Coupled quintessence - art. no. 043511

A new component of the cosmic medium, a light scalar field or "quintessence ," has been proposed recently to explain cosmic acceleration with a dynamic al cosmological constant. Such a field is expected to be coupled explicitly to ordinary matter, unless some unknown symmetry prevents it. I investigat e the cosmological consequences of a coupled quintessence (CQ) model, assum ing an exponential potential and a linear coupling. This model is conformal ly equivalent to Brans-Dicke Lagrangians with any power-law potential. I ev aluate the density perturbations on the cosmic microwave background and on the galaxy distribution at the present and derive bounds on the coupling co nstant from the comparison with observational data. A novel feature of CQ i s that during the matter dominated era the scalar field has a finite and al most constant energy density. This epoch, denoted as phi MDE, is a saddle p oint in the dynamical phase space. The phi MDE is responsible of several di fferences with respect to uncoupled quintessence: the multipole spectrum of the microwave background is tilted at large angles, the acoustic peaks are shifted, their amplitude is changed, and the present 8 Mpc/h density varia nce is diminished. The present data constrain the dimensionless coupling co nstant to \beta\less than or equal to 0.1 assuming Omega(m)=0.3 and a primo rdial fluctuation slope n(s)=1.