Quintessence and-the underlying particle physics theory

At present we know nothing about the nature of the dark energy accounting for about 70% of the energy density of the universe. One possibility is that the dark energy is provided by an extremely light field, the quintessence, rolling down its potential. Even though the underlying particle theory responsible for the present quintessential behaviour of our universe is unknown, such a theory is likely to have contact with supersymmetry, supergravity or (super)string theory. In these theories, there are plenty of scalar fields (moduli) which are gravitationally coupled to all the other degrees of freedom and have vacuum expectation values of the order of the Planck scale. We point out that, in theories which allow a consistent embedding of quintessence, the generic gravitational interaction of the moduli fields with the quintessence field gives rise to a contribution to the energy density from the moduli fields of the order of the critical energy density of the universe today. Furthermore, the interaction contribution can generically enhance the negativity of the equation of state. (C) 2003 Elsevier Science B.V. All rights reserved.