 of the moon, maybe? So today I want to talk to you about the accelerating universe. Understanding the accelerating universe really is meaning to understand the universe from beginning and end. But before we do that, let's acknowledge and pay our respects to the Aboriginal people of Australia and their elders past and present. And in doing that, let's think of what the elders in the past saw. They saw the Milky Way, the same Milky Way we see today. And they saw it as a giant emu in the sky. If you ever have the chance to go just outside of Sydney and one of the national parks there, there is indeed in these rocks an ancient carving of an emu. And you can see it up in the sky. The skies are something that belong to every human who has lived on planet Earth. It is something we share. And we realize this when we look, for example, in Europe, at some of the ancient paintings. This is the Cave of Lescombes in France, 17,300 years old. In it we see an asterism, which we call the Pleiades, or the Seven Sisters. Seven, actually six stars. It's one of the interesting features of it. In the sky, right next to the bull, or as we call it now, the constellation Taurus. The idea of the constellation Taurus dates back 17,300 years. The Pleiades, the Seven Sisters, how do the Aboriginal or many of the Aboriginal people of Australia refer to it? The Seven Sisters, despite being isolated from the Europeans by tens of thousands of years, the stars are stories that go back to the beginnings of humanity. And so when we look at the Pleiades today, a constellation in the sky, or an asterism in the sky, we're looking at the same thing that every human on planet Earth has seen. Why? Because they're visible to everyone. I grew up in Alaska, the very far north. And people in Patagonia and the very far south, they can all see this. Every human has shared those six stars that we refer to broadly around the world as the Seven Sisters. So let's now change gears and figure out what we've learned in the last 20,000 years. And to do that, we're going to take a tour of the universe. And to take that tour of the universe, I present to you our guide, the speed of light. Light travels very fast, seven and a half times around the Earth each second. That's 300,000 kilometers per second. And as Aditya was telling you, out and stealing my lines, when you look at the moon, and he has stolen my lines, he's seen my talk, when you look at the moon, you're looking one and a half seconds in the past. And so you are seeing things as they were. And the moon, one and a half seconds. What's that? What's that? It's not much. But when we saw Pluto recently, that was more than four hours. But let's just take a look at some other things in the solar system. The sun is much bigger than the Earth-Moon system, something we don't realize because the sun is so far away. It's eight light minutes in distance. And so when we look then at the most distant star, and I'm curious how many people saw this star out tonight, Alpha Centauri, that's the closest star in the sky. 4.3 light years in distance. Think what 4.3 light years is. That's light traveling 300,000 kilometers per second for 86,400 seconds a day for 365 days. 365 and a quarter days because we've got a leap year or two in 4.3 years. That's trillions upon trillions of kilometers. Deep horizons, the fastest spacecraft that we've yet constructed as humanity, would take 25,000 years to get to Alpha Centauri. The Earth and Alpha, sorry, the sun and Alpha Centauri are two stars in the Milky Way. The Milky Way is huge. It contains 100 billion stars. It is 100,000 light years across. We are orbited by an object which we can sort of see, and if we really squinted hard and got really dark outside, by two galaxies, the large and small Magellanic Cloud, which are more than 185,000 light years in distance. The light left before humans existed. That's how far you're looking in the past. You can see that, especially from a dark side in Canberra. They're fuzzy little blobs due south. And I encourage you to go out and look at them, because in a dark side, they actually look at least the large Magellanic Cloud like a real galaxy. They only contain 10 billion and 1 billion stars each. They're the most distant objects you can see with your eye easily in Australia. If you're in the Northern Hemisphere, the Andromeda Galaxy is 2 million light years, the most distant object you can easily see with your eye. All right, now let's zoom in. Let's look at a tiny piece of sky the size of the moon. And we're going to zoom in and see what the Hubble Space Telescope sees in this tiny patch of sky, 1 13 millionth of the entire sky. It sees, well, it sees back 12 billion years into the past. It sees 20,000 galaxies, each of these galaxies containing 100 billion stars like the Milky Way. There is a lot of stuff in the universe. One of the remarkable things about this picture is there are places where you can see almost nothing in optical light. Why? Because it turns out there's nothing there as far as the eye can see. If you look in microwaves, like what you would cook your vegetables with, you see this. This is the entire sky glowing. Why? Because 13.8 billion years ago, the universe was 3,000 degrees. It was almost as hot as the sun and glowing like the sun in all directions. We can't see that with optical light because there's just none of it there. We see it in the form of these microwaves. And before that, well, it turns out, that's the big bang. Now let's think about how we arrived at this. We did this by being able to measure things. Science is all about measurement and prediction. How do we measure things in the universe? Well, we don't have rulers that go to the nearest galaxies. We instead have to rely on how things appear. The further away an object is, the fainter it appears, or the smaller it appears. And we can use these physical facts to measure distances. But we can also measure motion. How? Through the Doppler shift. If you hear an ambulance coming towards you, the pitch of the ambulance is raised. As it passes, it fades. That's because sound is a wave. And the sound waves are compressed and raised in pitch as it's coming towards you, stretched as it goes by. Light is also a wave, slightly different wave than sound. But it too gets compressed as you're moving relative to the speed of light and gets stretched as it's moving away. Edwin Hubble in 1929 went out and measured velocities and distances for galaxies. And he found a remarkable thing. He found that the faster a galaxy was moving away from us, the further it was, the fainter it stars. And from that measurement in 1929, he announced to the world that means the universe is expanding. Why did he say that? Well, let's make a little cartoon. So here's my cartoon universe, which I am going to magically expand. Now, let's go through and put before and after on top of each other. So I'm going to take the before image, before I expanded and after. What do I see? From my reference point, nearby things, they moved a little bit. If I was measuring their velocities, it would be small. The distant things, they moved a lot. If I measured their velocities, it's fast. If you expand the universe, even a toy one, you see what Hubble saw, the further the distance, the faster the motion. Now, it turns out that Einstein's theory of general relativity, considered by most of science as being sort of the most elegant piece of science in the last 150 years. And we always ask whether or not it's all time. And then we get into a fight about Newton versus Einstein. You don't want to know about that stuff. And that, turns out, tells us a story that the universe, due to the way gravity works, should be expanding or contracting. It turns out, it tells us also that the universe should have this view from any point. So from no matter where you choose your reference point, you see the same thing. So Einstein's theory of general relativity is how we go out and predict things. Now, imagine a universe which is expanding. Now, I will trouble with this earlier. We're going to try it again and see if it works. Yeah, I'm going to try it again. Again, it did work once. Let's try it. I'm going to apply something here. No, no. Oh, that's amazing how history repeats itself. I didn't understand it last time, and I assumed it wouldn't happen again. I don't know. I'm going to try something. It's all unhappy now. I'm going to try it one more time. Maybe not the master of the universe, but definitely master of PowerPoint. So the universe is expanding. Let's put it in reverse. What happens? Everything in the universe gets on top of everything else. That's sort of guaranteed if the universe is expanding. What else could happen? The Big Bang, when everything in the universe is on top of everything else, almost guaranteed. So it turns out Einstein's theory of general relativity predicts more or less that to happen too. If the universe is in motion, there should be a time in the past where it begins. And the way we think about that in science is we draw graphs. So imagine the distance between us, the Milky Way, and some other distant galaxy is some distance where some distance departs separation. And of course, the time. What's the time? Well, it's 748. No, it's about 13.8 billion years after the Big Bang. How do I know that? Because if I measure how fast the universe is expanding, I could run the universe into reverse. How do I know how fast the universe is expanding? I measure distance. I measure that velocity. In relativity, that velocity is actually the stretching of space. It's a little different than a velocity. And so I measure that, and I run the universe back, and voila, the Big Bang. Now, I've cheated in this diagram. Why? Because that line is straight. And that means the universe is going the same speed. But we know from general relativity and just intuition if gravity is at play, gravity is going to attract everything and slow things down. If I throw a ball up in this lecture theater, gravity is going to slow it down. And it's eventually going to come to a halt unless I can throw it faster than the escape velocity of Earth, which turns out my arm isn't that good. So all I thought being able to measure how fast the universe is expanding now, so I figured out how old it is, would be the best thesis. And that's what I did in 1990, the fact that people had been doing it for 50 years before me and not got a definitive answer to not deter me. That's the beauty of being a scientist. Here I am, three years, 11 months and four days after I started, but I was not counting, showing my thesis advisor the answer. And I measured a whole bunch of distances and red shifts, as we call the velocities. And I got an answer. And that answer was that the Hubble constant, which is what we named the expansion rate of the universe, told us that the universe was about 14 billion years. But that was independent of gravity. I hadn't corrected for gravity. And the problem, of course, is gravity could make that number, because of the trajectory, be quite a bit younger than 14 billion years. Jeremy Mould here at Mount Strong Observatory had the keys to Hubble, the Hubble telescope. And his job was to measure this definitively. And the good news is, as he always likes to tell me, is I got the right answer in my thesis. Therefore, I got to keep a job here at ANU. But the question is, what about that gravity stuff? We're all ignoring it. And so I was brought here to think about gravity. And that gravity has profound implications for the universe. If the universe has very little gravity, then the universe just keeps expanding. It doesn't slow down at all. And the universe is infinite into the future. On the other hand, if there's a lot of stuff, then the universe will slow down and do the trajectory like a ball. And of course, all the universe is there, or have a beginning, a big bang. But only this one has the ganad kib, the big bang universe. So I came to Australia and was hired to go through and measure what the universe was doing now and compare it to the past. And we'll talk about that. So measuring the universe's past, imagine I can go through and measure distance and redshift or velocity for nearby objects. I'm not looking very far in the past, then. And then I do it to distant objects. I can compare if the universe hasn't changed its speed or its velocity, its expansion rate over time. Then I know the universe is going to keep going because gravity isn't slowing it down. On the other hand, if the universe is slowing down a lot, that is, it's expanding much faster in the past and has slowed down, then if it's doing it faster than this critical line, gravity wins. The universe is heavy and it ends. It's finite in the future. Doesn't go on forever. On the other side of that line, gravity loses. And the universe expands forever and exists forever without end. So if we can look far enough into the past, then we're all set. But it turns out to look a long ways away, you need to have something you can measure distances with that is very, very bright. And that's where type 1A supernovae come in. Type 1A supernovae are some of the biggest explosions in the universe, and they create, for example, the iron in the universe, the iron in these chairs, the wealth of Gina Reinhart, all created in these explosions. Gina has thus far declined to endow my own research portfolio into her wealth. But that's another story. So let's talk about these type 1A supernovae. Our son, I'm afraid, is running out of nuclear fuel slowly but surely in five billion years from now. It's going to puff up, destroy the earth, collapse down to a tiny little object called a white dwarf star. If our son were in a binary system, two of them, then things get much more complicated. The bigger the star does that process, except for it dumps all its material, or a lot of its material, into the second star. The second star, turns out, then gets kind of heavy. It gets all this stuff. It's like it went to McDonald's for a couple of billion years. And it gets a lot of energy from that. It gets bloaty, fat, all sorts of other things. And then eventually, as it starts running out of nuclear fuel, it's going to puff up. And when it does that, it will start dumping material onto the first star, the little white dwarf star. Now, a white dwarf star is a star about the size of our son, but collapsed down into something the size of the earth. And so as material falls onto it, that star becomes heavier and heavier, and its gravity comes stronger and stronger. And when it reaches a critical point, 1.383 times the mass of our son, gravity wins. And the whole thing tries to collapse down to a black hole, but fails, because it's made out of nuclear fuel, carbon and oxygen. And the whole thing blows up as a giant thermonuclear bomb, five billion times brighter than our son. It takes 20 days to reach its maximum brightness and then fades away. Now, this is a light bulb we can use. As part of my thesis before I came to Australia, I worked with a group in Chile that studied these things in detail. Literally on my way to moving to Australia, we talked about their new discovery of how to measure how many watts one of these things produce. And they're not all the same. There are ones that produce lots of iron and are bright and rise and fall slowly, and ones that produce a little bit of iron rise and fall quickly. And they're bright. 43 zeros with the one in front watts. That's how big a light bulbs they are. They're big. So on the way here, I talked to my colleagues in Chile. I have terrible sunburns. And we devised a project where we would use the modern technology, the digital technology we're all kind of used to in our phones and our computers to literally datamined 50 gigabytes of data back in 1995, which I can tell you was a lot of data back then, and find the rare explosions that are these type 1A supernovae. So essentially, looking for a needle of a haystack, we would take 1,000 plus images like this in a night. Each one is 8 megapixels about. And we would find objects look like this. And we would identify it by taking a before and after picture and looking for the changes, the differences. And so you can see nothing has become something here. And that something is a type 1A supernovae who or which was at a distance that light took more than 5 billion years to reach us. So that explosion using the best technology of the day in 1995, that explosion occurred before the Earth was formed. So we were able to do that. And I can tell you it was hard work. It took three years. And in 1998, we put it all and put our measurements onto that diagram, and here's what we got. And I looked at that diagram, and I said, OK, the nearby objects, only time to the dinosaurs are so back in time. They're all sensible, but these distant objects, they're not down here where the universe is slowing down. They're instead at the top where the universe was expanding slower in the past than it sped up, slower spading up. And using statistical analysis, we could say with 99.95% assurity, the universe had sped up. And it turns out 99.95 is not good enough for physicists. We're very picky. Fortunately, we were in mortal combat with a group out of Berkeley who got the same crazy answer scratching their heads saying, god, no one's going to believe us, which is the same thing we were saying. And eventually, we had to come clean, and they had to come clean. And we realized, both of us, that we had the same answer. And it turns out when you add 99.95 to 99.95, then you get 99.995. And that's still not good enough for physicists, but it's close. So people took us seriously, and eventually, more data came along, and that is why ourselves and the team from Berkeley were awarded the Nobel Prize. Obviously, I get a lot of credit for the Nobel Prize, but it really was a group of many, many people. 19 on our side, you will note, 19 males. And I'm sorry, that is the state of astronomy back in 1995. I'm proud to say that astronomy has moved to being about a third female now for the benefit of both men and women. And we're hoping to hit 50% at some point in my life and it's a slow process, but it's something that we in astronomy are particularly concerned about and taking lots of action. So what is pushing on the universe? Why is the universe speeding up? We need gravity not to slow the universe down, we need it to speed up. But it turns out Einstein, who's always on top of his game, came up with the answer. Back in 1917, he realized that if space itself had energy, that energy would cause gravity to push rather than pull. So we call that stuff dark energy and the problem is we need a lot of it. How much? 70% of the universe has to be this dark energy. 70%. The part I'm not gonna tell you tonight is we also have another problem. There's more gravity in the universe pulling than we can see atoms in the universe. 25% of the universe seems to be this stuff called dark matter that Ken Freeman up at Mount Stromo helped discover. Only 5% of the universe is the stuff we know, understand and can touch and see here on earth and through the universe. We have to make up 95% of the universe. Now that should concern you, because it sounds like we don't know what we're doing. But remarkably, this crazy universe, which concerned me a lot in 1998 when we published our paper, has been shown to be correct in every single experiment that we can do. And science is about predicting and we can predict in advance and have predicted in advance every experiment we've been able to do in the last 20 years. And these experiments aren't just one number, they're very complicated figures and curves that we need to reproduce. It is remarkable how well it works. So what's the future of the universe? Well, the future of the universe is a battle. It's a battle of domination, sort of WWF style. I'm sorry, I'm not allowed to say that anymore. That's not the World Wildlife Fund. It's something else now. WWA, I don't know what it is. Dark energy versus dark matter and atoms. Where dark matter and atoms always go together. Gravity that pulls versus gravity that pushes. The problem is dark matter and atoms. As the universe expands, we sit here and the universe gets bigger around us. That means we become less and less important over time. Dark energy, on the other hand, is part of space itself. Space gets bigger, it's like, fine, it'll be more me. And so it becomes stronger and stronger over time. And that tells us that the future of the universe is dark energy. The universe is getting bigger, faster and faster. And indeed, the more space expands, more dark energy can push. And that leads to the amazing thing, eventually, the creation of space between the galaxies happens faster than light can travel to it. When we look at galaxies in the future, the light's gonna travel through space and it's not gonna get to us. Why? Because space is created faster than the light can travel through it. All consistent with relativity, we can talk about it later on. We need to realize that you and me are not expanding. The Earth is not expanding. Why? That is the density of dark energy. There's not much of it. The density of Earth is much, much higher. 30 orders of magnitude higher. We live in a special part of the universe. We live in a place where the attractive gravity that allowed the Milky Way to form, allowed the Earth to form, took over the universe and collapsed us down 13 billion plus years ago and created where we live. It's the space between everything, the average part of the universe that is so thinly spread. Our part of the universe, the Earth, even the Milky Way, gravity is one, space is not expanding. Eventually, it turns out there's a sphere of gravitational influence that includes, for example, the Andromeda Galaxy. And our two galaxies are gonna merge together to form a super galaxy, but when we look out, well, the rest of the universe gets accelerated away. Now, if Lawrence Krauss was here, and he's probably listening just to taunt me later on, he would say, now Brian, you cannot predict the future unless you're God. And those of you who know Lawrence knows how strong he feels about God, so let's not go to that question. So it turns out you have to have infinite knowledge to be able to predict the future. So in practice, anything is possible. That same logic says we can all disappear in a puff of quantum mechanical magic right now, too. So be careful what you think your probabilities are. Dark energy can change in the future. We don't understand it very well. But unless dark energy suddenly disappears, and there's no reason to believe that's gonna happen any time soon, the universe will, in an ever increasing way, expand, fade away, and leave me and my colleagues up in Mount Stromloh unemployed. Thank you very much. I had intended to take questions. However, you are needed outside for a world record breaking attempt two. So this is the one for the whole continent. I think we've already broken one tonight.