Walk the Planck

Planck Collaboration

One of the recent sagas in cosmology began with the BICEP2 press conference announcing evidence of early cosmic inflation. There was some controversy since the press release was held before the paper was peer reviewed. The results were eventually published in Physical Review Letters, though with a more cautious conclusion than the original press release. Now the Planck team has released more of their data.¹ This new work hasn’t yet been peer reviewed, but it doesn’t look good for BICEP2.

As you might recall, BICEP2 analyzed light from the cosmic microwave background (CMB) looking for a type of pattern known as B-mode polarization. This is a pattern of polarized light that (theoretically) is caused by gravitational waves produced by early cosmic inflation. There’s absolutely no doubt that BICEP2 detected B-mode polarization, but that’s only half the challenge. The other half is proving that the B-mode polarization they saw was due to cosmic inflation, and not due to some other process, mainly dust. And therein lies the problem. Dust is fairly common in the Milky Way, and it can also create B-mode polarization. Because the dust is between us and the CMB, it can contaminate its B-mode signal. This is sometimes referred to as the foreground problem. To really prove you have evidence of B-mode polarization in the CMB, you must ensure that you’ve eliminated any foreground effects from your data.

When the BICEP2 results were first announced, the question of dust was immediately raised. Some researchers noted that dust particles caught in magnetic fields could produce stronger B-mode effects than originally thought. Others pointed out that part of the data BICEP2 used to distinguish foreground dust wasn’t very accurate. This is part of the reason the final results went from “We found inflation!” to “We think we’ve found inflation! (But we can’t be certain.)”

Dust effects seen by Planck (shaded region) compared with inflation results of BICEP2 (solid line).

The new results from Planck chip at that claim even further. Whereas BICEP2 looked at a specific region of the sky, Planck has been gathering data across the entire sky. This means lots more data that can be used to distinguish foreground dust from a CMB signal. This new paper presented a map of the foreground dust, and a good summary can be seen in the figure. The shaded areas represents the B-mode levels due to dust at different scales. The solid line represents the B-mode distribution due to inflation as seen by BICEP2. As you can see, it matches the dust signal really well.

The simple conclusion is that the results of BICEP2 have been shown to be dust, but that isn’t quite accurate. It is possible that BICEP2 has found a mixture of dust and inflation signals, and with a better removal of foreground effects there may still be a real result. It is also possible that it’s all dust.

While this seems like bad news, it actually answers a mystery in the BICEP2 results. The level of inflation claimed by BICEP2 was actually quite large. Much larger than expected than many popular models. The fact that a good chuck of the B-mode polarization is due to dust means that inflation can’t be that large. So small inflation models are back in favor. It should also be emphasized that even if the BICEP2 results are shown to be entirely due to dust, that doesn’t mean inflation doesn’t exist. It would simply mean we have no evidence either way.

It’s tempting to look at all this with a bit of schadenfreude. Har, har, the scientists got it wrong again. But a more accurate view would be of two rival sports teams playing an excellent game. BICEP2 almost scored, but Planck rallied an excellent defense. Both teams want to be the first to score, but the other team won’t let them cheat to win. And we get to watch it happen.

Anyone who says science is boring hasn’t been paying attention.