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We explore the cosmological implications of an ultralight pseudo Nambu-Goldstone boson. With global spontaneous symmetry breaking scale f10^18GeV and explicit breaking scale comparable to Mikheyev-Smirnov-Wolfenstein neutrino masses, M10^-3eV, such a field, which acquires a mass m_M^2/fH₀, 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, ₌0. 2, with a spatially flat universe, yielding H₀t₀1 consistent with limits from gravitational lens statistics.
Frieman et al. (Mon,) studied this question.
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