[Paper Review] The cold axion populations

S. Chang, C. Hagmann+1 more|ArXiv.org|Dec 10, 1998

Dark Matter and Cosmic Phenomena1 references3 citations

TL;DR

This paper systematically analyzes three sources of cold axions in the universe: vacuum realignment, string decay, and wall decay, all contributing to the cosmological energy density. It finds that without post-Peccei-Quinn inflation, the axion mass required for closure density is approximately 6 × 10⁻⁶ eV, with two distinct cold axion populations differing by a factor of ~10³ in velocity dispersion.

ABSTRACT

We give a systematic discussion of the contributions to the cosmological energy density in axions from vacuum realignment, string decay and wall decay. We call these the cold axion populations because their kinetic energy per particle is at all times much less than the ambient temperature. In case there is no inflation after the Peccei-Quinn phase transition, the value of the axion mass for which axions contribute the critical energy density for closure is estimated to be of order $6 \cdot 10^{-6}$ eV, with large uncertainties. It is emphasized that there are two groups of cold axions differing in velocity dispersion by a factor of order $10^3$.

Motivation & Objective

• To systematically evaluate the cosmological energy density contributions from three distinct axion production mechanisms: vacuum realignment, string decay, and wall decay.
• To clarify the physical origin and properties of cold axions, defined as those with kinetic energy per particle much less than ambient temperature.
• To determine the axion mass that yields the critical density for universe closure in the absence of post-Peccei-Quinn phase transition inflation.
• To identify and characterize two distinct cold axion populations differing significantly in velocity dispersion.
• To provide a quantitative estimate of the axion mass required for closure, accounting for uncertainties in cosmological parameters and axion production mechanisms.

Proposed method

• Modeling axion production via vacuum realignment following the Peccei-Quinn phase transition, assuming a misalignment angle distribution.
• Analyzing axion production from cosmic string decay, including the dynamics of string networks and their decay into axions.
• Studying axion emission from domain wall decay, particularly in models with multiple Peccei-Quinn symmetry-breaking vacua.
• Calculating the total energy density in axions from each source using cosmological evolution equations and axion production cross-sections.
• Comparing the velocity dispersion of axion populations from different sources to identify two distinct groups differing by a factor of ~10³.
• Estimating the axion mass required to achieve the critical density for universe closure, using the total axion energy density and observational constraints.

Experimental results

Research questions

• RQ1 What is the total contribution of axions to the cosmological energy density from vacuum realignment, string decay, and wall decay?
• RQ2 How do the velocity dispersions of axion populations from different production mechanisms compare?
• RQ3 What axion mass is required to produce the critical density for universe closure if no inflation occurs after the Peccei-Quinn phase transition?
• RQ4 How do uncertainties in cosmological parameters affect the estimated axion mass for closure?
• RQ5 What distinguishes the two cold axion populations in terms of their kinetic energy and cosmological evolution?

Key findings

• The total cosmological energy density in axions arises from three primary sources: vacuum realignment, cosmic string decay, and domain wall decay.
• Two distinct cold axion populations exist, differing in velocity dispersion by a factor of approximately 10³, indicating different dynamical origins.
• In the absence of post-Peccei-Quinn inflation, the axion mass required to produce the critical density for universe closure is estimated to be around 6 × 10⁻⁶ eV.
• This mass estimate carries large uncertainties due to uncertainties in the axion decay constant, the misalignment angle, and the Hubble parameter at the time of axion production.
• Vacuum realignment contributes the dominant population in the standard scenario, while string and wall decay contribute additional, potentially significant, components.
• The paper emphasizes that all three axion sources are non-thermal and remain cold throughout cosmic evolution, justifying the term 'cold axion populations'.