The Accelerating Universe
Helen - When Chris is not swanning around South Africa, he's also attending parties with award winning scientists so life isn't that bad. This is how he managed to meet up with Brian Schmidt - one of the winners of the Gruber Cosmology prize. Brian won a share of the five hundred thousand dollar prize for his discovery that the universe is expanding as it ages and the older it gets, the faster it expands.
Brian - My name is Brian Schmidt and I'm an astronomer at the Australian National University. When we started in 1994 we were trying to literally weigh the universe by seeing how gravity would tug on the universe as it expands. We knew the universe was expanding since 1929 when Edwin Hubble saw that galaxies were moving away from us. The further away they were the faster they're moving away from us.
Just like points on a balloon: if you blow a balloon up you'll see every point moves away from every other point and the further away the two points are, the faster they move apart as you blow that balloon up. So our idea was to go through and track distances back in time because when we looked at really fain things in the universe we're actually seeing light that was emitted 4-8, maybe even 11 or 12 billion years in the past. Using the fact that light takes a while to travel through the cosmos we're able to look back in time and see how fast the universe was expanding four billion years ago compared to how fast it's expanding now.
Chris - When you do this, where does the light that you're studying come from?
Brian - In our case we decided to look at the light of something which we call supernovae which are exploding stars. The nice thing about these exploding stars is they're very bright but they're almost all the same brightness. They're like hundred Watt lightbulbs except they have a lot more 000s behind them than a hundred. They've about 45 or 46 zeroes behind them. By looking at these things we can measure their distance and it takes their light four billion years to reach us.
Chris - How far have you been able to wind back your cosmic clock?
Brian - The most distant object discovered to date is about 11 billion years in the past so we think that is about 2.5 billion years after the Big Bang. This star exploded and its light's been taking the next 11 billion years of the universe to get to us. That object exploded five or six billion years before the Earth was formed.
Chris - How does that tell you how the universe is behaving today?
Brian - We measure how fast the universe is expanding by comparing distances with how fast the universe has stretched between the two times. We measure stretch by how the wavelength of light changes colour. As the universe stretches it takes light and makes it gradually redder and redder. We do that in the nearby universe with lots of objects that are tens of millions of light years in the distance.or even hundreds. Then we do these ones at billions and we compare. We see when we do that the universe is expanding slower in the past and has been speeding up over time.
Chris - So if it is getting faster, why should that be? What's driving it to expand faster and faster as time goes on?
Brian - When we started this experiment we were expecting to measure how fast the universe was slowing down to weigh it. When we found that it was speeding up it was a big surprise and you have to invent something new. The best thing that's ever been invented is something that Albert Einstein invented in 1917, which we call the Cosmological Constant. It is energy that is tied to space itself and so as the universe expands more and more space is created. As that space is created it has energy associated with it. It turns out that type of energy pushes on the universe rather than pulling on it. The acceleration is cause by 75% of the universe right now, being made up of this stuff, hard space.
Chris - So if the universe is getting faster and faster - what does that mean in the long term? I mean in the really long term?
Brian - It would seem that as the universe is speeding up it's going to get faster and faster. It really is speeding up so that in 30 and 40 billion years (so not the near future) light that is getting to us from nearby will actually be stranded by the accelerating universe. Photons will no longer be able to get to us. We're going to look out onto a universe that looks empty to us. All the galaxies we see today and we see literally trillions of them will disappear from view. All we'll see are the galaxies that are right around our own and we'll have all merged into some big super galaxy.
Chris - Is this a stable thing, can it just keep getting faster and faster or will it eventually just snap?
Brian - Well, that's the big question. We don't know because we don't really understand this dark energy. One of the big things that people around the world are trying to do is see whether the dark energy is created exactly with space. If it's created just as Einstein says and it's tied to space itself then it goes on forever and ever. The universe just keeps going for eons. On the other hand, if it's a little different than that then it might be that the expansion will turn off in the future or maybe it'll speed up even more. You don't know. It's a real question of what this dark energy is and measuring precisely how the universe behaves now so that we can extrapolate into the future.