My first Magazine Sky & Telescope - 01.2019 | Page 26

The Next Gravitational-Wave Revolution
All three projects started collecting pulsar timing data at
least a decade ago, and all have relatively similar capabilities
and sensitivities. An array’s frequency range depends on how
long it’s been operational; currently, the arrays span nanohertz
to millihertz wavelengths, with a sweet spot between 3 and
10 nHz, says Alberto Sesana (University of Birmingham, UK).
NANOGrav and EPTA observe many of the same Northern
Hemisphere pulsars, whereas PPTA concentrates on those vis-
ible from the Southern Hemisphere. Combined, they currently
watch roughly 75 pulsars, adding several new ones each year.
NANOGrav, EPTA, and PPTA are on the lookout for two
different kinds of sources. They can all catch the rumbles of
individual black hole binaries within several hundred million
light-years. But NANOGrav team member Scott Ransom
(National Radio Astronomy Observatory) says individual
sources probably won’t be their fi rst detection.
Instead, it’ll be the combined gravitational-wave signal of
all the inspiraling supermassive black hole binaries over time,
called the stochastic background. The stochastic background
is like a cacophony of voices in a football stadium, where
it’s impossible to distinguish any single conversation. The
contributing binaries will typically have black holes contain-
ing 100 million to 10 billion solar masses, with separations
of just a few thousandths of a light-year, and orbital periods
measured in years to decades.
Teasing out this background signal is an exceedingly dif-
fi cult task, because it consists of the superposition of gravita-
tional waves of different strengths and wavelengths coursing
through our corner of the galaxy from all directions. The
signal looks very different than waves from a specifi c binary,
which have a unique shape determined by the system’s char-
acteristics, including its distance (see facing page).
q HOW IT WORKS Gravitational waves ripple out from an inspiraling pair of supermassive black holes, slightly stretching and squeezing the
spatial dimensions that are perpendicular to the waves’ direction of motion (A, in 3D then with 2D cross sections). When these waves pass Earth
and nearby pulsars, they change the distance between each pulsar and Earth (B, as seen looking down on the crests in A). The white arrows in
the main graphic indicate how much the distance changes for each pulsar, determined by the angle with respect to the wave’s direction of motion.
As a pulsar’s distance oscillates, the arrival times of its signals change (see facing page). Because the pulsars lie at different distances from both
Earth and the waves’ source (white lines), different parts of the wave hit each pulsar at any given time. This difference means that each pulsar’s
timing shift probes a distinct slice of the gravitational wave pattern (C). By combining the changes in arrival times for many pulsars in different
parts of the sky, astronomers should be able to determine where the gravitational waves came from and what created them.
B
A
Direction of propagation
24
JA N UA RY 2 019 • SK Y & TELESCOPE
Gravitational
wave
Direction of
propagation
Pulsars probe different parts of wave
C