Neutron stars, gravitational waves, and dark matter

Dark matter’s gravitational interactions with itself and with normal matter are so pervasive that they shape the evolution of the universe. Frustratingly for physicists, dark matter’s nongravitational interactions—the clues that could reveal what dark matter is made of—are so feeble that lab experiments to measure them have yielded only upper limits. What’s more, the prevailing cosmological model, ΛCDM, can account for many of the large-scale structures of the universe by presuming that dark-matter particles interact only through gravity.

Spotting a weak interaction sometimes entails creating a strong perturbation. As Shuo Cao of Beijing Normal University and his colleagues propose in a new paper, the perturbation needed to reveal dark matter’s nongravitational behavior could come from the merger of two neutron stars. The gravitational waves produced by the inspiralling neutron stars pass inevitably through the dark-matter halo that encompasses the host galaxy and those of any other galaxies in the line of sight. If dark-matter particles interact nongravitationally with each other, the waves will experience a force akin to viscosity. And that viscosity will damp and retard the waves in a potentially measurable way.

Viscously damped gravitational waves from a neutron star merger would reach an observer after whatever burst of electromagnetic radiation the merger produces. Another source of delay arises between gravitational waves that reach the observer directly and those that take a longer path because an intervening galaxy, acting as a gravitational lens, has focused them into the line of sight.

Can those delays be measured with sufficient precision to yield the viscosity of dark matter? To determine the answer, Cao and his colleagues had to assume plausible values for their sought-after effect. Their starting point is the difficulty that ΛCDM faces in accounting for the number of dwarf galaxies and for the amount of dark matter at the centers of regular galaxies. The source of the discrepancy is not known. But if it’s due to dark matter interacting nongravitationally with itself, the magnitude of the discrepancy yields an estimate of dark matter’s putative viscosity. Another ingredient in the recipe is a plausible rate of neutron star mergers out to a redshift of 5.

The expected signal from viscosity-induced time delays is too weak for the currently operating gravitational wave detectors, LIGO and Virgo. But after running simulations under different conditions, Cao and his colleagues conclude that the signal is within the reach of two proposed new detectors: the ground-based Einstein Telescope and the space-based Big Bang Observer. (S. Cao et al., Mon. Not. R. Astron. Soc. 502, L16, 2021 .)