It's been more than three years since astronomers first detected gravitational waves – ripples in the very fabric of spacetime caused by some of the biggest cataclysms in the cosmos. Now, an international team of scientists has presented the full catalog of these events, gathered over the last few years across two observation runs.
The catalog contains a total of 11 gravitational wave events, four of which are being reported publicly for the first time. These events all begin as collisions between massive cosmic objects – mostly black holes, but in one case a pair of neutron stars – which are powerful enough to send ripples of energy spreading through the universe.
By the time those ripples reach us here on Earth, though, the distortions they create are minuscule, about a thousandth of the width of a proton. By beaming lasers down long tunnels and watching for disturbances, facilities like the Laser Interferometer Gravitational-wave Observatory (LIGO) in the US and the Virgo gravitational-wave detector in Europe are designed to detect even the tiniest ripples.
The four newly-described observations all took place in July and August 2017, towards the end of LIGO's second observing run. All four were the result of collisions between black holes, and some of them marked new records.
An event known as GW170729 has taken two top honors. Detected on July 29, 2017, this event is both the most distant and most massive gravitational wave source found so far. These two black holes smashed together 5 billion light-years away – in other words, 5 billion years ago – and released energy equivalent to the mass of five Suns.
Another new event, GW170818, was pinpointed more precisely than any other black hole merger. The two objects crashed into each other about 2.5 billion light-years from Earth, and the resulting waves were detected by both LIGO and Virgo, allowing astronomers to identify its position in the sky to within 39 square degrees. That precision is second only to the neutron star smashup detected just one day earlier, which was easier to pinpoint because it was accompanied by light and radio signals.
The more gravitational wave events detected, the more we can learn about black holes, gravity, and the formation of galaxies.
"We are learning things about the population (of black holes), such as how frequently binary black holes merge in the universe (once every few hundred seconds somewhere in the universe) and whether small (low mass) or large (high mass) black holes are more common – there are many more light black holes (around 5-10 times the mass of the Sun) in the universe than heavy black holes (around 30-40 times the mass of the Sun), but the heavy ones are 'louder' in gravitational-waves, and easier to 'hear' colliding," says Simon Stevenson, an author on one of the studies describing the work.
LIGO's next observing run is due to kick off in the first half of 2019, with more precision than ever before.
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