"The speed of light itself has not decayed at all, really. Rather, the obstacles (virtual particles) which absorb and then re-emit any given photon of light has increased with the increase in the Zero Point Energy in space, thus causing light to take longer to reach its final point of absorption – its destination. However, in between obstacles, the speed of light remains the same as the moment it was emitted from the atom. There is, in short, no such thing as ‘tired light.’ There is simply light being held up in its travel by denser mediums." A lay explanation can be found here:
http://www.setterfield.org/simplified.html
I'm surprised that Setterfield has been subjected to such a great shun of his work, seeing as though he is like a modern LeMaitre - creationist. I think it might have something to do with his adherence to zpe, eventhough he has much support to back up his inferences.
Though I don't fathom literal expansion, but rather light taking time to observably-"traverse" scales, the mechanics involved in his explanations are somewhat midway to opposing views.