New class of generalized coupling theories
  We propose a new class of gravity theories which are characterized by a
nontrivial coupling between the gravitational metric and matter mediated by an
auxiliary rank-2 tensor. The actions generating the field equations are
constructed so that these theories are equivalent to general relativity in a
vacuum, and only differ from general relativity theory within a matter
distribution. We analyze in detail one of the simplest realizations of these
generalized coupling theories. We show that in this case the propagation speed
of gravitational radiation in matter is different from its value in vacuum and
that this can be used to weakly constrain the (single) additional parameter of
the theory. An analysis of the evolution of homogeneous and isotropic
spacetimes in the same framework shows that there exist cosmic histories with
both an inflationary phase and a dark era characterized by a different
expansion rate.
