A powerful beam of microwaves could be fired into space to detect hypothetical dark-matter particles called axions. That is the proposal of Pierre Sikivie and Ariel Arza at the University of Florida, who hope to record a faint microwave “echo” from the dark matter thought to exist at higher concentrations in certain regions of the Milky Way.
Axions were postulated in the 1970s to overcome a problem with the strong nuclear force, but it was later realized that they could in fact make up much of the universe’s dark matter. Unlike some other dark-matter candidates, axions have unusually well-defined properties including a mass no smaller than about a millionth of an electronvolt (eV) – to make them compatible with the observed abundance of dark matter – and no larger than about 10-3 eV, given that experiments with inferior sensitivities have not discovered them.
Physicists have already tried to detect the pairs of photons that axions are predicted to decay into. One of the leading groups is the Axion Dark Matter Experiment (ADMX) at the University of Washington in Seattle. Led by Leslie Rosenberg, it uses an antenna sensitive to microwave photons at the energy of axions with a certain mass.