Towards singularity and ghost free theories of gravity
  We present the most general covariant ghost-free gravitational action in a Minkowski vacuum. Apart from the much studied f(R) models, this includes a large class of non-local actions with improved UV behavior, which nevertheless recover Einstein's general relativity in the IR. 
Tirthabir Biswas, Erik Gerwick, Tomi Koivisto, Anupam Mazumdar
gr-qc astro-ph.CO hep-ph hep-th