Tensor Modes in Pure Natural Inflation
  We study tensor modes in pure natural inflation (arXiv:1706.08522), a
recently-proposed inflationary model in which an axionic inflaton couples to
pure Yang-Mills gauge fields. We find that the tensor-to-scalar ratio r is
naturally bounded from below. This bound originates from the finiteness of the
number of metastable branches of vacua in pure Yang-Mills theories. Details of
the model can be probed by future cosmic microwave background experiments and
improved lattice gauge theory calculations of the theta-angle dependence of the
vacuum energy.
