Cosmological constraints on ultra-light axion fields

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. Their 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 mass around 10−33 eV, which means that the ULA field still rolls slowly at present with the equation of state w = −1 as dark energy. 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 3 × 10−33 eV if assuming the initial axion field ϕi = Mpl. We also investigate a general case by assuming ϕi ≤ Mpl, and find that the fitting results of ϕi/Mpl are consistent with or close to 1 for the datasets that we use.