QCD Axion Dark Matter with a Small Decay Constant

The QCD axion is a good dark matter candidate. The observed dark matter abundance can arise from misalignment or defect mechanisms, which generically require an axion decay constant $f_a\sim \mathcal{O}({10}^{11})$ GeV (or higher). We introduce a new cosmological origin for axion dark matter, parametric resonance from oscillations of the Peccei-Quinn symmetry breaking field, that requires $f_a\sim ({10}^8-{10}^{11})$ GeV. The axions may be warm enough to give deviations from cold dark matter in Large Scale Structure.