We discuss present and future cosmological constraints on variations of the fine structure constant $\alpha$ 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 $\alpha /{\alpha }_0=0.975\pm 0.020$ at 95% C.L., constraining the energy density in early dark energy to ${\Omega }_e<0.060$ 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.

• Received 12 April 2011

DOI:https://doi.org/10.1103/PhysRevD.84.023518