We explore the cosmological implications of an ultralight pseudo Nambu-Goldstone boson. With global spontaneous symmetry breaking scale and explicit breaking scale comparable to Mikheyev-Smirnov-Wolfenstein neutrino masses, , such a field, which acquires a mass , 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, , with a spatially flat universe, yielding consistent with limits from gravitational lens statistics.
- Received 17 May 1995
©1995 American Physical Society