Homing in on Axions?

Astronomer Fritz Zwicky first proposed dark matter in 1933 to explain the discrepancy between observed rotational velocities of galaxies in the Coma Cluster and expected rotational velocities based on the gravitational potential from the visible matter. Ever since, many studies have built a strong case for the existence of dark matter. Observations of the rotation curves of spiral galaxies, measurements of the temperature fluctuations in the cosmic background radiation, and gravitational-lensing images of distant galaxies clearly show that there is far more mass distributed uniformly throughout galaxies than what can be accounted for by the matter contained in the stars and dust. The question is not whether cold dark matter exists. The question is whether it consists of some type of particle and how such a particle might be detected and identified. The Axion Dark Matter eXperiment (ADMX) has now reported results from its latest search for one of the leading dark matter particle candidates—the axion [1]. Thanks to unprecedented detection sensitivity, ADMX has been able to probe, for the first time, the parameter space favored by both of the two most popular axion dark matter models. While the ADMX Collaboration has found no evidence for axions, the results represent a technical breakthrough in the ability to probe axion parameter ranges that were previously out of reach.