Hubble diagram of the Union 2.1 SN Compilation (red dots). Overplotted are the theoretical curves, representing the distance modulus as a function of the cosmological redshift in the polytropic-DM model, for Ω_M = 0.274 and several (acceptable) values of the polytropic exponent, namely, Γ = 0.541 (magenta), Γ = 0 (blue), and Γ = −0.377 (green), as compared to the corresponding quantity in the collisionless-DM (dust) case (dashed curve).