Collision of two 'dark matter stars' may have caused 'ripple' that reached Earth last year

Rob Waugh
·3 min read
illustration of Gravitational Waves.
Were the waves caused by colliding boson stars? (Getty)

A collision between two mysterious "dark matter stars" may have been behind a gravitational-wave "ripple" that reached Earth last year, researchers have suggested.

Gravitational waves are ripples in the fabric of spacetime that travel at the speed of light – and are produced by incredibly violent events like collisions between black holes or neutron stars.

But a signal picked up in 2020 may have had a far more mysterious origin, according to researchers at the Galician Institute of High Energy Physics (IGFAE) in Spain and the University of Aveiro in Portugal.

Instead of being produced by black holes, they suggest it might have been produced by a collision between boson stars – hypothetical objects made of "dark matter", which scientists believe makes up 27% of the universe.

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The research was published in Physical Review Letters.

Boson stars have never been detected, but the gravitational wave (GW190521) could have been created by a collision between two of the mysterious objects.

Dr Nicolás Sanchis-Gual, of the University of Aveiro, said: "Unlike regular stars, which are made of what we commonly known as matter, boson stars are made up of what we know as ultralight bosons.

"These bosons are one of the most appealing candidates for constituting what we know as dark matter.

"Boson stars are objects almost as compact as black holes but, unlike them, do not have a 'no-return' surface.

"When they collide, they form a boson star that can become unstable, eventually collapsing to a black hole, and producing a signal consistent with what [gravity wave detectors] LIGO and Virgo observed."

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Scientists have been able to detect and interpret gravitational waves since 2015, thanks to LIGO and Virgo.

To date, these detectors have already observed around 50 gravitational-wave signals – all of which match up to collisions between black holes and neutron stars.

The team compared the GW190521 signal to computer simulations of boson-star mergers, and found that these actually explain the data slightly better.

This result would not only involve the first observation of boson stars, but also that of their building block, a new particle known as ultra-light bosons.

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Such ultra-light bosons have been proposed as the constituents of what we know as dark matter.

Prof Carlos Herdeiro, from University of Aveiro, said "One of the most fascinating results is that we can actually measure the mass of this putative new dark matter particle, and that a value of zero is discarded with high confidence.

"If confirmed by subsequent analysis of this and other gravitational wave observations, our result would provide the first observational evidence for a long-sought dark matter candidate."

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