Cosmology with Ultra-light Pseudo-Nambu-Goldstone Bosons

We explore the cosmological implications of an ultra-light pseudo-Nambu-Goldstone boson. With global spontaneous symmetry breaking scale $f\simeq {10}^{18}$ GeV and explicit breaking scale comparable to MSW neutrino masses, $M\sim {10}^{-3}$ eV, such a field, which acquires a mass $m_\phi \sim M^2/f \sim H_0$, would have become dynamical at recent epochs and currently dominate the energy density of the universe. The field acts as an effective cosmological constant for several expansion times and then relaxes into a condensate of coherent non-relativistic bosons. Such a model can reconcile dynamical estimates of the density parameter, ${\mathrm{\Omega }}_m\sim 0.2$, with a spatially flat universe, and can yield an expansion age $H_0{t}_0\simeq 1$ while remaining consistent with limits from gravitational lens statistics.