Probing the Sound Speed of Dark Energy with a Lunar Laser Interferometer

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.

💡 Research Summary

The paper proposes a novel method to constrain the sound speed of dark energy, (c_s^2), by exploiting the unique capabilities of a lunar‑based laser interferometer, the Laser Interferometer Lunar Antenna (LILA). The sound speed governs how pressure perturbations in the dark‑energy fluid propagate; it determines whether dark energy remains smooth on sub‑horizon scales ((c_s^2\approx1)) or clusters on horizon‑scale modes ((c_s^2\ll1)). Existing cosmological probes (e.g., background expansion, ISW effect) only provide weak, integrated constraints on (c_s^2).

The authors first derive the linear perturbation equation for dark‑energy density fluctuations, \