Quintessential inflation: A tale of emergent and broken symmetries

TL;DR

This paper reviews quintessential inflation, a unified model of inflation and dark energy using a scalar field called the cosmon, emphasizing symmetries and their breaking in cosmological evolution and phenomenology.

Contribution

It provides a comprehensive overview of quintessential inflation, including its embedding in scalar-tensor theories, symmetry roles, and phenomenological implications for neutrino masses and symmetry breaking.

Findings

Cosmon field evolution linked to ultraviolet and infrared fixed points.

Symmetry breaking influences neutrino masses and structure formation.

Ricci couplings can trigger internal symmetry breaking in the early Universe.

Abstract

Quintessential inflation provides a unified description of inflation and dark energy in terms of a single scalar degree of freedom, the cosmon. We present here a comprehensive overview of this appealing paradigm, highlighting its key ingredients and keeping a reasonable and homogeneous level of details. After summarizing the cosmological evolution in a simple canonical case, we discuss how quintessential inflation can be embedded in a more general scalar-tensor formulation and its relation to variable gravity scenarios. Particular emphasis is placed on the role played by symmetries. In particular, we discuss the evolution of the cosmon field in terms of ultraviolet and infrared fixed points potentially appearing in quantum gravity formulations and leading to the emergence of scale invariance in the early and late Universe. The second part of the review is devoted to the exploration of the phenomenological consequences of the paradigm. First, we discuss how direct couplings of the cosmon field to matter may affect neutrinos masses and primordial structure formation. Second, we describe how Ricci-mediated couplings to spectator fields can trigger the spontaneous symmetry breaking of internal symmetries such as, but not limited to, global U(1) or $Z_2$ symmetries, and affect a large variety of physical processes in the early Universe.