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

TL;DR

The paper addresses constraining the dark energy sound speed $c_s^2$ by observing horizon-scale gravitational potential evolution with a lunar-based interferometer. It develops a framework using fluid and EFT descriptions to connect $c_s^2$ to observable strain spectra via a transfer function $T_\Phi(k/k_J)$ and a mapping $P_h(f)$. A cosmology-calibrated mock strain spectrum demonstrates that low-frequency power is enhanced for smaller $c_s^2$ and that high-frequency behavior becomes insensitive to $c_s^2$. Fisher forecasts show LILA can detect clustering dark energy or exclude broad model classes, especially when combined with Planck priors, establishing lunar interferometry as a powerful, complementary probe of cosmic acceleration.

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.

Figures (5)

• Figure 1: Mock strain power spectra illustrating enhanced low-frequency power for clustering dark energy.
• Figure 2: Fisher contour ellipses in the $(w,c_s^2)$ plane for a lunar laser interferometer, centered on the fiducial clustering dark energy model $(w,c_s^2)=(-1,10^{-2})$. Shown are the joint 2$\sigma$, 3$\sigma$, and 5$\sigma$ confidence regions obtained using the cosmology-calibrated mock strain power spectrum.
• Figure 3: Fisher contour ellipses in the $(w,c_s^2)$ plane for a lunar laser interferometer, centered on the fiducial clustering dark energy model $(w,c_s^2)=(-1,10^{-3})$. Shown are the joint 2$\sigma$, 3$\sigma$, and 5$\sigma$ confidence regions obtained using the cosmology-calibrated mock strain power spectrum.
• Figure 4: Fisher contour ellipses in the $(w,c_s^2)$ plane for a lunar laser interferometer, centered on the fiducial clustering dark energy model $(w,c_s^2)=(-1,10^{-2})$ with $w$ constrained within 3% of -1 using a Gaussian prior. Shown are the joint 2$\sigma$, 3$\sigma$, and 5$\sigma$ confidence regions obtained using the cosmology-calibrated mock strain power spectrum.
• Figure 5: Fisher contour ellipses in the $(w,c_s^2)$ plane for a lunar laser interferometer, centered on the fiducial clustering dark energy model $(w,c_s^2)=(-1,10^{-3})$ with $w$ constrained within 3% of -1 using a Gaussian prior. Shown are the joint 2$\sigma$, 3$\sigma$, and 5$\sigma$ confidence regions obtained using the cosmology-calibrated mock strain power spectrum.