The signals of doomsday II: Cosmological signatures of late time $SU(3{)}_c$ symmetry breaking

The only two gauge symmetries which remain unbroken today are $SU(3{)}_c$ and $U(1{)}_{EM}$. Both are crucial for our universe to appear the way it does and for our form of life to exist. Unless we are very special observers living at the very end of the cosmological symmetry-breaking chain, there is no reason to believe that these two symmetries will remain unbroken forever. In this paper, we investigate the cosmological observational signatures of a late-time $SU(3{)}_c$ symmetry breaking. We introduce a model with a new colored scalar field whose potential supports a first-order phase transition through nucleation of true-vacuum bubbles. We first calculate particle production due to vacuum mismatch across the expanding bubble wall, including both the scalar and the massive-gluon sectors. We then study the decays of the physical color-octet scalar and the massive gluons, and use Pythia to hadronize their decay products and determine the resulting photon and neutrino spectra. We further include frictional effects from the ambient medium, which slow the bubble wall to a subluminal terminal velocity and thereby open the possibility of an observable signal reaching us before the wall itself. In addition to the direct vacuum-mismatch contribution, we analyze thermal particle production sourced by frictional energy dissipation into the shocked medium and show that, for the benchmark scenarios considered here, this thermal channel can dominate by many orders of magnitude. The resulting high-energy photon and neutrino spectra constitute a long-range observational signature which, if ever observed, could be interpreted as a signal of cosmic doomsday.