Dr David Marsh, University of Cambridge and Dr Jon Kaufman, University of California, San Diego
Listen Now Download as mp3 from the show The Next Revolution in Astronomy: Gravitational Waves
In 2013, a group of American astronomers who work with a telescope called BICEP annouced they had found gravitational waves. How are these different to what LIGO have found? To spot the difference, David Marsh and Jon Kaufman take Graihagh Jackson back to the Big Bang by looking through the Naked Scientists archive...
Chris - In March this year, the U.S. BICEP team of astronomers claimed to have found the long sought evidence for cosmic inflation - one of the mechanisms that underpins what happened in a fraction of a second after the big bang, when the universe began. Inflations been theorised for decades and the results caused quite a big stir in the scientific community with talk of Nobel Prizes being awarded to the astronomers concernedÖ
Graihagh - The evidence Chris was referring to was an indirect detection of gravitational waves and it was found by a team of american astronomers using a telescope called BICEP. But these waves are different to the waves that have been announced this week and actually, the BICEP evidence has since been called into question by another satellite called Planck. So what the heck is going on here? How can these gravitational waves have the same name but could be different? And why didnít scientists just give them another name.
To spot the difference, we have to return to the big bang (I did promise some big bang, didnít I?) and and a theory called inflation - this explains how the universe went from being something teny, tiny tiny, to the massive expanse that our universe is todayÖ
David - So okay. My name is David Marsh and my title is Stephen Hawking Advanced Fellow/Senior Research Associate at the Centre for Theoretical Cosmology at the Department of Applied Mathematics & Theoretical Physics at the University of Cambridge.
Graihagh - I feel like Iíve got your postal address, yes? Thatís a big long title...
David - Thatís a big long title, yes.
Graihagh - Does that mean that your work with Stephen Hawking then?
David - I met him but I donít work with him at the moment, no.
Graihagh - Oh well. There you go then.
David - What inflation is, is an expansion of the universe but it expands at an accelerated pace. So each block of space at any given moment of time creates another block of the same size and then those two blocks each creates one block, and then you have four blocks, and then these four create another four, and then these eight create another eight, and so forth.
Graihagh - Your maths is much better than mineÖ
David - So itís an exponential expansion of space. Inflation is driven by some energy density and quantum fluctuation in this energy density, on a very small scale, gets stretched during inflation because space expands and everything gets stretched with it. So in the end, these quantum fluctuations can become as big as the universe.
Graihagh - A quantum fluctuation is a change in the amount of energy in a point in space for a really period of time. Why itís important is because these tiny changes in energy were the seed of matter and structure in our universe. In other words - these fluctuations created galaxies, stars, planets and, as it happens, gravitational waves. These tiny fluctuations have now been stretched with inflation and are absolutely huge, and pretty much undetectable, as David was just saying...
David - So in the end, these quantum fluctuations can become as big as the universe, or even bigger.
Graihagh - That a bit mindboggling.
David - It is quite, yes.
Graihagh - How do we know that theyíre there if theyíre on that sort of scale?
David - Yes, so thatís the thing. If we wouldnít have any observational evidence for it most of us would have just dismissed this as a theory. But now the funny thing is about this (04.12) is that in the theory of inflation, these stretched quantum fluctuations is what is responsible to the small temperature fluctuations that we see in different directions of the sky. So when we look at the sky at microwave wavelengths, we would see cosmic microwave background and it would seem almost completely smooth. Almost having the same temperature everywhere and so we can compute the temperatures to 2.7 kelvin so, -271 degrees.
Graihagh - Pretty cold then?
David - Cold, yes. And then you can compute it - first, second, third, forth, and at the fifth digit it starts to vary if you look in different directions of space. So at that level of accuracy or at that level of the temperature, it starts to fluctuate in different directions.
Graihagh - And you know whoís looking up at the sky to the cosmic microwave background for days on end trying to detect some evidence for inflation with gravitational waves? Meet Jon Kaufman, from the University of California, San Diego. The name of the telescope he uses is called BICEP, which makes Jon, BICEP man in my eyesÖ.
Jon - You know the inflation theory - the sort of bang behind the big bang - our best understanding for how the universe came to be. And so this would create some gravitational waves rippling throughout the universe. We donít look today using something like LIGO, we look about 13.8 billion years ago when these waves would have been much stronger at the surface of last scattering. This cosmic microwave background which is the transition between the very, very hot, dense, early universe and the universe thatís a lot more recognisable to what we see when we look out with our telescopes now.
Graihagh - In other words, these gravitational waves would have left an imprint on the CMB - hotter or colder patches - and this is what Jon is hunting for with the telescope BICEP, however, itís located in the rather inconvenient place, the south poleÖ
What you can hear is a video of Jon and colleagues heading out on a skidoo to the telescope. Theyíre 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 I like it down there. Itís so unique; itís really alien; you really feel like youíre on a different planet; thereís nothing recognisable. When youíre at the south pole; thereís no features; thereís no plants; thereís are animals; mean thereís not insects. Imagine a world where thereís no flies or spiders around.
Graihagh - What I want to know is why wereínt there any penguins?
Jon - Yes. Nothing can survive at the south pole.
Graihagh - So when youíre looking at the cosmic microwave background (the CMB), what are you looking for?
Jon - Well what our telescope does is we look for correlations on our patch of sky and we measure the temperature of the cosmic microwave background, the E mode polarisation, and the B mode polarisation. So this is a very different kind of telescope than what you would imagine when you think of a big telescope. Itís actually relatively small, you know, itís only a couple of metres high and about 30cms across the focal plane where all our detectors live, and itís not a big dish, itís a refractor kind of like what Galileo used to stare up at the sky. Just a couple of lenses and instead of an eyepiece itís, you know, these superconducting detectors, this advanced technology that we use, for example, for BICEP2. And so with BICEP we didnít detect anything, BICEP2 was already in the works, which was going to add about a factor of 10 in terms of sensitivity. So one year of BICEP2 measurement would be equal to 10 years of BICEP1 measurement.
Graihagh - And so this polarisation youíre looking for - whatís that?
Jon - How light is an electromagnetic radiation. It means that thereís an oscillating electromagnetic field and if it has a preferred direction that it likes to oscillate. If you imagine if you hold and string and you wip your hand up and down, you are creating a wave on the string thatís polarised in one direction - the up and down direction. If you wip your hand left and right, youíll create a polarised wave left and right. It just means that thereís a preferential axis to the wave that youíve generated.
Graihagh - Okay. And thatís what you detected?
Jon - Right. Only gravitational waves can create B mode polarisation at this time, at this cosmic microwave background time. And it became clear relatively early on 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, to just try to convince ourselves that what we were seeing wasnít real. And all sorts of crazy ideas of what this could be, including one of my favourites, which is the communication satellite that we use for internet at south pole might have been interfering with our telescope. So we ruled that out - we ruled everything we could think of out. You know there were no bad ideas and when finally everything was ruled out it looked like this must be on the sky and so, of course, that was very exciting. None of us - you know the excitement took a long time to build up because we were so convinced it was wrong, that there was nothing there. And so even as we were writing the paper it was just sort of - you know well, all right, we see this thing, it looks like itís real, letís just move on. You know, weíll keep going - weíll keep doing science. And then of course we published and we made the announcement. It was very exciting and it was starting to hit me that this is something that people might be interested in. You know, people, not just us. Articles starting popping up - New York Times and NPR, everything. All sorts of news sources from reputable to crazy ones. Weíre all starting to talk about it and there was so much interaction going on that I was just too excited. I just wanted to like shout it from the mountaintop - and then things got very interesting.
Graihagh - Things did indeed get interesting. Why? Because of dustÖyep, you heard me. Dust by supernovae, or comets and so on all create dust and this dust can create the same polarisation in the CMB as gravitational wavesÖ
Jon - When BICEP was being conceived, the idea was you look at the most empty patch of the sky, and the question, of course, is what to do you mean by empty. And what we looked at were maps of dust and you know, you say hereís a bit hole thatís visible 24/7 from the south pole - lets do that. And what was done up until our measurement was you looked at the intensity of the dust, you expected some amount of polarisation and you calculated the level at which this would contaminate the signal and this was way, way below anything that we had seen. So all these were models but it all looked like this dust level would be tiny. Well what happened afterwards is that the measurements from the PLANCK satellite came in and they showed that, contrary to what we we thinking, the less intensity of dust you have, actually, the higher the polarisation fraction of the dust, which means itís worse. Now it doesnít mean if you look through the centre of the galaxy would be better because thereís a lot of dust, but it does mean that out at the edges that weíre looking at, the dust polarisation fraction is a lot higher than anyone expected. With our result, it really injected a lot of excitement into the field from external sources and then with the PLANCK controversy, of course, everyone likes a controversy, everyone likes a battle.
Graihagh - They sure do! But gravitational waves from recent events, like black holes, have been detected. This is one of the final bits of evidence for Einsteinís theory of relativity so should Jon just pack his bags and be done? Nope, apparently not.
Jon - So these gravitational waves, the one that had experiments like BICEP and many others, are looking forÖ are pretty significant because they would be extreme evidence for the theory of inflation. And the theory of inflation is really - I mean thereís a lot of evidence for it already but it is one of the most important - again Iím a little biased - but one of the most important theories that I think humankind has ever developed in the sense that it is sort of why are we here. You know, why does the universe look like it does? Why is it susceptible to stars, and galaxies, and habitable worlds, and life? How did the universe get here and inflation answers that. And so while there is a significant amount of evidence for it, the smoking gun of this has not been detected and that is these gravitational wave.