Non-Standard Cosmological Models and the trans-Planckian Censorship Conjecture

TL;DR

This paper explores how non-standard cosmological models, including non-thermal histories and multiple inflation stages, can relax the constraints imposed by the trans-Planckian censorship conjecture on inflation parameters.

Contribution

It demonstrates that non-standard cosmologies can significantly raise the TCC bounds on inflationary parameters, impacting early universe models and observational prospects.

Findings

TCC bound on inflation Hubble parameter can be increased by three orders of magnitude in early matter domination.

Multiple inflation scenarios can raise the upper bound on inflation to near the Planck scale.

Non-standard models alleviate fine-tuning and enhance the viability of high-scale inflation.

Abstract

The trans-Planckian censorship conjecture (TCC) puts an upper bound on the life-time of de Sitter spacetimes. It has immediate consequences for inflationary cosmology. In the standard paradigm, the universe has experienced a single stage of inflation and follows a thermal history. Then, the TCC puts an upper bound on the Hubble parameter during inflation $H_{\rm inf}$ around $0.1$ GeV. Consequently, it implies a severe fine-tuning in initial condition for inflation and non-detection of primordial gravitational waves. In this note, we study non-standard cosmological paradigms with non-thermal history and/or multiple stages of inflations. It is motivated by string theory compactifications and axiverse scenarios in which the modulus/axion fields are effective in the early universe. In early matter domination the TCC bound on $H_{\rm inf}$ can be raised up to 3 orders of magnitude. In multiple inflationary scenarios the upper bound on the observable inflation can be raised up $10^{14}$ GeV to touch the Planck 2018 bound.