Generation of axions and axion-like particles through mass parametric resonance induced by scalar perturbations in the early universe

TL;DR

The paper tackles the origin of ultralight dark matter candidates such as axions and ALPs by proposing a novel production channel based on mass parametric resonance driven by scalar perturbations in the early universe. The authors map the light-field dynamics to a Mathieu equation for mode perturbations, with an instability band around $A_k Rightarrow 4$ corresponding to the $l=2$ resonance, under a phenomenological mass function $m(t)$ that transitions from 0 to its low-temperature value. They show that this mechanism can transfer significant energy into the light field, yielding a dark matter density potentially comparable to the misalignment scenario, while not generating additional isocurvature perturbations. The results imply a broader set of viable DM production pathways for axions/ALPs and potentially other light fields undergoing mass transitions in the early universe, with nonlinear effects likely enhancing production when $q$ grows toward unity.

Abstract

Axions and axion-like particles can be generated in the early universe through mechanisms such as misalignment production, thermal processes, and the decay of topological defects. In this study, we show that scalar perturbations in the early universe can produce a significant amount of these particles primarily through mass parametric resonance effects. Scalar perturbations induce temperature fluctuations during the particle mass transition era, e.g., during the QCD phase transition. These temperature fluctuations modulate the particle mass, transferring energy into the field through parametric mass resonance, a nonlinear process. This mechanism exhibits substantially unstable regions that could lead to explosive particle production. Notably, it does not generate additional isocurvature perturbations.

Figures (7)

• Figure 1: Mass as a function of time $t$.
• Figure 2: Correlation between the parameter $n$ of the mass function and wave vector $k_2$ for $m=10^{-5}\ \mathrm{eV}$ and $t_{2}=10^{8}\ \mathrm{eV}^{-1}$.
• Figure 3: Parameter $q$ in the Mathieu equation as a function of time $t$, with $l=2$, $m=10^{-5}\mathrm{eV}$, $t_2=10^{9}\mathrm{eV}^{-1}$, and $k=50/t_1$.
• Figure 4: Natural frequency $\omega$ and modulation frequencies $m\pm k\sqrt{t_1/3t}$ as functions of time $t$, with parameters $l=2$, $n=1$, $m=10^{-5}\mathrm{eV}$, $t_2=10^{9}\mathrm{eV}^{-1}$, and $k=5/t_1$.
• Figure 5: Natural frequency $\omega$ and modulation frequencies $m\pm k\sqrt{t_1/3t}$ as functions of time $t$, with parameters $l=2$, $n=2$, $m=10^{-5}\mathrm{eV}$, $t_2=10^{9}\mathrm{eV}^{-1}$, and $k=20/t_1$.