Merging black holes ripple space and time in
this artist's concept. Pulsar-timing arrays -- networks of the pulsing
cores of dead stars -- are one strategy for detecting these ripples, or
gravitational waves, thought to be generated when two supermassive black
holes merge into one.
Image credit: Swinburne Astronomy Productions. › Larger image
Throughout our universe, tucked inside galaxies far, far away, giant
black holes are pairing up and merging. As the massive bodies dance
around each other in close embraces, they send out gravitational waves
that ripple space and time themselves, even as the waves pass right
through our planet Earth.
Scientists know these waves, predicted by Albert Einstein's theory of
relativity, exist but have yet to directly detect one. In the race to
catch the waves, one strategy -- called pulsar-timing arrays -- has
reached a milestone not through detecting any gravitational waves, but
in revealing new information about the frequency and strength of black
hole mergers.
"We expect that many gravitational waves are passing through us all the
time, and now we have a better idea of the extent of this background
activity," said Sarah Burke-Spolaor, co-author of a new Science paper
published Oct. 18, which describes research she contributed to while
based at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
Burke-Spolaor is now at the California Institute of Technology in
Pasadena.
Gravitational waves, if detected, would reveal more information about
black holes as well as one of the four fundamental forces of nature:
gravity.
The team's inability to detect any gravitational waves in the recent
search actually has its own benefits, because it reveals new information
about supermassive black hole mergers -- their frequency, distance from
Earth and masses. One theory of black hole growth to hit the theorists'
cutting room floors had stated that mergers alone are responsible for
black holes gaining mass.
The results come from the Commonwealth Scientific and Industrial
Research Organization's (CSIRO) Parkes radio telescope in eastern
Australia. The study was jointly led by Ryan Shannon of CSIRO, and
Vikram Ravi, of the University of Melbourne and CSIRO.
Pulsar-timing arrays are designed to catch the subtle gravitational
waves using telescopes on the ground, and spinning stars called pulsars.
Pulsars are the burnt-out cores of exploded stars that send out beams
of radio waves like lighthouse beacons. The timing of the pulsars'
rotation is so precise that researchers say they are akin to atomic
clocks.
When gravitational waves pass through an array of multiple pulsars, 20
in the case of the new study, they set the pulsars bobbing like buoys.
Researchers recording the radio waves from the pulsars can then piece
together the background hum of waves.
"The gravitational waves cause the space between Earth and pulsars to stretch and squeeze," said Burke-Spolaor.
The new study used the Parkes Pulsar Timing Array, which got its start
in the 1990s. According to the research team, the array, at its current
sensitivity, will be able to detect a gravitational wave within 10
years.
Researchers at JPL are currently developing a similar precision
pulsar-timing capability for NASA's Deep Space Network, a system of
large dish antennas located around Earth that tracks and communicates
with deep-space spacecraft. During gaps in the network's tracking
schedules, the antennas can be used to precisely measure the timing of
pulsars' radio waves. Because the Deep Space Network's antennas are
distributed around the globe, they can see pulsars across the whole sky,
which improves sensitivity to gravitational waves.
"Right now, the focus in the pulsar-timing array communities is to
develop more sensitive technologies and to establish long-term
monitoring programs of a large ensemble of the pulsars," said Walid
Majid, the principal investigator of the Deep Space Network
pulsar-timing program at JPL. "All the strategies for detecting
gravitational waves, including LIGO [Laser Interferometer
Gravitational-Wave Observatory], are complementary, since each technique
is sensitive to detection of gravitational waves at very different
frequencies. While some might characterize this as a race, in the end,
the goal is to detect gravitational waves, which will usher in the
beginning of gravitational wave astronomy. That is the real exciting
part of this whole endeavor."
The ground-based LIGO observatory is based in Louisiana and Washington.
It is a joint project of Caltech and the Massachusetts Institute of
Technology, Cambridge, Mass., with funding from the National Science
Foundation. The European Space Agency is developing the space-based LISA
Pathfinder (Laser Interferometer Space Antenna), a proof-of-concept
mission for a future space observatory to detect gravitational waves.
LIGO, LISA and pulsar-timing arrays would all detect different
frequencies of gravitational waves and thus are sensitive to various
types of merger events.
Caltech manages JPL for NASA.
Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov
