3 Quintessence: A Review

Quintessence is a canonical scalar field introduced to explain the late-time cosmic acceleration. The cosmological dynamics of quintessence is reviewed, paying particular attention to the evolution of the dark energy equation of state w. For the field potentials having tracking and thawing properties, the evolution of w can be known analytically in terms of a few model parameters. Using the analytic expression of w, we constrain quintessence models from the observations of supernovae type Ia, cosmic microwave background and baryon acoustic oscillations. The tracking freezing models are hardly distinguishable from the Λ-cold-dark-matter model, whereas in thawing models the today's field equation of state is constrained to be w0 < −0.7 (95% CL). We also derive an analytic formula for the growth rate of matter density perturbations in dynamical dark energy models, which allows a possibility of putting further bounds on w from the measurement of redshift-space distortions in the galaxy power spectrum. Finally, we review particle physics models of quintessence—such as those motivated by supersymmetric theories. The field potentials of thawing models based on a pseudo-Nambu–Goldstone boson or extended supergravity theories have a nice property that a tiny mass of quintessence can be protected against radiative corrections.

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