A universal bound on the duration of a kination era

TL;DR

This paper establishes a universal upper limit of about 10 e-folds on the duration of a kination era in the early universe, based on primordial fluctuations and observational constraints on extra neutrino species.

Contribution

It demonstrates that primordial fluctuations induce an instability leading to a radiation-like component, setting a new bound on the duration of the kination era.

Findings

Generated higher Fourier modes dominate after ~11 e-folds

Current constraints imply a maximum of ~10 e-folds for kination

Provides the most stringent bound on pre-Big-Bang-Nucleosynthesis universe stiffness

Abstract

We show that primordial adiabatic curvature fluctuations generate an instability of the scalar field sourcing a kination era. We demonstrate that the generated higher Fourier modes constitute a radiation-like component dominating over the kination background after about $11$ e-folds of cosmic expansion. Current constraints on the extra number of neutrino flavors $\Delta N_{\rm eff}$ thus imply the observational bound of approximately 10 e-folds, representing the most stringent bound to date on the stiffness of the equation of state of the pre-Big-Bang-Nucleosynthesis universe.