Humanity's return to the moon could open up new windows in the universe. NASA is working to establish a permanent human presence on and around the moon by the end of the 2020s, via a program known as Artemis. That presence may eventually include radio telescopes on the moon's exceptionally quiet far side and, perhaps, even more, ambitious off-Earth science facilities.
For example, a recent study makes the case for building a gravitational-wave observatory on the moon. Gravitational waves are ripples in space-time created by the acceleration of massive objects. They were predicted by Albert Einstein's general theory of relativity in 1915 and first directly detected a century later, by the Laser Interferometer Gravitational-Wave Observatory (LIGO) consortium.
LIGO has detectors at two sites, one in Louisiana and one in Washington state. Each detector is an L-shaped structure with arms 2.5 miles (4 kilometers) long, with a laser at the center of the array. The laser shines light down each arm, and mirrors reflect the light back. If the light from one arm comes back a bit late, it's evidence of possible gravitational-wave-induced distortion.
The LIGO team has detected dozens of gravitational wave events to date, most of them caused by mergers between two black holes. But this is exacting work, and our noisy, active Earth makes spotting signals tough.
The moon, on the other hand, is an exceptionally quiet place, the new study notes.
Co-author Avi Loeb, an astronomer at Harvard University, said, the moon offers an ideal backdrop for the ultimate gravitational wave observatory since it lacks an atmosphere and noticeable seismic noise, which we must mitigate at great cost for laser interferometers on Earth. A lunar observatory would provide unprecedented sensitivity for discovering sources that we do not anticipate and that could inform us of new physics. GLOC could be the jewel in the crown of science on the surface of the moon.
GLOC is short for the Gravitational-wave Lunar Observatory for Cosmology, the name Loeb and study lead author Karan Jani propose for the moon facility. GLOC would be huge compared to LIGO and other detectors on Earth, featuring arms 25 miles (40 km) long. And it would be incredibly sensitive, capable of spotting gravitational-wave events in nearly 70% of the observable volume of the universe, the researchers calculated.
Jani, an astrophysicist at Vanderbilt University in Tennessee, said, by tapping into the natural conditions on the moon, we showed that one of the most challenging spectrum of gravitational waves can be measured better from the lunar surface, which so far seems impossible from Earth or space.
GLOC is just an idea at the moment, but Jani and Loeb said they hope to develop a pathfinder mission on the moon that would test GLOC's required technologies in the coming years. And if GLOC or something like it does end up getting built, it will pay scientific dividends for decades to come.
Jani said, unlike space missions that last only a few years, the great investment benefit of GLOC is it establishes a permanent base on the moon from where we can study the universe for generations, quite literally the entirety of this century.
The new study was published last month in the Journal of Cosmology and Astroparticle Physics.