Image by Shantaraj S.
Since 2015, astronomers have opened up a new way of studying the universe: listening to it with the help of gravitational waves. Albert Einstein’s general theory of relativity predicted these waves as distortions in spacetime. Massive objects like neutron stars and black holes create these distortions, which travel across the universe. Scientists detect them with the help of large observatories here on Earth.
But all that’s set to change. Scientists are now planning to use the far side of the Moon to detect gravitational waves. Last week, scientists from various institutions and universities across the world came together here at ICTS to discuss the scientific details of such a possible mission.
To detect gravitational waves, scientists need to place their observatories in an environment where there is little seismic vibration. While seismically quiet locations are available on Earth, the Moon offers them a much quieter environment. What’s more, it harbours some regions where the temperatures are as low as -240 ℃, some of the coldest regions in the Solar System. Scientists believe that these offer the ideal locations for detecting gravitational waves. When gravitational waves hit the Moon, it vibrates like a bell. Scientists can measure these vibrations by placing seismometers on the lunar surface.
Scientists are calling their mission the Lunar Gravitational-wave Antenna. It will consist of four ultra-sensitive seismometers spanning across a few kilometres each of which will measure the Moon’s vibrations independently. After gathering the data from each seismic detector, the scientists will club them together to decipher whether the vibrations occur due to moonquakes or meteors colliding with the Moon, or originate in astrophysical sources they are interested in studying.
Some astrophysicists argue that black holes created soon after the Big Bang must exist at very large distances from the Earth, but others question the existence of such black holes. “We don’t know if they exist; so we should look for them,” said Professor Jan Harms of Gran Sasso Science Institute, Italy. Detecting and studying these black holes will help scientists study the moments after the Big Bang, he said.
An open question in astrophysics is how black holes of masses millions of times than the Sun’s mass, which reside at the centres of galaxies, formed in the first place. Astrophysicists have observed black holes whose masses are a few times that of the Sun’s, which they call stellar-mass black holes, and supermassive black holes harboured by galaxies’ centres. They believe that other black holes with intermediate masses merged to form supermassive black holes. But they have not observed intermediate-mass black holes yet. The Moon-based antenna is ideally suited to detect these black holes.
Artist's impression of gravitational waves impacting the Moon. Image shared by Profesor Jan Harms, Gran Sasso Science Institute, Italy.
Prof Jan Harms explained how the mission will require assembling these ultra-precise seismometers here on Earth, placing them inside a rocket, sending them to the Moon, and placing the array at the desired spot without causing any damage. If scientists and engineers can pull off this ambitious project, it will be no less than an engineering feat.
There are several hurdles along the way, however.
Firstly, powering the observatory on the lunar poles, where sunlight hardly reaches, is a challenge. Although the technology to power it with nuclear fuel exists, there are open questions on how to adopt them in the case of the lunar array.
Secondly, the array consists of finely placed parts whose positions relative to each other are key to observing the gravitational waves from far-away sources. However, the rocket’s launch will shake up the entire assembled observatory. Jan explained, “The mass suspended inside our seismometers is around 10 Kg, and we have to keep that fixed with respect to the frame. So there can be no vibration of the mass against the frame because of the launch.” Although there are ways to prevent it from dismantling, scientists need to develop them further and tune them to suit their array’s launch.
If the observatory has reached the Moon safely, then there is an additional challenge of placing it at the desired location without damaging it or disturbing it. While scientists have tested methods to achieve this before, they need to re-test the mechanisms with respect to the lunar gravitational wave array.
The Indian Space Research Organisation (ISRO) is one of the few space agencies capable of launching lunar missions, as it has demonstrated with Chandrayaan-1 and Chandrayaan-2. The proposed gravitational wave observatory fits into its broad objective of contributing to science carried out from the lunar surface.
In collaboration with current Earth-based gravitational wave observatories and future space-based gravitational wave observatories, the lunar gravitational-wave array can offer extraordinary insights into the universe. But it comes with extraordinary challenges.
The author acknowledges the help of Professor Parameswaran Ajith and Aditya Kumar Sharma at ICTS.