Phys. Rev. Lett. 111, 021801 (2013)

What, wonders Julian Heeck, is the lifetime of the photon? It's not an outlandish question: adding a photon mass term to the Lagrangian of quantum electrodynamics would, admittedly, break gauge invariance and ruin renormalizability, but it's possible to write a gauge-fixed version of the Lagrangian by invoking the Stückelberg mechanism and allowing a photon mass. A photon whose mass is not zero could decay, possibly into neutrinos or particles hypothesized in extensions of the standard model, such as axions.

However, Heeck has for the moment ignored the decay products in favour of setting a model-independent limit on the lifetime of the photon — and does so using an existing set of data on the oldest photons in the visible Universe: measurements of the cosmic microwave background made by NASA's Cosmic Background Explorer. Assuming the photons to be free-streaming (effects of primordial plasma will be built into future work), Heeck sets a lower limit of about 3 years on the photon lifetime in its rest-frame, which corresponds to a lifetime of 1018 years for photons in the visible spectrum.