I will discuss the astrophysical and cosmological phenomenology of ULAs that makes them attractive, and methods to distinguish them from CDM observationally. However, if the axion global symmetry is an accidental symmetry descending from an exact discrete symmetry, then the problematic higher dimensional operators can be forbidden to very high order. Quantum gravitational effects are expected to break continuous global symmetries, and may spoil the axion potential. A single ULA with a GUT scale decay constant provides the correct relic density without fine-tuning. Ultralight axions (ULAs) with m~10^ eV are attractive dark matter candidates with novel properties that distinguish them from cold dark matter (CDM). Axions, having a perturbative shift symmetry, can have masses much smaller than other types of particles in a technically natural way.
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