Probing Non-Minimal Dark Sectors via the 21 cm Line at Cosmic Dawn

Observations of the hydrogen hyperfine transition through the 21 cm line near the end of the cosmic dark ages provide unique opportunities to probe new physics. In this work, we investigate the potential of the sky-averaged 21 cm signal to constrain metastable particles produced in the early universe that decay at later times, thereby modifying the thermal and ionization history of the intergalactic medium. The study begins by extending previous analyses of decaying dark matter (DM), incorporating back-reaction effects and tightening photon decay constraints down to DM masses as low as 20.4 eV. The focus then shifts to non-minimal dark sectors with multiple interacting components. The analysis covers two key scenarios: a hybrid setup comprising a stable cold DM component alongside a metastable sub-component, and a two-component dark sector of nearly degenerate states with a metastable heavier partner. A general parameterization based on effective mass spectra and fractional densities allows for a model-independent study. The final part presents two explicit realizations: an axion-like particle coupled to photons, and pseudo-Dirac DM interacting via vector portals or electromagnetic dipoles. These scenarios illustrate how 21 cm cosmology can set leading bounds and probe otherwise inaccessible regions of parameter space.

💡 Research Summary

The paper investigates how measurements of the sky‑averaged 21 cm hyperfine transition of neutral hydrogen during the end of the cosmic dark ages can be used to constrain exotic energy injection from metastable particles that decay long after they are produced in the early universe. The authors first revisit the classic decaying‑dark‑matter (DM) scenario, improving upon earlier work by incorporating back‑reaction effects—i.e., the fact that the efficiency with which decay products deposit energy into the intergalactic medium (IGM) depends on the evolving temperature and ionization state of the gas. Using state‑of‑the‑art energy‑deposition codes, they extend photon‑channel limits down to DM masses of 20.4 eV (the Lyman‑α transition energy), a regime previously inaccessible because low‑energy photons were thought to free‑stream without depositing significant energy.

The core of the analysis is a conservative bounding strategy based on the maximal absorption depth that ΛCDM predicts under the assumption of “maximally efficient” Lyman‑α coupling (i.e., the spin temperature follows the gas temperature). Any additional heating from decays can only reduce the absolute value of the brightness temperature δT_b, so the observed (or assumed) absorption depth provides an upper limit on the total injected power. This approach treats the astrophysical heating that would normally occur in ΛCDM as zero, thereby yielding the most stringent possible limits from 21 cm data alone.

Having established the updated single‑component constraints, the authors move beyond the minimal picture and consider dark sectors with multiple interacting components. They introduce a model‑independent parametrisation in terms of each component’s mass m_i, fractional density ξ_i ≡ Ω_i/Ω_DM, and decay rate Γ_i. The total energy‑injection rate is then Σ_i ξ_i Γ_i m_i e^{‑Γ_i t}. Two representative non‑minimal scenarios are explored:

1. Hybrid scenario – a stable cold‑dark‑matter (CDM) particle co‑exists with a sub‑dominant metastable species χ. Even if χ contributes only a tiny fraction (ξ_χ ≲ 10⁻³) of the total DM density, decay lifetimes of order τ ≈ 10 Gyr (Γ_χ ≈ 10⁻¹⁸ s⁻¹) are already excluded because the resulting heating would noticeably shallow the 21 cm absorption trough.

2. Nearly degenerate two‑component scenario – the lighter state χ₁ makes up essentially all of DM, while a heavier partner χ₂ decays into χ₁ plus a Standard‑Model photon or e⁺e⁻ pair. If the mass splitting Δm ≲ keV, the decay products are low‑energy and are absorbed with nearly 100 % efficiency. The 21 cm signal then constrains the combination ξ₂ Γ₂ Δm; values as small as ξ₂ ≈ 10⁻⁴ with Γ₂ ≈ 10⁻¹⁸ s⁻¹ are already ruled out.

To demonstrate the relevance of these generic results, the paper presents two explicit microscopic realizations:

• Axion‑like particle (ALP) coupled to photons – described by the interaction (g_{aγ}/4) a F_{\muν}\tilde{F}^{\muν}. The decay a → γγ injects two photons with energies set by the ALP mass. For m_a in the 30 eV–keV range, the 21 cm constraints are stronger than those from CMB spectral distortions, closing a portion of parameter space that would otherwise be viable.

• Pseudo‑Dirac fermion DM – two Majorana states ψ₁, ψ₂ that form a pseudo‑Dirac pair. ψ₂ can decay to ψ₁ plus a photon via a dipole operator or via a vector portal (e.g., a Z′ boson). The decay injects either a single photon or an e⁺e⁻ pair, both of which efficiently heat the IGM. The analysis shows that for dipole moments or portal couplings large enough to give Γ₂ ≈ 10⁻¹⁸ s⁻¹, the resulting heating would suppress the 21 cm absorption below the ΛCDM maximum, thereby excluding such parameter choices.

Overall, the study demonstrates that (i) incorporating back‑reaction and modern deposition efficiencies tightens existing DM‑decay bounds, (ii) the 21 cm line can probe DM masses an order of magnitude lower than previously thought, (iii) a simple effective‑parameter framework can capture a wide class of multi‑component dark sectors, and (iv) concrete models such as photon‑coupled ALPs and pseudo‑Dirac DM are already strongly constrained by current (or near‑future) 21 cm observations. The authors conclude that upcoming experiments (e.g., HERA, SKA, and improved global‑signal instruments) will be able to test lifetimes up to ∼10 Gyr for a broad range of masses, providing a powerful, complementary probe of dark‑sector physics beyond the minimal ΛCDM paradigm.