title: Towards singularity- and ghost-free theories of gravity.
abstract: 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 nonlocal actions with improved UV behavior, which nevertheless recover Einstein's general relativity in the IR.
authors.affiliation: Physics Department, Loyola University, Campus Box 92, New Orleans, Louisiana 70118, USA.
authors.forename: Tirthabir
authors.initials: T
authors.lastname: Biswas
authors.forename: Erik
authors.initials: E
authors.lastname: Gerwick
authors.forename: Tomi
authors.initials: T
authors.lastname: Koivisto
authors.forename: Anupam
authors.initials: A
authors.lastname: Mazumdar