  A nearly-massless, slowly-rolling scalar field $\phi$ may provide most of the
energy density of the current universe. One potential difficulty with this idea
is that couplings to ordinary matter, even if suppressed by the Planck scale,
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 $\phi$ to the pseudoscalar $F_{\mu\nu}\widetilde
F^{\mu\nu}$ 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.
