The Glow of Axion Quark Nugget Dark Matter: (IV) CMB Spectral and Anisotropy Signatures

TL;DR

This work investigates Axion Quark Nugget (AQN) dark matter as a macroscopic candidate that injects energy into the primordial plasma via baryon–antibaryon annihilation, potentially leaving observable CMB signatures. The authors modify the CLASS Boltzmann code to model the AQN-induced heat input and compute the resulting $μ$- and $y$-type spectral distortions and the optical depth, comparing these to annihilating or decaying DM scenarios. They find that CMB anisotropies are largely insensitive to AQN heating, while the spectral distortions lie within the reach of proposed missions such as PIXIE and Voyage 2050, with a notably larger $μ$-distortion than typical DM annihilation signals. Overall, the results support the viability of AQNs as an exotic DM candidate, offering a distinctive spectral footprint while remaining consistent with current cosmological constraints from Planck.

Abstract

Axion quark nuggets (AQNs) are macroscopic dark-matter candidates, with masses of the order of a few grams to a kilogram and sub-micron radius, thought to form at the Quantum Chromo Dynamic era through axion-induced charge separation. This framework naturally links the dark and visible matter abundances ($Ω_{\rm DM} \sim Ω_{\rm visible}$) and provides a mechanism for generating the baryon-antibaryon asymmetry where dark matter is composed of matter AQNs and antimatter AQNs. Although behaving as cold dark matter on cosmological scales, baryons annihilate with antimatter AQNs, producing ionizing high-energy photons. The resulting energy injection may imprint spectral distortions on the cosmic microwave background (CMB) and modify the reionization history. Using a modified version of the \texttt{CLASS} Boltzmann code we compute the impact of this energy injection on the $μ$ and $y$ spectral distortion parameters as well as on the optical depth. We find that (1) the CMB anisotropies remain essentially unaffected by baryon annihilation, and (2) the associated spectral distortion signatures lie within the sensitivity reach of proposed CMB spectral distortion missions. Finally, we discuss the similarities and differences between the AQN scenario and annihilating or decaying dark matter models.

Figures (4)

• Figure 1: Spectral distortion ($\Delta I_\nu$) contributions from Axion Quark Nugget (AQN) models compared to a dark matter annihilation model with $p_{\rm ann} = 4 \times 10^{-27}~\mathrm{cm^3\,s^{-1}\,GeV^{-1}}$. While the annihilation scenario exceeds Planck's upper bound and is observationally ruled out, its resulting distortions remain significantly smaller than those from AQN models. This illustrates that even observationally excluded levels of dark matter annihilation lead to weaker spectral distortion signatures than physically viable AQN scenarios, underlining the strong thermal visibility of AQN-induced distortions.
• Figure 2: Spectral distortions in the CMB predicted by the AQN model. Left: Projected sensitivities to $\mu$-type distortions. The solid red line shows the $\mu$-distortion spectrum from the AQN model, while the dashed and dash-dotted lines represent the expected detection thresholds of PIXIE ($\mu=5\times10^{-8}$) and Voyage2050 ($\mu=1\times10^{-9}$), respectively. Right: Predicted $y$-type distortion signal from the AQN model (solid blue), shown in comparison to the projected sensitivities of PIXIE ($y=5\times10^{-9}$, dashed green) and Voyage2050 ($y=1\times10^{-9}$, dash-dotted magenta). In both panels, the AQN signal remains above the lower bounds, indicating detectability within the safe observational region.
• Figure 3: Corner plots showing the posterior distributions for six cosmological parameters under four different models: the standard Basic Classy run, the AQN model with an average mass of 10g, and two dark matter annihilation scenarios with $p_{\rm ann} = 4 \times 10^{-28}~\mathrm{cm^3\,s^{-1}\,GeV^{-1}}$ and $p_{\rm ann} = 4 \times 10^{-30}~\mathrm{cm^3\,s^{-1}\,GeV^{-1}}$. Among these, only the red curve corresponding to the $p_{\rm ann} = 4 \times 10^{-28}$$\mathrm{cm^3\,s^{-1}\,GeV^{-1}}$ case exceeds the observational upper bound from Planck, while the others remain within the allowed region.
• Figure 4: Corner plots showing the posterior distributions for six cosmological parameters under three scenarios: the standard Basic Classy model, and three Axion Quark Nugget (AQN) models with nugget average masses of 10 g and 100 g and 1 kg. All cases remain consistent with the Planck upper limit on $p_{\rm ann}$, supporting the viability of AQN dark matter within current observational bounds.