In quintessence models, the dark energy content of the universe is described by a slowly rolling scalar field, the pressure and energy density of which obey an equation of state of the form p=wρ w is in general a function of time such that w < -1/3, in order to drive the observed acceleration of the Universe today. The cosmological constant model (ΛCDM) corresponds to the limiting case w=-1. In this paper, we explore the prospects of using the Lyman α forest to constrain w, using semi-analytical techniques to model the intergalactic medium (IGM). A different value of w changes both the growth factor and the Hubble parameter as a function of time. The resulting change in the optical depth distribution affects the optical depth power spectrum, the number of regions of high transmission per unit redshift and the cross-correlation coefficient of spectra of quasar pairs. These can be detected in current data, provided we have independent estimates of the thermal state of the IGM, its ionization parameter and the baryon density.
