Why it is worth placing a gravitational wave observatory on the Moon

Scientists first detected long-predicted gravitational waves (GWs) in 2015, and since then, researchers have been eager to create more advanced detectors. But Earth is warm and seismically noisy, and this will always limit the effectiveness of Earth-based detectors.

Is the Moon a good location for a new gravitational wave observatory? Maybe. Sending telescopes into space has yielded good results, and installing a GW observatory on the Moon may also prove successful, although this undertaking will obviously be very difficult.

Much of astronomy deals with light. The better we record it, the more we learn about nature. This is why telescopes like Hubble and Webb are in space. The Earth's atmosphere distorts the images captured by telescopes and even blocks some types of light, such as infrared. Space telescopes circumvented both of these problems and revolutionized astronomy.

Gravitational waves are not light, but detecting them still requires special sensitivity. Just as the Earth's atmosphere can introduce “noise” into telescope observations, Earth's seismic activity can create problems for gravitational wave detectors. The Moon has a great advantage over our dynamic, ever-changing planet: it has much less seismic activity.

Since the time of Apollo, we have known that seismic activity is observed on the Moon. But unlike Earth, most of its activity comes from tidal forces and tiny meteorite impacts. In addition, seismic activity on the Moon is weaker and much deeper than on Earth. This has attracted the attention of researchers developing Lunar gravitational wave antenna (LGWA).

LGWA developers have written a new work “Lunar Gravitational-Wave Antenna: Mission Research and Science Development” Lead author is Paramswaran Ajith, a physicist/astrophysicist at the International Center for Theoretical Science, Tata Institute of Fundamental Research, Bangalore, India. Ajith is also a member of the LIGO Science Collaboration.

A gravitational wave observatory (GWO) on the Moon will fill the gap in frequency coverage.

“Given the size of the Moon and the expected noise generated by the lunar seismic background, LGWA will be able to observe GWs of approximately 1 mHz to 1 Hz,” the authors write. “This will make LGWA the missing link between LISA-type space detectors with peak sensitivity of about a few millihertz and future ground-based detectors like Einstein telescope or Cosmic Explorer“

If built, LGWA will be a planetary-scale detector array. The unique conditions of the Moon will allow LGWA to open a wider window into the science of gravitational waves. The Moon has extremely low background seismic activity, which the authors describe as “seismic silence.” The absence of background noise will allow for more sensitive detections.

Also, in the constantly shadowed areas of the Moon, temperatures are very low. Detectors need to be cooled, and low temperatures make this task easier. The proposed LGWA will consist of four detectors located in a crater located at one of the lunar poles.

LGWA is an ambitious idea with potentially science-changing impact. Combined with telescopes that observe the entire electromagnetic spectrum, as well as neutrino and cosmic ray detectors – so-called multi-messenger astronomy – it can expand our understanding of a range of cosmic events.

LGWA will have unique capabilities to detect space explosions. “Only LGWA will be able to observe astrophysical events associated with white dwarfs, such as tidal disruptions and Type Ia supernovae,” explain the authors. They also note that only LGWA will be able to warn astronomers weeks or even months before the merger of compact solar-mass binaries, including neutron stars.

LGWA will also be able to detect lighter intermediate-mass binary black holes in the early Universe. The latter played a role in the formation of modern supermassive black holes at the centers of galaxies like ours. Astrophysicists have many questions related to black holes and their evolution, and LGWA should help answer some of them.

Mergers of double white dwarfs outside our galaxy are another thing that only LGWA will be able to “feel.” They can be used to measure the Hubble constant. Over the decades, scientists have obtained more accurate measurements of the Hubble constant, but discrepancies still remain.

LGWA will also tell us more about the Moon. Thanks to seismic observations, the internal structure of the Moon will appear in more detail than ever before. Scientists still do not know much about its formation, history and evolution. LGWA's seismic observations will also shed light on geological processes on the Moon.

LGWA's mission is still under development. Before implementing it, scientists need to know more about where they plan to place it. This is Soundcheck's preliminary mission.

In 2023, ESA included Soundcheck in its reserve pool of science activities for the Moon. Soundcheck will not only measure seismic surface displacements, magnetic fluctuations and temperature, but will also be a technology demonstration mission. “Validation of the Soundcheck technology focuses on deployment, inertial sensor mechanics and sensing, thermal management, and platform alignment,” the authors explain.

In astronomy, astrophysics, cosmology and related scientific fields, it always seems that we are on the cusp of new discoveries and new understandings of the Universe and how we fit into it. And it seems so because it is true. Our understanding of these areas is constantly improving, and the emergence and flowering of HS science exemplifies this, although it has only just begun. Less than ten years have passed since scientists discovered their first GW.

How will events develop further?

“Despite the well-developed road map for GW science, it is important to understand that exploration of our Universe with GW is still in its infancy,” the authors write in their paper. “In addition to the expected enormous impact on astrophysics and cosmology, this field holds a high potential for unexpected and fundamental discoveries.”