It happened 900 million years ago, long before the first invertebrates emerged on Earth. An 8.9 solar mass black hole swallowed a 1.9 solar mass neutron star. The violent binge triggered gravitational waves, ripples in space-time like those of a stone falling into a pond. On January 5, two detectors from the LIGO-Virgo collaboration, located in the United States and Italy, captured one of the waves produced in one of the last orbits before the merger. Ten days later, the same detectors recorded a second, similar signal, this time caused by the merger, 1 billion years ago, of a black hole and a neutron star of 5.7 and 1.5 solar masses, respectively. Astrophysicists predicted the existence of these types of binary systems decades ago, but now they have confirmed their existence, a discovery published in The Astrophysical Journal.

«The pairs of black holes and neutron stars were for astronomers the ‘missing binary systems’. With this discovery, we can finally begin to understand how many of these systems exist, how often they merge and why we have not yet seen examples in the Milky Way, “said Astrid Lamberts, a researcher at the Virgo collaboration, on Tuesday.

Predicted by Einstein in 1915, gravitational waves are the consequence of the most violent cosmic events. The detection of the first, in September 2015, won the Nobel Prize in Physics for researchers from the LIGO and Virgo consortiums. Those first gravitational waves were the result of the merger of two black holes. Two years later, the first were detected by the collision of two neutron stars. Neutron stars are the smallest and densest known. So much so that a teaspoon of its matter can weigh a billion tons.

Confirming the existence of binary systems of black holes and neutron stars will allow scientists to “deepen our understanding of the most extreme phenomena in the Universe” and test the fundamental laws of physics under extreme conditions that could never be reproduced in the universe.