Signals of Doomsday III: Cosmological signatures of the late time $U(1)_{EM}$ symmetry breaking

TL;DR

This paper investigates potential astrophysical signals from a late-time breaking of the electromagnetic symmetry, focusing on bubble nucleation, particle production, and observable high-energy photon and neutrino signatures.

Contribution

It introduces a model of a first-order phase transition in electromagnetism with detailed predictions of detectable astrophysical signals from such a transition.

Findings

Thermal channel produces a large burst of high-energy photons and neutrinos.

Signals can precede the bubble wall arrival, acting as observable precursors.

Potential detectability with current or future multi-messenger facilities.

Abstract

Of the universe's original gauge symmetries, only $SU(3)_c$ (quantum chromodynamics) and $U(1)_{\rm EM}$ (electromagnetism) remain unbroken today. There is, however, no reason to assume that these symmetries are permanent. This paper explores the potential astrophysical observational signatures of a late-time breaking of $U(1)_{\rm EM}$. We present a model with a new massive scalar field whose potential supports a first-order phase transition through the nucleation of true-vacuum bubbles. If the propagation of the bubble walls slows down due to interactions with the surrounding matter and radiation, these signals can reach us before the bubble wall itself arrives. Using the vacuum-mismatch method, we calculate the spectrum of particles produced by such a bubble until the terminal velocity is reached. In addition, we show that frictional dissipation at terminal wall velocity generates a large population of thermally produced scalars and massive photons, which continues even after the mismatch channel shuts off. We then use event generators to simulate the decays of the new scalar and the massive photon into Standard Model particles and obtain, as the final result, the energy spectra of photons and neutrinos. Since the dominant final decay products after hadronization and the decay of unstable particles are photons and neutrinos, they act as long-range signatures of the transition. We also estimate the possible lead time of these photon and neutrino signals relative to the arrival of the bubble wall itself, showing that even a modest subluminal wall velocity can in principle provide an observable precursor. For the conservative set of parameters used here, the thermal channel produces a macroscopically large burst of high-energy photons and neutrinos, which could in principle be detectable from sufficiently nearby bubbles with present or future multi-messenger facilities.