The so-called $f(X)$ hybrid metric-Palatini gravity, where $X=R+T$, with $T$ the stress-energy trace and $R$ the Ricci scalar, presents a unique viable generalization of the $f(R)$ theories within the metric-affine formalism. In this paper, the cosmology of the $f(X)$ theories is studied using the dynamical system approach. The method consists of formulating the propagation equation in terms of suitable (expansion-normalized) variables as an autonomous system. The fixed points of the system then represent exact cosmological solutions described by power law or de Sitter expansion. The formalism is applied to two classes of $f(X)$ models, revealing both standard cosmological fixed points and new accelerating solutions that can be attractors in the phase space. In addition, the fixed point with vanishing expansion rate is considered with special care in order to characterize the stability of Einstein static spaces and bouncing solutions.

• Received 30 July 2015

DOI:https://doi.org/10.1103/PhysRevD.92.064035