Non‐Standard Cosmological Models and The Trans‐Planckian Censorship Conjecture

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 Hinf 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 Hinf can be raised up to 3 orders of magnitude. In multiple inflationary scenarios the upper bound on the observable inflation can be raised up 1014 GeV to touch the Planck 2018 bound. 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 Hinf 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, the author studies 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 Hinf can be raised up to 3 orders of magnitude. In multiple inflationary scenarios the upper bound on the observable inflation can be raised up 1014 GeV to touch the Planck 2018 bound.