Gravitational wave method may solve dark matter boson mystery

11 June 2019

A team of astrophysicists led by Dr Tjonnie Li (Croucher Innovation Award 2018) of the Chinese University of Hong Kong (CUHK) has discovered a novel method that might verify the existence of hypothetical particles called ultralight bosons, a possible component of dark matter.

Their method uses observation of gravitational waves emitted by a smaller black hole orbiting a larger black hole. The findings have been published in Nature Astronomy.

Invisible dark matter makes up around 85 per cent of all matter in the universe. Scientists have strong evidence for its existence, yet the current Standard Model of particle physics does not offer any explanation for it.

One possibility is that dark matter is made of ultralight bosons. Being extremely light may allow the bosons to exhibit quantum mechanical effects on a large astronomical scale. As such, the particle could couple with black holes to create massive clouds that are intertwined with black hole properties.

In the new research, led by Li’s PhD student Otto Hannuksela, the team proposed using gravitational wave signals from supermassive black holes to detect or rule out the existence of ultralight boson particles.

Gravitational waves, ripples in the fabric of space-time, were first detected in 2015 by the Laser Interferometer Gravitational-wave Observatory (LIGO) Scientific Collaboration, comprising an international group of more than 1,000 scientists worldwide. The discovery opened a new era in scientific research of the universe.

When a smaller black hole orbits a more massive black hole with a cloud, its orbital trajectory is affected by the gravitational pull of the black hole and the friction of the cloud. Gravitational waves from such systems hence encode information on both the cloud and the black hole, which would allow scientists to study the ultralight boson in detail.

“Our research shows that a single gravitational wave measurement can be used to verify the existence of ultralight bosons by model selection, rule out alternative explanations for the signal, and measure the boson’s mass,” Li said.

The forthcoming space-based gravitational-wave detector Laser Interferometer Space Antenna (LISA), a European Space Agency and NASA collaboration which seeks to avoid much of the noise that limits its ground-based counterparts, may be able to discover the ultralight boson and write a new chapter in the understanding of fundamental particles of nature. According to Li, even more information could be gleaned from gravitational waves if a smaller black hole happened to be spiralling into the supermassive black hole.

“With LISA, we can track the orbit of the smaller black hole over timescales of years,” he said. As the smaller black hole orbits, it would pass through any bosonic cloud surrounding the larger black hole. This would alter the orbit, and consequently the gravitational wave signal of the smaller black hole. This could provide a completely independent measurement of the shape of the larger black hole’s bosonic cloud. If the shape of the cloud inferred from the small black hole orbit were consistent with the shape predicted from the large black hole’s geometry, this could verify the existence of the bosonic cloud and therefore the light boson hypothesis. Otherwise, it could rule out ultralight bosons.

Li and Hannuksela’s protocol could be used once LISA is ready for operation in 2034.

Dr Tjonnie Li received his BA and MSc in Natural Sciences from the University of Cambridge (2009), and his PhD in Physics from the Dutch National Institute for Subatomic Physics (2013). Prior to joining the Chinese University of Hong Kong (CUHK) in 2015, he spent two years at the California Institute of Technology as a Rubicon Postdoctoral Fellow. He is now an Assistant Professor in the Department of Physics at CUHK and a member of the LIGO Scientific Collaboration. Dr Li received a Croucher Innovation Award in 2018.

To view Dr Li’s Croucher profile, please click here