Constraining the runaway dilaton and quintessential dark energy

We investigate a form of quintessential dark energy for which the time evolution of a scalar dilaton field φ is given by |φ(a)|/mP ≡ α1 ln t + α2t = α ln a + β a2ζ , where a is the scale factor of a spatially flat FRW universe. This ansatz for φ is motivated by generic solutions of a scalar dilaton field in some effective string theory models in four dimensions, which also captures basic features of many models of quintessence. Using a compilation of current data from the cosmic microwave background (CMB) shift, baryon acoustic oscillations (BAO), suvernovae type Ia (SNIa) and legacy survey (SNLS), we impose observational constraints on the combination α + 2ζ and discuss the relation of our results to analytical constrains on various cosmological parameters, including the dark energy equation of state (EoS). With the input Ω0,m = 0.27, we find the constraint |α + 2ζ| = 0.313 (±0.294) at low redshift (z . 1). This constraint is stronger than the one obtained by simply demanding that the dark energy equation of state lies in the range −1.11 < wφ < −0.86 as inferred from a combination of BAO, SNIa and WMAP data at a 95% confidence level, which gives α + 2ζ = 0 (±0.5), and the one required simply to get a cosmic acceleration, |φ0|/mP ≡ m P | dφ d lna |a=a0 < 0.8 (with Ω0m > 0.24). By allowing a non-minimal coupling between dark matter and scalar dilaton (or quintessence), and by least squares curve fitting to observational results for the growth of structure (in the redshift range 0.15 < z < 3), we find Q = 0.27 ± 0.20, where Q ≡ 1 mP d lnA dφ and A(φ) measures the scalar field (dark) matter coupling. Further, with the input Ω0m = 0.27± 0.03, and using the best-fit values of the model parameters, it is found that cosmic acceleration began in the redshift range 0.6 < z < 0.9. The constraints of this model are shown to encompass the cosmological constant limit within 1σ error bars.