Firing lasers at the Moon to detect early-universe gravitational waves
A team of European researchers has suggested that the Moon’s orbit could be used as a gigantic detector for gravitational waves – ripples in the very fabric of spacetime itself. These waves, much smaller than those that existing detectors can pick up, could originate from the early universe.
Cosmic events involving huge masses, such as collisions between black holes, can unleash so much energy that they physically distort the spacetime continuum, resulting in ripples known as gravitational waves. While this phenomenon was first predicted by Albert Einstein over a century ago, gravitational waves weren’t directly detected until 2015.
To detect gravitational waves, facilities like LIGO and Virgo beam lasers down 4-km (2.5-mile) long tunnels and wait. The reasoning goes that, after reducing other environmental influences, any tiny change in this laser beam indicates that a gravitational wave has washed over it, literally distorting reality. That distortion might only be a thousandth the width of a proton, but these sensitive instruments can detect it.
Dozens of detections have been made over the years, but current technology can only pick up signals within certain frequencies. In the new study, researchers at UAB and IFAE in Spain and the University College London have proposed a new way to detect gravitational waves on much lower frequencies, using the Moon’s orbit around the Earth.
The Apollo astronauts left mirrors on the lunar surface, and observatories here on Earth continuously beam lasers up to them and measure how they reflect back. That allows scientists to track the Moon’s distance from Earth, accurate to within 1 cm (0.4 in). In a way that’s a much larger-scale version of existing gravitational wave detectors – but where LIGO’s lasers only travel 4 km, the average distance to the Moon is 384,400 km (238,855 miles).
The precision of our measurements to the Moon, plus the extra distance, plus the fact that the Moon takes 28 days to orbit Earth, all add up to make this method particularly sensitive to detecting gravitational waves in the microhertz band. These frequencies are beyond the capabilities of existing detectors, but are of particular interest to scientists.
It’s thought that microhertz gravitational waves would be coming from the very early universe, as it undergoes transitions between phases of high energy. Detecting and decoding these waves could reveal huge amounts of new information about a period of the universe’s history that’s tricky to study.
This isn’t the first time the Moon has been considered for a role in gravitational wave detection. Last year another team proposed that the lunar surface might make an ideal location for a future facility, thanks to its isolation from background interference. The main advantage of the new proposal, however, is that it doesn’t require a new facility to be built at all – existing techniques can just be repurposed.
The research was published in the journal Physical Review Letters.