Cosmological Constant Versus Quintessence

The mounting evidence that the universe is presently undergoing acceleratingexpansion has restored some credit to the scenarios with a nonvanishingcosmological constant. From the point of view of a theory of fundamentalinteractions, one may argue that a dynamical component with negative pressureis easier to achieve. As an illustration, the quintessence scenario is described andits shortcomings are discussed in connection with the nagging “cosmologicalconstant problem.”