Ten years ago last Friday, Australian astronomer Brian Schmidt, now Vice-Chancellor of the Australian National University, and two others, Adam Riess, and Saul Perlmutter won the Noble Prize in Physics for the discovery of the accelerated expansion of the Universe using distant supernova.
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The two competing teams, the High-z Supernova team and the Supernova Cosmology Project, used a specific type of supernova, type Ia, as a tool to measure how much the Universe was growing.
We have known since the 1920s that the universe has been expanding. Henriette Leavitt showed that a certain type of variable star, called Cepheids, had a relationship between how bright the stars get and how long it takes.
Edwin Hubble was then able to use them as an independent way to measure that the universe was expanding. Since Hubble could use the brightness and the time (or period), they could then figure out the true brightness. Once they knew how bright it should be, they compared it to how bright it appeared to be, and figured out how far away they were.
It was this measurement that showed the universe was growing. The idea was that since the Big Bang, all the galaxies and stars of the universe, the stuff, was slowing down the expansion. Over time, it would slow down the universe all together and have it stop expanding all together, and then slowly pull it all back, in a reverse Big Bang - the Big Crunch or Gnab Gib as Brian likes to say.
In the '90s, this is what the teams went out to measure - how slow the universe was growing, and then to work out when it would stop growing all together. They could then work backwards to see when the universe would end.
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Just like with Cepheids, they used even brighter objects, that could be seen further, a specific type of exploding star or supernova. The faster the supernova faded from maximum brightness, the fainter its peak brightness was. They were able to measure how bright it appeared, how bright it should be, and then therefore how far away the supernova or galaxy was.
After measuring dozes of supernova, they found that the more distance supernova were actually fainter than expected, implying they were further away than thought. The further they looked, the faster the universe was expanding, now slowing down. It was accelerating. Lots of work is now going into to figure out what is causing the universe to accelerate. Called dark energy, it is like gravity in reverse, making up about 70 per cent of the entire universe.
In the past few years, though, another puzzle has emerged. When we measure the speeds of the universe at different times, how fast it is growing a point in space, our measurements don't agree.
This past week, 10 years on, a team lead by Adam Riess (who won the Noble Prize with Brian Schmidt), showed that the differences in speeds was not likely a measurement error and now significant enough to pay attention to. The universe is doing something funnier. What is causing it or why, we do not know.
The more we study the universe and how it behaves, the crazier it gets.
- Brad Tucker is an astrophysicist and cosmologist at Mount Stromlo Observatory, and the National Centre for the Public Awareness of Science at ANU.
