Many physics calculations rely on the assumption that the ‘physical constants’ e.g. light speed, gravity, etc etc are, in fact, constant. Extremely accurate experimental procedures strongly suggest that they are. But the experiments can only be carried out on a ‘local’ scale, and in a very short timeframe (cosmologically speaking). Thus the possibility exists that some constants may vary at extreme distances and/or timescales. If so, current explanations of many cosmological phenomena may have to be completely re-evaluated.
Further reading: The Variability of Fundamental Constants (by Dr. Rupert Sheldrake)
In 2007, the MAGIC telescope array at La Palma, Canary Islands (Spain) found that high energy photons arriving from distant gamma ray bursts arrived at different times - the delay (max. of several minutes) being determined by the photon's wavelength (i.e. energy).