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 $T_{\text{rh}}$: $H_{\text{inf}}\mathrm{/}{M}_{\text{Pl}}\lesssim T_0\mathrm{/}{T}_{\text{rh}}$, 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 $w\approx -1\mathrm{/}3$, 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 $r\lesssim {10}^{-8}$, which can be realized in many string-inspired inflation models.