Comment on "Varying-G Cosmology with Type Ia Supernovae" [Am. J. Phys. 79, 57-62 (2011)]

In a recent paper, Dungan and Prosper claim that the Type Ia supernovae data alone are not enough to distinguish between the standard Lambda CDM model and other models with varying G. To substantiate this, they present two spatially flat variable G FRW models with Lambda = 0 that fit well the Type Ia supernova data. In these models they assumed that the energy momentum tensor of the matter distribution is conserved. We show that this assumption is inconsistent with variable G cosmology when Lambda is assumed to be constant, thus rendering the suggested models erroneous. (C) 2015 American Association of Physics Teachers.