In the mid-1980s, Bernard Schutz came up with a new solution to one of astronomy’s oldest problems: how to measure the distance from Earth to other objects in the cosmos. For generations, researchers have relied on an object’s brightness as a rough gauge for its distance. But this approach carries endless complications. Dim, nearby stars, for example, can masquerade as bright ones that are farther away.
Schutz, a physicist at the University of Cardiff, UK, realized that gravitational waves could provide the answer. If detectors could measure these ripples in space-time, emanating from interacting pairs of distant objects, scientists would have all the information needed to calculate how strong the signal was to start with — and so how far the waves must have travelled to reach Earth. Thus, he predicted, gravitational waves could be unambiguous markers of how quickly the Universe is expanding.
His idea was elegant but impractical: nobody at the time could detect gravitational waves. But, last August, Schutz finally got the opportunity to test this concept when the reverberations of a 130-million-year-old merger between two neutron stars passed through gravitational-wave detectors on Earth. As luck would have it, the event occurred in a relatively nearby galaxy, producing a much cleaner first measure than Schutz had dreamed. With that one data point, Schutz was able to show that his technique could become one of the most reliable for measuring distance. “It was hard to believe,” Schutz says. “But there it was.”
More mergers like that one could help researchers to resolve an ongoing debate over how fast the Universe currently is expanding. But cosmology is just one discipline that could make big gains through detections of gravitational waves in the coming years. With a handful of discoveries already under their belts, gravitational-wave scientists have a long list of what they expect more data to bring, including insight into the origins of the Universe’s black holes; the extreme conditions inside neutron stars; a chronicle of how the Universe structured itself into galaxies; and the most-stringent tests yet of Albert Einstein’s general theory of relativity. Gravitational waves might even provide a window into what happened in the first few moments after the Big Bang.
Researchers will soon start working down this list, with the help of the US-based Laser Interferometer Gravitational-Wave Observatory (LIGO), the Virgo observatory near Pisa, Italy, and a similar detector in Japan that could begin making observations next year. They will get an extra boost from space-based interferometers, and from terrestrial ones that are still on the drawing board — as well as from other methods that could soon start producing their own first detections of gravitational waves (see ‘The gravitational-wave spectrum’).
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Like many scientists, Schutz hopes that the best discoveries will be ones that no theorist has even dreamed of. “Any time you start observing something so radically new, there’s always the possibility of seeing things you didn’t expect.”
Spinning clues
For a field of research that is not yet three years old, gravitational-wave astronomy has delivered discoveries at a staggering rate, outpacing even the rosiest expectations. In addition to the discovery in August of the neutron-star merger, LIGO has recorded five pairs of black holes coalescing into larger ones since 2015 (see ‘Making waves’). The discoveries are the most direct proof yet that black holes truly exist and have the properties predicted by general relativity. They have also revealed, for the first time, pairs of black holes orbiting each other.
Researchers now hope to find out how such pairings came to be. The individual black holes in each pair should form when massive stars run out of fuel in their cores and collapse, unleashing a supernova explosion and leaving behind a black hole with a mass ranging from a few to a few dozen Suns.
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