ABSTRACT
Modern cosmological research has revealed a fascinating if puzzling picture of our universe. Only one third of the energy density of the universe is in the form of matter, most of which is of unknown composition that we term dark matter. The remaining two thirds of the energy density is accounted for by an even more mysterious dark energy, responsible for driving the acceleration of the universe’s expansion. What comprises the dark matter is one of the premier questions in all of science today. One candidate that has received much theoretical and experimental attention is the weakly interacting massive particle (WIMP), a heavy particle (10-1000 times the proton mass) arising from the theory of supersymmetry. To date however, no evidence for such particles has been seen either in large underground detectors, or at the Large Hadron Collider. Another compelling candidate is the axion, arising from the best-motivated theory to
explain the absence of charge-parity symmetry (CP)-violating effects in the strong interaction [Peccei & Quinn, 1977; Weinberg, 1978; Wilczek, 1978]. Unlike the WIMP, the axion is extremely light, perhaps a trillionth of an electron mass, and has extraordinarily weak interactions with radiation and matter.
