In 1916, Albert Einstein published the paper that predicted gravitational waves – ripples in the fabric of space-time resulting from the most violent phenomena in our distant universe, such as supernovae explosions or colliding black holes.
It took a century for Einstein’s theory to be proven when, in September 2015, the newly commissioned Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors observed gravitational waves that resulted from merging black holes approximately 1.3 billion light-years away. This watershed achievement earned three of LIGO’s founding members the 2017 Nobel Prize in Physics.
Today, more than 80 Australian researchers, amongst over 2000 scientists globally, have published data on the whole catalogue of gravitational-wave observations accumulated since September 2015. In total, 218 events have been recorded including three types of binary mergers: binary neutron star; neutron star–black hole; and binary black hole mergers.
Black holes have a gravitational pull so strong that nothing, not even light, can escape it. This makes them difficult to detect with conventional telescopes. They are characterised by their masses, measured in units equivalent to the mass of our Sun, and their spins.
According to lead Australian author, Christian Adamcewicz, from Monash University and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), many aspects of these black holes and the stars that form them remain a mystery.
“By observing the rapidly growing population of compact binary mergers through gravitational waves, thanks to our increasingly sensitive detectors, we’re uncovering vital clues about the lives and deaths of stars,” says Dr Ling Sun from the Australian National University and OzGrav. “Taking the 161 of the 218 mergers seen in the last decade, we’ve been able to decipher aspects of their behaviour from their masses,” Adamcewicz added.
“We found that most black holes have masses less than about 40 times that of our Sun. For a while, we’ve had this hypothesis that heavy black hole progenitors – the stars we would normally expect to turn into black holes heavier than 40 Suns – create supernovae so explosive that any evidence of them is annihilated. We’d never seen clear evidence for that previously, but this newly discovered drop off in our observations matches that prediction really well.”
He adds that “it’s not possible to test stuff on this scale in the lab, so, while we wait to collect the data we need, we rely on extrapolating and piecing together our knowledge from other areas. When you’re talking about the most extreme events in the Universe, these assumptions often break down. In this case, what we thought we would see with black holes in that mass range turned out to be accurate.” Sun shares a similar sentiment; “these cosmic collisions serve as natural laboratories, helping us piece together how black holes and neutron stars form, evolve, and interact across the Universe.”
