PARAMETERIZING AND MEASURING DARK ENERGY TRAJECTORIES FROM LATE INFLATONS

Bulk dark energy (DE) properties are determined by the redshift evolution of its pressure-to-density ratio, w(de)(z). An experimental goal is to decide if the DE is dynamical, as in the quintessence (and phantom) models treated here. We show that a three-parameter approximation w(de)(z; epsilon(s), epsilon(phi infinity), zeta(s)) fits well the ensemble of trajectories for a wide class of late-inflaton potentials V (phi). Markov Chain Monte Carlo probability calculations are used to confront our w(de)(z) trajectories with current observational information on Type Ia supernova, cosmic microwave background, galaxy power spectra, weak lensing, and the Ly alpha forest. We find that the best-constrained parameter is a low-redshift slope parameter, epsilon(s) proportional to (partial derivative ln V/partial derivative phi)(2) when the DE and matter have equal energy densities. A tracking parameter epsilon(phi infinity) defining the high-redshift attractor of 1 + w(de) is marginally constrained. zeta(s) is poorly determined, which characterizes the evolution of epsilon(s), and is a measure of partial derivative(2) ln V/partial derivative phi(2). The constraints we find already rule out some popular quintessence and phantom models, or restrict their potential parameters. We also forecast how the next generation of cosmological observations improve the constraints: by a factor of about five on epsilon(s) and epsilon(phi infinity), but with zeta(s) remaining unconstrained (unless the true model significantly deviates from Lambda CDM). Thus, potential reconstruction beyond an overall height and a gradient is not feasible for the large space of late-inflaton models considered here.