# Synopsis: Dark Matter Undetectable in Gravitational Waves

Calculations show that dark matter affects the propagation of gravitational waves but at a level far below the sensitivity of current detectors.

Astrophysicists propose that gravitational waves could be used to reveal properties of dark matter in much the same way that seismic waves are used to probe the structure of Earth’s interior. However, new calculations by Raphael Flauger of the University of California, San Diego, and Steven Weinberg of the University of Texas at Austin suggest that researchers are still some way from studying dark matter this way. They predict that, while dark matter could influence gravitational waves, the effects would be too small for near-future detectors to measure.

When light passes from vacuum into a material, it slows down because of interactions between the light wave and electromagnetic fields in the material. Flauger and Weinberg predict that something similar could happen when a gravitational wave encounters dark matter, causing the wave to slow. However, the predicted effect is miniscule: Interactions with dark matter would slow the speed of a gravitational wave with a wavelength the size of the Universe by roughly one part per million, while the speed of a gravitational wave akin to that detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015 would decrease by just one part in $1{0}^{45}$.

Flauger and Weinberg also found an additional effect for primordial gravitational waves, which originate from cosmic structural changes in the very early Universe. At that time, dark matter is thought to have moved at relativistic speeds and to have been strongly coupled to protons and neutrons, changing how it might have interacted with gravitational waves. Under these conditions, they predict that dark matter would both slow down a gravitational wave and decrease its amplitude. The duo says that both effects are currently undetectable. Researchers don’t yet know how to precisely measure the slow-down of a gravitational wave. And, so far, no one has detected primordial waves.

This research is published in Physical Review D.

–Christopher Crockett

Christopher Crockett is a freelance writer based in Montgomery, Alabama.

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