http://www.skyandtelescope.com.au 15binary, which have a unique shape determined by the system’s
characteristics, including its distance (see facing page).
In order to find the background signal, astronomers need
a lot of pulsar pairs, Sesana explains. When a gravitational
wave passes, it stretches space in one direction and squeezes
it perpendicular to that direction. “So if two pulsars are
observed at a 90° angle, the pulse will arrive later from one
pulsar and earlier from the other,” he says. By making many
such correlations between pulsar pairs, and also seeing
signals from pulsars close together on the sky being affected
the same way, the PTA teams will eventually be able to tease
out the stochastic background signal.
Adding to the complexity, team members have to
disentangle subtle gravitational-wave signals from myriad
sources of noise. For example, despite the fact that millisecond
pulsars beat with a precise regularity similar to humanity’s
best atomic clocks, individual pulsars exhibit slight jitters that
must be accounted for. Electrons in interstellar space also
slightly delay the arrival of low-frequency radio waves.
Another source of noise stems from the fact that
astronomers’ reference point for pulsar timing is not Earth’s
position but the Solar System’s centre of mass, called the
barycentre. The barycentre’s exact location has to be known
to incredibly high precision for this work, because an error of
just a few dozen metres changes a pulsar’s timing by several
nanoseconds. Slight errors in the barycentre’s position can
thus partially mimic the stochastic background’s signal. “Our
knowledge of the planetary motions in the Solar System is now
effectively a limiting factor for us, which is quite astonishing,”
says Ransom (see ‘The Solar System barycentre,’ page 17).
Astronomers debate whether they’re already seeing hints
of the stochastic background in their data, and one or more
of the PTAs will probably detect it within the next five years.
But the real payoff will come after scientists watch the signal
build up over time. By disentangling all the complexities in
the signal, scientists will learn about the distribution of black
hole masses and the eccentricity of binary orbits. Perhaps
more important, astrophysicists should be able to discern a
great deal about the rate of black hole mergers as a function
of redshift, which in turn will be a proxy for how the galaxy
merger rate has changed over cosmic history. In fact, the
failure to detect the background by now seems to be ruling
out the most optimistic models in which the collision of two
large galaxies always produces a black hole merger.Detecting individual sources
Detecting one or more individual black hole binaries remains
the ultimate goal of those who use pulsar timing arrays. “It
would be like detecting a continuous wave, or tone,” JPL’s
Lazio says. “Much like if you are standing close to somebody
at a party, you can hear that person’s voice.”
Based on infrared survey data and cosmological
simulations, Chiara Mingarelli (Flatiron Institute) and
colleagues estimate that there should be several dozen010200 500 1000 1500 2000 2500 3000 3500
Days since observing start(b) Stochastic background02040600 500 1000 1500 2000 2500 3000 3500Pulse arrival time minuspredicted arrival time (ns)Pulse arrival time minuspredicted arrival time (ns)Pulse arrival time minuspredicted arrival time (ns)Days since observing start(a) Ongoing individual source010200 500 1000 1500 2000 2500 3000 3500
Days since observing start(c) One-time eventSEXAMPLES OF SOURCES Different gravitational-wave sources
will create distinct signals in pulsar data, and each pulsar gives a
slightly different view of the signal. Shown here are three simulated
examples using the periods of three real pulsars (three colours):
an ongoing signal from a pair of billion-solar-mass black holes
lying about 140 million light-years away (a); a background signal
combining many sources (b); and the signal from a single event,
such as a black hole merger, passing Earth on day 1,500 (c). The
graphs show what the data look like after astronomers remove the
effects of each pulsar’s spin and other things that affect the signal
along its path from the pulsar to Earth.nHz sources within 730 million light-years of Earth. These
inspiralling black holes will be detectable for a long time in
human terms. “A typical binary will spend about 25 million
years in the PTA band, which is in stark contrast to LIGO
sources!” she says.
How long a binary is detectable depends on its mass,
Mingarelli explains. More massive binaries produce stronger
LEAH TISCIONE / (and thus more easily detectable) gravitational waves. But
S&T
, SOURCE: SARAH BURKE-SPOLAOR