Scientists turn to the moon to catch spacetime’s faintest music

The Greek philosopher Pythagoras explained the universe using the ‘Music of the Spheres’, orbs on which celestial objects moved in consonance with mathematical harmonies to create a cosmic symphony of sorts.

Today, astronomers are treated to this ethereal ‘music’ every time they eavesdrop on the universe using radio telescopes to unravel its mysteries. The bass hum they hear is a mix of the electromagnetic signatures of the most colossal objects in the universe — neutron stars (extremely dense remnants of massive stars that exploded), pulsars (rapidly rotating neutron stars that emit beams of electromagnetic radiation from their magnetic poles), and black holes.

Perhaps the most significant notes in this medley are gravitational waves, subtle wrinkles in the spacetime continuum caused by the abrupt movement of massive objects as in cataclysmic events like merging black holes or colliding neutron stars, bending space and time. These oscillations spread out as waves at the speed of light and their low rumble can be picked up by gravitational wave detectors, which measure how the waves stretch and compress spacetime between the objects they encounter.

Warping of spacetime

Curiously, gravitational waves are only powerful on large, cosmic scales. On smaller scales they are extremely weak — so weak that they are only able to alter the distance between the earth and the moon by less than the diameter of an atom! And the farther these waves travel, the weaker they become, so that by the time they reach the earth they are almost impossible to measure.

Astronomers build special instruments called interferometers that use laser light to detect gravitational waves. The Laser Interferometer Gravitational-wave Observatory (LIGO) in the US, for instance, has two L-shaped detectors, one in Louisiana and another in Washington. Each detector has a couple of 4-km-long arms. When a laser beam is sent down these arms, it is reflected back by mirrors; any delay in the reflection indicates that the light is being influenced by gravitational waves.

In 1916, Albert Einstein’s general theory of relativity made two predictions. One was that stars and galaxies, because of their mass, bend light as they warp spacetime in a phenomenon called gravitational lensing. This was experimentally proved in 1919. The second prediction was the existence of gravitational waves, which was debated at length in the decades that followed as scientists wondered if these were merely mathematical constructs devoid of physical reality. In fact, Einstein himself briefly questioned their existence in 1937, suggesting that they might be theoretical artefacts and not quite what he thought initially.

Anyhow, astronomers had to wait until 2015 before gravitational waves were picked up for the first time, when the LIGO detectors in the US recorded signals emanating from two colliding black holes 1.3 billion light years away. Suddenly, cosmologists, who could until then only study the universe through electromagnetic waves or particles, had a tool with which to observe the warping of spacetime that Einstein had predicted a century ago.

‘A cosmic raag’

To detect gravitational waves, a detector must be isolated from all vibrations that could potentially obscure the elusive signals. So even the best frontline gravitational wave observatories in the world — the two LIGO detectors in the US, the GEO600 in Germany, the Virgo in Italy, and the KAGRA in Japan — can only spot gravitational waves from flare-ups within 7 billion light-years from the earth.

This may be about to change as cosmologists look forward to opening a new window on the gravitational sky, on the moon. Researchers from the Vanderbilt Lunar Labs in the US plan to install a gravitational-wave detector, called the Laser Interferometer Lunar Antenna (LILA), on the lunar surface. LILA will study gravitational waves in the sub-hertz frequencies that cannot be observed by terrestrial detectors. 

The moon’s permanently shadowed polar regions offer ideal conditions to record gravitational waves.

“Gravity is a cosmic raag, and the moon lets us hear the notes that we cannot hear from any other place in this solar system,” Karan Jani, Director of the Vanderbilt Lunar Labs Initiative and a professor of physics and astronomy, electrical and computer engineering, and communication of science and technology at Vanderbilt University, said.

“The seismic noise (on the moon) is far lower than on earth, and a natural vacuum sits right above the surface, which means far less infrastructure is required to build the detector on the moon than at earth-based observatories.”

Recruiting the moon

Dr. Jani, who leads the international consortium that is building LILA, explained the project via email.

“The first phase, LILA Pioneer, can be built within this decade with the current lunar landers from American companies such as Blue Origin and Intuitive Machines, and from India’s Chandrayaan program. The next phase, LILA Horizon, will require astronauts on the lunar surface for deployment.”

Scientists have toyed with the idea of a moon-based gravitational-wave detector since the 1960s, when the Apollo missions and two robotic Soviet spacecraft placed five retro-reflectors on the lunar surface to reflect light back to the earth. By measuring the time light takes to travel between the moon and the earth, and knowing the speed of light, scientists have been able to calculate the earth-moon distance with great accuracy.

Such precise data has prompted some astronomers to believe that the earth-moon system itself could be a potential natural gravitational wave detector, as gravitational waves are constantly washing over the two-body system, generating small deviations in the moon’s orbit, which can be tracked.

“About every 15 minutes, a gravitational wave from the collision of two black holes sweeps through the earth, the moon, and even the sun,” Dr. Jani said. “The effect on the orbits of these bodies is so tiny that for practical purposes it is nonexistent. But what is scientifically interesting is that the moon can resonate with some of these incoming waves, which opens a new window for the gravitational-wave spectrum.”

Terra incognita

Ground-based observatories have a major handicap as they possess only a limited detection range. They are sensitive to gravitational waves within the 100 to 1,000 hertz band, which leaves the broader gravitational-wave spectrum unexplored. Other space-based interferometers such as the Laser Interferometer Space Antenna (LISA), scheduled for launch in the 2030s, may rectify this to an extent as they can be made large enough to be sensitive to signals at very low frequencies.

LISA consists of three satellites in a triangular formation that will trail the earth as the planet orbits the sun. The satellites will monitor their relative separations using lasers and sense the changes caused by passing gravitational waves so that they can be measured at lower frequencies. With an arm length nearly a million times more than LIGO’s, LISA will be able to record signals in the 0.1 millihertz to 0.1 hertz range.

The search for gravitational waves on other frequencies includes the world’s largest radio telescope arrays, the Square Kilometre Array (SKA) located in Australia and South Africa that scans the nanohertz frequency range, and the LIGO detectors in the centihertz frequency range. But the real challenge for scientists is to explore the uncharted decihertz gravitational-wave frequency range, which lies between the higher (10-1,000 Hz) band of ground-based observatories and the lower 0.1mHz-1 Hz band of LISA.

“Decihertz gravitational wave astronomy is a new frontier which will potentially open up in the next two decades,” Ajith Parameswaran of the International Centre for Theoretical Sciences, Bengaluru, said. “Besides LILA, there are many proposals for decihertz gravitational wave detectors,” he wrote in an email.

“These include the Japanese space-based DECi-hertz Interferometer Gravitational wave Observatory (DECIGO), the US-led TianGo space detector initiative and the Lunar Gravitational-wave Antenna (LGWA).”

Edge of time and space

Dr. Parameswaran also said Indian scientists are working on a different decihertz detector concept of their own. India’s Initiative in Gravitational-wave Observations (IndIGO) is a road map to build an advanced gravitational-wave observatory, LIGO-India, in Hingoli district in Maharashtra. When completed in 2030, it will join the global LIGO network and is expected to give a big boost to gravitational-wave astronomy in the country.

“There is no known technology that can access decihertz gravitational waves from the earth or in deep space, except building a detector on the Moon,” Dr. Jani said. “Gravitational waves come to us like the notes from various cosmic raags, each at a different pitch. SKA will pick up the deepest bass notes: the slow motions of massive black holes. LIGO in India and around the world listens to the high notes: the sharp bursts from colliding stars. And decihertz gravitational wave observatories such as LILA will bring the missing notes in between, so that for the first time humankind can hear the full cosmic symphony.”

Gravitational-wave astronomy is still in its infancy, but it is growing fast and promises unprecedented insights into the mysteries of the cosmos. By tapping the entire spectrum of gravitational waves, astronomers can peer back to the very edge of time and space. The decihertz range, for instance, can help in studying intermediate-mass black holes which are believed to be the building blocks of supermassive black holes found at the centres of galaxies.

It is even possible for scientists to use the entire Milky Way galaxy as an immense gravitational wave detector by monitoring pulsars. When gravitational waves sweep through the galaxy, they alter the earth-pulsar distance and, along with it, the pulsar frequencies. If astronomers can tune into these minute frequency changes, they will be able to ‘listen’ to gravitational waves from the early universe telling the story of its birth and evolution.

Prakash Chandra is a science writer.