12 September 2014 sky & telescopeNews Notes
map in May. Using this revised map, the
two new studies say the BICEP2 team
might have lowballed the amount of polar-
ization in their observations that comes
from Milky Way dust.
In other words, scientists can’t
conclude anything about where these
B-modes come from.Frequency Matters
However, here’s the #2 of the twofold prob-
lem: all the teams are extrapolating.Two new analyses suggest that observa-
tions heralded as evidence for the uni-
verse’s brief growth spurt (S&T: June 2014,
p. 10) don’t show what researchers thought
they did.
In March, BICEP2 team members
announced that they’d detected swirling
polarization patterns called B-modes in the
cosmic microwave background (CMB), the
afterglow from the universe’s birth. These
patterns should exist in the CMB if the
universe underwent a moment of exponen-
tial expansion called infl ation that lasted
roughly a nano-nano-nano-nanosecond.
But now, a team from the University
of California, Berkeley, and another team
from Princeton and New York University
are painting a diff erent picture. The teams
combined the BICEP2 data with a new map
of the polarized dust in the Milky Way,
created from observations by ESA’s Planck
satellite. Using these data, both teams say
they can’t distinguish whether the B-modes
detected by BICEP2 are in the CMB or in
the emission from dust fi lling our galaxy.
The problem is twofold. One, cosmolo-
gists observe from inside our galaxy. Look-
ing at the cosmos from inside the Milky
Way is like looking at a road through a
fogged, bug-spattered windshield. Observ-
ers must peel away these foreground
signals so that they can see the CMB.
At the frequency BICEP2 observed, the
three main signals are the CMB (which is
polarized at a level we’re trying to fi gure
out), dust in the Milky Way (also polar-
ized), and the cosmic infrared background
(CIB, unpolarized). The CIB is the sum of
infrared light from billions of unresolved,
dusty galaxies, and it suff uses the cosmos
much as the CMB does.
To tease out the CMB, cosmologists
must identify how much of the signal they
observe from a given part of the sky comes
from each source. The BICEP2 team used
a preliminary all-sky map of polarized dust
emission, taken from a Planck team mem-
ber’s conference presentation in April 2013.
But this map included the CIB — a
major problem, because the CIB is inte-grated light from dust in a whole bunch of
other galaxies. The CIB looks like emis-
sion from our galaxy’s dust, except it’s
unpolarized. When researchers sum all
the dusty emission together, it looks about
5% polarized. But remove the part that’s
unpolarized, and the fractional polariza-
tion of what’s left goes up, to an average of
roughly 10%.
The Planck team knew the CIB was a
problem and spent a year weeding it out.
They released a preliminary, CIB-less dustCOSMOLOGY I Infl ation Evidence Inconclusive
This preliminary map of polarized emission from the ESA’s Planck satellite includes the cosmic
infrared background, which damped the polarized signal from dust in the Milky Way.Once the Planck team subtracted the cosmic infrared background, the signal from polarized dust
in our galaxy became stronger. This signal might be the one that BICEP2 detected.PLANCK COLLABORATION (2)Fraction polarized0 20%Fraction polarized0 20%NN_layout.indd 12 6/23/14 12:20 PM