http://www.skyandtelescope.com.au 17human tension of each team wanting for itself the glory of
making the first confirmed detection.
“Gradually we are breaking down those barriers and
learning how to successfully work together and push science
forward,” says Michael Kramer (Max Planck Institute for
Radio Astronomy, Germany).
And the IPTA will only get better with the recent addition
of three new instruments: the Five-hundred-metre Aperture
Spherical Radio Telescope (FAST) in China, the MeerKAT
Radio Telescope in South Africa, and the Canadian Hydrogen
Intensity Mapping Experiment (CHIME) array in Canada.Looking ahead
In their quest to advance the field, scientists have devised
additional methods for detecting gravitational waves.
Astronomers will look for signs of gravitational waves in data
taken by the European Space Agency’s (ESA’s) Gaia satellite.
From its perch in space 1.5 million km beyond Earth, Gaia is
making extremely precise measurements of the positions and
motions of about 1 billion stars. Subtle shifts over many years
will indicate that Earth is bobbing on passing gravitational
waves, changing its position with respect to the stars.
“The drawback here is that Gaia’s data set will only be as
long as the mission, which is expected to be at most 10 years,”
Mingarelli says. “This limits the detection capabilities to
binaries with gravitational-wave periods of 5 years or less,
which is very restrictive.” A pair of billion-solar-mass black
holes with this period has “only” another 200,000 years
or so before it merges, she explains; those detectable withAt first glance, gravitational-wave
detection would seem to have very little
to do with planetary science. But when
it comes to precise timing of radio
pulses from millisecond pulsars, the
more stationary the reference point, the
easier. That’s why all the pulsar timing
arrays (PTAs) use the Solar System’s
centre of mass (the barycentre) rather
than Earth, which orbits the barycentre
at a speed of about 108,000 kph.
The barycentre is always located
inside or near the Sun, but it moves
around as the planets orbit our star. For
years, the PTAs used an ephemeris (the
table of coordinates, etc, for celestialbodies) calculated by JPL based on
the positions, velocities and masses of
the planets. But the PTA teams came
to realise that the JPL ephemeris is
not precise enough for pulsar timing,
where changes to the barycentre of a
few hundred metres in light travel time
add up to hundreds of nanoseconds.
Such errors can partially mimic the
effect of low-frequency gravitational
waves passing through our Solar
System, making it a limiting factor in the
teams’ ability to detect the stochastic
background.
PTA radio astronomers are now
working closely with JPL to refineits ephemeris, and the pulsar data
are actually helping to improve our
knowledge of the barycentre. “We have
been able to figure out a way to mostly
deal with the errors in the Solar System
ephemeris, albeit at a small cost to
our gravitational-wave sensitivity,”
says NANOGrav team member Scott
Ransom (NRAO). The Juno mission will
help further, he adds, by nailing down
the orbit and mass of Jupiter. Because
it’s so massive, the giant planet has a
big effect on the location of the Solar
System’s centre of mass, including in
its indirect gravitational effects on the
other outer planets.WBARYCENTRE The Sun and planets technically orbit their mutual centre of mass, called the solar system barycentre. The barycentre’s
location moves as the planets follow their elliptical orbits around the Sun. Sometimes it’s inside the Sun (diameter marked by yellow
circle), other times (including now) it lies outside the photosphere. The positive x-axis points in the direction of the vernal equinox.pulsar timing arrays, on the other hand, will be circling each
other for another 25 million years. Statistically speaking,
200,000 years is not much of a window to catch the binary’s
nanohertz gravitational waves.
And looking further afield, ESA is planning to launch the
Laser Interferometer Space Antenna (LISA) in the 2030s.
LISA will consist of three spacecraft orbiting the Sun in
an equilateral-triangle formation, each craft separated by
2.5 million kilometres. LISA is specifically tuned to catch the
spacetime ripples from merging black holes with masses of
roughly 10,000 to 10 million solar masses. ESA’s recent LISA
Pathfinder mission exceeded its performance goals, proving
LISA’s technological feasibility.
Taken together, these projects — ground-based
interferometers, pulsar timing arrays, Gaia, and LISA —
promise to usher in a revolutionary era of gravitational-
wave astronomy. By hearing gravitational rumbles across a
broad spectrum, scientists will piece together a story of the
universe’s most extreme objects, in a way that they could
not obtain by any other means.ROBERT NAEYE was editor in chief of AS&T’s US edition
MAP: GREGG DINDERMAN / from 2008 to 2014.
S&T
; BARYCENTER: LARRY MCNISH / CALGARY CENTER OF THE ROYAL ASTRONOMICAL SOCIETY OF CANADA
The Solar System barycentre
Unless we’re really lucky, it will probably take
10 to 15 years to build up enough data to
detect an individual binary.