Cosmological effects of the Galileon term in Scalar-Tensor Theories

Understand

We study the cosmological effects of a Galileon term in scalar-tensor theories of gravity.

• The subset of scalar-tensor theories considered are characterized by a non-minimal coupling $F(\sigma) R$, a kinetic term with arbitrary sign $Z (\partial \sigma)^2$ with $Z = \pm 1$, a potential $V(\sigma)$, and a Galileon term $G_3(\sigma, (\partial \sigma)^2) \square \sigma$.
• In addition to the modified dynamics, the Galileon term provides a screening mechanism to potentially reconcile the models with General Relativity predictions inside a Vainshtein radius.
• Thanks to the Galileon term, the stability conditions, namely ghost and Laplacian instabilities, in the branch with a negative kinetic term ($Z = -1$) are fulfilled for a large volume of the parameter space.