Cosmology with Ultralight Pseudo Nambu-Goldstone Bosons

We explore the cosmological implications of an ultralight pseudo Nambu-Goldstone boson. With global spontaneous symmetry breaking scale $f\simeq {10}^{18}\mathrm{GeV}$ and explicit breaking scale comparable to Mikheyev-Smirnov-Wolfenstein neutrino masses, $M\sim {10}^{-3\mathrm{}}\mathrm{eV}$, such a field, which acquires a mass $m_{\phi }\sim M^2/f\sim H_0$, would currently dominate the energy density of the Universe. The field acts as an effective cosmological constant before relaxing into a condensate of nonrelativistic bosons. Such a model can reconcile dynamical estimates of the density parameter, ${\Omega }_m\sim 0.2$, with a spatially flat universe, yielding $H_0{t}_0\simeq 1$ consistent with limits from gravitational lens statistics.