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 CDM dominated \Omega = 1 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 \Omega _ { \phi } \sim 0.1 which is small enough to be consistent with entry into the attractor prior to nucleosynthesis .