Explained: The Idea Of An Observatory On The Moon To Detect Gravitational Waves

Cosmic events of unimaginable magnitude are unfolding everywhere in the universe.

The “news” of these developments come to us, here on Earth, in a variety of forms.

For the scientific community, it is a matter of being at the right place at the right time — and hopefully with the right equipment — to receive this news of developments happening in distant reaches.

Two astrophysicists have proposed an idea that seeks to assemble this trio of right factors in order to detect objects and events hidden from plain view, in many cases even electromagnetic view.

Scientists Dr Karan Jani and Professor Abraham Loeb have suggested that a gravitational-wave observatory be put on the Moon to improve our ability to look deeper into space.

The Moon, they say, would make an excellent base for detection of gravitational waves.

The idea of gravitational waves originated in Albert Einstein's theory of general relativity.

It remained a prediction for a century (indirect evidence in 1974) until in 2015, it was confirmed with the Laser Interferometer Gravitational-wave Observatory (LIGO) — gravitational waves were detected from 1.3 billion years ago after two spiralling black holes had crashed into each other.

Since then, gravitational-wave detectors in the United States and Europe have been chasing gravitational signals from remote locations underground.

Together, they have detected scores, some 50 of them, in the last five years.

Future projects like the Einstein Telescope and Cosmic Explorer promise to help refine our abilities to probe further.

However, gravitational-wave detectors based on Earth, and even space, may not be enough in our quest for some of the more mysterious objects in space.

They have limitations that could be reduced or done away with if the detector was instead put to work on the Moon.

Gravitational-wave observatory on the Moon

The Laser Interferometer Gravity-wave Observatory (LIGO) and VIRGO experiments have been doing a fine job at detections in the 10-1000 Hz spectrum.

Future labs are set to shine a light on lower frequencies — around 5 Hz in the case of Earth-based observatories and down to milli-Hz in the case of the space-based Laser Interferometer Space Antenna (LISA).

However, there’s much to gain from tuning the frequency down further, to a spectrum of deci-Hz to 1 Hz.

The Moon is apt for this range.

“This frequency range tends to be too low for Earth-based detectors and too high for space missions,” write the authors in their paper.

“The universe offers a rich set of astrophysical sources in this regime.”

Detection of gravitational waves in this low-frequency domain could help one of the authors of the paper, Dr Jani, verify the existence of a cosmic object of his passion.

“I am very interested in intermediate-mass black holes. They are smaller than the monster black holes found at the centres of galaxies, and are a mysterious class with no confirmed existence. But with gravitational waves, we can detect any black holes,” says Dr Jani.

The low 0.1-5 Hz frequency range opens the doors to detecting these intermediate-mass black holes.

In addition, it’s possible to explore 30-80 per cent of the observable universe with the proposed “Gravitational-wave Lunar Observatory for Cosmology” (GLOC), vastly expanding the search area.

The Moon is apt for detection

The Moon lends itself better to detection of gravitational radiation in comparison to Earth.

For one, it is much quieter there than where we are.

Thanks to that, the lunar detector will not have to deal with geological rumblings on the Moon’s surface, which is especially important in the case of low-frequency detection.

“For less than 10 Hz, we start to have seismic noise on Earth that couples with the detector. The Moon is at least 1,000 times quieter than Earth,” says Dr Jani.

On the Moon, the size of the observatory can be scaled up.

Currently, the LIGO detectors have two arms at a length of 4 km each.

The arms are made up of over a metre-wide steel vacuum tubes arranged in an "L" shape and protected from the environment by concrete.

Dr Jani and Professor Loeb propose expanding the arms out 10 times to make the arm length 40 km on the Moon.

On Earth this extension is a challenge on accounts of both construction and cost.

Integral to the observatory is the creation and maintenance of a vacuum environment with an air pressure of one-trillionth of an atmosphere.

This vacuuming is necessary because, unlike light, sound doesn’t travel in vacuum, helping scientists keep it out of the LIGO operation.

Similarly, air currents, which would mess with the laser beam travelling in the tubes and critical to precise detection, can be kept out.

Temperature variations are prevented in vacuum, too.

Now imagine achieving this vacuuming across vastly longer tubes.

Thankfully, one quality of the Moon helps in this regard.

“The Moon provides you with a natural vacuum. So if you build a big LIGO, say an arm of 40 km, then the vacuum right above the lunar surface will naturally provide the condition,” Dr Jani tells me.

What if something goes wrong with the set up after it’s installed on the Moon?

In such a case, astronauts could be shipped to the Moon to carry out any fixes or upgrades necessary.

This advantage isn’t available if something had to be tweaked on a space-based gravitational-wave detector.

Under normal circumstances, the GLOC would be controlled remotely from a centre on Earth.

The Moon throws up its share of challenges too.

Cosmic rays and solar flares can affect the detector’s readings, if not more.

Lunar dust and especially, the blanket of debris on the surface called 'regolith' would similarly create difficulties.

When I posed these questions to Dr Jani, he said these challenges were real and needed to be tackled but that they would come in the way of any experiment that was to be carried out on the Moon.

“These challenges would hold for any base, not particular to the gravitational wave detector,” he said.

Despite these challenges, “we would still do a few orders of magnitude better on the Moon than on Earth,” he added.

How it improves our scientific understanding

The gravitational wave sources that could be detected in the proposed lunar lab can span a wide range of mass — the paper says subsolar dark matter candidates to stellar mass binaries to intermediate-mass black holes.

The optimal sensitivity of the GLOC would work better across this mass range in comparison to gravitational-wave detectors on Earth or in space.

The extent of the cosmological probe possible with the lunar gravitational-wave observatory exceeds that achievable with any electromagnetic telescope, with the exception of experiments in the cosmic microwave background — the leftover sound of the Big Bang.

Even here, Dr Jani says, “We are practically only a few redshifts behind the cosmic microwave background.”

Additionally, the researchers think there is a shot at relieving the “Hubble tension” — a pain in the neck in cosmology that refuses to go away.

There are two answers currently for how fast the universe is expanding — one given by cosmologists studying the early stages of the universe and another by astronomers peering into the later stages.

Dr Jani believes that data from the lunar gravitational-wave detector could help with this conundrum.

Besides the Hubble parameter, new data could help put the theory of general relativity to the test and explore physics beyond the Standard Model.

The Standard Model is our current framework for how the basic building blocks of matter interact under the sway of fundamental forces.

However, the Model doesn’t incorporate all the forces.

It fails to explain gravity. The desire to make gravity fit neatly into the Standard Model is a dream goal of the fundamental physics community.

Dr Jani is optimistic. “The farther you see in the universe, the more it throws you a surprise.”

Experiments on the Moon

The more advanced space agencies may be thinking of someday setting up a sustainable human lunar base. (This vision has already expanded to other objects like Mars.) The first step would need to be a return to the Moon.

After a dozen astronauts walked on the Moon during the Apollo era in the sixties and seventies, no human lunar spaceflight missions have taken off. Finally, close to half a century later, NASA is set to change that with its Artemis programme with a scheduled touchdown in 2024.

With eyes on a lunar outpost in the future, the Artemis mission will assess capabilities and lay the groundwork for future missions.

The outpost will have to be more than just a plot of land. In this context, evaluating what scientific experiments are possible on the Moon is important to identify. The proposal of the lunar gravitational-wave detector can be one of the candidates.

There are ideas for other kinds of telescopes to be set up on the Moon too. The Lunar Ultraviolet Cosmic Imager was one such telescope, developed by students of the Indian Institute of Astrophysics, that was slated to ride on board the Moon-bound spacecraft of the Bengaluru-based private space company TeamIndus. However, the lunar mission didn’t take off.

Housing a variety of telescopes across the electromagnetic and gravitational spectrum on the Moon will enable the creation of a “multi-messenger laboratory” — one that could provide a more complete picture of cosmic events unfolding far in the universe.

Because, if we are there to receive the cosmic news, we may as well try to get our hands on the full story.

And it appears that we’d be better placed to do that on the Moon.