Abstract
A nearly massless, slowly rolling scalar field $\ensuremath{\varphi}$ may provide most of the energy density of the current Universe. One potential difficulty with this idea is that couplings to ordinary matter should lead to observable long-range forces and time dependence of the constants of nature. I explore the possibility that an approximate global symmetry serves to suppress such couplings even further. Such a symmetry would allow a coupling of $\ensuremath{\varphi}$ to the pseudoscalar ${F}_{\ensuremath{\mu}\ensuremath{\nu}}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{F}}^{\ensuremath{\mu}\ensuremath{\nu}}$ of electromagnetism, which would rotate the polarization state of radiation from distant sources. This effect is fairly well constrained, but it is conceivable that future improvements could lead to a detection of a cosmological scalar field.
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