Brian Schmidt: The discovery of universal expansion

A 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?'