Observational constraints to a phenomenological $f\left( R,\nabla R\right) $-model

Understand

This paper analyses the cosmological consequences of a modified theory of gravity whose action integral is built from a linear combination of the Ricci scalar $R$ and a quadratic term in the covariant derivative of $R$.

• The resulting Friedmann equations are of the fifth-order in the Hubble function.
• These equations are solved numerically for a flat space section geometry and pressureless matter.
• The cosmological parameters of the higher-order model are fit using SN Ia data and X-ray gas mass fraction in galaxy clusters.