Restricting Fourth Order Gravity via Cosmology
  The cosmology of general fourth order corrections to Einstein gravity is
considered, both for a homogeneous and isotropic background and for general
tensor perturbations. It is explicitly shown how the standard cosmological
history can be (approximately) reproduced and under what condition the
evolution of the tensor modes remain (approximately) unchanged. Requiring that
the deviations from General Relativity are small during inflation sharpens the
current constraints on such corrections terms by some thirty orders of
magnitude. Taking a more conservative approach and requiring only that
cosmology be approximately that of GR during Big Bang Nucleosynthesis, the
constraints are improved by 4 - 6 orders of magnitude.
