Quintessence

Summary

Quintessence is a model of dark energy. In it, a scalar field that changes slowly in time drives the accelerated expansion of the universe. Unlike the cosmological constant, it has an energy density that can vary with time. Its equation of state parameter $w$ can vary too. The field has a potential energy $V(\phi)$. When the potential energy is much larger than the kinetic energy, the pressure is negative and close to minus the energy density. Observations of $w$ and of its time dependence are used to tell quintessence apart from a cosmological constant.