Matter perturbation in coupled scalar field cosmology

A universe containing a coupled scalar field would have different dynamical evolution compared to uncoupled cases. Significant differences are expected to emerge during matter dominated era, where scalar field density mimics how matter density evolves, by becoming subdominant. Analysis of the dynamics was investigated both analytically and numerically through phase plane method, and we obtained two attractor solutions which are compatible to a late time cosmic acceleration as the ending of matter dominated era. Ordinary scalar field triggers an acceleration with stable attractor solution which converges to ω_ϕ = ω_eff ≈ −1. Whereas, phantom scalar field allows negative values for its own energy (ω_eff ˂ −1 and Ω_ϕ ˂ 0); however this is not stable, therefore it will be caught by attractor solution which is stable to ω_ϕ = ω_eff ≈ −1. In addition, we found cosmological parameters generated from both analytical and numerical calculations, i.e. ω_ϕ, Ω_ϕ and ω_eff, by assuming a flat universe. The existence of a coupling constant Q between scalar field and matter induces different structure growth compared to uncoupled cases and standard cold dark matter (CDM) model. During matter dominated era, ordinary scalar field induces growth of structures so that it becomes faster than the standard model does, whereas phantom scalar field slows down the growth of structures. During scalar field dominated era (far in the future), we obtained that formed structures or density contrasts will decay in a similar way as in uncoupled cases, whereas following phantom scenario, the decay of formed structures occurs faster and more significant. This result is associated to background dynamics which shows that big rip will be the relevant fate of our universe.