Early dark energy from massive neutrinos -- a natural resolution of the Hubble tension

TL;DR

Neutrino decoupling provides a natural trigger for early dark energy by displacing the field just before matter-radiation equality, and various theoretical aspects of this proposal are discussed.

Abstract

The Hubble tension can be significantly eased if there is an early component of dark energy that becomes active around the time of matter-radiation equality. Early dark energy models suffer from a coincidence problem -- the physics of matter-radiation equality and early dark energy are completely disconnected, so some degree of fine-tuning is needed in order for them to occur nearly simultaneously. In this paper we propose a natural explanation for this coincidence. If the early dark energy scalar couples to neutrinos then it receives a large injection of energy around the time that neutrinos become non-relativistic. This is precisely when their temperature is of order their mass, which, coincidentally, occurs around the time of matter-radiation equality. Neutrino decoupling therefore provides a natural trigger for early dark energy by displacing the field from its minimum just before matter-radiation equality. We discuss various theoretical aspects of this proposal, potential observational signatures, and future directions for its study.

Figures (2)

• Figure 1: The integral $\tau(m_\nu/T_\nu(z))$ as a function of $z$ with $m_\nu=0.5$eV.
• Figure 2: The field as a function of redshift (red, solid). The blue dashed line shows the analytic prediction in equation \ref{['eq:phiin']}. We take $m_\nu=0.5$ eV, $\beta=4\times10^{-4}$, and $\lambda=10^{-75}$.