A Null Test of the Metric Nature of the Cosmic Acceleration
  We discuss the testable predictions of a phenomenological model in which the accelerated expansion of the universe is the result of the action of a non-gravitational force field, rather than the effect of a negative-pressure dark-energy fluid or a modification of general relativity. We show, through the equivalence principle, that in such a scenario the cosmic acceleration felt by distant standard candles like SNIa depends on the mass of the host system, being larger in galaxies than in rich clusters. As a consequence, the scatter in the observed SNIa Hubble diagram has mostly a physical origin in this scenario: in fact, the SNIa distance modulus is increasing, at fixed redshift, for SNe that are hosted in isolated galaxies with respect to the case of SNe hosted in rich galaxy clusters. Due to its strong dependence on the astrophysical environments of standard candles, we conclude that alternative non-gravitational mechanisms for the observed accelerated expansion of the universe can be interestingly contrasted against the standard metric interpretation of the cosmological acceleration by means of an environmental analysis of the cosmic structures in which SNIa are found. The possible absence of such environmental effects would definitely exclude non-gravitational mechanisms as responsible for the accelerated cosmological expansion and will therefore reinforce a metric interpretation. 
A.Buzzi (1), C.Marinoni (1), S.Colafrancesco (2) ((1) CPT, Universite   de Provence, France, (2) Osservatorio di Roma, Italy)