SkyandTelescope.com July 2014 21way rather than randomly. Many things can polarize the
cosmic microwaves, from Earth’s atmosphere to dust in
the Milky Way to gravitational lensing by distant irregu-
larities in the early universe. But most of this shows up as
radial polarization patterns known as E-modes. Some of
these had already been detected by other experiments.
The gravitational waves predicted to result from
exponentially infl ating space would result in diff erent,
pinwheel-shaped patterns called B-modes. These were pre-
dicted to be far weaker, perhaps so weak that they could
never be detected at all. Moreover, later foreground eff ects
should add B-modes of their own to confuse the picture.
The BICEP team chose a swath of sky in a region
known as the Southern Hole for its near total lack of
interstellar dust. They succeeded in mapping and remov-
ing the E-modes across the fi eld of view. Remaining were
B-mode patterns about a tenth as strong, and only a hun-
dredth as strong as the familiar warm and cool patches in
the microwave background — which themselves diff er in
temperature by only a few parts per hundred thousand.
But the B-mode swirls were clear nonetheless. They
overpowered the expected B-mode contamination from
foreground gravitational lensing (as shown in the map
and simulation at right) with a statistical signifi cance
of 5.3 sigmas. That means there’s only a 1-in-10-million
chance that the strong patterns in the map are a statistical
fl uke. And the swirls are strongest at an apparent width
of a couple degrees on the sky, just about what infl ation
theory predicted.
The strength of the early gravitational waves themselves
is denoted by a single number, called r. It stands for ratio.
It’s the ratio of the gravitational-wave distortions of space
itself to fl uctuations of the material in the space. It’s also
called the tensor-to-scalar ratio, for the two types of distor-
tions that these two diff erent eff ects create. Expect to hear
a lot about r. It bodes to become as big a deal in the 21st
century as the Hubble constant was for much of the 20th.
The BICEP team announced that r is 0.2 ± 0.06, which
is staggeringly large. “This has been like looking for a
needle in a haystack, but instead we found a crowbar,”
said co-leader Clem Pryke (University of Minnesota). By
comparison, the next-generation project that is being
planned was intended to detect an r as small as 0.001, two
hundred times weaker.
But already there are signs of complications — maybe
problems, or maybe breakthroughs. The Planck science
team had already announced fi nding an upper limit to
r of 0.11, working instead from the temperature fl uctua-
tions and on smaller angular scales. The apparent confl ict
could be a statistical fl uke. Or maybe r itself changed as
infl ation progressed, due to an eff ect that theorists call
“running.” In fact, the infl ationary paradigm requires
something like this. Something had to make infl ation
slow and stop, leaving our bubble of ordinary space-time
expanding with the fi xed quota of matter and energy we
STANDING OUT The complete BICEP2 polarization map
(top) shows much stronger B-mode patterns on the sky
than could be created by noise and the expected foreground
contamination, shown in the simulation below it. The signal
stands out with a statistical signifi cance of 10 million to one.In the excitement over cosmic infl ation, several other big
physics fi rsts in the BICEP discovery went less noticed.- If the discoverers are right, the BICEP2 map is the fi rst
direct observation of gravitational waves. Albert Einstein
predicted them in 1916 from his general theory of relativity,
but until now we’ve only had indirect evidence for them, in
the energy they steal from orbiting pulsars. - This is the fi rst confi rmation that gravity is quantized.
Physicists assumed it must be, but no one has seen gravi-
tons before. “I think this is the only observational evidence
we have that actually shows that gravity is quantized,” says
cosmologist Ken Olum (Tufts University). “It’s probably the
only evidence we will ever have.” - This is a detection of Hawking radiation, which Stephen
Hawking predicted in 1974. Hawking radiation is usually
associated with the evaporation of black holes, in the form
of particles emitted at a hole’s event horizon. But ordinary
space also has horizons, a diff erent one centered on each
point. These horizons are everywhere, so Hawking radia-
tion should come from every point in space. Today the
cosmic horizons are huge and their Hawking radiation is
utterly insignifi cant. But in the universe’s fi rst instants, the
horizons were tiny and sharply curved. The gravitational
waves seen by BICEP are their Hawking radiation.
BICEP2 COLLABORATION−0.300.3Right ascensionRight ascensionPolarization temperature(microkelvins)DeclinationBICEP2 Map−65$−60$−55$−50$Expected Foreground Contamination3 h 2 h 1 h 0 23 h 22 h 21 h3 h 2 h 1 h 0 23 h 22 h 21 hMore Physics Firsts