Demonstration That Differential Length Changes of Optical Cavities Are a Sensitive Probe for Ultralight Dark Matter

Measurements of differential length oscillations of Fabry-Perot cavities provide a sensitive and promising approach to searching for scalar ultralight dark matter (ULDM). The initial demonstration sets direct lower bounds that are 1 to 2 orders of magnitude lower for two model ULDM distributions-a standard galactic halo and a relaxion star bound to Earth-ranging over a decade of ULDM mass and Compton frequency. The demonstration suggests how a much higher sensitivity to a much larger ULDM mass range can be obtained.