy and the acceleration makes a small bit, enough to actually measure as well a bit. So, what do you end up with? Well, you end up with a universe that looks exactly flat when we compare data. It is within .5 percent under almost any set up assumptions. The universe is very close to Omega matter = 1. So, you have the cosmic microwave background that says the universe is flat - Omega is 1. You have 30% from the large scale structure in gravity so that gives you essentially a universe that is 70% mystery matter. The same stuff, the supernovae you are seeing and so the latest numbers are this, we are in a universe that’s only 5% atoms, 69% dark energy, 26% dark matter. It is a messy universe that should not be this way, if you were a reductionist and you wanted to break things down to simplicity. So, the density of the universe, well, we know the Hubble constant quite well. It’s very close to 70 plus or minus about 3 right now. And so we have a universe made up of these fractions of things. But we can also see how many photons there are for example. You get that from the black body so that’s something that George Smoot measured. But he talked about gravity waves, so I'm going to talk about George’s Nobel Prize here. You can also go through and look at things like neutrinos. The best measurement of the mass of the neutrino, at least the upper mass, now comes from astrophysics. Probably make a couple of physicists mad there. But if you have a universe full of neutrinos, you don’t know how many there are compared to the photons due to the physics in the early universe, if they get heavier and heavier the sum of them, then that affects the large-scale structure. And right now our measurements of galaxies tell us that the mass neutrino or the sum of them must be below .25 eV. So, our cosmological model, well, you start with these initial fluctuations from the Big Bang inflation. You’ve got a universe made up of all this stuff, and that agrees with essentially every measurement we’ve made in the universe. And that is what science is all about, having a theory which predicts things. Now, there’re some problems which I’ll talk about in a second. The only newly arising issue is that Adam Riess, my co-Nobel prize winner, has measured very accurately now the Hubble constant and he is claiming that there may be a discrepancy between the local value of the Hubble constant and that which you get from the cosmic microwave background. I'm not sure yet, I'd call it 50/50, maybe. Worthwhile looking at, it needs more attention and it’s a very hard measurement. So, dark matter, dark energy inflation. Those are our three problems with that model. We don’t know what inflation is, we don’t know what dark matter is, we don’t know what dark energy is. We know how they behave, we just don’t know why they are there. The big questions we have to look for is what is dark matter? What is dark energy? Why do neutrinos have mass – that’s an interesting one as well, not exactly astrophysics. Why is the universe not just full of photons? The equations of the early universe say everything should have been annihilated, we should not have any atoms or dark matter. What is dark matter? And what is inflation? Just to give you a sense of how astronomers can answer what seems imponderable – how could you figure out what happened when the universe was 10^-35 seconds old? Well, down in South Pole we had a bit of excitement last year, where people measured what they thought was the imprint of gravitational waves in the cosmic microwave background through what we call B-modes which are essentially the cross product of the polarisation – very exciting. Unfortunately, it’s a hard measurement because the universe is full of dust which is also polarised. Here’s the part measurement and it’s measurement of polarisation. They were looking down on this area and it turns out the polarisation there was stronger than their signal. So shame on them – no. Probably shouldn't have done the press release. But it is good they are out there trying to make this measurement. This is profound because if and when we detect this, we are going to get a constraint when the universe was 10^-35 seconds old, our best chance to understand inflation and what the Big Bang is. So, I will leave you with Einstein’s to my mind greatest quote: Excellent to use when your politician comes up to you and asks you, ”What on earth are you doing?” Tell him that. Thank you very much.

Brian Schmidt (2016)

State of the Universe

Brian Schmidt (2016)

State of the Universe

Abstract

13.8 Billion years ago, something happened - the Big Bang - which set in motion our expanding Universe. Through the systematic process of science, humanity has managed in a very short time to piece together a comprehensive story of our Universe. In 2016 the vital statistics of the universe - its composition, size, density, shape, and age – are known to remarkable accuracy. But the job is nowhere near done. In my lecture, I will discuss what we know, and what we don't know about the Universe, and try to guess likely new areas for discovery in the years and decades to come.

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