Screened Axio-dilaton Cosmology: Novel Forms of Early Dark Energy

TL;DR

This paper investigates a multi-field axio-dilaton dark energy model with screening mechanisms, showing it can produce early dark energy effects consistent with cosmological observations and influence structure growth.

Contribution

It introduces a novel screening mechanism in axio-dilaton models that allows for significant early dark energy without conflicting with solar system tests.

Findings

Early dark energy can reach up to 10% of total density.

Screening reduces dilaton-matter couplings, enabling compatibility with local tests.

Enhanced Hubble friction from dilaton-matter coupling suppresses structure growth.

Abstract

We study the cosmology of multi-field Dark Energy, using a well-motivated axio-dilaton model that contains the minimal number of fields to have the 2-derivative sigma-model interactions that power-counting arguments show naturally compete with General Relativity at low energies. Our analysis differs from earlier, related, studies by treating the case where the dilaton's couplings to matter are large enough to require screening to avoid unacceptable dilaton-mediated forces in the solar system. We use a recently proposed screening mechanism that exploits the interplay between stronger-than-gravitational axion-matter couplings with the 2-derivative axion-dilaton interactions to suppress the couplings of the dilaton to bulk matter. The required axion-matter couplings also modify cosmology, with the axion's background energy density turning out to resemble early dark energy. We compute the properties of the axion fluid describing the rapid oscillations of the axion field around the time-dependent minimum of its matter-dependent effective potential, extending the usual formalism to include nontrivial kinetic sigma-model interactions. We explore the implications of these models for the Cosmic Microwave Background and the growth of structure and find that for dilaton potentials of the Albrecht-Skordis form (itself well-motivated by UV physics), successful screening can be consistent with the early dark energy temporarily comprising as much as 10% of the total density in the past. We find that increasing the dilaton-matter coupling decreases the growth of structure due to enhanced Hubble friction, an effect that dominates the usual fifth-force effects that amplify structure growth.