Quintessence and gravitational waves

We investigate some aspects of quintessence models with a nonminimally coupled scalar field and in particular we show that it can behave as a component of matter with $-3\lesssim P/\rho \lesssim 0.$ We study the properties of gravitational waves in this class of models and discuss their energy spectrum and the cosmic microwave background anisotropies they induce. We also show that gravitational waves are damped by the anisotropic stress of the radiation and that their energy spectrum may help to distinguish between the inverse power law potential and supergravity motivated potential. We finish with a discussion on the constraints arising from their density parameter ${\Omega }_{\mathrm{GW}}.$