An international team of researchers led by the Chinese University of Hong Kong’s Department of Physics has discovered a novel method that may verify the existence of new hypothetical dark matter particles – ultralight bosons – by observing gravitational waves emitted from a small black hole orbiting a larger one.

Dark matter refers to invisible matter that makes up around 85% of all the matter in the universe. Astronomers worldwide have strong evidence for the existence of dark matter, yet the current Standard Model of Particle Physics does not offer any support for it. Nonetheless, it is proposed that dark matter could be composed of cloud-like ultralight bosons that exhibit quantum mechanical effects to surround black holes. The 2015 discovery of gravitational waves – ripples in the fabric of space-time – has offered a new approach to detect and study dark matter and its ultralight bosons.

The research by CUHK’s astroparticle and cosmology professor Tjonnie Li, a member of the team who discovered gravitational waves, and his PhD student Otto Hannuksela shows that gravitational waves could be used to study black holes and their cloud systems and further ascertain the existence of ultralight bosons.

Professor Tjonnie Li, Assistant Professor from the Department of Physics of CUHK and his PhD student Otto Hannuksela. Photo: Handout

Prof Li noted that when a small black hole orbited a more massive one with a cloud, its orbital trajectory would be affected by the gravitational pull of the bigger black hole and by the friction of the cloud. His study showed gravitational waves from such systems could encode information on both the cloud and the black holes, which would allow scientists to study ultralight bosons in detail.

“Our research shows that a single gravitational wave measurement can be used to verify the existence of ultralight bosons by model selection to measure the boson’s mass,” he said at a press conference.

Li is pinning hopes on forthcoming gravitational wave detectors, such as the launch of the European Space Agency’s Laser Interferometer Space Antenna, to look for ultralight bosons and subsequently prove the existence of dark matter.

Li’s team consists of members from across the world who use gravitational waves in several other projects, including gravitational lensing, testing of general relativity, neutron star equation, inference of cosmological parameters as well as supernovae explosion mechanisms.