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.

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