Heavy photons are too light to be behind dark matter

SADLY, dark matter is not made of light. That may sound obvious, but many physicists were hoping that photons - particles of light - could help us to piece together the nature of the mysterious stuff thought to make up 85 per cent of the universe's matter.

Instead, readings from Vitor Cardoso of the Technical University of Lisbon in Portugal and colleagues seem to have quashed this idea.

Some theories had hinted that "heavy photons", hypothetical versions of the more familiar massless particles, might be dark matter. According to that idea, the heavy photon would have a small amount of mass and might carry an unknown fundamental force that allows it to interact only with ordinary photons - effectively hiding it from the visible world.

In that case, heavy photons passing close to black holes would have noticeable effects, says Cardoso.

When most particles with mass get too near to a black hole, they fall in, never to be seen again. Photons with no mass can skirt past danger if they are on the right trajectory. But a photon with a very tiny "in between" mass can enter into an orbit of the spinning black hole and steal some of its angular momentum. If conditions are right, this process can continue until orbiting particles slow the hole down so much that it stops spinning.

Cardoso and colleagues calculated how long photons of given masses would take to sap a black hole's spin. Then they examined data on the ages and rotation speeds of eight supermassive black holes. The age of the oldest spinning black holes effectively puts an upper limit on the photon's mass. If it does exist, the heavy photon must be lighter than 10^-20 electronvolts - an extreme improbability - the team concluded (arxiv.org/abs/1209.0465).

"We are constraining and putting aside what is not dark matter," Cardoso says.

Alfred Goldhaber of Stony Brook University in New York says that if black holes have charged plasma swirling around them, a photon's slowed movement through the plasma could make it behave as if it has mass, ruining the calculations. Cardoso counters that this apparent mass only affects things at a subatomic level - the photon's real mass is what matters at the scale of the black hole.

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