LILA Horizon: Lunar GW Detection

• LILA Horizon is an advanced lunar-based gravitational-wave detection initiative that exploits the Moon’s low seismic activity and a 40 km interferometric baseline for enhanced sensitivity.
• It employs resonant amplification from lunar normal modes and state-of-the-art optical systems to detect millihertz to decihertz frequency signals from diverse astrophysical and cosmological events.
• Its design bridges the observational gap between terrestrial and space detectors, enabling precise studies in fundamental physics and planetary science.

LILA Horizon is the advanced phase of the Laser Interferometer Lunar Antenna (LILA) program, representing a major initiative in lunar-based gravitational-wave (GW) detection. By exploiting the Moon’s unique seismic and environmental conditions, LILA Horizon aims to achieve GW sensitivity extending to the cosmological horizon in the millihertz to decihertz frequency range. The facility’s objectives, technological innovations, detection capabilities, and comparative advantages reflect its anticipated impact on both fundamental physics and planetary science.

1. Scientific Objectives and Motivations

LILA Horizon is designed to enable detection of both astrophysical and cosmological GW sources across the millihertz to decihertz band. The primary objectives are:

• Increase GW Sensitivity to the Cosmological Horizon: Allow detection of signals from distant and faint sources, such as inspiraling neutron star and black hole binaries, supermassive black hole mergers across cosmic distances, and potentially the primordial GW background from the early universe (Creighton et al., 25 Aug 2025).
• Survey of Astrophysical Sources: Observe inspirals analogous to well-studied events (e.g., GW150914, GW170817) throughout the final year of evolution, as well as continuous signals from known pulsars and populations of white dwarf binaries (Creighton et al., 25 Aug 2025).
• Exploration of Primordial and Exotic Physics: Enable constraints on early universe phenomena and potential deviations from General Relativity, including possible signatures of dark matter candidates (Jani et al., 15 Aug 2025).

This extended reach is contrasted with the LILA Pioneer phase, which focuses on nearer sources and employs a shorter baseline suitable for initial engineering demonstrations.

2. Technical Design and Innovations

The technical foundation of LILA Horizon consists of several key advancements:

• Interferometric Baseline: Uses a 40-kilometer triangular configuration (opening angle ~60°) as opposed to the 5-kilometer right-angled configuration of Pioneer (Creighton et al., 25 Aug 2025). This results in greater strain sensitivity, as measured displacement scales directly with baseline length ($\delta x \propto h L$).
• Optical Components and Laser Systems: Employs unsuspended optical elements anchored to the lunar surface on a monolithic bench. Later mission stages may integrate suspended elements for enhanced sensitivity (Creighton et al., 25 Aug 2025). Laser systems of up to 500 mW power, frequency stabilization, and electro-optic modulators minimize technical sources of noise.
• Resonant Amplification via Lunar Normal Modes: Takes advantage of the Moon’s normal mode resonances, which amplify mechanical responses and thus incoming GW signals. Relevant parameters are the effective length-scale and quality factor:

$$L_\mathrm{eff} \approx 10^3\,\mathrm{m}\,(f/\mathrm{Hz})^{-1} \ Q \sim 10^2\,(f/\mathrm{Hz})^{-2/3}$$

These factors enhance GW sensitivity in the targeted band (Creighton et al., 25 Aug 2025).

• Thermal Noise Limitation: Across most frequency domains, the design aims for sensitivity limited only by thermal Brownian noise in the optics. The projected thermal noise is modeled as:

$$S_x(f) = 1.2 \times 10^{-33}\,\mathrm{m}^2/\mathrm{Hz}\,\left( \frac{T}{300\,\mathrm{K}} \right) (f/\mathrm{Hz})^{-1}$$

3. Gravitational-Wave Detection Capabilities

LILA Horizon is positioned to observe a diverse array of GW sources in a frequency range inaccessible to terrestrial and space-based detectors:

• Inspirals and Mergers: Tracks stellar-mass black hole and neutron star inspirals, enabling long-duration pre-merger monitoring and early warning for electromagnetic counterpart searches (Creighton et al., 25 Aug 2025).
• Supermassive Black Hole Binaries: Extends detection to binaries of mass $\sim 3 \times 10^6\,M_\odot$ at distances $\sim$ 10 Gpc.
• Continuous Sources: Sensitive to quasi-monochromatic GW emission from known galactic pulsars and white dwarf binaries.
• Primordial GW Background: May resolve a cosmological GW background, providing insight into early universe physics.
• Complementarity: Fills the observational gap between terrestrial detectors (above $\sim$ 10 Hz) and LISA (below $\sim$ 0.1 Hz), enabling multi-band GW astronomy from a single lunar site (Jani et al., 15 Aug 2025).

The Moon’s ground acts as a resonant amplifier when excited by GWs, further increasing sensitivity at low frequencies.

4. Signal Response and Noise Modeling

The fundamental strain sensitivity, accounting for resonant amplification, is characterized by:

• Inertial and Resonant Response:

$S_h(f) = \frac{S_x(f)}{L^2 \sin^2\theta}$

where $L$ is the interferometer baseline and $\theta$ is the opening angle.

• Mode-Specific Transfer Functions:

$$T_m(f) = \left[ \left( \frac{f_m}{f} - \frac{f}{f_m} \right)^2 + \frac{1}{Q^2} \right]^{-1}$$

Incorporated into the complete sensitivity function:

$$S_h(f) \approx \left[ \frac{S_x(f)}{L^2 \sin^2\theta} \right] \cdot \frac{f_0}{f} \Big/ T_0(f) \cdot \left[ 1 + \left( \frac{25 L_\mathrm{eff}^2}{R_\mathrm{Moon}^2} \sum_{m \neq 0} T_m(f) \right) \right]^{-1}$$

$L_\mathrm{eff}$ and $Q$ enhance response near lunar normal mode frequencies.

• Optimization: Laser phase noise, stray light, and temperature drifts are further mitigated by onboard stabilization techniques, leveraging the lunar vacuum and averaging over stable temperature windows.

5. Comparative Assessment with Terrestrial Detectors

Relative to LIGO, Virgo, and KAGRA (optimized for above 10 Hz), and LISA (below 0.1 Hz), LILA Horizon achieves unique coverage:

• Lunar Advantages: The Moon’s negligible seismic activity and natural vacuum reduce environmental noise sources that critically limit Earth-based detectors in the target band (Creighton et al., 25 Aug 2025).
• Sensitivity: With its extended baseline and resonant response, LILA Horizon is projected to reach strain sensitivities competitive with spaceborne proposals and unapproachable on Earth at millihertz to decihertz frequencies.
• Technical Challenges: Lunar deployment requires precise module placement, dust mitigation, and integration with lunar terrain infrastructure. These aspects are addressed in the phased development plan, utilizing U.S. Artemis, CLPS, and international partnerships (Jani et al., 15 Aug 2025).

6. Mission Development Strategy and Collaborative Framework

LILA Horizon follows the pioneer phase, which is tasked with initial demonstrations using a 3–5 km station on the lunar surface. LILA Horizon is characterized by:

• Triangular Interferometric Configuration: With a 40 km baseline, this phase capitalizes on the Moon’s seismic quietness for maximum sensitivity (Creighton et al., 25 Aug 2025).
• International and Cross-Disciplinary Collaboration: LILA mobilizes U.S. university, industry, government laboratory, and international expertise across optics, seismic instrumentation, quantum sensing, and lunar engineering.
• Alignment with Lunar Programs: Deployment and operations are integrated with CLPS and Artemis, ensuring technology support and infrastructure readiness (Jani et al., 15 Aug 2025).

7. Long-Term Prospects and Scientific Implications

The extended capability of LILA Horizon has several implications:

• Cosmological Reach: Enables population studies and precision cosmology with GW sources at high redshift, contributing sub-percent constraints to cosmological parameters.
• Fundamental Physics: Facilitates joint multi-band measurements with terrestrial and space-based GW observatories, opening opportunities for testing gravity theories and searching for exotic phenomena (e.g., axion clouds, primordial backgrounds) (Jani et al., 15 Aug 2025).
• Planetary Science Synergy: Provides unprecedented data for characterizing the lunar interior via normal mode resolution, advancing planetary differentiation and formation scenarios.
• Future Expansion: Successful LILA Horizon operation may motivate networks of lunar GW detectors, further improving coverage and event localization. Developments in cryogenic optics and suspended systems could yield additional sensitivity improvements (Creighton et al., 25 Aug 2025).

A plausible implication is increased collaboration between astrophysics and planetary science communities, catalyzed by the dual-use character of LILA as both GW observatory and geophysical laboratory.

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In sum, LILA Horizon advances lunar-based gravitational-wave science by integrating extended baselines, resonant amplification, and optimized optics to achieve cosmological-level sensitivity in the millihertz to decihertz band. The program’s phased development, technical innovations, and alliance with lunar exploration initiatives position it to make pivotal contributions to multi-messenger astrophysics and planetary science over the coming decades.