Observational constraints on interacting quintessence models

We determine the range of parameter space of an Interacting Quintessence Model that best fits the recent WMAP measurements of Cosmic Microwave Background temperature anisotropies. We only consider cosmological models with zero spatial curvature. We show that if the quintessence scalar field decays into cold dark matter at a rate that brings the ratio of matter to dark energy constant at late times, the cosmological parameters required to fit the CMB data are: dark energy density Ωₓ=0.43±0.12, baryon fraction Ωb=0.08±0.01, slope of the matter power spectrum at large scales nₛ=0.98±0.02 and Hubble constant H₀=56±4 km/s/Mpc. The data prefers a dark energy component with a dimensionless decay rate parameter c²=0.005 and noninteracting models are consistent with the data only at the $99.9%$ confidence level. Using the Bayesian Information Criteria we show that this extra parameter fits the data better than models with no interaction. The quintessence equation of state parameter is less constrained; i.e., the data sets an upper limit wₓ≤-0.86 at the same level of significance. When the WMAP anisotropy data are combined with supernovae data, the density parameter of dark energy increases to Ωₓ0.68 while c² augments to 6.3×10^-3. Models with quintessence decaying into dark matter provide a clean explanation for the coincidence problem and are a viable cosmological model, compatible with observations of the CMB, with testable predictions. Accurate measurements of baryon fraction and/or of matter density independent of the CMB data, would support/disprove these models.