A near-Earth asteroid provides clues to a dark matter theory

A hypothetical “fifth force” could attract asteroids, if it exists. But asteroid Bennu shows no signs that its orbit has been altered by such a force. This puts a ceiling on how strong a possible fifth force could be, physicists report Sept. 20 Physics of communications.

Scientists know of four forces in the universe: electromagnetism, gravity, and the strong and weak nuclear forces. But some scientists suspect that a fifth force may exist. For one thing, a fifth force could help explain a great unsolved puzzle of physics: the identity of the unknown source of mass in the entire universe known as dark matter.

So scientists are looking for unknown forces. And Bennu is a particularly suitable target, realized physicist Yu-Dai Tsai and colleagues. The asteroid’s trajectory was closely monitored by NASA’s OSIRIS-REx spacecraft, which collected a sample of the asteroid and returned it to Earth in 2023 (SN: 15.2.24). “Bennu has been tracked with amazing precision over a long period of time, so even small deviations from its predicted path can provide evidence for new physics,” says Tsai, of Los Alamos National Laboratory in New Mexico.

In physics, forces go hand in hand with particles transmitting the force. For example, photons, particles of light, mediate electromagnetism. If there is a fifth force, its corresponding particles may play the role of dark matter. Bennu’s data allowed scientists to search for a fifth force associated with particles that are very light, perhaps a millionth of a trillionth of an electron volt. (For comparison, an electron has a mass of about 500,000 electron volts.)

Measurements of additional asteroids could help scientists expand the search to cover a range of possible masses. So while the space rocks returned by Bennu can tell us about the matter that makes up the objects in our solar system, its trajectory, and that of asteroids like it, can tell us about the dark side of matter.