It's difficult to detect giltational waves. Modern optical astronomy has been around for hundreds of years. Our ability to detect waves is limited. Powerful events such as the mergers of stellar black holes can only be detected by LIGO. They can only detect waves with a narrow range of frequencies. We can observe many of the waves produced at lower frequencies. Imagine raising a telescope and only being able to see light that is a few shades of purple.

Several methods have been proposed to observe a wider range of gravitational effects. The proposed LISA observatory should be able to detect waves that are less than 10 hertz. The NANOGrav project is trying to detect extremely slow nanohertz gravitational waves. Both of these frequencies will teach us a lot about the universe.

The ability to detect frequencies is missing. These are waves that take a long time to make a complete oscillation. It's a range that could help us understand the big bang. In the early moments of the big bang, the universe experienced a brief moment of rapid expansion known as early cosmic inflation. We haven't been able to prove the inflation theory is needed to solve the big bang problems. Microhertz waves could be the solution. The theory says that early inflation should have created ripples in the universe. The universe should echo like a bell as it rings with them. A team of sceptics think they know how to detect microhertz waves.

Gravitational waves could shift the Moon’s orbit. Credit: D. Blas and A. C. Jenkins

If there are waves from early inflation, everything is being disrupted. There are stars, asteroids, and the Earth and Moon. The key lies here. The Moon should be shifted slightly as the Earth-Moon system passes through it. The effect would be most dramatic at a Frequency equal to the Moon's orbital period, which is about 28 days. The range is right in the microhertz range.

You need to be able to track the Moon's position with extreme precision. We can already do that. Thanks to the Apollo missions, we can measure the Moon's position by shining lasers at it. The team wants to make a series of measurements over time to look for shifts in the waves. They propose a similar project. The signals from the binaries pulsars could be used to detect microhertz waves.

Right now it is just an idea, but it is a good one. Most of the tools we need exist. We might be able to solve one of the largest mysteries in the universe with a careful look at the Moon.

Bridging the microHz Gap in the Gravitational-Wave Landscape with Binary Resonances is a reference.