The quintessence field as a perfect cosmic fluid of constant pressure

We study the cosmology of a quintessence scalar field which is equivalent to a non-barotropic perfect fluid of constant pressure. The coincidence problem is alleviated by such a quintessence equation-of-state that interpolates between plateau of zero at large redshifts and plateau of minus one as the redshift approaches to zero. The quintessence field is neither a unified dark matter nor a mixture of cosmological constant and cold dark matter, because the quintessence density contrasts decay monotonously on sub-horizon scales and the squared sound speeds of quintessence perturbations do not vanish. What a role does the quintessence play is dynamic dark energy. Though the clustering of quintessence decays drastically, it could remarkably impact the growth rate of the density perturbations of non-relativistic matters at early stage.