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

Abstract

Only two gauge symmetries remain unbroken today: $SU(3)_c$ and $U(1)_{\rm EM}$. Both are crucial to our universe as it is and to our form of life. Unless we are special observers at the end of the cosmological symmetry-breaking chain, there is no reason they must remain unbroken forever. We investigate the cosmological signatures of late-time $SU(3)_c$ breaking in our model with a new colored scalar whose potential supports a first-order phase transition through true-vacuum bubble nucleation. We analyze three physically distinct production mechanisms of the expanding bubble wall. The first is direct nonthermal production from vacuum mismatch across the accelerating wall in the scalar and massive-gluon sectors. The second arises from frictional dissipation into a shocked layer. Including the finite-temperature effective potential, we show that our benchmark critical temperature $T_c$ is below the massive-gluon and scalar masses; an equilibrated relativistic broken-phase thermal bath would therefore restore $SU(3)_c$. We formulate a subcritical thermal freeze-out estimate below $T_c$, where massive broken-phase particles can still be produced but only with Boltzmann suppression. The third is a non-equilibrium wall--matter transition-radiation channel: ambient baryonic matter crossing the relativistic wall can radiate broken-phase massive gluons and scalar excitations. This requires no thermal bath above $T_c$ and can also continue after the wall reaches terminal velocity. We study the decays of the physical color-octet scalar and massive gluons, using \texttt{Pythia} to hadronize their decay products and determine the resulting photon and neutrino spectra. If the wall reaches a subluminal terminal velocity, these particles can arrive before the wall. The resulting high-energy spectra are a long-range observational signature that, if observed, could signal cosmic doomsday.