[论文解读] Recurrent Axinovae and their Cosmological Constraints

Patrick J. Fox, Neal Weiner+1 more|arXiv (Cornell University)|Feb 1, 2023

Dark Matter and Cosmic Phenomena参考文献 69被引用 11

一句话总结

本文分析在 axion minihalos 中的 recurrent axion star explosions (axinovae)，并从可能较大的衰变分数将暗物质转化为 relativistic axions 导出对 axion-like particles 的宇宙学界限。

ABSTRACT

Axion-like dark matter whose symmetry breaking occurs after the end of inflation predicts enhanced primordial density fluctuations at small scales. This leads to dense axion minihalos (or miniclusters) forming early in the history of the Universe. Condensation of axions in the minihalos leads to the formation and subsequent growth of axion stars at the cores of these halos. If, like the QCD axion, the axion-like particle has attractive self-interactions there is a maximal mass for these stars, above which the star rapidly shrinks and converts an $\mathcal{O}(1)$ fraction of its mass into unbound relativistic axions. This process would leave a similar (although in principle distinct) signature in cosmological observables as a decaying dark matter fraction, and thus is strongly constrained. We place new limits on the properties of axion-like particles that are independent of their non-gravitational couplings to the standard model.

研究动机与目标

• Motivate the study of axion-like dark matter with post-inflation symmetry breaking leading to small-scale density enhancements.
• Investigate formation of axion minihalos and cores (axion stars) within these halos.
• Assess how recurrent axinovae convert dark matter into relativistic axions and constrain model parameters.
• Derive bounds on axion mass and self-coupling independent of Standard Model couplings.
• Compare constraints to existing bounds from other cosmological and astrophysical probes.

提出的方法

• Model the white-noise-like isocurvature spectrum of axion perturbations with a cutoff at small scales (k0 ~ Hosc) to seed minihalos.
• Assume NFW minihalo profiles and compute central axion star formation, growth, and explosive decay (axinovae).
• Use formation and growth timescales from gravitational and self-interactions to estimate decay rate df_decay/dt.
• Relate axion self-coupling to macroscopic axion-star maximal mass and explosion fraction κ ≈ 0.1.
• Translate cumulative axinova energy release into a bound on the decay fraction of dark matter after matter-radiation equality (comparable to decaying DM constraints).
• Explore parameter space (ma, fa) and oscillation temperature to identify excluded regions.

Figure 1: The exclusion region from axinovae for different assumptions for axion parameters, see text for more details. Existing limits from the black hole superradiance are shown in grey. The green region is the most conservative bound using a constant rate of $d{\rm log}M/dt$ at $M_{*}^{\rm max}\l

实验结果

研究问题

• RQ1 How does post-inflation axion perturbations generate dense axion minihalos and axion stars?
• RQ2 What is the rate and impact of recurrent axinovae on the dark-matter energy budget?
• RQ3 What regions of axion parameter space (ma, fa) are excluded by cosmological constraints on the decay of axion dark matter?
• RQ4 How do choices of oscillation temperature and axion growth dynamics affect the bounds?
• RQ5 How do these bounds compare with other constraints like black-hole superradiance?

主要发现

• Axion minihalos form from enhanced small-scale fluctuations after matter-radiation equality and host central axion stars.
• Axion stars can grow to a maximal mass M*max and explode (axinovae), converting a significant fraction of mass into relativistic axions.
• The cumulative decay of axion stars after equality is constrained to be less than about 2.62% of the total dark matter density, akin to decaying DM bounds (2σ).
• The decay rate depends on ma, fa, structure mass M0 (or Mpeak) and redshift zc, with early formation enhancing the bound.
• Different growth models (constant d log M/dt vs power-law M ∝ t1/α) yield conservative to strongest exclusions in(ma, fa) parameter space.
• The resulting excluded regions are shown in Fig. 1 and are independent of axion couplings to Standard Model fields, relying only on gravity and self-interaction.

Figure 2: The exclusion plot of axion parameters from axinovae. The colored region represents the exclusion region assuming the axion star growth $M\propto t^{0.2}$ , where the exclusion is independent of $T_{\rm osc}$ . The gray regions are the existing limits from the black hole superradiance. Das