Gravitational waves and neutron stars

Astrophysicist Professor Andrew Melatos was one of 100s of international scientists who found the first evidence of the existence of gravitational waves. The three senior physicists behind the momentous discovery received the 2017 Nobel Prize in Physics.

Einstein predicted the existence of such waves about 100 years earlier, when formu- lating his general theory of relativity. The first detected gravitational wave arose when two orbiting black holes, together the size of about 65 suns, coalesced some 1.3 billion light years away.

“Researchers more-or-less gave up looking for the tiny waves until the advent, among other things, of extremely good lasers and computers,” Professor Melatos said. “Even today, my research remains computationally limited. In this field, more is always better.”

Gravitational waves are produced when a massive object accelerates or is disturbed. They carry energy and radiate from the ‘event’ that caused them, rather like ripples from the point a stone enters a pond.

Non-trivial geometry of neutron vortices pinned to the misaligned array of flux tubes in a neutron star arises naturally due to the “frustration” of the system caused by the competing forces on the superfluid. The vortices do not crystallise into a rectilinear array as they do when the rotation and magnetic axes are aligned. Instead, they settle into a complex, partially polarised, interlocked pattern, called a "vortex crystal";. This figure depicts the deformation and pinning of vortices (shaded in red) in a vortex crystal due to interactions with flux tubes (shaded in blue) which are tilted 90° with respect to the rotation axis. (a) Close-up snapshot of “vortex spike”. (b) A pinned vortex.    (Drummond L. V., Melatos A., 2018, Monthly Notices of the Royal Astronomical Society, 475, 910).

Non-trivial geometry of neutron vortices pinned to the misaligned array of flux tubes in a neutron star arises naturally due to the “frustration” of the system caused by the competing forces on the superfluid. The vortices do not crystallise into a rectilinear array as they do when the rotation and magnetic axes are aligned. Instead, they settle into a complex, partially polarised, interlocked pattern, called a "vortex crystal";. This figure depicts the deformation and pinning of vortices (shaded in red) in a vortex crystal due to interactions with flux tubes (shaded in blue) which are tilted 90° with respect to the rotation axis. (a) Close-up snapshot of “vortex spike”. (b) A pinned vortex.

(Drummond L. V., Melatos A., 2018, Monthly Notices of the Royal Astronomical Society, 475, 910).

The gravitational wave detected in the Nobel Prize-winning work was vanishingly small at 10-19 metres. Enhancing the sensitivity of the detection process is a key challenge to finding more gravitational waves and thereby learning about the nature of gravity and the immense events in the universe that generated them.

Professor Melatos provided input on theoretical aspects and computer modelling to the successful project for about 10 years before the gravitational wave was detected. At the outset, he used supercomputing provided

by a consortium of Victorian universities. He then made the seamless transition with the formation of MASSIVE 8 years ago.

He now draws on the supercomputing provided by MASSIVE to design, write and test algorithms (some not unlike those found in mobile phones) to extract a delicate signal from all the noise caused by interfering vibra- tions from sources as wide-ranging as ocean waves, traffic, construction and even the wind.

“From my experience with supercomputing, when the overall service provision doesn’t work, it’s rarely the hardware. It’s all the rest, like management of data. In addition to providing reliable hardware, MASSIVE does a fantastic job of support and data manage- ment,” Professor Melatos said.

A second project that relies on MASSIVE supercomputing relates to neutron stars, which form at the end of a star’s life when

there is insufficient fuel to support the star against gravity.

The star collapses and explodes to form a super nova. Most of the resulting material is scattered across the universe, but some of it forms an extremely dense neutron star—one teaspoon of which is said to weigh 1 billion tonnes!

With the help of MASSIVE, the Melatos lab is conducting computer-intensive simula- tions of the core material of the neutron star, seeking answers to such questions as: Is it a superfluid? Is it like liquid helium which, at absolute zero, has no friction and could flow down a pipe forever?