[Update: Over the months following the announcement, doubt was cast over exactly what BICEP2 saw, and now it seems that the signal announced by BICEP2 is consistent with polarisation produced by galactic dust. See here.]
I’m actually in hiding and silence for a week. It is Spring Break and I have locked myself away in a seaside town to do some writing, as I did last year. But I must break my silence for a little while. Why? Well there’s been a really great announcement in physics today and while being very happy that it is getting a lot of press attention – and it should since the result is very important and exciting – I’ve been stunned by how confusingly it has been reported in several news reports. So I thought I’d say a few things that might help.
But first, let me acknowledge that there’s a ton of coverage out there and so I don’t need to point to any press articles. I will just point to the press release of the BICEP2 collaboration (yes, that’s what they’re called) here, and urge you once you’ve read that to follow the link within to the wealth of data (images, text, graphs, diagrams) that they provide. It’s fantastically comprehensive, so knock yourself out. The paper is here.
I keep hearing reports saying things like “Scientists have proved the Big Bang”. No. The Big Bang, while an exciting and important result for modern cosmology, is very old news. (You can tell since there’s even a TV comedy named after it.) This is not really about the Big Bang. This is about Inflation, the mechanism that made the universe expand rapidly from super-tiny scales to more macroscopic scales in fractions of a second. (I’ll say more about the “super-tiny” below).
I also hear (slightly more nuanced) reports about this being the first confirmation of Inflation . That’s a point we can argue about, but I’d say that’s not true either. We’ve had other strong clues that Inflation is correct. One of the key things that pops out of inflation is that it flattens out the curvature of universe a lot, and the various observations that have been made about the Cosmic Microwave Background over the years (the CMB is that radiation left over from when the universe was very young (about 380,000 years old – remember the universe is just under 14 billion years old!)) have shown us that the universes is remarkably flat. Another previous exciting result in modern cosmology. Today’s result isn’t the first evidence.
So what is today’s exciting news about then? The clue to the correct characterization of what all this means is in the title of the press release: “First Direct Evidence of Cosmic Inflation”. It’s the “Direct” part that’s important, and it connects to more excitement too. Let me explain. By examining the Cosmic Microwave Background (CMB), the BICEP2 people were able to find something very important. If there was a period of inflation in the universe then the stuff left over (matter and radiation) ought to have an imprint of the gravitational waves (actual ripples in spacetime) that accompanied inflation. We’ve never seen these before, and this is precisely what they found! That imprint is in the from of a certain type of polarisation of the CMB. Yes, the same kind of polarization you play with when you use polarizing sunglasses. Light from the sun gets dodged around a bit by the molecules of our atmosphere, and we see the imprint of that as polarization… Well the imprint on the light of the CMB from gravitational waves in the very early universe makes a particular type of polarization too. That’s what they found.
This is hugely exciting since it is a direct (ish) detection of physics connected to the super-tiny scales many of us have been thinking about, and (through the gravitational waves) to much earlier than the 380K years when the CMB radiation first appeared. Those tiny scales are knocking on the door of the kinds of things I work on – quantum gravity. The very young universe had both quantum physics and spacetime physics (gravity) doing important things together and the combination of the two “quantum gravity” is what we hope to understand and put to the test one day, whether it be string theory or some other approach we may or may not have thought of. So this is exciting, since we’ve finally got some physics that is a bit more directly connected to those scales than we usually have access to.
Finally, there are questions like whether this is somehow “bigger than the Higgs” in terms of discoveries. I just think that’s a bit of a silly discussion, but let me engage slightly. On the one hand, I’d say we’d have been more surprised if we’d not found the Higgs, since there are so many things about the Standard Model of particle physics, the internal consistency of how quantum field theory (quantum mechanics for particle physics) works, and various other things concerning particles having mass, that depend upon something like the Higgs existing. On the other hand, there’s a lot more that we don’t understand about what constrains models of the very early universe (especially when knocking on the door of quantum gravity), and perhaps how cleanly we’d be able to see signatures of mechanisms like inflation, so I think we were less bound to having to find today’s signatures than we were to finding direct evidence of the Higgs mechanism. So it is great to learn (and more cleanly than most people were expecting) more evidence that inflation is on the right track. It could have been a messier story that took longer to sort out.
But you can run it both ways. You could say that my previous paragraph means the Higgs is a bigger result since we kind of needed it for full consistency of lots of things we already knew, or you could say it means the Higgs is a smaller result since we kind of needed it for full consistency of lots of things we already knew. Pick.
I return to my assertion that it’s silly to try to rank recent discoveries in this way, not the least because it plays into that tedious and damaging “what’s the next big thing” or “what has science done for me lately” attitude our culture likes to go in for.
Anyway – hurrah for physics and Congratulations to the BICEP2 team!
(Returning to quiet hiding mode… although there’s be some short posts every now and again.)