I had considered writing about BICEP2 a little while ago, but wasn’t sure how much of what I knew was in the public domain; so thought I would wait a little while. It all seems to be out in the open now, so I thought I might write a brief post. To be clear, though, I’m not a cosmologist, so may not get this quite right. Also, I’m going to write this at a level that I think will be understandable. However, anyone who thinks something needs correcting, feel free to do so through the comments.
The background to this issue is that there are a number of cosmological conundrums. For example, the universe appears homogeneous. It looks the same in all directions. However, different parts of the sky are not causally connected, so how can they have the same basic properties? Similarly, the universe appears to be flat, but that’s a tricky concept to explain, so I won’t (to be clear, it’s not because I don’t think my readers could understand this; it’s because I don’t think I can explain it properly 🙂 ). A solution to these conundrums is that the universe – when very young – underwent a very short period of very rapid expansion : called inflation. Prior to this, everything was connected. Afterwards, different regions were no longer causally connected, but now had the same basic properties.
A signature of this inflation is primordial gravitational waves (for those interested in climate science, these are not the same as gravity waves) that would be imprinted on the cosmic microwave background (CMB) radiation – the remnant radiation from the Big Bang. To detect these gravitational waves requires detecting a polarization signal in the CMB. This is what the BICEP2 team announced that they had done a few months ago. To determine this, however, they had to remove the influence of gravitation lensing (as the CMB radiation travels through the universe, mass can introduce polarization) and they have to remove the polarization from foreground – i.e., from dust in our own galaxy. They did this and presented a result that suggested that they had detected (at very high significance) polarization in the CMB and, hence, the first evidence for primordial gravitational waves (indicative of an early period of inflation).
This result was hailed by everyone I knew who worked in this area – including those who were associated with competing teams. It was seen as a remarkable discovery; possibly the discovery of the century (apart from maybe the Higgs Boson). Noone I knew wanted this to be wrong. I even asked a seminar speaker whether or not they thought the foreground contamination could be an issue, and their response was that the signal was so strong that all it could do was reduce the significance a little.
Well, it now seems that that was wrong. It appears that the BICEP2 team may have done their foreground correction using a figure from a conference presentation given by someone on a competing team. They didn’t – it seems – realise that the data in the figure had been smoothed so as to reduce the apparent foreground in the relevant region. It now seems that there is a good chance that their signal is simply polarization from material in our own galaxy, and not a signature of primordial gravitational waves. I don’t think this is yet certain, but it is a little disappointing if true.
There are, however, a number of interesting aspects to this issue. Firstly, noone I know of is – publicly at least – suggesting that the BICEP team did anything nefarious. They were maybe a little sloppy. Took a bit of a risk; maybe they should have waited to be more certain. However, everyone recognises that this was an extremely exciting discovery. Career defining. Furthermore, it’s a great illustration – in my view – of how science works. Despite everyone being extremely excited by this announcement, there were still people delving into the details and checking what was being presented. Within a matter of weeks, there were already hints that there might be a problem. I don’t think that the BICEP team have yet conceded that the measurement is purely foreground, but I’m sure they will if that is what the additional analysis suggests. It’s still possible, of course, that they will still have a small signal of primordial gravitational waves. So, it could still end up being an exciting discovery; just not quite as significant/robust as – at first – thought.
In a sense, this is why I get frustrated by suggestions that scientists (climate scientists in particular) are involved in some kind of conspiracy or suffering from groupthink. It’s very difficult for incorrect results to persist. Anything interesting is going to be checked by clever people who will almost certainly find a problem, if there is something to be found. I appreciate that the BICEP2 issue is only one example and we don’t yet know the final outcome, but I think it is still an illustration of the scientific process in action. There may be negative aspects to this whole issue but – from a scientific perspective – the BICEP2 announcement has done no damage. Even if we don’t actually have evidence for inflation, we’ve learned something and, presumably, understand aspects of this topic more now than we did before.