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We propose a new class of quintessence models in which late times oscillations of a scalar field give rise to an effective equation of state which can be negative and hence drive the observed acceleration of the universe. Our ansatz also describes a new form of dark matter which we call frustrated cold dark matter (FCDM). FCDM inhibits gravitational clustering on small scales and could provide a natural resolution to the core density problem for disk galaxy halos.
Sahni et al. (Tue,) studied this question.