A Simple Varying-alpha Cosmology

We investigate the cosmological consequences of a simple theory in which the electric charge $e$ is allowed to vary. The theory is locally gauge and Lorentz invariant, and satisfies general covariance. We find that in this theory the fine structure 'constant', $\alpha $, remains almost constant in the radiation era, undergoes small increase in the matter era, but approaches a constant value when the universe starts accelerating because of the presence of a positive cosmological constant. This model satisfies geological, nucleosynthesis, and CMB constraints on time-variation in $\alpha $, while fitting simultaneously the observed accelerating universe and the recent high-redshift evidence for small $\alpha $ variations in quasar spectra. Further tests of this model, involving stellar spectra and the E\"otvos experiment, are proposed.