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≃10¹⁸GeV and explicit breaking scale comparable to Mikheyev-Smirnov-Wolfenstein neutrino masses, M∼10^-3eV, such a field, which acquires a mass m_φ∼M²/f∼H₀, 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, Ω_m∼0.2, with a spatially flat universe, yielding H₀t₀≃1 consistent with limits from gravitational lens statistics.