On 14 September 2015, scientists at the Laser Interferometer Gravitational-Wave Observatories (LIGO) made the first direct detection of a gravitational wave (GW). This wave, represented by a chirp of sound less than a second long, was humanity's first verifiable glimpse into the symphony of GW frequencies detailing the history of our universe. Since that first LIGO detection, enthusiasm for GW research has exploded, with projects being put forward for detectors like ground-based Cosmic Explorer (CE) and the European Einstein Telescope, and space projects like ESA's Laser Interferometer Space Antenna (LISA). While these ground-based and space GW detectors can expand observation of the GW spectrum beyond LIGO, no upcoming detector can measure GWs in the frequency range of deciHz to ~5Hz. The proposed Laser Interferometer Lunar Antenna (LILA) promises to fill this gap.
Being unable to observe this range might seem a small oversight in the face of this new era of science, but this limitation hampers human understanding of important cosmic events across the history of the universe. Access to the deciHz to ~5Hz band grants insight into astrophysical phenomena like Type Ia supernovae progenitors, binary neutron star mergers, intermediate-mass-black holes, and physics beyond the Standard Model. The technology necessary for terrestrial or space observation of the deci-Hz band will not exist for at least 2-3 decades, if such development is even possible. This delay is even more significant considering what existing technologies could achieve when constructed on the lunar surface. Deeper understanding of the history of the universe being just out of reach is a hard enough pill to swallow; choosing to refrain from understanding within reach is almost unreasonable.
Why the Moon?
Access to the deci-Hz to ~5Hz band cannot yet be found on the ground or in space, but studies have demonstrated it is guaranteed to be found on the Moon. The Moon is almost perfectly suited for GW detection. While Earth's seismic noise is too loud to observe any frequency lower than 10Hz, lunar seismic studies suggest that the Moon's background noise is much quieter than Earth's. Space observatories like LISA do not have to account for seismic noise, but quantum limitations prevent them from surveying frequencies higher than 0.1Hz.
Reduced seismic noise is far from the only advantage of a lunar GW observatory. Large scale interferometers like LIGO require an ultra-high vacuum environment, to protect against complicating factors like external noise and temperature variation that would interfere with the experiment. The Moon's near-vacuum atmosphere, lower gravity, and expansive surface area make it significantly easier to establish the conditions necessary for GW observation. The proposed LILA will consist of three end-stations in a triangular shape, separated by a few km; likely straddling the edge of a large crater. This positioning not only takes less preparation than a terrestrial observatory, it also does not require the land clearance and negotiations required to establish such a large facility on Earth.
As GW science inspires innovation in multi-messenger astrophysics and the NASA Artemis program renews interest in establishing scientific operations on the lunar surface, LILA promises to unveil some of the most expansive stories and songs of the evolution of our universe. Building such a detector not only promises insight into the 70% of the observable volume of our universe within the deci-Hz to ~5Hz band, it sets a new precedent for conducting space science on the lunar surface.