Gravitational Wave Search Finds Tantalizing New Clue
An international team of scientists may be close to detecting faint ripples in space-time that fill the universe.
Pairs
of black holes billions of times more massive than the Sun may be circling one
another, generating ripples in space itself. The North American Nanohertz
Observatory for Gravitational Waves (NANOGrav) has spent more than a decade using
ground-based radio telescopes to look for evidence of these space-time ripples created
by behemoth black holes. This week, the project announced the detection of a
signal that may be attributable to gravitational waves, though members aren't
quite ready to claim success.
Gravitational
waves were first theorized by Albert Einstein in 1916, but they weren't
directly detected until nearly a century later. Einstein showed that rather
than being a rigid backdrop for the universe, space is a flexible fabric that is
warped and curved by massive objects and inextricably linked with time. In
2015, a collaboration between the U.S.-based Laser Interferometer
Gravitational-wave Observatory (LIGO) and the Virgo interferometer in Europe
announced the first direct detection of gravitational waves: They were emanating
from two black holes - each with a mass about 30 times greater than the Sun - circling
one another and merging.
In a
new paper published in the January 2021 issue of the Astrophysical Journal
Supplements, the NANOGrav project reports the detection of unexplained fluctuations,
consistent with the effects of gravitational waves, in the timing of 45 pulsars
spread across the sky and measured over a span of 12 1/2 years.
Pulsars
are dense nuggets of material left over after a star explodes as a supernova.
As seen from Earth, pulsars appear to blink on and off. In reality, the light
comes from two steady beams emanating from opposite sides of the pulsar as it
spins, like a lighthouse. If gravitational waves pass between a pulsar and
Earth, the subtle stretching and squeezing of space-time would appear to
introduce a small deviation in the pulsar's otherwise regular timing. But this
effect is subtle, and more than a dozen other factors are known to influence
pulsar timing as well. A major part of the work done by NANOGrav is to subtract
those factors from the timing data for each pulsar before looking for signs of
gravitational waves.
LIGO
and Virgo detect gravitational waves from individual pairs of black holes (or
other dense objects called neutron stars). By contrast, NANOGrav is looking for
a persistent gravitational wave "background," or the noiselike combination of
waves created over billions of years by countless pairs of supermassive black
holes orbiting one another across the universe. These objects produce gravitational
waves with much longer wave lengths than
those detected by LIGO and Virgo - so long that it might take years for a
single wave to pass by a stationary detector. So while LIGO and Virgo can
detect thousands of waves per second, NANOGrav's quest requires years of data.
As
tantalizing as the latest finding is, the NANOGrav team isn't ready to claim
they've found evidence of a gravitational wave background. Why the hesitation? In
order to confirm direct detection of a signature from gravitational waves,
NANOGrav's researchers will have to find a distinctive pattern in the signals
between individual pulsars. According to Einstein's theory of general
relativity, the effect of the gravitational wave background should influence the
timing of the pulsars slightly differently based on their positions relative to
one another.
At this point,
the signal is too weak for such a pattern to be distinguishable. Boosting the
signal will require NANOGrav to expand its dataset to include more pulsars
studied for even longer lengths of time, which will increase the array's
sensitivity. NANOGrav is also pooling its data with those from other pulsar timing
array experiments in a joint effort by the International Pulsar Timing Array, a
collaboration of researchers using the world's largest radio telescopes.
"Trying to
detect gravitational waves with a pulsar timing array requires patience," said Scott
Ransom with the National Radio Astronomy Observatory and the current chairperson
of NANOGrav. "We're currently analyzing over a dozen years of data, but a
definitive detection will likely take a couple more. It's great that these new
results are exactly what we would expect to see as we creep closer to a
detection."
The NANOGrav team discussed their findings at a press
conference on Jan. 11 at the 237th meeting of the American Astronomical
Society, held virtually from Jan. 10 to 15. Michele Vallisneri and Joseph
Lazio, both astrophysicists at NASA's Jet Propulsion Laboratory in Southern
California, and Zaven Arzoumanian at NASA's Goddard Space Flight Center in
Maryland are co-authors of the paper. Joseph Simon, a researcher at University
of Colorado Boulder and the paper's lead author, conducted much of the analysis
for the paper as a postdoctoral researcher at JPL. Multiple NASA postdoctoral
fellows have participated in the NANOGrav research while at JPL. NANOGrav is a
collaboration of U.S. and Canadian astrophysicists. The data in the new study
was collected using the Green Bank antenna in West Virginia and the Arecibo
dish in Puerto Rico before its recent collapse.
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