Recently, a method has been proposed to use GPS satellites to detect dark matter, which will also hopefully help us explain what it is.
Dark matter, that is material that gravitationally affects the things we can see but is not itself visible, was proposed by Vera Rubin in the 1960s to explain the speed at which galaxies rotate. There is thought to be more than five times as much dark matter in the universe as ordinary matter.
Despite scattered doubts about the details, most astrophysicists agree that galactic behavior cannot be explained any other way. However, the quest to find the missing matter has proven frustrating, with nothing but hints thought to account for a small portion of the total mass.
"Despite solid observational evidence for the existence of dark matter, its nature remains a mystery," says Professor Andrei Derevianko, of the University of Nevada, Reno. "Some research programs in particle physics assume that dark matter is composed of heavy-particle-like matter. This assumption may not hold true, and significant interest exists for alternatives."
Efforts to find dark matter have involved searching either for massive compact halo objects (MACHOs) or for Weakly Interacting Massive Particles (WIMPs). The former are black holes, brown dwarves or other large objects in the galactic halo that emit little or no light, while the latter are particles that have mass, but don't interact with the universe in ways we have been able to detect.
In the course of this year, Derevianko and Profesor Maxim Pospelov of the University of Victoria, British Columbia, have presented the idea of using GPS satellites at major physics conferences. The work relies on Einstein's prediction that gravity affects time, so that a clock in a gravitational field slows down. It also assumes that whatever we are looking for, it is not a MACHO.
"Our research pursues the idea that dark matter may be organized as a large gas-like collection of topological defects, or energy cracks," Derevianko said. "We propose to detect the defects, the dark matter, as they sweep through us with a network of sensitive atomic clocks. The idea is, where the clocks go out of synchronization, we would know that dark matter, the topological defect, has passed by. In fact, we envision using the GPS constellation as the largest human-built dark-matter detector."
Andrei Derevianko, University of Nevada, Reno. As a massive object passes an atomic clock it should slow it down slightly, leading to detectable difference with other clocks, which might experience the same effect before or afterwards.
Having received positive responses to their proposal, the pair have now published the idea in Nature Physics.
Derevianko is examining data from the atomic clocks in 30 GPS satellites, but has no results to report so far. He anticipates being able to pick up any variations larger than a billionth of a second triggered by mass passing by as the Earth sweeps around the galaxy.
Atomic clocks have previously been used in an effort to determine whether the fundamental constants of the universe, such as the speed of light, are changing.