Cosmological phase transition

A cosmological phase transition is a physical process, whereby the overall state of matter changes together across the whole universe. The success of the Big Bang model led researchers to conjecture possible cosmological phase transitions taking place in the very early universe, at a time when it was much hotter and denser than today.

Any cosmological phase transition may have left signals which are observable today, even if it took place in the first moments after the Big Bang, when the universe was opaque to light.

Cosmological first-order phase transitions
Phase transitions can be categorised by their order. Transitions which are first order proceed via bubble nucleation and release latent heat as the bubbles expand.

As the universe cooled after the hot Big Bang, such a phase transition would have released huge amounts of energy, both as heat and as the kinetic energy of growing bubbles. In a strongly first-order phase transition, the bubble walls may even grow at near the speed of light. This, in turn, would lead to the production of a stochastic background of gravitational waves. Experiments such as NANOGrav and LISA may be sensitive to this signal.

Shown below are two snapshots from simulations of the evolution of a first-order cosmological phase transition. Bubbles first nucleate, then expand and collide, eventually converting the universe from one phase to another.

Examples
The Standard Model of particle physics contains three fundamental forces, the electromagnetic force, the weak force and the strong force. Shortly after the Big Bang, the extremely high temperatures may have modified the character of these forces. While these three forces act differently today, it has been conjectured that they may have been unified in the high temperatures of the early universe.

Strong force phase transition
Today the strong force binds together quarks into protons and neutrons, in a phenomenon known as color confinement. However, at sufficiently high temperatures, protons and neutrons disassociate into free quarks. The strong force phase transition marks the end of the quark epoch. Studies of this transition based on lattice QCD have demonstrated that it would have taken place at a temperature of approximately 155 MeV, and would have been a smooth crossover transition.

This conclusion assumes the simplest scenario at the time of the transition, and first- or second-order transitions are possible in the presence of a quark, baryon or neutrino chemical potential, or strong magnetic fields. The different possible phase transition types are summarised by the strong force phase diagram.

Electroweak phase transition
The electroweak phase transition marks the moment when the Higgs mechanism first activated, ending the electroweak epoch. Just as for the strong force, lattice studies of the electroweak model have found the transition to be a smooth crossover, taking place at 159.5±1.5 GeV.

The conclusion that the transition is a crossover assumes the minimal scenario, and is modified by the presence of additional fields or particles. Particle physics models which account for dark matter or which lead to successful baryogenesis may predict a strongly first-order electroweak phase transition.

Phase transitions beyond the Standard Model
If the three forces of the Standard Model are unified in a Grand Unified Theory, then there would have been a cosmological phase transition at even higher temperatures, corresponding to the moment when the forces first separated out. Cosmological phase transitions may also have taken place in a dark or hidden sector, amongst particles and fields that are only very weakly coupled to visible matter.