Conservation laws in theories with universal gravity/matter coupling

Understand

In this paper, we investigate the conservation laws of different type of particles in theories with a universal gravity/matter coupling.

• The result brings new insights about previous studies on universal gravity/matter theories.
• Especially, the paper demonstrates that for perfect fluids, there is an equivalence between the assumption $\L_m=-\epsilon$, where $\epsilon$ is the total energy density; and the assumption that the matter fluid current is conserved ($\nabla_\sigma (\rho u^\sigma)=0$, where $\rho$ is the rest mass density).
• However, the main result is given in the general case where one does not make any assumption on the conservation of the matter fluid current.