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.
