Cosmological perturbations in non-local higher-derivative gravity
  We study cosmological perturbations in a non-local higher-derivative model of
gravity introduced by Biswas, Mazumdar and Siegel. We extend previous work,
which had focused on classical scalar perturbations around a cosine hyperbolic
bounce solution, in three ways. First, we point out the existence of a
Starobinsky solution in this model, which is more attractive from a
phenomenological point of view (even though it has no bounce). Second, we study
classical vector and tensor perturbations. Third, we show how to quantize
scalar and tensor perturbations in a de Sitter phase (for choices of parameters
such that the model is ghost-free). Our results show that the model is
well-behaved at this level, and are very similar to corresponding results in
local $f(R)$ models. In particular, for the Starobinsky solution of non-local
higher-derivative gravity, we find the same tensor-to-scalar ratio as for the
conventional Starobinsky model.
