12 AUSTRALIAN SKY & TELESCOPE February | March 2019
THE NEXT GRAVITATIONAL-WAVE REVOLUTION by Robert NaeyeTHE US-BASED LIGO PROJECT and its European
compatriot, Virgo, will forever be hailed for opening up the
field of gravitational-wave astronomy. LIGO and Virgo are
tuned to a relatively high-frequency band of the gravitational-
wave spectrum, giving them the ability to hear chirps coming
from the death spirals of neutron stars and relatively low-
mass black holes.
But despite their success, both instruments are deaf to the
greatest of cosmic cataclysms: the inspiral and merger of two
supermassive black holes. In this sense, gravitational-wave
science right now can be likened to the era when astronomers
could study only visible light.
Fortunately, radio astronomers will soon be opening a
new window in the gravitational-wave spectrum, enabling
scientists to catch the collisions of much larger objects. Using
pulsars scattered across the galaxy, teams based in the US,
Europe and Australia have been patiently collecting data for
about a decade to look for ripples from supermassive black
holes. The international community is rife with optimism
that the first detections will be made in the next few years.
“If the universe holds no surprises for us, we should be
detecting gravitational waves relatively soon,” says radio
astronomer Joseph Lazio (Jet Propulsion Laboratory).Employing nature’s best clocks
LIGO and Virgo each detect gravitational waves by measuring
the minuscule difference a passing wave creates in the length
of each site’s two arms. The facilities use an infrared laser
as a yardstick, bouncing it off mirrors in the arms multiple
times. The beam-bouncing effectively makes the arms more
than 1,100 kilometres long, and the arm lengths and mirror
reflectivities together determine which wavelengths can
be detected: roughly 60 to 15,000 km, corresponding to
frequencies of 5 kHz to 20 Hz. This is the ‘sweet spot’ for
catching waves from the final inspiral and mergers of low-
mass binaries, which contain objects with about one solar
mass to a few hundred solar masses.
But what about binaries consisting of black holes with
millions or even billions of solar masses? Virtually every
large galaxy has at least one monster black hole lurking in
its core, and when large galaxies coalesce, their respective
black holes should gravitationally sink to the centre of the
combined galaxy, lock onto each other, and orbit a common
centre of gravity.
At first, the holes draw closer by interacting with stars
through a process called dynamical friction, a kind of
gravitational braking. Once the black holes are about a light-
year apart, their encounters with the stars that cross their
paths rob them of angular momentum and help their orbit
shrink further. Eventually, they’ll venture within a fraction
of a light-year of each other, at which point the loss of energy
via gravitational-wave emission will drive them together.
These gravitational waves will have wavelengths on the
order of a few to tens of light-years, growing shorter as theblack holes approach each other. If scientists wanted to
build a LIGO-like instrument to catch these low-frequency
spacetime distortions, they would need to construct galaxy-
size detectors.
Fortunately, there’s a much cheaper alternative. In the
late 1970s, Soviet astrophysicist Mikhail Vasilievich Sazhin
and American physicist Steven Detweiler conceived the idea
of timing pulsars. Pulsars are Nature’s most precise clocks,
neutron stars that spin with near-perfect regularity, beaming
radio pulses our way. And those that spin hundreds of times
per second, with rotation periods of 1 to 30 milliseconds,
are the best clocks of all. Radio astronomers have discovered
nearly 300 such millisecond pulsars, spread across the sky at
distances of thousands of light-years.
Gravitational waves from inspiralling supermassive black
hole binaries radiate outward at light speed, stretching and
squeezing spacetime over cosmological distances. As these
waves ripple through our galaxy, they subtly shift Earth’s
position with respect to the millisecond pulsars, so that the
pulsars appear like buoys bobbing on a turbulent sea. The
regular beats from some pulsars will arrive slightly early and
others will arrive slightly late. By timing millisecond pulsars
in different directions over many years, radio astronomers
should be able to detect these irregularities and which
direction the waves are coming from. But the effect is so tiny
that an individual pulsar’s signal might shift by only about 10
nanoseconds over decades of observation.In the background
Three teams have taken up this challenge. The North
American Nanohertz Observatory for Gravitational Waves
(NANOGrav) times pulsars using three US radio telescopes;
the European Pulsar Timing Array (EPTA) uses five telescopes
distributed across Europe; and the Parkes Pulsar Timing Array
(PPTA) employs the venerable Parkes Telescope in New South
Wales (see map on page 16).If scientists wanted to build a LIGO-like
instrument to catch these low-frequency
spacetime distortions, they would need
to construct galaxy-size detectors.