It’s long been recognised that if such stars had any bumps – or “mountains” – on its surface, this would create a wobble that could spawn a gravity wave. Because the gravitational field of a neutron star is so intense, one might expect the star to be pulled into a perfect sphere. But that’s not necessarily the case. Four years ago, Australian scientists showed that if a neutron star had an orbiting companion star from which it pulled off matter, this matter could accumulate at the poles, propped up by the star’s magnetic field to prevent it from flattening out. In that way, mountains several kilometres across but only a few tens of centimetres high could develop, having about the mass of the planet Saturn.
Glampedakis and colleagues say that there is another way to build mountains on neutron stars which doesn’t rely on their having a companion star to cannibalise – and this points towards the second sought-after phenomena in fundamental physics. It supposes that the incredibly high density of the star could squash its atoms not into neutrons but into a sea of the still more fundamental particles of which atomic nuclei are made: quarks.
It’s not known if this “quark matter” can really exist. Some hope that it might be sighted in the Large Hadron Collider particle accelerator at CERN in Geneva, but a better bet could be to search for its signature in neutron stars – which would then in fact be quark stars, most probably with a core of quark matter coated with ordinary matter such as neutrons.
The researchers say that this quark matter will most likely form pairs, and as a consequence they can become stirred by vortices. The vortices will create something like a magnetic force that will make the star’s interior lumpy, with some bits more dense than others. In effect, these lumps would act like “internal mountains”, again producing a wobble that stimulates gravitational waves.
The calculations of Glampedakis and colleagues suggest that the Einstein Telescope might be able to detect gravitational waves produced this way. If, for example, the well-studied Vela and Crab pulsars – both produced by supernovae, the latter being the origin of the Crab Nebula – have cores made of paired quark matter, they should radiate strong enough gravitational waves to be just about observable. Depending on the details, they might even be spotted by upgrades of LIGO. In either case, this would not only vindicate Einstein but also offer a window on this weird, extreme form of matter.