My thoughts. The 8-page PDF report is interesting reading. I note here from the report. “Along with the electroweak (EW) and QCD transitions, known to be crossovers in the Standard Model (SM) [1, 2], one or more first order phase transitions may have taken place in the first second after inflation. They are indeed commonly predicted in several motivated extensions of the SM, such as extra-dimensional [3], confining [4, 5], or supersymmetric models [6], and solutions to the strong CP [7, 8], flavour [9], or neutrino mass problems [10]. Independently of where they come from, such early universe phase transitions may have far reaching consequences through the possible cosmological relics they can leave behind, e.g. primordial black holes [11–21], topological defects [22–26], magnetic fields [27–33], dark matter (DM) [34–43], the baryon asymmetry [44–56], together with a background of gravitational waves (GW) [57–66], to cite just a few. As the universe expands, sitting in its lowest free energy vacuum, another vacuum may develop at a lower energy due to the fall in temperature, eventually triggering a PT. If a PT is first order then it proceeds via the nucleation of bubbles of broken phase into the early universe bath (see e.g. [67, 68] for reviews), analogously to the PT of water to vapor. Bubble walls that expand with ultrarelativistic velocities store a lot of energy, locally much higher than both the bath temperature and the scale of the PT.” My note, the timeline provided here is "in the first second after inflation", so a post-inflation universe evolution model presented. This period overlaps with the quark-gluon plasma phase of the BB model, overlaps the 4 stages for the cosmic fireball said to create the universe and CMBR we see today. The Big Bang model of the Universe.,
https://astronomy.swin.edu.au/~gmackie/BigBang/universe.htm, my note the 4 stage fireball evolution to explain the CMB is found in the URL too. "The First Few Minutes: It takes GUTs ... and Quantum Physics" "A Fireball in four parts." "Heavy particle era", "Light particle era", "Radiation era", "Matter era". My note, given the approach of the 8-page PDF report for bubbletrons in the early universe, I think some of these bubbles with different vacuum energies, continue to evolve into other universes. A particular bubble evolved into our universe so we are here today to read this report. The bubble walls expand much faster than c velocity too, “Bubble walls that expand with ultrarelativistic velocities”.
ref - Bubbletrons,
https://arxiv.org/abs/2306.15555, 27-June-2023. "In cosmological first-order phase transitions (PT) with relativistic bubble walls, high-energy shells of particles generically form on the inner and outer sides of the walls. Shells from different bubbles can then collide with energies much larger than the PT or inflation scales, and with sizeable rates, realising a `bubbletron'..."
Edit, my bad about ultrarelativistic velocities and c. Wikipedia defines,
"Ultrarelativistic limit" In physics, a particle is called ultrarelativistic when its speed is very close to the speed of light c.",
https://en.wikipedia.org/wiki/Ultrarelativistic_limit
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