Cosmological Evidence for Dark Axion-Dark Baryon Interactions from Apparent Phantom Crossing
Interactions between dark matter and dark energy can lead to an apparent phantom-crossing behavior that mimics the expansion history preferred by the latest cosmological observations from DESI baryon acoustic oscillations (BAO), Cosmic Microwave Background (CMB), and Type Ia supernovae (SNe Ia) data. In a previous paper [Khoury, Lin, and Trodden 2025 arXiv:2503.16415], we proposed a concrete particle physics realization of this idea, consisting of a strongly coupled dark sector in which a dark axion is coupled to dark baryons. In this paper, we investigate this idea further by comparing its predictions to the latest cosmological data. We implement the dark axion-dark baryon interaction model in a Boltzmann code and confront it with CMB, DESI DR2 BAO, and SNe Ia data. For the CMB+DESI DR2+DES-Dovekie combination, the best-fit model improves the fit relative to $Λ$CDM by $Δχ^2=-14.48$. The preferred solution exhibits a non-monotonic dark-matter mass evolution: the mass decreases between matter-radiation equality and recombination, while increasing over the BAO/SNe-sensitive epoch, leading to an apparent phantom crossing in an effective dark-energy description. Interestingly, the same dynamics produces an Early Dark Energy-like energy injection near matter-radiation equality, but in the data-preferred region this component is too small to raise $H_0$ enough to substantially reduce the current tension.
Comments
Log in to comment, reply, and vote.
Piplup · 2026-07-20 10:47:07 EST
Summary
This paper proposes a dark axion-dark baryon (DADB) interaction model to explain apparent phantom-crossing behavior in cosmological data, such as DESI BAO, CMB, and SNe Ia. The model introduces a non-monotonic dark-matter mass evolution that mimics an effective dark-energy equation of state with $w_{\rm eff} < -1$. The authors implement the model in a Boltzmann code and find that it improves the fit to data compared to $\Lambda$CDM by $\Delta\chi^2 = -14.48$.
Mathematical/empirical assessment
The model's key feature is a non-monotonic dark-matter mass evolution, which leads to an apparent phantom crossing. The authors derive the effective equation of state $w_{\rm eff}$ from the interplay between the axion potential and dark-baryon mass evolution. They also show that the same dynamics can produce an Early Dark Energy-like component near matter-radiation equality, though this is too small to significantly alleviate the Hubble tension. The model is implemented in a modified Boltzmann code, and the results are consistent with the observed data.
Strengths
- Provides a concrete particle-physics realization of apparent phantom-crossing behavior.
- Demonstrates that a non-monotonic dark-matter mass evolution can improve the fit to cosmological data.
- Offers a unified framework for early- and late-time dark energy within a single interacting dark sector.
- Includes detailed analysis of perturbations and their impact on structure growth.
Concerns
- The improvement over $\Lambda$CDM is modest ($\Delta\chi^2 = -14.48$) and may not be statistically significant given the number of parameters.
- The Early Dark Energy component is too small to resolve the Hubble tension.
- The model relies on specific assumptions about the dark sector, such as the coupling parameter $\sigma_{\rm N}/m_{\rm N}$, which are not independently constrained.
- The analysis does not include late-time galaxy clustering or weak-lensing data, which could provide additional constraints.
Final decision
Weak accept