Cosmological constraints on ultra-light axion fields

TL;DR

This paper investigates ultra-light axion fields with a mass around 10^-33 eV as a form of dark energy, using cosmological data and MCMC methods to constrain their properties and initial conditions.

Contribution

It provides new constraints on the mass and initial field value of ultra-light axions acting as dark energy using observational data and MCMC analysis.

Findings

ULA mass around 3x10^-33 eV for initial field at M_pl

Constraints on initial field value phi_i/M_pl close to 1

Consistent results across multiple cosmological datasets

Abstract

Ultra-light axions (ULAs) with mass less than 10^-20 eV have interesting behaviors that may contribute to either dark energy or dark matter at different epochs of the Universe. Its properties can be explored by cosmological observations, such as expansion history of the Universe, cosmic large-scale structure, cosmic microwave background, etc. In this work, we study the ULAs with a mass around 10^-33 eV, which means the ULA field still rolls slowly at present with the equation of state w=-1 as dark energy. In order to investigate the mass and other properties of this kind of ULA field, we adopt the measurements of Type Ia supernova (SN Ia), baryon acoustic oscillation (BAO), and Hubble parameter H(z). The Markov Chain Monte Carlo (MCMC) technique is employed to perform the constraints on the parameters. Finally, by exploring four cases of the model, we find that the mass of this ULA field is about 3x10^-33 eV if assuming the initial axion field phi_i=M_pl. We also investigate a general case by assuming phi_i< M_pl and find that the fitting results of phi_i/M_pl are consistent with or close to 1 for the datasets we use.