Pulsar Timing Arrays

THE 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