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Gravitational Waves from Big Bang Detected
A curved signature in the cosmic microwave background light
provides proof of inflation and spacetime ripples
Mar 17, 2014 |By Clara Moskowitz
BICEP2 measurement of gravitational waves via
CMB B-mode polarization
Proof of gravitational waves created by cosmic inflation is
shown here in this image of the cosmic microwave background
radiation collected by the BICEP2 experiment
at the South Pole. The proof comes in the form of a signature
called B-mode polarization, a curling of the orientation, or
polarization, of the light, denoted by the black lines on the
image. The color indicates small temperature fluctuations in the
cosmic microwave background that correspond to density fluctuations
in the early universe.
BICEP2 Collaboration
Physicists have found a long-predicted twist in light from the big
bang that represents the first image of ripples in the universe
called gravitational waves, researchers announced today. The
finding is direct proof of the theory of inflation, the idea that
the universe expanded extremely quickly in the first fraction of a
nanosecond after it was born. What’s more, the signal is coming
through much more strongly than expected, ruling out a large class
of inflation models and potentially pointing the way toward new
theories of physics, experts say.
“This is huge,” says Marc Kamionkowski, professor of physics and
astronomy at Johns Hopkins University, who was not involved in the
discovery but who predicted back in 1997 how these gravitational
wave imprints could be found. “It’s not every day that you wake up
and find out something completely new about the early universe. To
me this is as Nobel Prize–worthy as it gets.”
The Background Imaging of Cosmic Extragalactic Polarization 2
(BICEP2) experiment at the South Pole found a pattern called
primordial B-mode polarization in the light left over from just
after the big bang, known as the cosmic microwave background (CMB).
This pattern, basically a curling in the polarization, or
orientation, of the light, can be created only by gravitational
waves produced by inflation. “It looks like a swirly pattern on the
sky,” says Chao-Lin Kuo, a physicist at Stanford University, who
designed the BICEP2 detector. “We’ve found
the smoking gun evidence for inflation and we’ve also produced the
first image of gravitational waves across the sky.”
Such a groundbreaking finding requires confirmation from other
experiments to be truly believed, physicists say. Nevertheless, the
result has won praise from many leaders in the field. “There’s a
chance it could be wrong, but I think it’s highly probable that the
results stand up,” says Alan Guth of the Massachusetts Institute of
Technology, who first predicted inflation in 1980. “I think they’ve
done an incredibly good job of analysis.” The BICEP2 detectors found a surprisingly strong signal of
B-mode polarization, giving them enough data to surpass the
“5-sigma” statistical significance threshold for a true discovery.
In fact, the researchers were so startled to see such a blaring
signal in the data that they held off on publishing it for more
than a year, looking for all possible alternative explanations for
the pattern they found. Finally, when BICEP2’s successor at the same location, the Keck Array,
came online and began showing the same result, the scientists felt
confident. “That played a major role in convincing us this is
something real,” Kuo says.
The cosmic microwave background is a faint glow that pervades
the entire sky, dating back to just 380,000 years after the big
bang. Before then the baby universe was too hot and dense for light
to travel far without bumping into matter. When it cooled to the
point that neutral atoms could form, light was freed to fly through
space unimpeded, and it became the CMB. This
glow was discovered accidentally 1964 by Arno Penzias and Robert
Wilson, who initially mistook it for interference caused by pigeon
droppings on their antenna. Eventually, the scientists realized
they had discovered an imprint from the primordial universe, a
finding that won them the 1978 Nobel Prize in Physics. “It’s
amazing what’s come out of the CMB,” Wilson
says. “Initially I didn’t realize anywhere near how much
information there might be coming from it. From my point of view,
it’s been a wonderful ride.”
BICEP2 uses about 250 thumbnailsize
polarization detectors to look for a difference in the CMB light from a small patch of sky coming through its
telescope in two perpendicular orientations. The instrument
collected data between January 2010 and December 2012 at the
Amundsen–Scott South Pole Station, where the cold, dry air offers
especially stable viewing conditions. Another experiment there, the
South Pole Telescope, reported finding B-mode polarization last
year, although the signal it saw was at a different angular scale
across the sky and was clearly due to the known process of
gravitational lensing (a warping of light caused by massive
objects) of the CMB by large galaxies, rather
than the primordial gravitational waves seen here.
BICEP2 has plenty of competition in
searching for B-mode polarization in the CMB:
other projects include the Atacama B-mode Search (ABS) led by
Princeton University; the POLARBEAR
experiment led by the University of California, Berkeley; the
high-altitude balloon–borne E and B Experiment (EBEX) run by the
University of Minnesota; the Cosmology Large Angular Scale Surveyor
(CLASS) led by Johns Hopkins University; and numerous others. Given
that the BICEP2 team saw such a clear signal,
these searches should easily confirm the results if they are real.
“Right now it’s just the tip of the iceberg,” Kamionkowski says.
“In the coming years we’re going to be able to extract a huge
amount from these measurements. It’s a great thing, not just for
the guys who found it but also for the people they scooped” because
the different experiments should collect complementary data.
The BICEP2 researchers have reported a
surprisingly large number for r, the ratio of the gravitational
wave fluctuations in the CMB to the
fluctuations caused by perturbations in the density of matter. This
value was previously estimated to be less than 0.11 based on
all-sky CMB maps from the Wilkinson Microwave
Anisotropy Probe (WMAP) and the Planck satellite. BICEP2’s value, however, is around 0.20. “Everything
hinges on this little r,” Guth says, “and this measurement changes
things quite a bit. In fact, the models that looked like they were
ruled out last week are now the models that are favored this week.”
Such a high value of r, for instance, indicates that inflation
began even earlier than some models predicted, at one trillionth of
a trillionth of a trillionth of a second after the big bang.
The timing of inflation, in turn, tells physicists about the
energy scale of the universe when inflation was going on.
BICEP2’s value of r suggests that this was
the same energy scale at which all the forces of nature except
gravity (the electromagnetic, strong and weak forces) might have
been unified into a single force—an idea called grand unified
theory. The finding bolsters the idea of grand unification and
rules out a number of inflation models that do not feature such an
energy scale. “This really collapses the space of plausible
inflationary models by a huge amount,” Kamionkowski says. “Instead
of looking for a needle in a haystack, we’ll be looking for a
needle in a bucket of sand.” Grand unified theories suggest the
existence of new fields that act similarly to the Higgs field
associated with the Higgs boson particle discovered in 2012. These
new fields, in turn, would indicate that other, heavier Higgs boson
particles also exist, although with masses so high they would be
impossible to create in any traditional particle accelerator. “This
measurement is allowing us to use the early universe as a lab for
new physics in energy ranges that are otherwise inaccessible to
us,” Kamionkowski says.
Link: http://www.scientificamerican.com/article/gravi…