Observational constraints on quintessence: Thawing, tracker, and scaling models

For two types of quintessence models having thawing and tracking properties, there exist analytic solutions for the dark energy equation of state w expressed in terms of several free parameters. We put observational bounds on the parameters in such scenarios by using the recent type Ia supernovae, cosmic microwave background, and baryon acoustic oscillations data. The observational constraints are quite different depending on whether or not the recent baryon acoustic oscillations data from BOSS are taken into account. With the BOSS data the upper bounds of today's values of w (=w₀) in thawing models is very close to -1, whereas without this data the values of w₀ away from -1 can still be allowed. The tracker equation of state w₍₀₎ during the matter era is constrained to be w₍₀₎<-0.949 at 95% C.L. even without the BOSS data, so that the tracker models with w away from -1 are severely disfavored. We also study observational constraints on scaling models in which w starts to evolve from 0 in the deep matter era and show that the transition to the equation of state close to w=-1 needs to occur at an early cosmological epoch. In the three classes of quintessence models studied in this paper, the past evolution of the Hubble parameters in the best-fit models shows only less than a 2.5% difference compared to ΛCDM.