Astrophysical observations show that dark matter makes up most of the “stuff” in the universe but so far it has eluded capture. Researchers around the world have been looking for it in various forms. The JILA team focused on ultralight dark matter, which in theory has a teeny mass (much less than a single electron) and a humongous wavelength–how far a particle spreads in space–that could be as large as the size of dwarf galaxies. This type of dark matter would be bound by gravity to galaxies and thus to ordinary matter.
Ultralight dark matter is expected to create tiny fluctuations in two fundamental physical “constants”: the electron’s mass, and the fine-structure constant. The JILA team used a strontium lattice clock and a hydrogen maser (a microwave version of a laser) to compare their well-known optical and microwave frequencies, respectively, to the frequency of light resonating in an ultra-stable cavity made from a single crystal of pure silicon. The resulting frequency ratios are sensitive to variations over time in both constants. The relative fluctuations of the ratios and constants can be used as sensors to connect cosmological models of dark matter to accepted physics theories.