Sky and Telescope - May 2015 - 12

News Notes

ESA / PLANCK

COSMOLOGY I Inﬂation Signal Reduced to Dust

This map from ESA's Planck satellite shows the patch of the Southern Hemisphere sky analyzed
by the BICEP2/Keck projects (white outline, 400 square degrees). Colors represent emission from
dust (orange is more, blue less) and lines represent the orientation of the Milky Way's magnetic
ﬁeld. The knot of red to the upper right of the BICEP2 ﬁeld is the Small Magellanic Cloud.

The long-awaited analysis of swirly
polarization patterns called B-modes
aﬃrms that these signals, purportedly
from the universe's brief but stupendous
spurt of inﬂationary growth, are probably
instead from dust in our galaxy.
Last March, researchers with the
BICEP2 and Keck Array experiments at
the South Pole reported the detection of
B-modes in their cosmic microwave background (CMB) observations (S&T: June
2014, p. 10). These B-modes, if in fact in
the CMB itself, would come from spacetime ripples called gravitational waves
that were created by the hypothesized 10
nano-nano-nano-nanoseconds (10 -35 s) of
inﬂationary growth.
12

May 2015 sky & telescope

Yet skepticism soon replaced euphoria
when two other teams using data from the
Planck spacecraft suggested the signals
might instead be from dust in the Milky
Way (S&T: Sept. 2014, p. 12). Aligned with
our galaxy's magnetic ﬁeld, interstellar
dust grains produce polarized emission
of the same pattern and angular scale
(a couple of degrees) as the primordial
B-modes cosmologists are hunting for.
To settle the debate, the BICEP2/Keck
and Planck teams combined forces (and
data) in a joint analysis. On January 30th
the teams announced that the analysis
shows that dust in the Milky Way can
completely explain the B-modes detected
by the South Pole experiments. At most,

gravitational waves from inﬂation could
make up only half of the observed signal.
Planck's full data set was crucial in
this investigation because it obviates the
need to extrapolate. Both BICEP2 and
the Keck Array focused on a frequency
of 150 GHz, favorable for CMB studies
but challenging for dust identiﬁcation.
The Planck satellite, on the other hand,
observed nine frequency bands, with
seven of those - 30, 44, 70, 100, 143, 217,
and 353 GHz - including polarization
measurements. Using those data, astronomers can directly see how dust emission
changes from frequency to frequency.
Emission from our galaxy's dust is
strongest at 353 GHz (25 times stronger than it is at 150 GHz, in fact). And
because Planck reveals the relationship
between emission strengths at diﬀerent frequencies, the researchers could
use Planck's exquisite 353-GHz dust
map to analyze the detected B-modes.
They carefully compared, combined, and
cross-analyzed the observations in order
to calculate the implied ratio of how big
the spacetime ripples were compared
with the ordinary density ﬂuctuations in
the material ﬁ lling space, a ratio called r.
Basically, r measures the strength of the
gravitational waves and how energetic
inﬂation was. A higher r means more
energy behind inﬂation.
The teams calculated an upper limit
of r < 0.12, which agrees with the upper
limit of r < 0.11 from Planck's 2013 results
(those included only the ﬁrst 15.5 months
of satellite data). These upper limits favor
simpler forms of inﬂation, with an energy
scale of 2 × 1016 gigaelectron volts. Paired
with Planck's full data (more on that
next month), the limit shows cosmologists should focus on "slow-roll" inﬂation
models, in which the potential energy that
drives inﬂation decreases slowly (well, for
inﬂation), like a ball rolling down a gentle
hill. A lower energy scale would also be
friendlier to string theories, which seek to
unite quantum mechanics and gravity.
■ CAMILLE M. CARLISLE

Sky and Telescope - May 2015

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