Gauge Invariant Perturbation Analysis for Quintessence with an Exponential Potential

Adiabatic perturbations in the cosmology of a quintessential scalar field with exponential potential gravitationally coupled to radiation/matter are investigated in a gauge invariant formalism. The main question addressed in this paper is if the inhomogenous fluctuations can be ignored as often argued and the result is that this is not always the case. Consequences for microwave background radiation and structure formation in the matter dominated era are considered.