Is Trans-Planckian Censorship a Swampland Conjecture?

TL;DR

This paper critically examines the trans-Planckian censorship conjecture (TCC), highlighting conceptual issues and its implications for the landscape of quantum gravity theories, questioning whether TCC functions as a swampland conjecture.

Contribution

The paper introduces multiple versions of the TCC, analyzes their conceptual problems, and discusses how these variations impact observable predictions and the classification of theories in quantum gravity.

Findings

Different TCC versions lead to observable differences.

Violation of TCC in another universe can exclude a theory.

TCC's role as a swampland conjecture remains uncertain.

Abstract

During an accelerated expansion of the Universe, quantum fluctuations of sub-Planckian size can be stretched outside the horizon and be regarded effectively classical. Recently, it has been conjectured that such horizon-crossing of trans-Planckian modes never happens inside theories of quantum gravity (the trans-Planckian censorship conjecture, TCC). We point out several conceptual problems of this conjecture, which is in itself formulated as a statement on the restriction of possible scenarios in a theory: by contrast a standard swampland conjecture is a restriction of possible theories in the landscape of the quantum gravity. We emphasize the concept of swampland universality, i.e. that a swampland conjecture constrains any possible scenario in a given effective field theory. In order to illustrate the problems clearly we introduce several versions of the conjecture, where TCC condition is imposed differently to scenarios realizable in a given theory. We point out that these different versions of the conjecture lead to observable differences: a TCC violation in another Universe can exclude a theory, and such reduction of the landscape restricts possible predictions in our Universe. Our analysis raises the question of whether or not the trans-Planckian censorship conjecture can be regarded as a swampland conjecture concerning the existence of UV completion.