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Cosmology with Ultra-light Pseudo-Nambu-Goldstone Bosons
We explore the cosmological implications of an ultra-light
pseudo-Nambu-Goldstone boson. With global spontaneous symmetry breaking scale
GeV and explicit breaking scale comparable to MSW neutrino
masses, eV, such a field, which acquires a mass , would have become dynamical at recent epochs and currently
dominate the energy density of the universe. The field acts as an effective
cosmological constant for several expansion times and then relaxes into a
condensate of coherent non-relativistic bosons. Such a model can reconcile
dynamical estimates of the density parameter, , with a
spatially flat universe, and can yield an expansion age
while remaining consistent with limits from gravitational lens statistics.Comment: 15 pages (including 2 figs.), uuencoded compressed postscript; also
available on www at http://www-astro-theory.fnal.gov/ under Publication