Quintessence, Flat Potential and String/M Theory Axion

A slow-rolling scalar field (Quintessence) has been proposed as the origin of accelerating universe at present. We discuss some features of quintessence, particularly those related with the {\it flatness} of its potential. We distinguish two types of quintessences, a metrical quintessence which couples to the particle physics sector {\it only} through the mixing with the spacetime metric and a non-metrical one without such property. It is stressed that these two types of quintessences have quite different features in regard to the fine tuning of parameters required for the flat potential, as well as different implications for the quintessence-mediated long range force. The fine tuning argument strongly suggests that non-metrical quintessence should correspond to the Goldstone boson of an almost exact nonlinear global symmetry whose explicit breaking by quantum gravity effects is highly suppressed. It is briefly discussed whether the radion field in higher dimensional theories can be a candidate for metrical quintessence. We finally discuss the possibility that string/$M$ theory axion corresponds to a Goldstone-type quintessence, and argue that a certain combination of the heterotic $M$ theory or Type I string theory axions can be a good candidate for quintessence if some conditions on the moduli dynamics are satisfied.