In an expanding universe model. In addition to galaxies getting fainter with distance, they are also thought to get bigger. Thus the surface brightness of the object should decrease with distance. However, in this model why wouldn't galaxies remain the same size as its only space that's expanding between galaxies? Why are galaxies expecting to get bigger?
I read about this here Universe is Not Expanding After All, Controversial Study Suggests
and in this paper Evidence for a Non-Expanding Universe: Surface Brightness Data From HUDF, and this more recent paper UV surface brightness of galaxies from the local Universe to z ~ 5.