My first Magazine Sky & Telescope - 01.2019

THE NEXT GRAVITATIONAL-WAVE REVOLUTION by Robert Naeye T 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 kilometers (680 miles) long, and the arm lengths and mirror reﬂ ectivities together determine which wave- lengths can be detected: roughly 60 to 15,000 km, corre- sponding to frequencies of 5 kHz to 20 Hz. This is the “sweet spot” for catching waves from the ﬁ nal 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 center of the combined galaxy, lock onto each other, and orbit a common center of gravity. At ﬁ rst, the holes draw closer by interacting with stars through a process called dynamical friction, a kind of gravi- tational 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 the black holes approach each other. If scientists wanted to build 22 JA N UA RY 2 019 • SK Y & TELESCOPE a LIGO-like instrument to catch these low-frequency space- time distortions, they would need to construct galaxy-size detectors. Good luck getting that through Congress! 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 Mother 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 mil- liseconds, 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 inspiraling 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 direc- tion 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 U.S. radio telescopes; the European Pulsar Timing Array (EPTA) uses ﬁ ve telescopes distributed across Europe; and the Parkes Pulsar Timing Array (PPTA) employs the venerable Parkes Telescope in New South Wales, Australia (see map on page 26). he American-based LIGO project and its European com- patriot, Virgo, will forever be hailed for opening up the ﬁ eld of gravitational-wave astronomy. LIGO and Virgo are tuned to a relatively high-frequency band of the gravita- tional-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 astrono- mers could only study 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 U.S., 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 ﬁ rst 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).

My first Magazine Sky & Telescope - 01.2019 | Page 24