Varying-Alpha Cosmologies with Potentials

We generalize the Bekenstein-Sandvik-Barrow-Magueijo (BSBM) model for the variation of the fine structure 'constant', $α,$ to include an exponential or inverse power-law self-potential for the scalar field $$ which drives the time variation of $α$, and consider the dynamics of $ϕ$ in such models. We find solutions for the evolution of $ϕ$ or $α$ in matter-, radiation- and dark-energy-dominated cosmic eras. In general, the evolution of $ϕ$ is well determined solely by either the self-potential or the coupling to matter, depending on the model parameters. The results are general and applicable to other models where the evolution of a scalar field is governed by a matter coupling and a self-potential. We find that the existing astronomical data stringently constrains the possible evolution of $α$ between redshifts $z\simeq 1-3.5$ and the present, and this leads to very strong limit on the allowed deviation of the potential from that of a pure cosmological constant.

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