A gravitational wave observatory on the moon may “hear” 70% of the observable universe

Gravitational wave astronomy will revolutionize our understanding of the cosmos. In just a few years it has vastly improved our understanding of black holes, but it is still a scientific field in its youth. This means that there are still serious limitations to what can be observed.

Currently, all gravitational observatories are based on Earth. This makes the detectors easier to build and maintain, but it also means that the observatories are plagued by background noise. Observatories like LIGO and Virgo work by measuring the shift in distance between the mirrors when a gravitational wave passes through the observatory. This shift is extremely small. For mirrors that are 4 kilometers apart, the displacement is only a fraction of the width of a proton. The vibrations of a truck driving down a nearby road shift the mirrors much more. Therefore, LIGO and Virgo use black hole merging statistics and models to distinguish a real signal from a false one.

Theoretical observation area for GLOC. Photo credit: Jani, et al

Due to the terrestrial background noise, current observatories focus on the high-frequency gravitational waves (10-1000 Hz) that are generated by the merging of black holes. It was discussed building a space-based gravitational wave observatory like LISA that would observe low frequency gravitational waves such as those generated by early cosmic inflation. But many gravitational waves are in the middle range. In order to recognize this, a current study proposes the construction of a gravitational wave observatory on the moon.

The moon has long been a sought-after place for astronomers. Optical telescopes on the moon wouldn’t suffer from atmospheric blurring, and unlike space-based telescopes like Hubble and Webb, they wouldn’t be limited by the size of your launcher. Most of the ideas proposed were very hypothetical, but with a human return to the moon in view in the next decade, they become fewer. NASA is already investigating the construction of a radio telescope on the distant lunar surface. Building a lunar gravitational wave observatory would be significantly more difficult, but not impossible.

This current study proposes a Gravitational Wave Moon Observatory for Cosmology (GLOC). Rather than worrying about the construction of such an observatory, the study instead focuses on the sensitivity and observation limits of such an observatory. As you might expect, a lunar observatory would not suffer from the background vibrations that disrupt Earth observatories. As a result, it could have a baseline four times longer than LIGO. This would give it a range of gravitational wave frequencies as low as a tenth of a Hertz. This would allow him to observe everything from binary mergers of stellar mass to those of medium mass black holes.

But it could also observe the same kind of mergers as LIGO and Virgo at a much greater distance. Distances so far that the gravitational waves are strongly redshifted. If GLOC were built, it would be able to use distant fusion events to measure the rate of cosmic expansion over billions of years. This would perhaps be its greatest strength because it would allow us to measure the Hubble parameter over much of cosmic history. We would eventually learn whether cosmic expansion is part of the structure of spacetime or whether it varies in time and space.

Of course, the GLOC proposal is purely hypothetical at this point. It will be at least decades before we can build such an observatory. But this study shows that building such a telescope would be worthwhile.

Relation: Jani, Karan and Abraham Loeb. “Gravitational Wave Moon Observatory for Cosmology.” Journal of Cosmology and Astroparticle Physics 2021.06 (2021): 044.

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