Theoretical bounds on the tensor-to-scalar ratio in the cosmic microwave
  background
  Tensor modes in the cosmic microwave background are one of the most robust
signatures of inflation. We derive theoretical bounds on the tensor fraction,
as a generalization of the well-known Lyth bound. Under reasonable assumptions,
the new bounds are at least two orders of magnitude stronger than the original
one. We comment on a previously derived generalization, the so-called
Efstathiou-Mack relationship. We also derive a new absolute upper bound on
tensors using de Sitter entropy bounds.
