# Comment to the CPT-symmetric Universe: Two possible extensions

## TL;DR

This paper explores two extensions of the CPT-symmetric universe model, analyzing analytic continuations across the Big Bang that suggest the universe could have originated with negative temperature or experienced symmetry breaking at the Big Bang.

## Contribution

It introduces two novel scenarios of analytic continuation across the Big Bang, expanding the theoretical framework of CPT-symmetric universe models.

## Key findings

- Initial universe may have had negative temperature.
- Big Bang could be a symmetry-breaking bifurcation point.
- Two different analytic continuation scenarios are proposed.

## Abstract

In Ref.1 (L. Boyle, K. Finn and N. Turok, CPT-Symmetric Universe, Phys. Rev. Lett. {\bf 121}, 251301 (2018)) the antispacetime Universe was suggested as the analytic continuation of our Universe across the Big Bang singularity in conformal time. We consider two different scenarios of analytic continuation. In one of them the analytic continuation is extended to the temperature of the system. This extension suggests that if such analytic continuation is valid, then it is possible that the initial stage of the evolution of the Universe on our side of the Big Bang was characterized by the negative temperature. In the second scenario, the analytic continuation is considered in the proper time. In this scenario the Big Bang represents the bifurcation point at which the $Z_2$ symmetry between the spacetime and antispacetime is spontaneously broken.