The Moon is an Ideal Spot for a Gravitational Wave Observatory

In the coming years, multiple space agencies will be sending missions (including astronauts) to the Moon’s southern polar region to conduct vital research. In addition to scouting resources in the area (in preparation for the construction of a lunar base) these missions will also investigate the possibility of conducting various scientific investigations on the far side of the Moon.

However, two prominent scientists (Dr. Karan Jani and Prof. Abraham Loeb) recently published a paper where they argue that another kind of astronomy could be conducted on the far side of the Moon – Gravitational Wave astronomy! As part of NASA’s Project Artemis, they explain how a Gravitational-wave Lunar Observatory for Cosmology (GLOC) would be ideal for exploring GW in the richest and most challenging frequencies.

The paper, titled “Gravitational-Wave Lunar Observatory for Cosmology,” recently appeared online and is being considered for publication. Whereas Dr. Karan Jani is an astrophysicist from Vanderbilt University and a member of the LIGO Scientific Collaboration, Prof. Abraham Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University and the director of the Harvard-Smithsonian Center for Astrophysics (CfA).

Originally predicted by Einstein’s Theory of General Relativity, GWs are essentially ripples in space-time caused by the merger of massive objects like black holes or neutron stars. The first confirmed GW event happened in 2016, where scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) announced GWs coming from Markarian 231 – a binary system of black holes over 1.3 billion light-years away.

Since then, with more detectors coming online, collaborations between observatories around the world, and improvements in the technology and methodology, astronomers have detected a total of 56 candidate events. Using these events, astrophysicists have been able to conduct tests of General Relativity which have helped

“The time is ripe to explore what science programs could be best pursued from the lunar surface.” -Professor Abraham Loeb

What’s more, astronomers have found many instances where GW astronomy could succeed where conventional methods fall short. As Prof. Loeb told Universe Today via email:

“Traditionally astronomy was all about detecting light by telescopes. Some environments, like the nuclei of galaxies or star-forming regions, are obscured behind opaque clouds of gas and dust. Others, like black holes with no matter around them, do not emit any light. Gravitational waves offer a glimpse at these environments that we could have never probed before. Their strongest sources are mergers of black holes, which provide a new testbed for Albert Einstein’s theory of gravity because they are the most extreme structures of spacetime that the theory predicts.”

The concept at work here is similar to what’s involved with space telescopes, or what astronomers hope to accomplish with lunar radio astronomy – i.e. operating beyond interference. For space telescopes – like Hubble, TESS, Gaia, and others – operating outside of Earth’s atmosphere means being able to gather light that is not subject to wavelength distortion without the need for adaptive optics.

The situation is similar when it comes to interferometers and gravitational waves. Basically, an interferometer relies on two or more merging sources of light in order to create an interference pattern, which is then analyzed by photodetectors to note any sudden changes. When an interferometer intercepts gravitational waves, the ripples cause measurable distortions that scientists use to determine the nature and distance of the source.

Unfortunately, interferometers have to be extremely sensitive since gravitational waves are very difficult to detect, which makes them vulnerable to interference. For one, the arms of a detector need to be kept in a state of vacuum in order to eliminate possible interference for air molecules and seismic events (aka. earthquakes) will also result in false positives.

But on the Moon, which is geologically inactive and there is no atmosphere to speak of, interference would be virtually non-existent. As Prof. Loeb explained:

“The Moon has an extremely low seismic noise, since it has no geological activity. This allows it to probe a frequency range of gravitational waves that is two orders of magnitude smaller than can be accessed from Earth. The situation is analogous to building a radio telescope instead of an optical telescope. The Moon also has no atmosphere, so its surface already has levels of vacuum far lower than the vacuum tubes of the LIGO and Virgo instruments on Earth.”

As for what a Gravitational-Wave Lunar Observatory for Cosmology (GLOC) on the far side of the Moon could reveal, that’s where things get really interesting. On Earth, scientists are limited when it comes to what kinds of mergers they can detect. On the Moon, says Loeb, an observatory could access domains that GW astronomers currently have no insight into:

“The new frequency range allows us to detect intermediate-mass black holes (between stellar-mass objects formed from the collapse of stars – currently probed by LIGO-Virgo and supermassive objects formed at the centers of galaxies – to be probed by the space observatory LISA) through most of the volume of the observable universe.”

Already, scientists have proposed using gravitational waves to study the interiors of black holes, supernovae, locate dark matter, and measure the expansion of the cosmos (aka. the Hubble Constant). This last possibility is especially tantalizing since scientists have been gradually reducing the level of uncertainty they have with their measurements for over a century.

At the same time, scientists have had to deal with a discrepancy (known as the “Hubble tension”) where the reduction of uncertainties with cosmic expansion has not been paralled by a reduction between different measurements. “The orbit of the Moon allows GLOC to pinpoint the host galaxies of merging black holes and neutron stars,” added Dr. Jani. “This is crucial to solving the Hubble tension.”

Another compelling reason why Dr. Jani and Prof. Loeb recommend the creation of GLOC is because of NASA’s (and other space agencies) plan for lunar exploration in the coming years. In addition to sending astronauts back to the Moon for the first time since the Apollo Era (by 2024), NASA also hopes to create a program of “sustainable lunar exploration” beyond that.

This will include an orbiting space habitat that will allow for regular trips to the lunar surface (the Lunar Gateway) and infrastructure on the surface that will facilitate long-term exploration missions (the Artemis Base Camp). For this reason, says Loeb, now is the perfect time to contemplate the kind of infrastructure we want to build there based on what would offer the best scientific returns:

“The time is ripe to explore what science programs could be best pursued from the lunar surface. In the past, scientists contemplated radio, UV and X-ray telescopes because of the lack of an atmosphere. We are suggesting an exciting new possibility for a large scale science project, which we hope the scientific community will endorse. 

This raises another exciting aspect about plans for space exploration in this decade and the next. In addition to going back to the Moon to stay in the 2020s and building the infrastructure that will take to Mars by the 2030s (and beyond), future missions will enable the types of scientific experiments that are challenging here on Earth. In this respect, exploring more of our Solar System will allow to explore more of the Universe!

Further Reading: arXiv