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 si milar or equal to 10(18) GeV and explicit breaking scale comparable to Mikheyev-Smirnov-Wolfenstein neutrino masses, M similar to 10(-3) eV, such a field, which acquires a mass m(phi) similar to M(2)/f, similar to H-0, would currently dominate the energy density of the Universe. The field acts as an effective cosmological constant before relaxing i nto a condensate of nonrelativistic bosons. Such a model can reconcile dynamical estimates of the density parameter, Omega(m) similar to 0.2 , with a spatially flat universe, yielding H(0)t(0) similar or equal t o 1 consistent with limits from gravitational lens statistics.