The species scale and the refined TCC bound in time-dependent backgrounds of string theory

TL;DR

This paper introduces a refined version of the TCC conjecture linked to the species scale, predicting shorter de-Sitter lifetimes and bounding the cosmological constant, supported by string theory cosmological solutions.

Contribution

It proposes the refined TCC as a new cutoff in quantum gravity, relating it to the species scale and analyzing its implications for cosmology and singularities.

Findings

Refined TCC predicts shorter de-Sitter spacetime lifetimes.

Supports from cosmological solutions of string compactifications.

Both TCC and refined TCC exclude the big-bang singularity.

Abstract

The species scale is the energy scale at which quantum corrections to Einstein's theory of gravity become significant. In many cases, this scale corresponds to the string mass scale, which can be much lower than the Planck scale in the weak coupling limit. In this note, we explore a variant of the TCC conjecture, which we refer to as the refined TCC. This version is related to the species scale as a new cutoff in quantum gravity, and we study whether stringy modes remain stringy and never exceed the Hubble horizon. The refined TCC predicts a shorter lifetime for de-Sitter spacetime and imposes a bound on the cosmological constant. We present supporting evidence for the refined TCC from cosmological solutions of different compactifications of various string theories. Furthermore, it is argued that both the TCC and the refined TCC are incompatible with the big-bang singularity. Therefore, if the TCC and the refined TCC hold, the big bang singularity must be ruled out. In scenarios such as string gas cosmology, where the TCC is realized, the spacetime background remains completely regular.