SCIENCE - Speed of light may not be fixed, scientists suggest; Ephemeral vacuum particles induce speed-of-light fluctuations

Two new articles challenge established wisdom about the nature of vacuum. In one article scientists have identified a quantum level mechanism for interpreting vacuum as being filled with pairs of virtual particles with fluctuating energy values. As a result, the inherent characteristics of vacuum, like the speed of light, may not be a constant after all, but fluctuate.

http://www.sciencedaily.com/releases/2013/03/130325111154.ht