I examine how two specific examples of modified gravity explanations of cosmic acceleration help us understand some general problems confronting cosmological tests of gravity: how do we distinguish modified gravity from dark energy if they can be made formally equivalent? how do we parameterize deviations according to physical principles with sufficient generality, yet focus cosmological tests ...

In this paper we obtain the conservation equations of momentum-energy a relativistic fluid in flat space-time. Three mechanisms for energy and momentum transferring are considered, which are: perfect fluid, stress viscosity heat transfer mechanisms. The effect magnetic field is ignored paper. Also, connection non-relativistic seen paper, so, can be modified with influences.The method deriving a...

Modifications of general relativity provide an alternative explanation to dark energy for the observed acceleration of the Universe. Modified gravity theories have richer observational consequences for large scale structures than conventional dark energy models, in that different observables are not described by a single growth factor even in the linear regime. We examine the relationships betw...