Researchers have proposed a plethora of other ghost-free variations on de Rham’s original theme. In 2011, for instance, cosmologists Sayed Fawad Hassan of Stockholm University and Rachel Rosen of Columbia University in New York proposed combining two types of gravitons, one massive and the other massless, in one model. However, this would require a Universe where space is made of two overlapping fabrics that interact with each other.
At the Cambridge meeting, several cosmologists including de Rham independently presented a series of models in which interplay between the two fabrics could naturally set space-time accelerating. This would generate the dark-energy effect that astronomers have observed through an alternative mechanism that does not require any vacuum energy.
The key to whether such theories will hold up will be to calculate if they make testable predictions to distinguish massive gravity from standard cosmology, says de Rham. Such experiments could soon be carried out within the Solar System, because massive-gravity models predict a gravitational field between Earth and the Moon that is slightly different to that of ordinary gravity. This would create a detectable difference of one part in 1012 in the precession of the Moon’s orbit around Earth.
Experiments that fire lasers back and forth between Earth and mirrors left on the Moon currently measure the distance between the two bodies and that angle with an accuracy of one part in 1011. “We are just on the edge of being able to test massive gravity,” says de Rham.
Until such experimental evidence is found, however, some remain skeptical of the entire massive-gravity idea. Viatcheslav Mukhanov, a cosmologist at the Ludwig-Maximilians-University in Munich, Germany, says that although he was initially attracted to the theory of massive gravity for its simplicity, bolting on new space-times and adding extra gravitons makes it too contrived. “I think the dark energy problem will require a more elegant solution,” he says.
But elegance is a matter of taste, says Wyman. “If they can settle on a unique compelling model that explains dark energy, I think people will have to take notice,” he says. “What happens in the next few months will decide if the theory has any relevance for the real world, or if it is just a flash in the pan.”