Many extensions of the standard model predict the existence of hidden sectors that may contain unbroken Abelian gauge groups. We argue that in the presence of quantum decoherence photons may convert into hidden photons on sufficiently long time scales and show that this effect is strongly constrained by CMB and supernova data. In particular, Planck-scale suppressed decoherence scales D ∝ ω 2 / M Pl (characteristic for noncritical string theories) are incompatible with the presence of even a single hidden U(1). The absence of photon decoherence in this simple standard model extension complements other strong bounds derived from solar, reactor, and atmospheric neutrinos.