Scalar-Vector-Tensor Gravity Theories
  We construct the consistent ghost-free covariant scalar-vector-tensor gravity
theories with second order equations of motion with derivative interactions. We
impose locality, unitarity, Lorentz invariance and pseudo-Riemannian geometry
as the fundamental terms. In the tensor sector we require diffeomorphism
invariance, whereas we allow the vector sector to be gauge invariant or not.
The resulting Lagrangians consist of new genuine couplings among these fields
with at most two derivatives per field. They propagate five physical degrees of
freedom in the gauge invariant case and six degrees of freedom if the gauge
invariance is broken. In the corresponding limit of the free general functions
in the Lagrangian, one recovers the generalized Proca theories. These
scalar-vector-tensor theories will have important implications for cosmological
and astrophysical applications, among which we mention the application to
inflation and generation of primordial magnetic fields, new black hole and
neutron star solutions, dark matter and dark energy.
