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 $H_{\inf }$ which is solely determined by the reheating temperature $T_{\mathrm{rh}}$: $H_{\inf }/M_{\mathrm{Pl}}\lesssim T_0/T_{\mathrm{rh}}$, where $T_0$ is the photon temperature today. The upper limit can be saturated by a postinflationary 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 $r\lesssim {10}^{-8}$, which can be realized in many string-inspired inflation models.

• Received 9 November 2019
• Accepted 22 June 2020

DOI:https://doi.org/10.1103/PhysRevD.102.021301