[Paper Review] Inhomogeneous Energy Injection in the 21-cm Power Spectrum: Sensitivity to Dark Matter Decay

Yitian Sun, Joshua W. Foster+3 more|arXiv (Cornell University)|Dec 18, 2023

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper develops a complete framework for modeling inhomogeneous energy injection from dark matter decay into the 21-cm power spectrum, accounting for spatially varying ionization, baryon density, and photon propagation. It demonstrates that upcoming HERA observations will constrain dark matter decay lifetimes to ∼10²⁸ s—surpassing all existing limits in the sub-GeV range—highlighting 21-cm cosmology as a uniquely sensitive probe of exotic energy injection during cosmic dawn.

ABSTRACT

The 21-cm signal provides a novel avenue to measure the thermal state of the universe during cosmic dawn and reionization (redshifts $z\sim 5-30$), and thus to probe energy injection from decaying or annihilating dark matter (DM). These DM processes are inherently inhomogeneous: both decay and annihilation are density dependent, and furthermore the fraction of injected energy that is deposited at each point depends on the gas ionization and density, leading to further anisotropies in absorption and propagation. In this work, we develop a new framework for modeling the impact of spatially inhomogeneous energy injection and deposition during cosmic dawn, accounting for ionization and baryon density dependence, as well as the attenuation of propagating photons. We showcase how this first completely inhomogeneous treatment affects the predicted 21-cm power spectrum in the presence of exotic sources of energy injection, and forecast the constraints that upcoming HERA measurements of the 21-cm power spectrum will set on DM decays to photons and to electron/positron pairs. These projected constraints considerably surpass those derived from CMB and Lyman-$α$ measurements, and for decays to electron/positron pairs they exceed all existing constraints in the sub-GeV mass range, reaching lifetimes of $\sim 10^{28}\,\mathrm{s}$. Our analysis demonstrates the unprecedented sensitivity of 21-cm cosmology to exotic sources of energy injection during the cosmic dark ages. Our code, $\mathtt{DM21cm}$, includes all these effects and is publicly available in an accompanying release.

Motivation & Objective

• To model the impact of spatially inhomogeneous energy injection from dark matter decay on the 21-cm power spectrum during cosmic dawn and reionization.
• To account for the full complexity of energy deposition, including dependence on gas ionization, baryon density, and photon propagation.
• To forecast the sensitivity of upcoming HERA 21-cm power spectrum measurements to dark matter decay into photons and electron-positron pairs.
• To demonstrate that inhomogeneous effects significantly alter the predicted 21-cm signal and must be included for accurate constraints.
• To release a publicly available code, DM21cm, implementing the full inhomogeneous treatment for community use.

Proposed method

• Develops a new simulation framework, DM21cm, that couples inhomogeneous dark matter decay energy injection with the 21-cm signal evolution using a modified version of 21cmFAST.
• Incorporates redshift-dependent energy deposition efficiency that varies with local gas ionization fraction and density, reflecting realistic astrophysical conditions.
• Models photon propagation and attenuation through the intergalactic medium, accounting for energy loss and spatial anisotropy in deposition.
• Uses small time steps (Δz/(1+z) = 0.002) to ensure convergence of brightness temperature and power spectrum features under strong injection scenarios.
• Performs systematic convergence tests across multiple redshifts and injection scenarios to validate numerical stability and accuracy.
• Employs likelihood-based forecasting to project HERA sensitivity to dark matter decay parameters, including lifetime, mass, and decay channel.

Experimental results

Research questions

• RQ1 How do spatial inhomogeneities in dark matter decay energy injection—driven by local gas density and ionization—affect the 21-cm power spectrum?
• RQ2 What is the relative importance of inhomogeneous emission versus inhomogeneous deposition efficiency for different decay channels (photons vs. electrons)?
• RQ3 To what extent do photon propagation effects and energy deposition anisotropies alter the predicted 21-cm signal morphology compared to homogeneous models?
• RQ4 How do upcoming HERA measurements constrain dark matter decay lifetimes, and how do these constraints compare to existing limits from CMB and Lyman-α data?
• RQ5 What role does the interplay between astrophysical background processes and exotic energy injection play in degeneracy breaking for dark matter parameter estimation?

Key findings

• The 21-cm power spectrum is highly sensitive to inhomogeneous energy deposition from dark matter decay, with morphology strongly shaped by local gas conditions and ionization state.
• For decays to photons, spatial inhomogeneity in energy deposition efficiency dominates the signal morphology, while for electron-positron decays, inhomogeneity in emission tracking the DM distribution is most critical.
• HERA is projected to constrain dark matter decay lifetimes to ∼10²⁸ seconds for sub-GeV dark matter, significantly improving over existing limits.
• Constraints from 21-cm power spectrum measurements exceed those from CMB and Lyman-α observations, particularly for late-time energy injection scenarios like decay or p-wave annihilation.
• The framework reveals a non-trivial degeneracy between X-ray luminosity and low-energy cutoff in astrophysical modeling, which is broken when inhomogeneous deposition is correctly modeled.
• Convergence tests confirm numerical stability, with power spectrum changes below 0.1% when reducing time step from Δz/(1+z) = 0.002 to 0.001, validating the fiducial simulation setup.