Trans-Planckian censorship, inflation and excited initial states for perturbations

TL;DR

The paper discusses how the trans-Planckian censorship conjecture constrains inflationary models to very low energy scales, but suggests that excited initial states for perturbations could potentially circumvent these restrictions.

Contribution

It demonstrates the robustness of TCC bounds and proposes excited initial states as a possible way to reconcile inflation with these constraints.

Findings

TCC implies extremely low inflation energy scale

Standard inflation predicts negligible tensor-to-scalar ratio

Excited initial states may allow inflation to evade TCC bounds

Abstract

The recently proposed trans-Planckian censorship conjecture (TCC) seems to require that the energy scale of inflation is significantly lower than the Planck scale $(H_\text{inf}<10^{-20} \Mpl)$. This, in turn, implies that the tensor-to-scalar ratio for inflation is negligibly small, \textit{independent} of assumptions of slow-roll or even of having a single scalar field, thus ruling out inflation if primordial tensor modes are ever observed. After demonstrating the robustness and generality of these bounds, we show that having an excited initial state for cosmological perturbations seems to be a way out of this problem for models of inflation.