A weakly coupled scalar field $\Phi$ with a simple exponential potential $V=M_P^4\exp(-\lambda\Phi/M_P)$ where $M_P$ is the reduced Planck mass, and $\lambda > 2$, has an attractor solution in a radiation or matter dominated universe in which it mimics the scaling of the dominant component, contributing a fixed fraction $\Omega_\phi$ (determined by $\lambda$) to the energy density. Such fields arise generically in particle physics theories involving compactified dimensions, with values of ...Expand abstract
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31 pages, uses RevTex, 19 figures, revised and accepted to Phys. Rev.