Science Interviews


Fri, 26th Feb 2016

Didn't BICEP detect gravitational waves?

Dr David Marsh, University of Cambridge and Dr Jon Kaufmann, University of California, San Diego

Listen Now    Download as mp3 from the show Gravitational Waves: Discovery of the Decade?

If you cast your mind back to early 2014, you might recall news that a group BICEP2 Telescope of American astronomers, working with a telescope called BICEP, were studying the sky to find remnants of gravitational waves from the big bang… To much fanfare, they published their findings reporting the discovery of these ripples. Unfortunately, subsequent tests showed that what they were seeing was actually down to space dust. But their search continues though, because they’re trying to find and use these waves to probe what happened during the earliest moments in the history of the Universe, following the Big Bang; this is a period known as “inflation” but it’s further back in time than traditional telescopes can see, which is why we need gravity waves to probe it. Graihagh Jackson...

Graihagh - If you’re anything like me and love a bit of boogying you might be dancing around or jiggling around in your seat.  And you know what, if you are, you're making gravitational waves. Pretty cool, huh?

David - Yes it is - pretty cool.  Undetectably small but, nevertheless, you’re creating gravitational waves.

Graihagh - Yes, David Marsh is right; they’re so small we can’t even detect them.  It’s only the ripples from massive events that we can pick up on, like two black holes colliding.  But what about the biggest event in the history of the universe - the big bang.

Jon - Oh yes, absolutely so the inflation theory - the sort of bang behind the big bang.  This would send gravitational waves rippling throughout the universe.

Graihagh - But how would Jon Kaufman look 13.8 billion years ago into the past?  Well, with something called the CMB - the cosmic microwave background.  This is a faint glow of light that fills the universe with nearly uniform intensity; it’s the residual heat of creation, the afterglow of the big bang, streaming through space these last 13.8 billion years.  Much like the heat from a sun warmed rock re-radiating the heat at night. Notice how I say though nearly uniform intensity.  David Marsh again…

David - When we look at the sky at microwave wavelengths, we would see cosmic microwave background and we would see it almost having the same temperature everywhere. So we can compute the temperature - it’s 2.7 kelvin, -271 degrees.

Graihagh - Pretty cold then?

David - Cold, yes.  And then you can compute it - first, second, third, forth, and then at the fifth digit you see that it starts to vary, so at that level of temperature, it starts to fluctuate.

Graihagh - Temperature fluctuations only after the fifth decimal place - that’s absolutely bonkers.  But these tiny changes in residual heat are critical to the study of the universe because it’s the fossil imprint of those bang big light particles that tell us about another fluctuation; the ripple that gravitational waves made in the fabric of space time.  But members of the BICEP team like Jon Kaufman have to go to great lengths to detect them - like the South Pole…

What you can hear is a video of Jon and colleagues heading out on a skidoo to the telescope.  They’re wearing so much clothing that not a bit of flesh is visible, despite it being a beautiful, blue-skied and sunny day.

Jon - It can be gruelling but what our telescope does is we look for the temperature of the cosmic microwave background, the E mode polarisation, and the B mode polarisation.

Graihagh - And so this polarisation you’re looking for - what’s that?

Jon - How light is an electromagnetic radiation.  It means that there's an oscillating electromagnetic field and if it has a preferred direction that it likes to oscillate.  If you imagine you hold a string and you wip your hand up and down, you are creating a wave on the string that’s polarised in one direction - the up and down direction.  If you wip your hand left and right, you’ll create a polarised wave left and right.  It just means that there’s a preferential axis to the wave that you’ve generated.

Graihagh - And that’s what you detected?

Jon - Right. Only gravitational waves can crate B mode polarisation at this cosmic microwave background time.  And it became clear that we were seeing something and none of us believed it.  We all expected it was some sort of systematic contamination,  and so we set about, for a long time, just trying to convince ourselves that what we were seeing wasn’t real.  All sorts of crazy ideas of what this could be, including one of my favourites, which is the communication satellite might have been interfering with our telescope, so we ruled that out.  Finally everything was ruled out; it looked like this must be on the sky.  And then off course, we published and articles starting popping up - New York Times and NPR and everything.  All sorts of news sources and then things got very interesting.

Graihagh - And do you know why it got interesting - because of seemingly something very uninteresting - dust.  Yep, you heard me right - dust.  Things like comets, supernovae, all leave dust in their wake and this dust can imitate the light polarisation sign that Jon is looking for.  But when another satellite called PLANCK came along, it suggested that it's not really quite that simple.  Jon and the BICEP team continue to look for this polarisation and hints of gravitational waves down at the south pole.  But now the LIGO team have found them, should Jon just give up the ghost… Now that it’s not longer a ghost…

Jon - So these gravitational waves, the ones that experiments like BICEP are looking for are pretty significant because they would be evidence for the theory of inflation.  And the theory of inflation is one of the most important theories, I think, humankind has ever developed, in the sense that it is the sort of “why are we here,” and inflation answers that.  And so while there's a significant amount of evidence for it, the smoking gun of this has not been detected and that is these gravitational waves.



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