New study of merging black holes casts doubt on dark matter origin story
It had been proposed that merging black holes observed with gravitational waves formed not from dying stars but in the very earliest moments of our Universe. However, a new analysis has provided evidence against this hypothesis and the possibility that dark matter consists entirely of ‘primordial’ black holes.
All matter was created in the first few moments following the Big Bang. It has long been speculated that black holes, extremely dense regions of space with a gravitational pull so strong not even light can escape, could also have formed in this primordial maelstrom. These primordial black holes may hold the solution to the ‘dark matter’ problem of astronomy, providing an invisible source of gravity throughout space and time.
Black holes can also form when massive stars collapse under their own weight at the end of their life. Astronomers expect that the Universe contains a large population of these ‘stellar’ black holes, which until recently could not be observed directly. The new field of gravitational wave astronomy has given us a unique new window onto the physics of black holes.
The first direct detection of gravitational waves was made in September 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo observatory. These waves are ripples in the fabric of space-time, emitted by a pair of black holes spiralling into each other and eventually merging to form one large black hole. The waves travel across space unimpeded and can be detected on Earth with highly sensitive detectors.
A recent study led by a University of Edinburgh researcher has used measurements of black hole masses from LIGO-Virgo to test the primordial or stellar origin of these gravitational wave sources. The team used advanced statistical techniques to confront different predictions for the observed masses against the data, comparing scenarios in which all sources are either primordial or stellar black holes. They found that a model in which all black holes were primordial failed to match the detailed shape of the black hole mass distribution, with simple stellar merger predictions strongly favoured.
As a by-product of their analysis, the team was also able to rule out 100% of the dark matter being primordial black holes, with such a model producing many more merger events than observed. The new study instead limits this fraction to at most 7%. This result suggests that dark matter must be a new particle or force not predicted by standard physics.
Dr Alex Hall, a Senior Research Associate in the School of Physics and Astronomy who led the study, said:
Our results are strong evidence against all of the observed merging black holes being primordial in origin, and instead suggest that at least some of the sources must be the stellar black holes of standard astrophysics. Our work also demonstrates the remarkable power of this new data set when analysed within a rigorous statistical framework. With hundreds of new detections expected in the next few years, the potential of using gravitational waves to study the physics of the early Universe is extremely exciting.
The study has been published in Physical Review D.