Cosmological evolution in vector-tensor theories of gravity
  We present a detailed study of the cosmological evolution in general
vector-tensor theories of gravity without potential terms. We consider the
evolution of the vector field throughout the expansion history of the universe
and carry out a classification of models according to the behavior of the
vector field in each cosmological epoch. We also analyze the case in which the
universe is dominated by the vector field, performing a complete analysis of
the system phase map and identifying those attracting solutions which give rise
to accelerated expansion. Moreover, we consider the evolution in a universe
filled with a pressureless fluid in addition to the vector field and study the
existence of attractors in which we can have a transition from
matter-domination to vector-domination with accelerated expansion so that the
vector field may play the role of dark energy. We find that the existence of
solutions with late-time accelerated expansion is a generic prediction of
vector-tensor theories and that such solutions typically lead to the presence
of future singularities. Finally, limits from local gravity tests are used to
get constraints on the value of the vector field at small (Solar System)
scales.
