Detecting Gravitational Waves

This week a team of astronomers announced that the BICEP2 radio telescope in Antarctica had detected evidence of primordial gravitational waves, residual ripples in the fabric of space from the big bang.

These findings are very exciting because they may reveal what happened during a period called "inflation" shortly after the birth of the Universe.

To find out more Chris Smith spoke to one of the BICEP2 team Clement Pryke from the University of Minnesota. 

Chris - So, kick off first of all and tell us what are gravitational waves.

Clem - Well, gravitational waves are distortions in the fabric of space-time which propagate through space with the speed of light. They can come in various different flavours. As you said, the kind that we are announcing a discovery of this week, are primordial gravitational waves which come from the birth of the universe. There are also gravitational waves potentially produced by compact objects in the nearby universe, but that's a different story.

Chris - So, why has it taken so long to spot these waves? What's special about them that makes them difficult to detect?

Clem - Well, the gravitational waves that we're seeing - what we're doing is we're seeing the imprint of them on the pattern of the cosmic microwave background as it was kind of written at 400,000 years after the beginning. So, in the first 400,000 years, the universe was a hot dense plasma. As it expanded and cooled, eventually, it decoupled and we see the cosmic microwave background coming to us from that time. What we're seeing is a subtle imprint, an additional imprint on that pattern caused by the gravitational waves from that super-early time kind of written in that pattern of 400,000 years. And the only plausible source of those gravitational waves that we're seeing, they fit exactly the predictions for gravitational waves coming from the very, very early universe. So, trillion, trillion, trillionth of a second after the beginning.

Chris - So, to sort of put this into a nutshell, you have a point source which explodes in this catastrophically powerful explosion - the Big Bang. This is impenetrable to our current ways of looking with standard telescopes. And so at the moment, it's something of a black box. We don't know what's gone on in there. It does however produce energy and light which the echoes of which are still knocking around in the universe today and we call that the cosmic microwave background. And you're able to then read out of that signal that's still around today. These waves which you can then use to infer what must have happened in that first 400,000 years or so.

Clem - Right. So, studying the pattern that comes from 400,000 years is a well developed science at this point. So, we've been studying that for several years. The temperature and isotropy, the temperature ripples as it were, just the differences in brightness from place to place on the sky and then measuring the polarization pattern. And to start with, we were measuring the kind of generic version of polarisation which occurs just because you have small deviations in density. That's called E mode polarisation.  So, the big deal this week is the first detection of B mode polarisation. The B mode is the kind of swirliness of the polarisation pattern. If you think of a bunch of little vectors on the sky and then if you decompose the pattern, if you can see a kind of swirl in it, that's the signature of the gravitational waves. As I said, those gravitational waves come from the very, very beginning. It's a way of seeing back further. As you said, we can't see back using light any further that 400,000 years, but we can use the gravitational waves to get information about earlier times still.

Chris - What can they tell you and what are they telling you about what happened during that first 400,000 years or so?

Clem - What this week's discovery is all about is the first trillion, trillion, trillionth of a second, right? And in that period of time, there's this theory, this hypothesis that the universe hyperinflated, expanded essentially superluminally, for a tiny fraction of a second by an enormous factor. That process of inflation sets up the initial conditions for the subsequent hot Big Bang. So, this is essentially kind of a preface before the hot Big Bang. And there's been strong suspicion that this inflation actually did occur based on various observations about those initial conditions. But there was an additional prediction from that theory that the inflation process should have injected gravitational waves into the fabric of space time. But they'd never been seen. And so, that's the big deal this week, that we actually see a new prediction from inflation. So, this is kind of a smoking gun that inflation is actually the correct theory for the birth of the universe.

Chris - I mean, having a look online and Peter Coles is Professor of Theoretical Astrophysics at Sussex University. He's got a blog and he says that he is slightly sceptical of what you're finding because he's saying, if you look at your results, you've only found this in one particular frequency regime of microwave, the polarisation, the twist. Why haven't you found this in others or have you just not looked in other frequencies?

Clem - So, we're very convinced the signal that we've found is true and on the sky, but then the question is, what is it? Is it really the primordial signal that we think we're seeing or could it potentially be foreground emission from our own galaxy. So, while our galaxy emits rather weakly at these wavelengths, it still does a little bit and we're down at a fantastic level of sensitivity now. So, the concern is that since we've only really achieved really high sensitivity at one frequency at 150 gigahertz, potentially, we don't have a guard that this signal isn't galactic foreground because the foreground would have a different frequency spectrum than the thermal emission from the cosmic microwave background. If you have multiple frequencies, you can discriminate different kinds of signal that you can rule out foregrounds. Now, we have ruled out foregrounds to reasonably good confidence, but he's right that it's not as high as one would like.

Chris - And the European Space Agency have spent an absolute arm and a leg sending the Planck Mission into space to make these sorts of measurements. They haven't found this yet or at least they haven't said so. So, why did your telescope in Antarctica find it, apart from the fact that you're obviously very clever and they didn't?

Clem - So we stuck our necks out further. So, we're a small ground based experiment and highly targeted. So, we designed the experiment specifically to look for these primordial gravitational waves. And had we not found them then we would've basically been empty handed. Whereas the Planck Mission which is vastly more expensive as you say is much more general. So, it can do all sorts of science with the standard kind of CMB anisotropies and also, with galactic science, and a lot of things. It's a very general experiment. And so basically, the reason that we're first is just because we decided to target this very specific niche science and go after it very aggressively.