Five years after the first discovery of gravitational waves that allow us to listen to extraordinary things in the universe like black holes and neutron stars colliding, an international team of scientists, including those from The University of Western Australia and OzGrav ARC Centre of Excellence, are continuing their hunt for new discoveries and insights into the Universe.
Using super-sensitive, kilometre-sized LIGO detectors in the United States, and the Virgo detector in Europe, the team has witnessed explosive collisions of black holes and neutron stars. However, recently, the scientists have been looking for something quite different: the elusive signal from a solitary, rapidly spinning neutron star.
Neutron stars are super dense stars, with a density similar to squashing a sky scraper into a grain of sand. When a neutron star spins at hundreds of revolutions per second and isn't perfectly spherical, it will wobble about a bit and omit a faint humming sound. Spinning neutron stars make gravitational waves if they are not perfectly round. Scientists call this a continuous gravitational wave.
"Neutron stars, which are composed of matter collapsed in on itself like a giant atomic nuclei, have to be one of the most exotic stars in the Universe."
Dr Carl Blair
The faint humming, like the constant buzz of a faraway bee, is much more difficult to detect so the spinning neutron stars have proved elusive.
Scientists have used a few different strategies to detect them, such as honing in on a particular direction to spot them and listening keenly to try to pick out any buzzing sounds in the background, but so far they are yet to unravel the mysteries of the neutron star.
One of the lead researchers, UWA postdoctoral fellow and OzGrav researcher Dr Carl Blair said gravitational waves were being used to probe the most exotic objects in the Universe.
"Neutron stars, which are composed of matter collapsed in on itself like a giant atomic nuclei, have to be one of the most exotic stars in the Universe," Dr Blair said.
"We don't know that much about neutron stars because they're so small and strange. We want to understand if they are hard or soft, and if they emit energy in the form of gravitational waves when they are spinning fast.
"While there is no evidence yet for continuous gravitational waves from neutron stars, limits have been placed on how wobbly a neutron star is from the fact that we haven't measured gravitational waves from them yet."
In addition, recent studies announced by the international research team -including the US/international LIGO Scientific Collaboration, European Virgo Collaboration and Japanese KAGRA Collaboration have focused on pulsars.
These are neutron stars that act as cosmic lighthouses, beaming out copious energy in the form of radio waves. Pulsars are like giant spinning magnets, except they're billions of times stronger than the ones stuck to your fridge. So strong, in fact, that the magnetic field distorts the shape of the neutron star, and may lead to a tell-tale hum of continuous gravitational waves.
While recent studies have not picked up anything, by not hearing the hum of continuous gravitational waves, some theoretical predictions about the nature of neutron stars have been shown to be wrong.
Scientists estimate that there are billions of neutron stars in the Milky Way with a faint murmur of continuous gravitational waves. Further studies have therefore taken an "ears wide open" approach, combing through the LIGO and Virgo data for any hint of a signal.
The results so far suggest that these murmurings are extremely quiet and out of the detectors range. However, as detector technology becomes more advanced and sensitive, the first ever detection of continuous gravitational waves could soon become a reality.
