The Gruber Prize for Cosmology

07 June 2009

Interview with

Robert Kennicutt, Director of the Institute of Astronomy at the University of Cambridge

Ben - Also in the news this week, the winners of the Gruber Prize for Cosmology have been announced.  This is the tenth anniversary of the Cosmology Prize, and this year it's shared between three winners:

Wendy Freedman - director of the Observatories of the Carnegie Institution of Washington in Pasadena, California

ArtistJeremy Mould, professorial fellow at the University of Melbourne School of Physics.

And Robert Kennicutt, Director of the Institute of Astronomy at the University of Cambridge in England.

They're sharing the $500,000 prize for leading the teams that measured the value of the Hubble constant - the rate at which the universe has been expanding since the Big Bang.

We're joined by Professor Robert Kennicutt now. Hi, Robert. Thanks for joining us.

Robert - Thank you. Happy to be here.

Ben - You must be really pleased to see your work recognised by such a prestigious award.

Robert - Indeed. The phone call was quite a surprise but the work was very influential. It was done about ten years ago. I think the organisers of the prize were waiting to see if we'd gotten the right answers or not. In any case, it was delightful to have it recognised in the way it was.

Ben - I can imagine. What is so important about this number, the Hubble constant.

Robert - Yeah. The measurement actually ties into the whole series of experiments that your listeners have probably heard about over the last decade to try to characterise the expansion history of the universe. Our measurement actually provides some very basic numbers: how large is the universe? What is the distance scale? In fact it actually provides a measurement of the age of the universe. Moreover when you combine it with other measurements of the supernova, high red shift, it actually was the work that provided this strong evidence for dark energy in the universe and this expansion. These experiments all tie together and yield consistent results. This was one of the cornerstones of that series of experiments.

Ben - Without this research we wouldn't be asking the questions that we're asking today. It really was a major step forward in understanding out universe.

Robert - That's right. I can give a simple analogy. The measurements that established the existence of dark energy actually tell us how the expansion rate of the universe has changed. It actually slowed down and has sped up over time whereas we're measuring the absolute scale. It's actually a difficult measurement. For example, if you've looked at the television in the last couple of days you've seen the images of Barack Obama standing next to Nicholas Sarkozy and Gordon Brown. You can instantly  when you look at the screen tell which of these people is the tallest. Obama is the tallest and you can even get, when they stand next to each other, get some idea whether he's 20% taller or 10% taller. It's very difficult when looking at the monitor to have some idea how tall these people actually are; whether Obama's six feet tall or six-and-a-half, five foot eight and so on. That's our experiment - really to see by measuring relatively nearby galaxies and providing the yardstick next to these galaxies to measure the exact distance, the quantitative distances and the scale of this whole expansion.

Ben - Will it also help us to understand what the ultimate fate of the universe will be. If we can understand its history then surely we can predict a little bit more about its future.

Robert - Indeed. Prior to this work these distances were only known to a factor of 2 accuracy. The best measurements of the time gave an age of the universe, derived from the expansion, was younger than the ages of the oldest stars we could measure using other techniques. The result of these measurements, combined with the more distant supernova cosmology experiments, actually led to several independent measurements of the cosmic age scale. We've actually determined the problem and remarkably these measurements have come into almost exact concordance and it's almost unsettling, in fact. Scientists, especially astronomers, are not used to precise agreement. The term "precision  cosmology," which is a new term is something we're still getting used to. The result of that is - the implication of that is the universe of the future's actually going to speed up its expansion over time as this dark energy becomes more and more important a force in driving the expansion of the universe.

Ben - So how did your teams actually measure it? It seems like a difficult thing to come up with.

The PleiadesRobert - It actually boils down fairly mundane elements. What we want to do is survey distances to galaxies. We want to measure how far they are away in miles and kilometres precisely. The key instrument that enabled this breakthrough was the Hubble Space Telescope. The key technique we use is a class of variable pulsating stars called cepheid variable stars whose brightness can be measured accurately when you measure their period of oscillations. Hubble actually had the resolution to be able to identify these stars in galaxies at distances of up to 50 or 100,000,000 light years. A light year is six trillion miles. These are vast distances. We essentially laid out twenty benchmarks in distances in the local universe. We were able to measure not only the local expansion but how that expansion actually changes with distance. We live in an over-dense part of the universe that has affected the local expansion. We had to calibrate that out as well. So you can think of this sort of as being surveyors laying out benchmarks and establishing a scale. The result was accurate to about 10% at the time. Since then other techniques using cosmic microwave background have repeated their measurements completely independently and the two sets of results actually agree within a few per cent. We're quite sure we've got it right.

Ben - That's astounding. So finally, the prize money shared between the three of you - will that go into more scientific research or are you planning a well-deserved treat?

Robert - You're not the first person to ask. These important decisions take time, you must understand.

Ben - Of course!

Robert - I'm not going to buy a supercar. I'm not going to give it all to charity but we are going to spend some of it on one major party for the team.

Ben - Wonderful. Just in case you happen to be doing the invites at the moment you can write to The Naked Scientists at Cambridge University. But no, of course, we wouldn't expect an invite...

Robert - I don't know. I'm a fan of the show so who knows!?

Ben - Wonderful. I'll keep an eye on the post. Thank you so much for joining us, Robert. That was Professor Robert Kennicutt, he's the Director of the Institute of Astronomy at Cambridge University and along with Wendy Freedman and Jeremy Mould he's just been awarded the Gruber Foundation's Cosmology Prize for measuring the rate of expansion of the universe.

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