Sky & Telescope - January 2019 - 4

SPECTRUM

by Peter Tyson

The How and the Why

The Essential Guide to Astronomy
Founded in 1941 by Charles A. Federer, Jr.
and Helen Spence Federer

WHEN IT COMES TO ASTROPHYSICAL studies these days, it's often

hard to decide what's more amazing: how scientists do them, or
why scientists do them. Both can boggle the mind.
Take the search for low-frequency gravitational waves. LIGO
detected high-frequency gravitational waves from the merger of
small black holes (S&T: Jan. 2018, p. 10). But LIGO can't observe the longer
waves emanating from the most massive black holes of all: supermassive black
hole binaries. These are pairs of behemoths each millions to billions of times
the mass of our Sun that, after the galaxies that host them merge, wind up
orbiting each other and eventually combining. As they circle and when they collide, they generate these low-frequency gravitational waves, which radiate outward at the speed of light and ripple the fabric of spacetime across the universe.
These waves have extremely long wavelengths, with
a billion seconds (>30 years) between peaks. And their
inﬂuence when they pass objects like Earth is exceedingly
subtle: Even a "strong" gravitational wave will cause an
object to stretch or shrink by just one part in a quadrillion.
Facing such challenges, researchers have devised an ingenious solution for how to detect these ripples, as Bob Naeye
Artist's concept of a
describes beginning on p. 22. They rely on the exquisitely
pulsar pulling in matstable timing of radio ﬂashes from pulsars, a variety of
ter from a nearby star
neutron star that whirls like a lighthouse beacon, dispatching its beams into the ocean of space. So predictable are these celestial clocks that
astronomers can time, often decades into the future, when radio signals from the
most stable pulsars will reach Earth to within a few hundred nanoseconds.
By monitoring across the galaxy a scattering of such cosmic clocks - which
together form a pulsar timing array - astronomers can use the Milky Way itself
as a detector. Consider it a kind of Galaxy Positioning System. The idea is to
watch for pulses that arrive at our radio telescopes ever so slightly earlier or ever
so slightly later than forecast. In this way, radio astronomers are striving to use
pulsars as stellar buoys to reveal the swells of passing gravitational waves.
Why go to all this trouble? Spacetime is full of ripples, warps, and holes that
we can't see but we can "hear." Successfully doing so will open up a whole new
area of astronomy, allowing us to address questions we can't address any other
way. How do galaxies merge and grow over cosmic time? What characterizes the
ultra-exotic environments around supermassive black hole binaries? In such
environments, we'll be operating at the limits of our understanding. What we
ﬁnd will doubtless challenge us with entirely new, unforeseen discoveries that will lead to even more remarkable
hows and whys.

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Sky & Telescope - January 2019

Table of Contents for the Digital Edition of Sky & Telescope - January 2019