Gravitational waves are notoriously difficult to detect. Although modern optical astronomy has been around for centuries, gravitational wave astronomy has only been around since 2015. Even now our ability to detect gravitational waves is limited. Observatories such as LIGO and Virgo can only detect powerful events such as the mergers of stellar black holes or neutron stars. And they can only detect waves with a narrow range of frequencies from tens of Hertz to a few hundred Hertz. Many gravitational waves are produced at much lower frequencies, but right now we can’t observe them. Imagine raising a telescope to the night sky and only being able to see light that is a few shades of purple.
Naturally, astronomers would like to observe a wider range of gravitational “colors,” and several methods have been proposed. Space-based gravitational-wave telescopes such as the proposed LISA observatory should be able to detect millihertz waves, for example. There are also projects trying to detect extremely slow nanohertz gravitational waves such as NANOGrav which studies radio signals from fast-rotating pulsars. Both of these frequency ranges will have much to teach us about the universe.
But what’s missing is the ability to detect microhertz frequencies. These are gravitational waves that take several weeks to make a complete oscillation. It is a range that could prove crucial to our understanding of the big bang. According to the standard model of cosmology, in the early moments of the big bang, the universe experienced a brief moment of super-rapid expansion known as early cosmic inflation. The inflation theory is needed to solve several of the problems with the big bang, but we haven’t been able to prove it. Microhertz gravitational waves could be the solution. According to the theory, early cosmic inflation should have created microhertz gravitational waves across the cosmos. The universe should still be ringing with them, like the fading echo of a bell. Now a team of astronomers thinks they know how microhertz waves could be detected.
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If there really are gravitational waves from early inflation, then everything is being jostled by them. Stars, asteroids, even the Earth and Moon. And therein lies the key. As these gravitational waves pass through the Earth-Moon system, they should shift the orbit of the Moon very slightly. This effect would be most dramatic at a frequency equal to the Moon’s orbital period, which is about 28 days. Right in the microhertz frequency range.
The catch is you’d need to be able to track the Moon’s position with extreme precision. But we can already do that. Thanks to the Apollo missions, we have reflectors placed on the Moon, and by shining lasers at them, we can measure the Moon’s position to within a centimeter. The team proposes making a series of measurements over time to specifically search for shifts from gravitational waves. They also propose a project similar to NANOGrav. Pulsars orbiting a companion with a period of several weeks would also be sensitive to microhertz gravitational waves, and measuring the signals from these binary pulsars might detect microhertz waves.
Right now it’s just an idea, but it’s a good one. And most of the tools we need already exist. Perhaps in the near future, a careful look at the Moon could allow us to solve one of the largest mysteries in cosmology.
Reference: Blas, Diego, and Alexander C. Jenkins. “Bridging the microHz Gap in the Gravitational-Wave Landscape with Binary Resonances.” Physical Review Letters 128.10 (2022): 101103.