"Scientists will have to wait a while longer to find out what kicked off the Big Bang.
Last
spring, a team of astronomers who go by the name of Bicep announced
that they had detected ripples in space-time, or gravitational waves,
reverberating from the first trillionth of a trillionth of a trillionth
of a second of time — long-sought evidence that the expansion of the
universe had started out with a giant whoosh called inflation.
The
discovery was heralded as potentially the greatest of the new century,
but after months of spirited debate, the group conceded that the result
could have been caused by interstellar dust, a notion buttressed by
subsequent measurements by the European Space Agency’s Planck satellite
that the part of the sky Bicep examined was in fact dusty.
Now
a new analysis, undertaken jointly by the Bicep group and the Planck
group, has confirmed that the Bicep signal was mostly, if not all,
stardust, and that there is no convincing evidence of the gravitational
waves. No evidence of inflation.
“This
analysis shows that the amount of gravitational waves can probably be
no more than about half the observed signal,” Clem Pryke of the
University of Minnesota said Friday in an interview.
“We
can’t say with any certainty whether any gravity wave signals remain,”
Dr. Pryke added.
“Obviously, we’re not exactly thrilled, but we are
scientists and our job is to try and uncover the truth. In the
scientific process, the truth will emerge.”
When
the galactic dust is correctly subtracted, the scientists said, there
was indeed a small excess signal — a glimmer of hope for inflation fans?
— but it was too small to tell if it was because of gravitational waves
or just experimental noise.
The
Bicep/Planck analysis was led by Dr. Pryke, one of the four Bicep
principal investigators.
Brendan Crill, of the California Institute of
Technology and a member of Planck, acted as a liaison between the
groups. They had planned to post their paper Monday, but the data was
posted early, apparently by accident. It was soon taken down, but not
before it set off an outburst of Twitter messages and hasty news releases.
A paper is to be posted to the Bicep website and has been submitted to the journal Physical Review Letters.
But
it will be far from the final word. A flotilla of experiments devoted
to the cause are underway, studying a thin haze of microwaves, known as
cosmic background radiation, left from the Big Bang, when the cosmos was
about 380,000 years old. Among them is a sister experiment to Bicep
called Spider, led by Bill Jones of Princeton and involving a
balloon-borne telescope that just completed a trip around Antarctica, as
well as Bicep’s own Keck Array and the recently installed Bicep3.
At
stake is an idea that has galvanized cosmologists since Alan Guth of
the Massachusetts Institute of Technology invented it in 1979. Inflation
theory holds that the universe had a violent and brief surge of
expansion in the earliest moments, driven by a mysterious force field
that exerted negative gravity.
It would explain such things as why the
universe looks so uniform and where galaxies come from — quantum dents
in the inflating cosmos.
Such an explosion would have left faint corkscrew swirls, known technically as B-modes, in the pattern of polarization of the microwaves. So, however, does interstellar dust.
The
Bicep group — its name is an acronym for Background Imaging of Cosmic
Extragalactic Polarization — is led by John M. Kovac of the
Harvard-Smithsonian Center for Astrophysics; Jamie Bock of Caltech; Dr.
Pryke; and Chao-Lin Kuo of Stanford. They have deployed a series of
radio telescopes at the South Pole in search of the swirl pattern.
Their
second scope, Bicep2, detected a signal whose strength was in the sweet
spot for some of the most popular models of inflation, leading to a
sensational news conference attended by Dr. Guth and Andrei Linde, two
of the founding fathers of inflation.
But
that was before critics raised the dust question. Moreover, that result
was contrary to a previous limit on the strength of gravitational waves
obtained by the Planck satellite, which has scanned the entire
microwave sky in search of the Big Bang’s secrets.
Planck
observed the microwaves in nine frequencies, making it easy to
distinguish dust. Bicep2 had only one frequency and lacked access to
Planck’s data until last fall, when the two groups agreed to work
together.
Dr.
Bock of Caltech, in an interview at the end of what he called a long,
stressful day, characterized the result as “no detectable signal.”
“I’m not discouraged,” he went on. “We’re going to have to have better data to get a definitive answer.”
In
an email, Paul J. Steinhardt, a Princeton cosmologist who was a founder
of inflation but turned against it in favor of his own theory of a
cyclic bouncing universe, said the new results left cosmologists back
where they were before Bicep.
But
Dr. Linde noted that there was evidence in the new analysis for a
gravitational wave signal, albeit at a level significantly lower than
Bicep had reported. “This is what all of us realized almost a year ago,
and it did not change,” he said in an email.
The
earlier Planck result limiting gravitational waves, he said, had
inspired a firestorm of theorizing, in which he and others produced a
whole new class of theories relating not just to inflation, but to dark energy as well.
“So yes, we are very excited, and no, the theory did not become more contrived,” he said.
Max
Tegmark, an M.I.T. expert on the cosmic microwaves, said, “It’s
important to remember that inflation is still alive and well, and that
many of the simplest models predict signals just below this new limit.”
The next few years will be interesting, he said.
Michael
Turner, a cosmologist at the University of Chicago, said he could
appreciate the frustration of his colleagues, who have been wandering in
the wilderness for nearly four decades looking for clues to the
Beginning.
“Inflation
is the most important idea in cosmology since the hot Big Bang,” he
said. “It is our Helen of Troy, launching a thousand experiments.”"
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