Scaling solutions as Early Dark Energy resolutions to the Hubble tension

TL;DR

This paper explores scalar field models like Quintessence, K-essence, and axion, demonstrating how their energy density peaks around matter-radiation equality can serve as early dark energy solutions to the Hubble tension, with K-essence and axion models fitting data better.

Contribution

It shows that a broad class of scalar field models can produce early dark energy effects around equality, offering potential resolutions to the Hubble tension.

Findings

K-essence and axion models fit observational data better than Quintessence.

Scalar field energy density peaks around equality can address the Hubble tension.

Quintessence models often have incompatible sound speeds for EDE requirements.

Abstract

A wide class of scalar field models including Quintessence and K-essence have the attractive property of tracker regimes, where the energy density stored in the field evolves so as to mimic that of the dominant background component for a period of time. During this evolution, for a brief period of time there is an increase in the energy density of the field as it spirals in towards it's attractor solution. We show that when the peak of this energy density occurs around the epoch of equality, we can address a key requirement of early dark energy (EDE), postulated as a solution to the Hubble tension. In particular we demonstrate how this can occur in a wide class of Quintessence, axion and K-essence models, before showing that the Quintessence models suffer in that they generally lead to sound speeds incompatible with the requirements of EDE, whereas the K-essence and axion models can do a better job of fitting the data.