Neutron star “mountains” will cause ripples in space-time

The gravitational pull from the fast-spinning neutron star mountains creates ripples in space-time known as gravitational waves. The Laser Interferometer Gravitational-Wave Observatory (LIGO) searches for such waves. Credit: Charles Horowitz
The collapsed dead stars known as neutron stars are a trillion times denser than lead, and their surfaces are poorly characterized. Nuclear theorists have investigated the mountain-building mechanisms at work on the solar system’s moons and planets. Some of these mechanisms suggest that neutron stars likely have mountains.
The neutron star’s “mountain” would be much larger than any star on Earth. So massive that the gravitational pull from these mountains alone can cause tiny vibrations, or ripples, in the fabric of space-time.
Mountains, or non-axisymmetric deformations of rotating neutron stars, efficiently radiate gravitational waves. In a study published in Physical Review D, nuclear theorists at Indiana University consider similarities between mountains in neutron stars and features on the surfaces of objects in our solar system.
Both neutron stars and certain moons, such as Jupiter’s moon Europa and Saturn’s moon Enceladus, have thin crusts over deep oceans, but Mercury has a thin crust over a large metallic core. Thin sheets are generally likely to wrinkle. Europa has linear features, Enceladus has tiger-like stripes, and Mercury has curved, step-like structures.
Neutron stars with mountains may have similar types of surface features that can be discovered by observing continuous gravitational wave signals. The Earth’s innermost core is anisotropic, and its shear modulus is direction-dependent.
If the material in the neutron star’s crust also has anisotropy, a mountain-like deformation will occur, increasing in height as the star rotates faster. Such surface features could explain the maximum spin observed in neutron stars and the minimum possible deformation of radio-emitting neutron stars known as millisecond pulsars.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is currently looking for ripples that these mountains may create. This research will lead to the search for space-time oscillations known as continuous gravitational waves. These waves are so weak that they can only be detected with very detailed and sensitive searches carefully tuned to the expected frequencies and other signal characteristics.
The first detection of continuous gravitational waves will open a new window on the universe and provide unique information about neutron stars, the densest astronomical objects after black holes. These signals may also provide sensitive tests of fundamental natural laws.
Further information: JA Morales et al., Anisotropic neutron star crusts, mountains in the solar system, and gravitational waves, Physical Review D (2024). DOI: 10.1103/PhysRevD.110.044016. For arXiv: DOI: 10.48550/arxiv.2309.04855
Provided by the U.S. Department of Energy
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