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Probing the sound speed of dark energy with a lunar laser interferometer

Alfredo Gurrola, Robert J. Scherrer, and Oem Trivedi

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Phys. Rev. D 114, 023507 – Published 6 July, 2026

DOI: https://doi.org/10.1103/gpyr-9tvw

Abstract

The sound speed of dark energy encodes fundamental information about the microphysics underlying cosmic acceleration, yet remains essentially unconstrained by existing observations. We demonstrate that a lunar-based laser interferometer, such as the proposed Laser Interferometer Lunar Antenna (LILA), can directly probe the sound speed of dark energy by measuring the real-time evolution of horizon-scale gravitational potentials. Operating in the ultra-low-frequency gravitational band inaccessible from Earth, LILA is sensitive to scalar metric perturbations sourced by dark energy dynamics. Using both fluid and effective field theory descriptions, we develop a complete framework linking dark energy sound speed to observable strain signatures. We construct a likelihood pipeline and Fisher forecasts, showing that LILA can either detect clustering dark energy or exclude broad classes of models with unprecedented sensitivity. This establishes lunar interferometry as a novel and powerful probe of the physics driving cosmic acceleration.

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Phys. Rev. D 114, 023507– Published 6 July, 2026
  • Received 7 February 2026
  • Accepted 2 June 2026
Article part of CHORUS
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