The current best-fitting picture of the statistical properties of the Universe, from horizon scales to typical galactic separations, is the so-called Λ Cold Dark Matter scenario. This 6-parameter model assumes the existence of two accelerated expansion eras taking place in the very early and the very late Universe: Inflation and Dark Energy.
Since inflation and Dark energy share many essential properties, it seems natural to seek for the unification of these two stages into a common framework. This is the main idea behind the so-called Quintessential inflation paradigm, where the inflaton and dark energy fields are identified with a single scalar component, the cosmon (for a review see 2112.11948 [astro-ph.CO]).
Quintessential inflation involves generically an intermediate cosmological epoch in which the energy density of the Universe is dominated by the kinetic energy density of the cosmon field, the kination era. Interestingly, the combination of a long lasting period of kination with non-minimal couplings to gravity may lead to the spontaneous symmetry breaking of internal symmetries, opening a new way to discriminate quintessential inflation from standard scenarios.
In a recently published article in the Journal of Cosmology and Astroparticle Physics, Javier Rubio and collaborators carried out 3+1 classical lattice simulations to follow the symmetry breaking pattern and subsequent non-linear evolution of a Z2 symmetric spectator field in Quintessential Inflation, demonstrating for the first time the formation of spatially-localized field configurations separated by domain walls, as displayed in the attached figure [Have a look to these cool videos too!].
The gradient energy density immediately after the transition represents a non-negligible fraction of the total energy budget, steadily growing to equal the kinetic counterpart. Combined with the aforementioned period of kination, this observation allows for the generic onset of radiation domination for arbitrary self-interacting potentials, significantly extending previous results in the literature. The formation of domain walls may lead additionally to the production of a detectable gravitational waves’ backgrounds, an aspect to be studied by the COSTAR group in subsequent publications.
Read more in “Hubble-induced phase transitions on the lattice with applications to Ricci reheating", Dario Bettoni, Asier Lopez-Eiguren and Javier Rubio, JCAP 01 (2022) 01, 002.