The species scale and the refined TCC bounds

Abstract

The species scale ${\Lambda }_s$, 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, or the higher-dimensional Planck scale, which can be much lower than the Planck scale in the weak coupling limit. In this note, we explore two variants of the trans-Planckian censorship conjecture (TCC), which we refer to as the refined TCC conjectures. These versions are related to the species scale as a new cutoff in quantum gravity, and we study whether stringy or quantum modes remain stringy or quantum and never exceed the Hubble horizon. The refined TCCs predict a shorter lifetime for de-Sitter spacetime and impose a bound on the cosmological constant. We also show that one of these conjectures violates some decelerating expansions as opposed to the standard TCC bound. We present supporting evidence for the refined TCCs where its consequences are compatible with other swampland results. The most clear consequence of it is the lower bound for $|\frac{{\Lambda }_s^{\prime }}{{\Lambda }_s}|>\frac{1}{\sqrt{(d-1)(d-2)}}$ which is expected from other swampland criteria. 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. In addition to this line of reasoning, we argue for a lower bound on the $\frac{d(1/H)}{dt}$ which excludes the de-Sitter spacetime, which is a regular spacetime.

arXiv comment8 pages, 1 figure