Cosmology of hybrid metric-Palatini f(X)-gravity

Understand

A new class of modified theories of gravity, consisting of the superposition of the metric Einstein-Hilbert Lagrangian with an $f(\cal R)$ term constructed \`{a} la Palatini was proposed recently.

• The dynamically equivalent scalar-tensor representation of the model was also formulated, and it was shown that even if the scalar field is very light, the theory passes the Solar System observational constraints.
• Therefore the model predicts the existence of a long-range scalar field, modifying the cosmological and galactic dynamics.
• An explicit model that passes the local tests and leads to cosmic acceleration was also obtained.