Universal Upper Bound on the Inflationary Energy Scale from the
  Trans-Planckian Censorship Conjecture
  We study the constraint on the inflationary energy scale from the recently
proposed Trans-Planckian Censorship Conjecture (TCC). We find a universal upper
bound on the inflationary Hubble expansion rate $H_\text{inf}$ which is solely
determined by the reheating temperature $T_\text{rh}$:
$H_\text{inf}/M_\text{Pl}\lesssim T_0/T_\text{rh}$, where $T_0$ is the photon
temperature today. The upper limit can be saturated by a post-inflationary
oscillatory stage with the critical equation-of-state parameter $w\approx-1/3$,
or by an inflation model with multiple stages. In the lowest reheating
temperature required for big bang nucleosynthesis, the upper bound on the
tensor-to-scalar ratio $r$ at the CMB scales is $r\lesssim10^{-8}$, which can
be realized in many string-inspired inflation models.
