New analyses suggest that observations heralded as evidence for the universe’s brief growth spurt don’t conclusively show what researchers thought they did.
Hubbub is a-bubbling in cosmology right now. In March, the BICEP2 team announced that they’d detected swirling polarization patterns called B-modes in the cosmic microwave background (CMB), the leftover radiation from the universe’s birth. These patterns should exist in the CMB if the universe underwent a moment of exponential expansion called inflation that lasted roughly a nano-nano-nano-nanosecond.
The announcement set off fireworks-level excitement and speculations about a Nobel prize for the theorists who first proposed inflation, if the result was confirmed.
But now, two other teams have combined the BICEP2 data with the latest release from the Planck mission and are painting a different picture. Both these teams say they can’t distinguish whether the B-modes the BICEP2 team detected are in the CMB or in the emission from dust filling our own galaxy.
This blog is long because the issue is complicated and deserves careful treatment. But if you take one thing away from this article, let it be this: we don’t know what the correct answer is yet. But we soon might. The back-and-forth we’re experiencing now is simply how science works.
The problem is twofold. One, we’re stuck in a galaxy. Looking at the cosmos from inside the Milky Way is like looking at a road through a fogged, bug-spattered windshield. Observers have to peel away all this “foreground” stuff so that they can see the CMB.
At the frequency BICEP2 observed, the three main signals we care about are the CMB (which is polarized at a level we’re trying to figure out), dust (also polarized), and the cosmic infrared background (CIB, unpolarized). The CIB is the sum of infrared light from billions of unresolved, dusty galaxies, and it suffuses the cosmos in much the same way that the CMB does.
To tease out the CMB’s signal, cosmologists must identify how much of the signal they observe from a given part of the sky comes from each source. To do this, the BICEP2 team used a preliminary all-sky map of polarized dust emission, taken from a conference presentation given by a Planck team member in April 2013.
But this map included the CIB. Because the CIB is just integrated light from dust in a whole bunch of galaxies, it looks like the dust in the Milky Way — except it’s not polarized. If you look at the whole dusty signal together, it looks like about 5% of the emission is polarized. But if half the signal in there is unpolarized and you remove that part, the “fractional polarization” of what’s left goes up, maybe to 10% (I’m using rough numbers here).
The Planck team knew the CIB was a problem and spent a year weeding it out. They released a preliminary CIB-less dust map several weeks ago.
It’s this second map that the two new papers are using. With the revised polarized dust map, a team from University of California, Berkeley, and another team from Princeton and New York University say the BICEP2 team might have lowballed the amount of polarization that comes from the Milky Way’s dust. In other words, we can’t conclude anything about where the B-modes BICEP2 sees come from.
Frequency Matters in Cosmology
There’s a big BUT here, the #2 of the twofold problem: all the teams are extrapolating.
The Planck polarization map is at 353 GHz, where the dust emission is strong. But BICEP2 observed at 150 GHz. So cosmologists have to take a three-step approach, explains Planck scientist Charles Lawrence (JPL).
First, they need to know how much of the galactic dust emission at 353 GHz is polarized. Second, the strength of the galactic dust signal is different at different frequencies, so they need to correctly deduce what the signal looks like at 150 GHz, where the dust emission is weaker. Third, they need to correctly split that polarization signal into its two types, E-modes and B-modes.
The BICEP2 team extrapolated to 150 GHz using a 353-GHz, CIB-tainted map of the section of sky that they observed. The two other teams extrapolated using a 353-GHz map that’s clean of the CIB but doesn’t include the BICEP2 field of view. The Planck team hasn’t released the data for regions near the north and south galactic poles because the observations of those sectors are ridiculously hard to analyze. And BICEP2 looked at the Southern Hole.
The Planck team is taking so long because the scientists are working on a cosmic scale. The Planck satellite observed the CMB to high precision in order to measure the numbers that characterize the universe, things related to its expansion speed and density and so forth. The team has to correlate all these observations and calculations with one another before they’re finished.
“It’s not like you can have a result over here on one side that is inconsistent with another on the other side, and then say, ‘Oh whoops, we didn’t notice that,’” Lawrence says, laughing. It all has to fit together. “This is just hard work and it has to be done right. So it takes as long as it takes.”
The Planck data are important in part because they’ll obviate the need to extrapolate. Planck measured polarization at 30, 44, 70, 100, 143, 217, and 353 GHz. In the units that the cosmologists use to make their maps, the CMB signal doesn’t change as you look in different frequencies. But the dust signal does. So if researchers can look at how the signal changes as they move between frequencies, they can effectively wipe the dust off their cosmic windshield.
Several sources have reported that Planck’s results will be out in October. The real deadline is the first week of December, because that’s when the next Planck conference is scheduled. They might be out earlier, in part or in total, but the conference deadline is drop-dead, Lawrence says. These results will include the temperature and polarization data from the full mission; the previous release in 2013 included only the first half of the temperature data. It’s too soon to say whether the upcoming release will include Planck’s version of direct B-mode measurements.
The fact that the BICEP2 result hinges on Planck’s data has upped the stakes. “There’s one thing that everybody agrees on, and that is we have to be right,” Lawrence stresses. “We cannot afford to be wrong about this. And if it takes a little bit longer, and we say ‘Well, what about—? Or have we checked this?’ or so on, then that’s going to happen. Until that’s done, it’s not done.”
That’s life in the messy universe.
M.J. Mortonson and U. Seljak. “A joint analysis of Planck and BICEP2 B modes including dust polarization uncertainty.” Posted to arXiv.org May 22, 2014.
R. Flauger, J.C. Hill, and D.N. Spergel. “Toward an Understanding of Foreground Emission in the BICEP2 Region.” Posted to arXiv.org May 28, 2014.
Planck Collaboration. “Planck Intermediate Results. XIX. An Overview of the Polarized Thermal Emission from Galactic Dust.” Posted to arXiv.org May 5, 2014.
BICEP2 Collaboration. “BICEP2 I: Detection of B-mode Polarization at Degree Angular Scales.” Posted to arXiv.org March 18, 2014.