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Trans-Planckian Censorship Conjecture (TCC) in Expanding Universes

Establish that in any expanding universe consistent with quantum gravity, sub-Planckian regions cannot exit the cosmological horizon and become frozen, thereby preventing trans-Planckian fluctuations from becoming physically measurable.

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Background

The paper discusses the de Sitter conjecture and emphasizes a more plausible formulation known as the Trans-Planckian Censorship Conjecture (TCC). TCC demands that sub-Planckian modes never cross the horizon and become classical, a restriction with significant implications for cosmology.

Consequences highlighted include bounds on scalar potential gradients and an upper limit on the lifetime of de Sitter phases, which the paper later uses to address the "why now" problem in cosmology.

References

Another Swampland conjecture, is the de Sitter conjecture [Obied:2018sgi] or a more plausible version of it known as the "trans-Planckian censorship conjecture (TCC)" [Bedroya:2019snp], which states that in an expanding universe regions smaller than Planck scale cannot exit the horizon and freeze, as sub-Planckian fluctuations are unphysical and should not become physically measurable.

— Swamplandish Unification of the Dark Sector (2402.00981 - Vafa, 1 Feb 2024) in Section 2 (Relevant Swampland Principles)