31 Mar 2006 | Shin-ichi Nojiri, Sergei D. Odintsov
The paper reviews various modified gravity theories as gravitational alternatives to dark energy, focusing on $f(R)$, $f(G)$, and $f(R,G)$ gravity, as well as models with non-linear gravitational coupling and string-inspired models with Gauss-Bonnet-dilaton coupling. These theories are shown to pass Solar System tests and have rich cosmological structures, capable of describing late-time cosmic acceleration and transitions from deceleration to acceleration. The theories can naturally explain the coincidence problem as a manifestation of the universe's expansion and predict the existence of dark fluid with inhomogeneous equations of state. The paper also discusses the possibility of using modified gravity to solve the hierarchy problem in high energy physics.The paper reviews various modified gravity theories as gravitational alternatives to dark energy, focusing on $f(R)$, $f(G)$, and $f(R,G)$ gravity, as well as models with non-linear gravitational coupling and string-inspired models with Gauss-Bonnet-dilaton coupling. These theories are shown to pass Solar System tests and have rich cosmological structures, capable of describing late-time cosmic acceleration and transitions from deceleration to acceleration. The theories can naturally explain the coincidence problem as a manifestation of the universe's expansion and predict the existence of dark fluid with inhomogeneous equations of state. The paper also discusses the possibility of using modified gravity to solve the hierarchy problem in high energy physics.