Extended Cuscuton as Dark Energy
  Late-time cosmology in the extended cuscuton theory is studied, in which
gravity is modified while one still has no extra dynamical degrees of freedom
other than two tensor modes. We present a simple example admitting analytic
solutions for the cosmological background evolution that mimics $\Lambda$CDM
cosmology. We argue that the extended cuscuton as dark energy can be
constrained, like usual scalar-tensor theories, by the growth history of matter
density perturbations and the time variation of Newton's constant.
