Axions : Theory and Cosmological Role

TL;DR

This paper surveys axions as a solution to the strong CP problem and their cosmological role, emphasizing $F_a$-dependent phenomenology and the link to dark matter. It analyzes invisible axion models (KSVZ/DFSZ), astrophysical bounds, and experimental searches (helioscopes, haloscopes, lasers), and develops a thorough framework for axion cosmology, including PQ scalar evolution, isocurvature perturbations, and non-Gaussianity. It distinguishes scenarios with PQ breaking after versus before inflation, detailing axionic strings and domain walls (with $N_{ m DW}=1$ and $N_{ m DW} eq 1$), and reports on the latest simulations and resulting bounds on $F_a$ and model-building constraints. The SUSY extension is explored via stabilization mechanisms (Models A–C) and the cosmology of axinos and saxions, with implications for dark matter and high-scale physics, including connections to the 125 GeV Higgs and string axions/axiverse.

Abstract

We review recent developments on axion cosmology. Topics include : axion cold dark matter, axions from topological defects, axion isocurvature perturbation and its non-Gaussianity and axino/saxion cosmology in supersymmetric axion model.

Figures (10)

• Figure 1: Potential of the PQ scalar.
• Figure 2: Evolution of the axionic string networks from the simulation Hiramatsu:2010yu. The panels show the time slices at $t= 4t_c$, $9t_c$, $16t_c$ and $25t_c$ where $t_c$ is the cosmic time corresponding to the critical temperature $T_c$. The spatial scale shows a comoving length in unit of the horizon size at $t_\mathrm{end}=25t_\mathrm{c}$.
• Figure 3: Time evolution of the length parameter $\xi$ from the simulation Hiramatsu:2010yu.
• Figure 4: Differential energy spectrum of radiated axions between $t_1=12.25t_\mathrm{c}$ and $t_2=25t_\mathrm{c}$. Green (red) bars correspond to statistical errors alone (statistical and systematic errors). The comoving wavenumber $k$ is in units of $(2t_c)^{-1}$. Note that the horizon scale corresponds to $k \sim 3$. The scale in $y$-axis is arbitrary.
• Figure 5: Evolution of the string-domain wall networks for $N_{\rm DW} =1$. The white lines correspond to the position of strings, while the blue surfaces correspond to the position of the center of domain walls. "tau" in each panel is the conformal time which is related to the cosmic time as ${\rm tau} = \sqrt{t\eta}$.