Cosmologies in Horndeski's second-order vector-tensor theory

Understand

Horndeski derived a most general vector-tensor theory in which the vector field respects the gauge symmetry and the resulting dynamical equations are of second order.

• The action contains only one free parameter, $\lambda$, that determines the strength of the non-minimal coupling between the gauge field and gravity.
• We investigate the cosmological consequences of this action and discuss observational constraints.
• For $\lambda<0$ we identify singularities where the deceleration parameter diverges within a finite proper time.