Gravitational wave detectors could unlock dark matter mystery

The same instruments used in the Nobel Prize-winning discovery of gravitational waves caused by colliding black holes could help unlock the secrets of dark matter, scientists, including one of Indian origin, say. Dark matter is a mysterious and as-yet-unobserved component of the universe. Its nature remains unknown, but scientists estimate that it is five times as abundant as ordinary matter throughout the universe.

"The nature of dark matter is one the greatest mysteries in physics," said Emanuele Berti, associate professor at University of Mississippi (UM) in the US. "It is remarkable that we can now do particle physics - investigate the "very small" - by looking at gravitational- wave emission from black holes, the largest and simplest objects in the universe," Berti said. The research, published in the journal Physical Review Letters, details calculations by a global team of scientists which show that gravitational-wave interferometers can be used to indirectly detect the presence of dark matter.

Calculations show that certain types of dark matter could form giant clouds around astrophysical black holes, said researchers, including UM graduate student Shrobana Ghosh. If ultralight scalar particles exist in nature, fast- spinning black holes would trigger the growth of such scalar "condensates" at the expense of their rotational energy. This will produce a cloud that rotates around the black hole, now more slowly-spinning, and emits gravitational waves, pretty much like a giant lighthouse in the sky, they said.

"One possibility is that dark matter consists of scalar fields similar to the Higgs boson, but much lighter than neutrinos," said Paolo Pani, scientist at UM. "This type of dark matter is hard to study in particle accelerators, such as the Large Hadron Collider at CERN, but it may be accessible to gravitational-wave detectors," Pani said. The team studied gravitational waves emitted by the "black hole plus cloud" system.

Depending on the mass of the hypothetical particles, the signal is strong enough to be detected by the Laser Interferometer Gravitational-wave Observatory in the US, and its European counterpart Virgo, as well as by the future space mission Laser Interferometer Space Antenna. "Surprisingly, gravitational waves from sources that are too weak to be individually detectable can produce a strong stochastic background," said Richard Brito, who led the study.

"This work suggests that a careful analysis of the background in LIGO data may rule out - or detect - ultralight dark matter by gravitational-wave interferometers," Brito said.