CMB Implications of Multi-field Axio-dilaton Cosmology

TL;DR

This paper explores multi-field axio-dilaton models motivated by string theory, analyzing their cosmological evolution and implications for CMB anisotropies, matter spectra, and structure growth using modified Boltzmann codes.

Contribution

It provides a detailed study of axio-dilaton cosmologies, deriving fluctuation equations, and identifying viable parameter spaces with significant matter-scalar interactions.

Findings

Certain couplings yield viable cosmologies consistent with observations.

Large matter-scalar interactions are possible in these models.

Axion potentials can influence cosmological evolution under specific conditions.

Abstract

Axio-dilaton models are among the simplest scalar-tensor theories that contain the two-derivative interactions that naturally compete at low energies with the two-derivative inter-actions of General Relativity. Such models are well-motivated as the low energy fields arising from string theory compactification. We summarize these motivations and compute their cosmological evolution, in which the dilaton acts as dark energy and its evolution provides a framework for dynamically evolving particle masses. The derivative axion-dilaton couplings play an important role in the success of these cosmologies. We derive the equations for fluctuations needed to study their implications for the CMB anisotropy, matter spectra and structure growth. We use a modified Boltzmann code to study in detail four benchmark parameter choices, including the vanilla Yoga model, and identify couplings that give viable cosmologies, including some with surprisingly large matter-scalar interactions. The axion has negligible potential for most of the cosmologies we consider but we also examine a simplified model for which the axion potential plays a role, using axion-matter couplings motivated by phenomenological screening considerations. We find such choices can also lead to viable cosmologies.