[This item has been revised and updated.]
In March, scientists representing an experiment called BICEP2 announced at a news conference at Harvard that they had detected gravitational waves from a violent inflationary event at the dawn of time. The scientists had built a telescope at the South Pole and stared into the polar sky to discern the subtle polarization of light that can be caused, in theory, by these primordial gravitational waves rippling through the cosmic microwave background radiation (CMB).
This seemingly affirmed the theory of “cosmic inflation,” which posits that, in the initial moments of its existence, the universe grew exponentially larger in a kind of hyper-expansion that has given us this very big and glamorous and sparkly cosmos. Inflation, the theorists said, puts the bang in the big bang. (Then the universe calmed down into a more stately process of expansion.)
But there was a second, far more prosaic, and downright boring explanation for the BICEP2 observation: Maybe they were seeing merely the effects of dust in the foreground, within our own galaxy. Dust can also polarize light.
The BICEP2 team considered that possibility, and used existing estimates for dust when making the calculations. The scientists felt that their signal was so strong that it was exceedingly unlikely that dust could have caused it.
But now comes Planck, a European satellite, with its own dust map. The paper, signed by a very long list of authors, was submitted over the weekend to the physics pre-print site.
It is not good news for the BICEP2 team.
The Planck satellite has detected galactic dust, or more precisely the effects of galactic dust, at levels that could potentially explain away the entirety of the alleged cosmological signal reported by BICEP2 scientists at their big March 17 news conference at Harvard.
The dust-only possibility is something that the BICEP2 team itself acknowledged after the big controversy erupted this spring. The two teams will now collaborate and come up with a dust map based on the combined data. So the story isn’t over — the Planck data don’t rule out the possibility that there’s a cosmological signal there.
But “maybe” doesn’t count as a discovery. (That sound you hear in the background is from Nobel Prizes being put on hold.)
See this from the Planck paper’s abstract:
This level is the same magnitude as reported by BICEP2 over this range, which highlights the need for assessment of the polarized dust signal even in the cleanest windows of the sky. The present uncertainties are large and will be reduced through an ongoing, joint analysis of the Planck and BICEP2 data sets.
Bill Jones, a Princeton professor who is part of the Planck team and a co-author of the new paper, tells me by e-mail:
“… a scientific claim of this sort, evidence of a fundamental connection between gravity (general relativity) and quantum physics, requires overwhelming evidence. It isn’t a question as to whether Planck or anybody else can exclude the hypothesis that there might be a cosmological signal. The point is that there isn’t any evidence to suggest that a cosmological signal exists. With the Planck data, we now understand much better how to go about acquiring the sort of data needed to provide such evidence.”
Here’s Paul Steinhardt, a Princeton professor who helped invent inflationary cosmology but later turned against the theory, saying that it does not make testable predictions and is thus not truly scientific (I had asked him this morning, “what’s the headline here?”):
Planck is publishing their second official analysis of dust in recent months – the first specifically excluded the BICEP2 and other high altitude regions and the second (this one) specifically includes it. So, the new Planck satellite paper makes certain and more precise the earlier data and suspicions suggesting that dust contributes a lot of B-mode to the BICEP2 region specifically and that claims of primordial gravitational waves were premature. In sum, thus far: Dust, Not Gravitational Waves
By e-mail, Chao-Lin Kuo, a professor at Stanford and one of the BICEP2 leaders, offers this response to the new Planck results:
This paper confirms that the polarization level of dust is higher than most of the pre-Planck models, including the ones we considered in the BICEP2 paper. A preliminary analysis of this data published in May already indicated this. Since then we have taken a step back regarding the cosmological interpretations, as reflected in the published version of the BICEP2 paper in Physical Review Letter.
On the other hand, this paper does not rule out a substantial contribution from gravitational waves at r~0.1, where many interesting theories predict. Cross-correlation study with BICEP2 will provide a much more powerful measurement. As you know the BICEP2 team is already formally collaborating with the Planck collaboration.
I also want to emphasize is the fact that BICEP2 validated our whole experimental approach. We now have receivers operating at a different frequency that is much less sensitive to dust emission. We are deploying BICEP3, which will further improve the sensitivity in this channel. So basically if BICEP2 result doesn’t turn out to be a grand slam after all, we are still playing with the bases loaded.
Also by e-mail, John Kovac, another BICEP2 team leader, weighs in on the Planck paper:
This paper confirms in more detail the trend which was already indicated by the first Planck papers on dust polarization which appeared in May — that Planck sees generally significantly more B-mode signal from dust in the cleanest regions away from the galaxy than the typical predictions of the pre-Planck models which we considered in our BICEP2 paper. We alluded to this trend in the “Note Added” to our final published version of our paper which appeared in PRL in June.
However unlike BICEP2, the Planck signal-to-noise on B-mode polarization in our region is quite low. (This is because their sensitivity is spread out across the whole sky). We are working on a joint analysis with Planck which cross-correlates our maps — this will be a much more powerful way of determining the exact level of B-modes from polarized dust in our region, and comparing it to the total level of B-modes detected in our extremely sensitive BICEP2 and Keck Array 150 GHz maps.
We are also recording additional high signal-to-noise data at 95GHz with the Keck Array. We are focused on these new more powerful analyses and datasets, which should give a much more constraining answer within the next several months.
And here’s another comment from a BICEP2 team leader, Caltech’s Jamie Bock:
BICEP2 adds statistical power for detecting dust, so a direct comparison between BICEP2 and Planck does a significantly better job in determining the fraction of dust and any remaining CMB signal. So while we can say the dust level is significant, we really need to wait for the joint BICEP2-Planck paper that is coming out in the fall to get the full answer.
Finally, here’s the perspective of David Spergel, chair of the Princeton astrophysics department:
“This is good news and bad news for gravitational wave detection. Good news is that there are regions of the sky with low dust polarization signal. The bad news is that BICEP2 did not look at one of these regions. There were two possible explanation for BICEP2’s measurement: they are seeing primordial gravity waves or they are seeing dust. If they were seeing primordial gravity waves, then Planck’s higher frequency measurements would not see a signal. If BICEP is observing primarily dust, then we can predict what Planck will see. Planck reported detecting exactly this predicted level: it looks like dust. Too bad — gravitational waves would be much more exciting.”
After peer review, BICEP2 revises the original paper for final publication and acknowledges that dust could explain the signal.
Here’s an excellent explainer in Quanta from Natalie Wolchover that scales from the specific details to the broader picture.
Here’s the Nature story by Ron Cowen, who has had excellent coverage of BICEP2 from the get-go.