Kinematical and dynamical aspects of ghost-matter cosmologies
  We consider the kinematical and dynamical evolution of Friedmann universes
with a mixture of non-interacting matter and a ghost-like field, in a scenario
analogous to that advocated by the Quintom model. Assuming that the
conventional matter dominates today, we find that the ghost component can bring
the future expansion and the past contraction of the model to a finite halt.
Moreover, at the moment the expansion or contraction stops, we find that the
tendency of the universe is to bounce back and re-collapse or re-expand.
Therefore, the presence of a (never dominant) ghost-field with negative density
could, in principle, drive the universe into an eternal cycle of finite
expansion, collapse, and re-expansion. Our study outlines the key features of
such a scenario and provides a simple condition for it to occur. We also derive
an autonomous set of differential equations and then employ dynamical-system
techniques to identify two families of fixed points, with and without spatial
curvature respectively. The members of the first family correspond to coasting
universes and are stable in the Lyapunov sense. Those of the latter family are
unstable repellers when their matter satisfies the strong energy condition and
Lyapunov stable in the opposite case.
