This is why we need to put a gravitational wave observatory on the moon

Scientists discovered the first long-predicted gravitational wave in 2015, and since then researchers have been hungry for better detectors. But Earth is warm and seismically noisy, and that will always limit the effectiveness of Earth-based detectors.

Is the moon the right place for a new gravitational wave observatory? It could be. Sending telescopes into space worked well, and putting a GW observatory on the moon could do the same, although the proposal is clearly very complex.

Most of astronomy is about light. The better we can sense it, the more we learn about nature. That’s why telescopes like Hubble and JWST are in space. Earth’s atmosphere distorts telescope images and even blocks some light, such as infrared. Space telescopes avoid both problems and have revolutionized astronomy.

Gravitational waves are not light, but detecting them still requires extreme sensitivity. Just as Earth’s atmosphere can introduce ‘noise’ into telescopic observations, Earth’s seismic activity can cause problems for gravitational wave detectors. The moon has a major advantage over our dynamic, ever-changing planet: there is much less seismic activity.

We have known since the Apollo days that the moon exhibits seismic activity. But unlike Earth, most of its activity is related to tidal forces and small meteorite impacts. Most of the seismic activity is also weaker and much deeper than that of Earth. That caught the attention of researchers who developed the Lunar Gravitational-wave Antenna (LGWA).

The LGWA developers have written a new article, “The Lunar Gravitational-wave Antenna: Mission Studies and Science Case,” and posted it on the website. arXiv preprint server. The lead author is Parameswaran Ajith, a physicist/astrophysicist from the International Center for Theoretical Science, Tata Institute of Fundamental Research, Bangalore, India. Ajith is also a member of the LIGO Scientific Collaboration.

A gravitational wave observatory (GWO) on the moon could bridge a gap in frequency coverage.

“Given the size of the moon and the expected noise produced by the moon’s seismic background, the LGWA could detect GWs from approximately 1 MHz to 1 Hz,” the authors write. “This would make the LGWA the missing link between space-borne detectors such as LISA with peak sensitivities around a few millihertz and proposed future terrestrial detectors such as Einstein Telescope or Cosmic Explorer.”

If built, the LGWA would consist of an array of planetary-scale detectors. The unique conditions on the moon will allow the LGWA to open a larger window into the science of gravitational waves. The moon has extremely low background seismic activity that the authors describe as “seismic silence.” The lack of background noise will allow for more sensitive detections.

The moon also has extremely low temperatures in its permanently shadowed regions (PSRs). Detectors must be super cooled, and the cold temperatures in the PSRs make that task easier. The LGWA would consist of four detectors in a PSR crater at one of the moon’s poles.

The LGWA is an ambitious idea with a potentially groundbreaking scientific payoff. Combined with telescopes that observe the entire electromagnetic spectrum and with neutrino and cosmic ray detectors – so-called multi-messenger astronomy – this could increase our understanding of a whole range of cosmic events.

The LGWA will have some unique capabilities for detecting cosmic explosions. “Only LGWA can observe astrophysical events involving WDs (white dwarfs), such as tidal disturbance events (TDEs) and SNe Ia,” the authors explain. They also point out that only the LGWA can warn astronomers weeks or even months in advance of the merger of compact solar-mass binary stars, including neutron stars.

The LGWA will also be able to detect lighter binary stars with an intermediate black hole (IMBH) in the early universe. IMBHs played a role in forming today’s supermassive black holes (SMBHs) at the heart of galaxies like ours. Astrophysicists have many unanswered questions about black holes and how they evolved, and the LGWA should help answer some of them.

Double White Dwarf (DWD) mergers outside our Milky Way are something else that only the LGWA will be able to observe. They can be used to measure the Hubble constant. In recent decades, scientists have obtained more refined measurements of the Hubble constant, but discrepancies still exist.

The LGWA will also tell us more about the moon. The seismic observations will reveal the moon’s internal structure in more detail than ever before. There are many scientists who still don’t know anything about its formation, history and evolution. LGWA’s seismic observations will also illuminate the moon’s geological processes.

The LGWA mission is still under development. Before it can be implemented, scientists need to know more about where they want to place it. That’s where the preliminary Soundcheck mission comes in.

In 2023, the ESA selected Soundcheck in its Reserve Pool of Science Activity for the moon. Soundcheck will not only measure surface seismic displacement, magnetic fluctuations and temperature, it will also be a technology demonstration mission. “The validation of the Soundcheck technology focuses on implementation, inertial sensor mechanics and readout, thermal management, and platform leveling,” the authors explain.

In astronomy, astrophysics, cosmology and related scientific endeavors, it always seems as if we are on the cusp of new discoveries and a new understanding of the universe and how we fit into it. The reason it always seems that way is because it’s true. People are getting better at it, and the rise and blossoming of GW science is an example of that, even though we’re just getting started. Less than ten years have passed since scientists discovered their first GW.

Where will things go from here?

“Despite this well-developed roadmap for GW science, it is important to realize that exploration of our universe through GWs is still in its infancy,” the authors write in their paper. “In addition to the immense impact expected on astrophysics and cosmology, this field offers a great opportunity for unexpected and fundamental discoveries.”