Shin'ichi Nojiri

Ghost-free F(R) gravity with Lagrange multiplier constraint

Abstract We propose two new versions of ghost-free generalized F ( R ) gravity with Lagrange multiplier constraint. The first version of such theory for a particular degenerate choice of the Lagrange multiplier, corresponds to mimetic F ( R ) gravity. The second version of such theory is just the Jordan frame description of mimetic gravity with potential. As we demonstrate, it is possible to realize several cosmological scenarios in such theory. In particular, de Sitter solutions may also be found.

Cosmological Bound from the Neutron Star Merger GW170817 in scalar-tensor and

Abstract We consider the evolution of cosmological gravitational waves in scalar–tensor theory and F ( R ) gravity theory as typical models of the modified gravity. Although the propagation speed is not changed from the speed of light, the propagation phase changes when we compare the propagation in these modified gravity theories with the propagation in the ΛCDM model. The phase change might be detected in future observations.

F(R)

Abstract Energy conditions can play an important role in defining the cosmological evolution. Specifically acceleration/deceleration of cosmic fluid, as well as the emergence of Big Rip singularities, can be related to the constraints imposed by the energy conditions. Here we discuss this issue for f ( R ) gravity considering also conformal transformations. Cosmological solutions and equations of state can be classified according to energy conditions. The qualitative change of some energy conditions when transformation from the Jordan frame to the Einstein frame done is also observed.

gravity theories

Maximal mass of relativistic star is one of the quantities which may distinguish modified gravity theories because it is strongly influenced by the short-range behavior of gravitation. We derive modified Tolman-Oppenheimer-Volkoff equations for the de Rham-Gabadadze-Tolley (dRGT) massive gravity with the flat reference metric. We find that new constraint equations appear and that the maximum masses in the minimal model of dRGT massive gravity become smaller than those in the general relativity.