Since astronomers discovered in the late 1990s that the Universe's expansion is accelerating, researchers have struggled to explain not only the nature of the hypothetical entity — dubbed dark energy — that's causing the acceleration but also why the acceleration is so weak.
One of their best guesses is that dark energy is an inherent property of the vacuum of space. Particle physics predicts the existence of such vacuum energy, but also that it should be a whopping 10120 times larger than what is needed to explain the acceleration observed by astronomers. If dark energy were that large, the Universe would have been ripped apart long before stars and galaxies ever formed.
In 2010, Claudia de Rham, a cosmologist at Case Western Reserve University in Cleveland, Ohio, and her colleagues came up with the surprising suggestion that dark energy could be the vacuum energy if most of it were swallowed up by the hypothetical ‘graviton’ particle...
Most physicists have assumed that the graviton would be massless like the photon, so that the reach of gravity could extend across the Universe. “We know that gravity is long-range because we feel gravity from the Sun — and that sets a bound on how large the graviton could be,” says de Rham. She and her colleagues realized, however, that if the graviton were given a tiny mass of less than 10–33 electronvolts, it could still fit with all astronomical observations. (By comparison, neutrinos, the particles with the smallest-known non-zero mass, have masses of the order of 1 electronvolt, and the electron has a mass of about 511,000 electronvolts.)
A graviton that is massive — as opposed to massless — would earn its heft by swallowing up almost all of the vacuum's energy, leaving behind just a small fraction as dark energy to cause the Universe to accelerate outwards.