In New Eyes on the Universe I mentioned a few of the experiments that were attempting to find B-mode polarisation in the cosmic microwave background (CMB). I thought that Planck might be able to tease out the B-mode pattern, or perhaps POLARBEAR. But I thought the signal would be so difficult to discern that I didn’t mention all the different experiments. I didn’t mention Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2), for example. But today – 17 March 2014 – BICEP2 announced the discovery of the B-mode polarisation pattern in the CMP. And the signal appears to be clear. If the discovery holds up – if POLARBEAR, Planck or any of the other experiments confirm the finding – then it’s profound. The initial CMB studies enabled us to probe the universe when it was 380,000 years old; with B-mode investigations we have a probe that takes us back to when the universe was a trillionth of a trillionth of a trillionth of a second old.
What is BICEP2?
BICEP2 is a telescope based at the South Pole. Antarctica might seem a strange place to put a telescope, but if you’re going to study the CMB then it’s probably the best place on Earth. The South Pole has one of the driest environments on the planet, so the absorption of microwaves by atmospheric water vapour is minimised. (You can’t observe the CMB from England, say: it’s simply too wet.) Its stable weather patterns and the altitude – about 3000 meters – also help. And average winter temperatures of -58°C are useful: BICEP2 uses bolometers – sensitive devices for measuring EM radiation through the heating of a material – that are best operated and maintained in cold conditions.
Putting a telescope into space, which was the approach taken by NASA and ESA with the COBE, WMAP and Planck missions, is even better than siting it at the South Pole. But space missions have their drawbacks, of course, not least of which is cost. A microwave observatory at the South Pole is as close as one can get to the conditions in space without actually launching a satellite.
The original BICEP telescope observed from 2006-2008. The upgraded BICEP2, deployed in 2009, used the same principles, but had improved optics and detectors. BICEP2 was very much more sensitive than BICEP1.
What is B-mode polarisation?
Light from the sky is polarised: you can check this by wearing polarised sunglasses. The polarisation arises from the way that atoms in the atmosphere scatters light towards us. As with visible light, microwaves can be polarised. And radiation that was scattered towards us from atoms that existed when the universe was young can cause the CMB to be polarised. This so-called E-mode polarisation of the CMB was detected as long ago as 2002.
In addition to E-mode polarisation there is a polarisation mode called the B-mode. The two modes look very different. The E-mode pattern is symmetric – look at it in a mirror nothing changes. The B-mode pattern has a swirling aspect, a handedness – look at it in a mirror and it appears to change. B-mode polarisation arises not from scattering but from the passage of a photon through a gravitational wave: at any point the wave squeezes and stretches space in one direction, then stretches and squeezes it.
Why should be expect the CMB to contain a B-mode polarisation pattern?
Inflation is the idea that the cosmos expanded at an exponential rate for a fleeting instant when the universe was a trillionth of a trillionth of a trillionth of a second old. Under inflation, the universe is thought to have expanded from a quantum size to something the size of a melon. And that process would have generated gravitational waves. Those primordial waves would still be propagating across the universe, but would be too feeble for us to detect directly. However, those primordial waves would have polarised the CMB and left a B-mode pattern. The pattern would be exceedingly difficult to detect, but it should be possible. And that is precisely what BICEP2 claims to have done.
It’s possible that some form of “contamination” might generate the patterns detected by BICEP2 – perhaps it was dust in our galaxy, or flaws in the telescope, or gravitational lensing from distant galaxies. But the BICEP2 team have taken huge care to rule out those effects. It appears to be a solid result. By far the simplest explanation for the observations is that BICEP2 have seen microwave polarisation caused by primordial gravitational waves.
Why is this discovery so important?
The discovery of the B-mode is of huge importance for a variety of reasons.
- It’s a clear, if indirect, detection of gravitational waves.
- It’s clear evidence for inflation.
- It’s a clear indication that gravity and quantum physics must somehow “hang together”: when we see the B-mode we are seeing the effects of quantum gravity
Now that astronomers know where to look, a plethora of experiments are going to be investigating the B-mode – and helping us to understand how the universe came into being.