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

TL;DR

This paper proposes using a lunar laser interferometer like LILA to directly measure the sound speed of dark energy, providing new insights into cosmic acceleration physics through low-frequency gravitational wave observations.

Contribution

It introduces a novel lunar-based gravitational wave detection method to constrain dark energy sound speed, linking theoretical models to observable signals.

Findings

LILA can detect or constrain dark energy clustering.

The framework connects dark energy sound speed to measurable strain signatures.

Forecasts show unprecedented sensitivity for probing dark energy physics.

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.