2018ApJ...867...50C

Probing the speed of light is an important test of general relativity, but the measurements of c using objects in the distant universe have been almost completely unexplored. In this paper, we propose an idea to use the multiple measurements of galactic-scale strong gravitational lensing systems with Type Ia supernovae acting as background sources to estimate the speed of light. This provides an original method to measure the speed of light using objects located at different redshifts that emitted their light in a distant past. Moreover, we predict that strongly lensed Type Ia supernovae observed by the Large Synoptic Survey Telescope (LSST) would produce robust constraints on Δc/c at the level of 10^–3. We also discuss whether future surveys such as LSST may succeed in detecting any hypothetical variation of c predicted by theories in which fundamental constants have a dynamical nature.