In Real Science

Nicholas J Albertini

Physicists Charles Horowitz and Kai Kadau have found surprising evidence that neutron stars could radiate much more energy in the form of gravitational waves than previously theorized. In their report to Physical Review Letters, the scientists describe findings from simulations that they ran on a Los Alamos supercomputer.
The simulations of a rotating neutron star suggest that the strength of neutron degenerate matter could allow large mountains to be generated on the surface of such a star during its formation from a collapsing stellar core.
When a star’s core collapses during a supernova explosion, if the collapsing matter is heavy enough, it will get squeezed so hard that the electrons of its atoms get pushed into the atoms’ nuclei. When this happens, the electrons combine with protons to form neutrons.
This compression leaves only neutrons in the atoms, which all come together under the force of the collapsing matter’s gravity to form what is called a neutron degenerate. This form of matter is extremely dense, at about 5×10^17 kg/m^3.
These simulations demonstrate that this form of matter should be strong enough to allow for mountains with masses approaching that of the earth to remain on the surface of the star after collapse.
If such large mountains can remain on a spinning neutron star, they will distort the curvature of spacetime as the star rotates and produce gravitational waves – or waves of curvature in spacetime that travel at the speed of light and distort the lengths of objects and the flow of time when they pass by.
The researchers suggest that the energy released from spinning neutron stars with such mountains, energy in the form of gravitational radiation, could be up to 100 times more than previously thought possible.
This prediction has astronomically verifiable implications. As a spinning neutron star produces gravitational waves in this way, it will slow down. If the star has such a large mountain and produces stronger waves, it will slow down even faster. Also, physicists and astronomers now have gravitational wave detectors with which to observe this phenomenon, such as the laser interferometer gravitational-wave observatory built by Caltech and MIT.
LIGO is currently in operation with two detectors on either side of the country, and it has successfully detected many gravitational signals. There is an advanced version of this device currently in the works.
Other gravitational observatories have been proposed as well, including a satellite system and, recently, a method that uses observations of pulsars. Even if the waves produced by mountains on spinning neutron stars are outside of the frequency range of such detectors, rotating stars with such deformities would slow down faster than otherwise expected.
It will be interesting to see what kind of evidence can be found for this prediction in the near future. With so much astronomical data collected from pulsars, the evidence may already be out there.