deci-Hz Gravitational Wave Observations on the Moon and Beyond

This document summarizes talks and discussions from the workshop “deci-Hz Gravitational Wave Observations on the Moon and Beyond” that took place at Johns Hopkins University between September 1 and September 3, 2025. The workshop focused on experimental proposals to observe gravitational waves in the deci-Hz band, including lunar detectors, laser interferometers in space, and atom interferometry; gravitational wave sources in the deci-Hz frequency band; and the multi-messenger and multi-band astronomy that would be enabled by these observations.

💡 Research Summary

The document is a comprehensive report of the “deci‑Hz Gravitational Wave Observations on the Moon and Beyond” workshop held at Johns Hopkins University from September 1–3, 2025. Its purpose is to bring together experts from three emerging experimental avenues—lunar‑based detectors, space‑based laser interferometers, and atom‑interferometry—to assess the feasibility of observing gravitational waves (GWs) in the deci‑hertz band (0.1 Hz to 1 Hz). This frequency range sits between the high‑frequency ground‑based detectors (LIGO‑Virgo‑KAGRA, 10–1000 Hz), the milli‑hertz space mission LISA (0.1–1 mHz), and the nanohertz pulsar‑timing arrays (PTAs). It is rich in astrophysical and cosmological sources that are otherwise inaccessible: intermediate‑mass black‑hole (IMBH) mergers, high‑eccentricity compact binaries, double white‑dwarf or neutron‑star inspirals, and primordial stochastic backgrounds from inflation or phase transitions.

The workshop began with a review of the Gravitational‑Wave International Committee (GWIC). While GWIC has successfully coordinated ground‑based and space‑based projects, its current roadmaps give only passing mention to the deci‑Hz regime. Participants argued that a dedicated GWIC sub‑committee, a specific deci‑Hz roadmap, and targeted funding streams are needed to give this band the strategic priority it deserves.

The second session focused on the limitations of next‑generation ground‑based observatories such as Cosmic Explorer (CE) and the Einstein Telescope (ET). Although CE’s 20–40 km arms and advanced signal‑recycling will push sensitivity an order of magnitude beyond Advanced LIGO, seismic and Newtonian gravity gradient noise still impose a hard low‑frequency cutoff near 1 Hz. Consequently, key science cases—IMBH mergers, residual orbital eccentricity, and precise localization of “dark sirens” (GW events without electromagnetic counterparts)—remain out of reach.

The third session presented three concrete deci‑Hz concepts:

1. Lunar Detectors –
   LGWA (led by Jan Harms) proposes to install ultra‑stable laser interferometers on the Moon’s surface, exploiting the Moon’s low gravity, negligible atmosphere, and extremely low seismic activity to measure minute deformations of the lunar body resonances induced by passing GWs. The LILA concept (Karan Jani, Volker Quetschk, James Tripp) adds a complementary approach based on monitoring lunar geoid variations. Technical challenges include thermal cycling of lunar regolith, micrometeoroid impacts, and long‑term instrument stability in a harsh vacuum environment.

2. Space‑Based Laser Interferometers –
   DECIGO (presented by Kento Komori) envisions a constellation of triangular spacecraft with arm lengths of several thousand kilometres, operating in the 0.01–10 Hz band. Its multi‑triangle design enables cross‑correlation for directional sensitivity and stochastic‑background subtraction. Critical R&D items are high‑power, low‑noise lasers, drag‑free control, and inter‑satellite ranging at picometer precision.
   GW‑Space‑2050 (Daniele Vetrugno) is a longer‑term vision for a 2050‑class mission that would push laser power, optical coating durability, and deep‑space communication to new levels, aiming for a factor‑10 improvement over DECIGO.

3. Atom Interferometry –
   MAGIS (Jason Hogan) uses laser‑cooled atoms in free fall to sense spacetime strain via phase shifts in matter‑wave interferometers. Because atom interferometers are fundamentally limited by quantum projection noise rather than photon shot noise, they can achieve lower white‑noise floors than laser interferometers in the same band. The primary hurdles are scaling the atom cloud to kilometre‑scale baselines, maintaining ultra‑high vacuum, and achieving long‑duration free‑fall in orbit.

Scientific discussions highlighted the unique opportunities offered by the deci‑Hz band. IMBH binaries (10³–10⁵ M⊙) merge at frequencies precisely in this window, providing the first direct observations of the “mass gap” between stellar‑mass and supermassive black holes and shedding light on seed formation in the early universe. High‑eccentricity binaries, which retain measurable eccentricity only at low frequencies, could be identified, revealing dynamical formation channels in dense star clusters or galactic nuclei. Early‑stage double white‑dwarf or neutron‑star inspirals would be observable weeks to months before entering the LIGO band, enabling pre‑merger electromagnetic alerts and refined multi‑messenger campaigns.

On the cosmology front, a deci‑Hz detector would be sensitive to primordial stochastic backgrounds that peak above the PTA band, potentially uncovering signatures of inflationary tensor modes, first‑order phase transitions, or cosmic‑string networks. Because the astrophysical foreground from compact binaries is weaker at 0.1–1 Hz than at nanohertz frequencies, the deci‑Hz window offers a cleaner view of the early‑universe GW spectrum.

The workshop concluded with a consensus that a coordinated, multi‑pronged approach is essential. Lunar detectors provide a low‑noise, low‑cost testbed; space interferometers deliver the long baselines needed for high‑sensitivity strain measurements; atom interferometers bring quantum‑limited performance. By integrating these technologies, the community can build a robust deci‑Hz network that bridges the gap between LISA and ground‑based observatories, dramatically improving sky localization for dark sirens, enhancing parameter estimation for all sources, and opening a new discovery space for fundamental physics.

Finally, the authors call for immediate actions: (i) formation of a GWIC deci‑Hz sub‑committee, (ii) allocation of seed funding from NSF, ESA, and national agencies for technology demonstrators, (iii) establishment of an international data‑sharing platform, and (iv) launch of a path‑finder mission (e.g., a small lunar interferometer or a CubeSat‑based atom interferometer) within the next five years. If these steps are taken, a full‑scale deci‑Hz observatory could be operational by the mid‑2030s to early‑2040s, ushering in a new era of gravitational‑wave astronomy.