Constraining variations in the fine structure constant in the presence of early dark energy

We discuss present and future cosmological constraints on variations of the fine structure constant α induced by an early dark energy component having the simplest allowed (linear) coupling to electromagnetism. We find that current cosmological data show no variation of the fine structure constant at recombination with respect to the present-day value, with α/α0=0.975±0.020 at 95% C.L., constraining the energy density in early dark energy to Ωe<0.060 at 95% C.L. Moreover, we consider constraints on the parameter quantifying the strength of the coupling by the scalar field. We find that current cosmological constraints on the coupling are about 20 times weaker than those obtainable locally (which come from Equivalence Principle tests). However forthcoming or future missions, such as the Planck Surveyor and the CMBPol satellite, can match and possibly even surpass the sensitivity of current local tests.