What 50 gravitational-wave events reveal about the Universe

Nature
Illustration of two black holes orbiting each other

The merging of two black holes releases gravitational waves.Credit: Science Photo Library/Alamy

Astronomers observed 39 cosmic events that released gravitational waves over a 6-month period in 2019 — a rate of more than one per week. The bounty, described in a series of papers published on 28 October, demonstrates how observatories that detect these ripples — usually created by the merging of two black holes — have dramatically increased their sensitivity since the first identification was made in 2015. The growing data set is helping astronomers to map how frequently such events have happened in the Universe’s history.

Gravitational waves are ripples in the fabric of space-time that are released by accelerating masses, in particular when two massive objects spiral into each other and merge. Their detailed properties provide numerous tests of Albert Einstein’s general theory of relativity, including some of the strongest evidence to date for the existence of black holes. And through gravitational waves, astronomers have gained a new way of observing the cosmos, next to electromagnetic waves and cosmic rays.

Infographic: Cosmic clashes. Line chart showing number of events observed by gravitational-wave detectors since 2015.

The latest data release describes events observed during half of the third observation run of the Laser Interferometer Gravitational-Wave Observatory (LIGO) — a pair of twin detectors based in Hanford, Washington, and Livingston, Louisiana — and its European counterpart Virgo, near Pisa, Italy. It is the collaboration’s second catalogue of events, following one published in December 2018 describing their first 11 detections. In all, the observation network has now observed 50 gravitational-wave events (see ‘Cosmic clashes’).

Most of the events are mergers of two black holes. The detectors have also caught sight of a handful of collisions between two neutron stars and at least one merger of one neutron star and one black hole. Mergers that involve neutron stars are especially interesting to astrophysicists because they are expected to release ordinary light as well as gravitational waves, which was confirmed in a merger of neutron stars seen in August 2017. A few of the most spectacular events in the catalogue had already been described in papers. Those include the largest black-hole merger yet and the most ‘lopsided’ one — in which two black holes of vastly different masses collided.

One surprising discovery is in the masses of the black holes involved in the mergers. Astrophysicists expected a sharp cut-off, with no black holes weighing more than 45 times as much as the Sun. “Now we’re seeing that it’s not so sharp,” says Maya Fishbach, a LIGO researcher at Northwestern University in Evanston, Illinois. The catalogue includes three events with outlier masses, including one announced in September with a black hole of 85 solar masses.

The wealth of data has now enabled LIGO–Virgo researchers to roughly estimate the rate at which black-hole mergers happen in an average galaxy. That rate appears to have peaked around eight billion years ago, following a period in which stars were forming — and some were later turning into black holes — at a particularly high rate, says Fishbach.

The catalogue also provides information on how the black holes spin, which holds the key to understanding how the objects came to orbit each other before they merged. It shows that, in some binary systems, the two black holes have misaligned axes of rotation, which would imply that they formed separately. But many other binaries appear to have roughly aligned axes of rotation, which is what astrophysicists expect when the two black holes began their lives as a binary star system. Two schools of thought in astrophysics have each favoured one of the two scenarios, but it now appears that both were correct, Fishbach says.

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