Structure Formation with a Self-Tuning Scalar Field

A scalar field with an exponential potential has the particular property that it is attracted into a solution in which its energy scales as the dominant component (radiation or matter) of the Universe, contributing a fixed fraction of the total energy density. We study the growth of perturbations in a cold dark matter dominated Ω0ex0ex=0ex0ex1 universe with this extra field, with an initial flat spectrum of adiabatic fluctuations. The observational constraints from structure formation are satisfied as well, or better, than in other models, with a contribution to the energy density from the scalar field Ω_φ~0.1 which is small enough to be consistent with entry into the attractor prior to nucleosynthesis.