Einstein’s general theory of relativity predicts that enormous disturbances in space, such as the collisions of massive black holes or neutron stars, cause gravitational waves.
“They’re ripples in spacetime that travel at the speed of light,” explains astrophysicist Paul Lasky. “If I move a really large mass around quickly, then just like when a duck swims along a pond, the water ripples out in a little tail at its back. Similar thing with gravity … and that’s the thing that we detect.”
Gravitational waves were directly observed for the first time on 14 September, 2015, a century after Einstein published his theory. Two black holes merged – one was about 36 solar masses, and the other the size of about 29 suns – and when the ripples reached the Earth, they caused the two 4km-long arms of the two LIGO gravitational wave detectors in the US to record what might be the most subtle measurement ever, a movement one thousand times smaller than the nucleus of a proton.
When researchers were satisfied that gravitational waves had caused the nano-measurement, astrophysicists celebrated the world over. A new era of astronomy had become possible. Gravitational waves could now be used to observe distant and violent events that could not be detected by other means.
Dr Lasky was particularly excited when, in 2017, the first merger of neutron stars was observed. Neutron star mergers are “the biggest collisions in the universe”, he said at the time. “Our best estimate is that a little bit more than a full mass of the Earth equivalent of gold was created. Heavy elements like gold are hard to create in any other environment. It is very cool.”
But the information recorded by existing gravitational wave detectors was incomplete. Researchers could detect the waves caused as the two neutron stars approached each other, but not what happened when the stars collided.
“We never saw gravitational waves from after the system merged,” Dr Lasky says. “We’re not sensitive to those gravitational waves; the frequency is too high.”
Missing piece of the puzzle
NEMO – Neutron Star Extreme Matter Observatory – is an Australian proposal for a gravitational wave detector that’s able to supply this missing information. It would be able record the neutron star merger itself, “and maybe waves for one second after that”.
“Those gravitational waves are very, very high frequency,” Dr Lasky explains. “The wavelength is a lot shorter, and that would allow us to probe the insides of these neutron stars immediately after they’re born.”
He’s co-led a paper that makes the case for how and why NEMO should be built in Australia. The study was co-authored by the Australian Research Centre of Excellence for Gravitational Wave Discovery (OzGrav). It coincides with an Astronomy Decadal Plan mid-term review by the Australian Academy of Sciences that identified NEMO as a priority goal.
The case for an Australian base
NEMO is intended to supplement the data that would be supplied by future gravitational wave detectors. Detectors that would increase the sensitivity of LIGO by a factor of 10 are now on the drawing board – and if NEMO is built, the case could be made for a third-generation detector to be located in Australia.
These detectors are expected to cost at least $1 billion, and to have arms that are 40km long, Dr Lasky says. They would not be operational until at least 2035.
NEMO is a relatively modest proposal – it would have four kilometre-long arms and cost between $50 million and $100 million. It could also be built within a decade, allowing scientists to test developing technology.
Dr Lasky has a particular interest in merging neutron stars. After the 2017 neutron star merger was detected, he co-led a paper describing what happened to the neutron stars immediately after they collided.
“There are two possibilities,” he said at the time. “Either it immediately formed a black hole, which I don’t think is very likely. Or it could have formed what we call a hyper-massive neutron star. And this would be about two times the mass of our sun – it would last for approximately a few hundred milliseconds before collapsing to a black hole.
“I think a hyper-massive neutron star survived the collision,” he said. “It lasted for a few hundred milliseconds, and during that time a huge magnetic field got wound up,” launching a highly-energetic gamma ray burst. “And then the neutron star collapsed to form a black hole.”
NEMO would allow astrophysicists to test this hypothesis in future events.
“Neutron stars are an end-state of stellar evolution,” he says. “They consist of the densest observable matter in the universe.”
Although neutron stars are only about 20km across, they contain nearly 1.5 times the mass of our sun. If we could take a sugar cube of neutron star matter, it would weigh about 100 million tonnes on Earth.
“Such conditions are impossible to produce in the laboratory,” Dr Lasky says. “Theoretical modelling of the matter requires extrapolation by many orders of magnitude beyond the point where nuclear physics is well understood.”
NEMO could, theoretically, allow researchers to isolate and identify the quarks that are believed to be the smallest building blocks of matter. OzGrav is a joint research project led by Swinburne University, with nodes at the University of Melbourne, Monash, Adelaide, Western Australia and ANU.