Quintessence Field as a Perfect Cosmic Fluid of Constant Pressure

We study the cosmology of a quintessence scalar held 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 Geld is neither a unified dark matter nor a mixture of cosmological constant and cold dark matter,relying on the facts that the quintessence density contrasts of sub-horizon modes would undergo a period of late-time dechne and the squared sound speeds of quintessence perturbations do not vanish.What a role does the quintessence play is dynamic dark energy,its clustering could remarkably reduce the growth rate of the density perturbations of non-relativistic matters.