One of the great unanswered questions in science is whether the universe is filled with gravitational waves and if so, whether we can spot them. This question comes directly from Einstein’s theory of general relativity which assumes that the fabric of the cosmos is able to warp, bend and vibrate like a rubber sheet.

The bending and warping effectively causes gravity, the effects of which we can measure in detail. The vibrating is gravitational waves but physicists have yet to see this directly. However, they are hugely confident that gravitational waves must permeate the universe and have spent hundreds of millions of dollars building machines to spot them, so far unsuccessfully.

in recent years, a number of scientists have begun to point out that there may be much cheaper ways of finding gravitational waves. One idea is to study pulsars since the precise signals they send out must “shimmer” when gravitational waves pass by.

Another idea is to look at the Earth itself. The thinking is that it must vibrate like a bell when gravitational waves pass by. So there could be signs of this vibration in the data collected by the global network of sensors set up to measure seismic vibrations around the planet.

Indeed, earlier this year Michael Coughlin at Harvard University in Cambridge and his pal Jan Harms at the National Institute for Nuclear Physics (INFN) in Florence, Italy, crunched this data looking for evidence of these waves. They found none, a result that places strong limits on the energy density of gravitational waves in our neighbourhood. In fact, the data improved the limits from laboratory experiments by an extraordinary nine orders of magnitude.

Now Coughlin and Harms say it is possible to do even better. One problem with the Earth’s seismic data is that it is filled with background noise caused by tectonic movement, the constant churn of the oceans and by atmospheric fluctuations. So a better place to look for evidence would be somewhere that has no oceans, no atmosphere and no tectonic movement of the crust.

Just such a place isn’t too far away: the Moon. And it turns out that planetary geologists gathered a significant amount of seismic data from our nearest neighbour during the 1970s, thanks to a network of seismic recorders left on the surface by various Apollo missions.

That gave Coughlin and Harms an idea. These guys have crunched the seismic data from the Moon looking for the tell-tale evidence that gravitational waves have passed through. And again they found no evidence, a result that places even stronger limits on the kind of waves that might exist.

First, some background about the waves themselves. These vibrations are caused by some of the most extreme events in the universe, such as the collision between black holes, the death of stars in powerful explosions and so on.

Astrophysicists expect the waves to have a wide range of frequencies ranging at the highest level of thousands per second to very low frequency waves of only one every 10,000 years or so. And of course different detectors are required for different frequency ranges.

These detectors look for the way gravitational waves distort the very fabric of space-time, squeezing and stretching it as they pass by. However, this effect is tiny. Astrophysicists expect the waves to stretch space by distances comparable to the size of a proton. That’s why they are so hard to detect.

But one way to make this easier is to look for resonant effects. If the waves coincide with the resonant frequency of an object, they would cause it to ring like a bell and so be easier to detect. So that is exactly what Coughlin and Harms have hunted for in the seismic data from the Moon.

This data comes from a network seismometers left by Apollos 12, 14, 15 and 16 between 1969 and 1972. The network remained active until 1977 when NASA switched it off.

Together, these instruments form an equilateral triangle with sides of roughly 1100 kilometres. Coughlin and Harms study data gathered between July 1975 and March 1977, which was originally used to look for moonquakes, of which some 12,000 were identified. It also helped planetary geologists determine the inner structure of the Moon, which turns out to have a solid inner core surrounded by a fluid outer core and a partially molten layer above this.

But this data should hold other clues about the seismic environment. In particular, Coughlin and Harms looked for the signature of a gravitational waves in the form of seismic activity at all of the detectors at the same time.

Since they know the sensitivity of these detectors, the absence of this kind of signal places important limits on how active gravitational waves can be. “We find an upper limit approximately three orders of magnitude smaller than the previous best limits in the frequency range of interest,” they say.

And by looking at correlations between the seismic activity on the Moon and on Earth they can rule out chance coincidences with greater certainty. This also places strong limits on gravitational wave activity, although not quite strong as the Moon data alone.

That is an interesting result that fills in an important part of the jigsaw regarding the nature of gravitational waves. The lunar seismic data places the best limits yet on the activity of gravitational waves for frequencies in the range of about one per second. Interestingly, the limits at other frequencies are significantly the stronger than these so there is clearly room for improvement in this frequency range.

That will not come from analysing lunar seismic data. The current work relies on technology developed in the 1960s and 70s to tackle a problem that cosmologists are wrestling with in the 21st century.

It is hard to imagine that Apollo’s engineers would ever have expected their seismometers to still be useful in 2014 but it is testimony to the quality of their work that this data is relevant today. The Moon landings represent one of humankind’s great achievements and this work shows just how hard it is to determine the benefits of these kinds of missions when the data they generate is still being used 40 years later.

All this obviously raises the question of whether a new generation of lunar seismic measurements are now needed. Coughlin and Harm say that a modern network would improve the quality of the data and hence the strength of the limits but probably not by more than an order of magnitude. So by this measure alone, it might be hard to justify the cost.

In the meantime, astrophysicists will continue to await the first direct detection of gravitational waves with bated breath.

Ref: arxiv.org/abs/1409.4680 : Constraining The Gravitational Wave Energy Density Of The Universe In The Range 0.1 Hz To 1 Hz Using The Apollo Seismic Array