Titans of Science: Brian Schmidt
PodcastsThe Naked ScientistsTitans of Science: Brian SchmidtOne of the great minds behind the discovery of accelerated universal expansion and dark energy. . . 06 August 2024In this edition of Titans of Science,. . .
PodcastsThe Naked ScientistsTitans of Science: Brian SchmidtOne of the great minds behind the discovery of accelerated universal expansion and dark energy. . . 06 August 2024In this edition of Titans of Science, the man who co-discovered the accelerating expansion of the Universe and gave us dark energy; the Nobel laureate Brian Schmidt. . . Brian Schmidt: The discovery of universal expansionBrian SchmidtA century ago, the star gazer Edwin Hubble shook the world of astronomy when he presented findings showing that the Universe wasn’t standing still: it was growing; blowing up like a balloon. But he could only look so far in space, because his measurements relied on a class of stars called Cepheid variables, which throb in brightness with a regular rhythm that depends on how large they are. So the rate of their pulsations told him how big they were, which in turn enabled him to work out, based on their brightness, how far off they were. And that enabled him to show that the distances to farther off objects were growing with time. In other words, the Universe was growing. What would happen, though, if scientists turned to far more powerful light sources, that were always the same brightness, but could be seen over far vaster distances? Would they hold true to Hubble’s observations of a gently expanding Universe. This was the question that Brian Schmidt and his colleagues asked. They used the dramatic supernova explosions - cosmic firecrackers as he puts it - that mark the end of a star’s life. These huge blasts are transiently the brightest lights in the Universe and can be seen like beacons, from literally billions of light years away. And what they reveal is that the Universe isn’t just growing, its growth is speeding up, powered by an entity we don’t understand, we can’t measure, and which appears to come out of nowhere, but which must account for the vast majority of what is out there. That entity is Dark Energy. These observations were a seismic revelation in cosmology, and they earned their discoverers the 2011 Nobel Prize. Brian takes up the tale of how he came to be working on this problem in the first place, and where it ultimately led him…Brian - We were a little confused at the time. We had been struggling to measure how fast the universe was expanding, something we knew about since the time of Hubble. The problem we had is we had two groups trying to measure it and they were getting very different answers. And one of the answers favoured by a lot of sensible people seems to be in discord with the age of the universe because if you measure how fast the universe is expanding, you can then run the universe in reverse and find out when everything was on top of everything else. That's the time of the Big Bang. So I came in at a time with a lot of uncertainty and was very keen therefore to measure the expansion rate of the universe with a new method, Type II supernovae. That was a little different than what everyone else was using. Chris - What had Hubble done then? Because you came at this armed with the knowledge and his observations from many, many decades before. What did he show?Brian - So Hubble went out in 1929 and measured how bright objects were and he compared galaxy stars, stars and galaxies. And he wanted to see what the relative distances of galaxies were by comparing how bright their stars were. So the further an object is, the fainter at stars are going to appear. And he also used data from someone named Vesto Melvin Slipher. Most people have never heard of this guy, but he's half the deal. And Slipher had measured how much the light from these galaxies had been stretched by their motions, or at least what they thought were the motions. And what he mysteriously found in 1917 was that every object was stretched red-ward, indicating the doppler shift was saying they were moving away from us. And that was a big surprise. But Hubble realised that in an expanding universe that that's what you get. Everything's moving away from you. And the further something is, the faster it's moving, which is what he saw when he compared his brightnesses of stars and the distances that he'd figured out from them, with Slipher's redshifts or motions of galaxies. Chris - So we had an idea that the universe was getting bigger, things were moving apart. Did we have any idea as to how fast they were doing that at the time?Brian - So at the time there were two numbers and in astronomers units, which are kilometres per second per megaparsec, which I don't expect your listeners to understand. The numbers came as about 50 and maybe 90, which corresponds if you just run the universe in reverse with that number to an age of the universe between 15 billion years or 10 billion years. And we were pretty sure at the time that the oldest stars were at least 13 billion years old. Chris - Did they think then if they saw things, distant objects moving away from them, that space was infinite and these things were moving away from us through space? Or did they actually have a concept that the space between us and those distant objects was getting bigger, making them appear to be moving away? When did that kind of change or view shift?Brian - So we knew since people thought through Einstein's theory of general relativity, which says that gravity curves space, that a heavy universe would be curved onto itself in the shape of a sphere and a light universe would be curved away from itself in the shape of a saddle. And then the 'just right' universe, the one that had just the right amount of stuff in it, would actually be flat and kind of look like what we think of life looking like here on Earth. So it was the big question, 'oh, wouldn't it be great to be able to go and measure the shape of the universe?'Brian Schmidt: How dark energy was discoveredBrian SchmidtIn this edition of Titans of Science, Nobel laureate Brian Schmidt discusses how his work led to the discovery of our universe expanding at an accelerated rate, and how dark energy was theorised to be responsible. . . Chris - Why were there those two different numbers if people were just measuring objects moving away from us, seeing how far the universe appeared to be stretched in the interim, why did that give two values that were quite disparate?Brian - Turns out it's really, really hard to measure distances in space. You can't just put a ruler down. And so we had photographic plates. Photographs are good for looking at things, but they're terrible for measuring things accurately. We had not developed precision methods for measuring distances. We had Cepheid variable stars, which were by far in a way the best thing going back to Hubble, but they only are bright enough to be seen in the very nearest parts of the universe where the universe really isn't expanding because there's too much gravity around us and everything else. So it really took the development of technology to allow us to start measuring the numbers accurately. And so that happened right at the time I did my PhD thesis. I was born at the right time. I had access to the first digital data on stars and I used the brightest stars there are, exploding stars called supernovae, to measure the distances. So Type 1A supernovae are these giant thermonuclear firecrackers and they are very well behaved. It turns out no matter how you blow them up, they kind of do the same thing. They're just firecrackers. It's like a piece of TNT and it doesn't matter if you light it with a fuse or you kick it with a hammer. When it blows up, it's still a big piece of TNT. So they are remarkably uniform compared to most things in the sky. And it turns out that just by measuring how long they take to explode, that tells you to within 5% in measuring distance, how bright they are. Chris - Were you looking for the light or were you looking for other things that come out from supernovae things like the x-rays that they blast out. What was your signal that you were looking for when you were doing this work to spot them and then record the distance?Brian - So this has changed quite dramatically. In 1990, we had amateurs. There was a reverend in Australia who very famously could memorise the nighttime sky and just point his telescope and say 'there's a new star there. ' And he would phone them in and we would look at them and sure enough he was, he was right. Then we got digital cameras, new technology. We went away from photographs, which were a real pain to go and find exploding stars because you had to go develop it and weeks pass. So these digital cameras allowed us to use computers and start scanning all the nearby galaxies. So we were out using optical light looking for new stars that appeared around nearby and progressively more distant galaxies. Chris - And when you say the appearance of new stars, these are stars that appear because they are blowing themselves up and becoming very bright. Brian - That's correct. So the star was there, it just was a billion times fainter. And so when you look at it, you couldn't see anything but just black and then suddenly it's as bright as a billion stars and it really shines at that point. Chris - This gives you a wealth of data so you can go well beyond Hubble. When did it become apparent what the real number was then? Where, between those two extremes that were born out of the inaccuracies of the recording measures that they were using decades ago, what the real number for the expansion of the universe was?Brian - So in 1993 for my thesis, I measured a number that was almost directly in between. The number was 73, which gave an age of the universe of about 14 billion years. It made everyone unhappy. The people who got the high number thought I was wrong, the people who got the low number thought I was wrong. And I would say I was not absolutely convinced I had nailed it, but at least it was a good try. The thing that really broke, I think, the deadlock was the Hubble Space Telescope technology that allowed us to go through and go out an order of magnitude further. That allowed us to calibrate a whole bunch of supernovae using them. And then it became very clear that the number it turns out was around 70. So I got I think a gold star, and that number 70 has been refined over time. Now with the James Webb Space Telescope, the Hubble for many years. Adam Riess, my colleague, has really spent 20 years doing this. And he gets 73 and a half plus or minus a very small number, which in itself is very interesting. Chris - The implications of that number are that not only is the universe getting bigger, but as it gets bigger, it gets bigger, faster. It's blowing up. How did that become apparent?Brian - In 1993, I had done that work to measure how fast the universe is expanding now. That technology that allowed us to start measuring the Hubble constant accurately, these digital cameras, suddenly allowed us to start looking at supernovae, not just millions of years in the past, but billions upon billions of years in the past. So suddenly what became possible using Type 1A supernovae was to measure how fast the universe was expanding nearby, like the last 10, 50 million years. And then compare it to objects that were 10 billion light years in some cases. And so we were able to then see what the universe was doing and was it slowing down a little bit or slowing down a lot or not slowing down at all? That's what we thought, you know, were the possibilities in 1993. Chris - What did you think? Did you think that it was just a continuous rate of growth then, or were you prepared to accept that actually we could have been wrong? It's gone through various phases over its lifetime. What was your thinking?Brian - So the theory at the time was pretty straightforward. The heavier the universe, the more it slows down over time. And so we were going to measure how much the universe slowed down and the more the universe slowed down over the time that we looked at it, the heavier the universe was. And so we're essentially just going to weigh the universe and by weighing the universe we could actually figure out was the universe going to slow down so much that in the future it would stop, go in reverse, and we would have, as I always like to refer via Douglas Adams, the 'Gnab Gib'. The Big bang in reverse. Or was it going to expand forever and not slow down enough? So it just kept on going for eternity. Chris - Did it kind of concern you then that when you began to get the numbers in it didn't seem to fit the 'big bang in reverse' type model? Did you think we're getting something wrong or did you think actually this is really exciting, it's not going to go into reverse?Brian - Our first object that we got, we got really good data on it and it was showing that the universe was expanding slower in the past and sped up. And that bothered me because that was not a reasonable answer, but it was one object and we're like, 'okay, we're going to get more objects, see what happens'. And at the end of 1997, Adam Riess emailed me a figure of the preliminary results on where we were up to that time. And it very clearly showed that this was not an anomaly, but all the objects seemed to be showing that the universe was expanding slower in the past and had sped up. So that was not me yelling eureka immediately, quite the opposite. It was, 'alright, we have screwed up something we need to go through systematically and do everything from scratch with me doing the stuff you did and you doing the stuff I did' and making sure everyone, everything had a separate completely independent channel on it. And so that took a few months as you might imagine, but after that couple months, the numbers didn't move around at all. And so we had to live with it by the beginning of 1998. You know, we weren't going to make this go away.