- How the accelerating expansion of the Universe was discovered
- What it’s like to make a discovery that changes the course of modern physics
- How such a discovery can affect a scientific career
Also see: A Nobel Achievement (part I)
At the beginning of the 1990s, the biggest question in astronomy was probably: what is the future of the Universe? Is it going to collapse? Or will it expand forever? Nobody knew.
At the time many astronomers were looking at type Ia supernovae, which are the extremely bright explosions that occur when two stars in a binary system merge. This type of supernova always produces a similar amount of light, so we know how far away they are by how bright they look from Earth. And because they are so bright, we can often see supernovae even when they are really distant. This all means that we can use type Ia supernovae to find out about the past and future of the Universe; by comparing the apparent distance predicted by their brightness to their actual distance, it is possible to determine whether the expansion of the Universe has decelerated since the explosion occurred.
I was working as part of a team that was trying to do just that. In 1995 and 1996, team member Adam Riess collected our observations on the brightness of ten distant supernovae, and he and team leader Brian Schmidt compared their distances and their brightnesses.
They came up with the result that the expansion of the Universe was not decelerating. This was very surprising as we had expected that all the matter in the Universe is pulled together by gravity, leading to a decelerating expansion. But then in December 1997, Adam said to us: “Those distant supernovae are too far away. It’s like something has pushed them away from us. Could it be that the expansion of the Universe is actually accelerating?”
This sparked a heated discussion — via email as the team was distributed all over the world! Brian was in Australia, Adam was on the west coast of the US, we had people on the east coast, we had people in Hawaii. And I was in Germany working with the data from the ESO telescopes. We would send an email in the evening and get up in the morning to find out about a number of other issues. But in the end, Adam and Brian could prove that there was no obvious mistake in the analysis.
So, we decided we would have to submit a paper presenting our results, and we were sure that someone else would tell us what was wrong. But this didn’t happen. There were some people who didn’t believe us, but they were in the minority, and they couldn’t prove we were wrong.
I’m not sure we were fully aware at the time what a big deal this discovery was. The fact that the expansion of the Universe is accelerating means that there must be some invisible “thing” in the Universe driving the expansion, causing the objects in it to flow apart faster than we would expect even for a universe without matter. The calculations tell us that this “thing” must be about three quarters of the energy content of the Universe. In a way, this was like discovering three quarters of the Universe that people had no idea existed.
We were helped by another discovery made around the same time, related to the Cosmic Microwave Background (CMB). People were using the CMB to study the geometry of the Universe. The tiny temperature fluctuations in the CMB indicated that the geometry of spacetime is flat, which requires a specific amount of matter and energy. Einstein’s famous equation E=mc2 tells us that mass (matter) and energy are equivalent. But determinations of the amount of matter and energy known to exist in the Universe made up just 25% of the amount required by a flat Universe. In other words, 75% of the matter and energy was missing.
This value matched perfectly with our discovery of the extra energy component that makes up three quarters of the matter/energy content of the Universe. It was the combination of the two discoveries at almost the same time that convinced most people.
This new component is now called dark energy. But more than twenty years later, we still have no idea what dark energy actually is! There is no physical explanation for it, but astronomers all over the world are working to find one.
This discovery certainly affected my career. All of a sudden, I became one of the best-known observational cosmologists in Europe, which came with its pros and cons.
As one of only two Europeans on the High-z Supernova Search Team, I got invited to many, many conferences here in Europe to present the result, and was asked to write major review papers on it. This took a lot of time out of my research.
And then in 2011, Adam Riess and Brian Schmidt won the Nobel Prize in Physics for this research — they each won a quarter, and Saul Perlmutter of the Supernova Cosmology Project won the other half. We all went along for the Nobel Prize celebrations, which was an amazing experience.
But in the long run, I decided that I didn’t want to be part of large collaborations any more. The High-z Supernova Search Team wasn’t that big — around 25 people — but there were still so many teleconferences and meetings. I just felt tired of all that. I wanted to do things that I could be recognised for directly, rather than being a member of a team. I wanted to create something that people could recognise as coming from me.
I’m still doing cosmology, though not the same type any more. That kind of research now requires large teams of hundreds of people. I’ve started to pick smaller problems again — things that I can do with students, to solve some of the smaller questions that we have about supernovae. It’s interesting to come from a big stage, from a place where the whole world pays attention to you, to go back to smaller problems that are not necessarily seen by everybody, and maybe not even seen as interesting by a lot of people. But that’s OK.
Numbers in this article
|25||Percentage of the Universe that could be accounted for in 1990|
|75||Percentage of the Universe that was found to be “missing”|
|1990||Year that the future of the Universe became one of the biggest questions in astronomy|
|2011||Year that the Nobel Prize in Physics was awarded for the discovery of the accelerating expansion of the Universe|
Biography Bruno Leibundgut
After a Physics degree and a PhD in Astronomy, Swiss astronomer Bruno Leibundgut found himself in the United States for two postdoctoral positions. Returning to Europe in 1993, Bruno started working at ESO in a group that defined how the VLT would be operated. After a couple of years he became Deputy VLT Programme Scientist, then in 1999 moved on to building up the data quality control group, connected to the archive. Bruno was Head of Office for Science for eight years, then Director for Science for six years, before closing the circle by becoming VLT Programme Scientist four years ago.