21 cm forest one-dimensional power spectrum as an indirect probe of dark matter particles and primordial black holes

TL;DR

This paper investigates the 21 cm forest 1D power spectrum as a novel, small-scale probe of dark matter during the epoch of reionization. It combines DM annihilation/decay and PBH Hawking radiation as exotic energy injection sources, with a multi-scale simulation framework linking large-scale 21cmFAST fields to small-scale halos and DarkHistory-based heating, and forecasts SKA sensitivity via a Fisher matrix analysis. The authors show that, under low astrophysical X-ray heating, the 21 cm forest can yield order-of-magnitude improvements over current limits, potentially constraining DM annihilation to $\langle\sigma v\rangle \lesssim 10^{-31}\,{\rm cm^3 s^{-1}}$ for $m_\chi\sim10$ GeV and DM decay to $\tau \gtrsim 10^{30}$ s, while probing PBHs with $M_{PBH}\sim10^{15}$ g at $f_{PBH}\sim10^{-13}$ and extending sensitivity to higher PBH masses. However, these constraints are highly degenerate with the level of astrophysical heating, requiring independent priors on $f_X$ from other probes. The study highlights the 21 cm forest as a powerful complementary tool to global and power-spectrum 21 cm measurements, capable of probing small-scale DM physics and expanding PBH parameter space, provided that astrophysical heating is well constrained. The results advocate a multi-probe observational strategy to fully exploit DM diagnostics in the early universe.

Abstract

Understanding the nature of dark matter (DM) particles remains a pivotal challenge in modern cosmology. Current cosmological research on these phenomena primarily utilizes cosmic microwave background (CMB) observations and other late-time probes, which predominantly focus on large scales. We introduce a novel probe, the 21 cm forest signal, which can be used to investigate DM properties on small scales during the epoch of reionization, thereby addressing the gap left by other cosmological probes. Annihilation and decay of DM particles, as well as Hawking radiation from PBHs, can heat the intergalactic medium (IGM). This heating suppresses the amplitude of the 21 cm forest 1D power spectrum. Therefore, the 1D power spectrum provides an effective method for constraining DM properties. However, astrophysical heating processes in the early universe can also affect the 21 cm forest 1D power spectrum. In this work, we assess the potential of using the SKA to observe the 21 cm forest 1D power spectrum for constraining DM properties, under the assumption that astrophysical heating can be constrained reliably by other independent probes. Under low astrophysical heating conditions, the 1D power spectrum could constrain the DM annihilation cross section and decay lifetime to $\langleσv\rangle \sim {10^{-31}}\,{\rm cm^{3}\,s^{-1}}$ and $τ\sim {10^{30}}\,{\rm s}$ for ${10}\,{\rm GeV}$ DM particles, and probe PBHs with masses $\sim {10^{15}}\,{\rm\,g}$ at abundances $f_{\mathrm{PBH}} \simeq 10^{-13}$. These constraints represent improvements of 5-6 orders of magnitude over current limits. Furthermore, the 21 cm forest 1D power spectrum has the potential to exceed existing bounds on sub-GeV DM and to probe PBHs with masses above $10^{18}\,{\rm g}$, which are otherwise inaccessible by conventional cosmological probes.

Figures (7)

• Figure 1: Thermal history of IGM under different exotic energy injection scenarios. Black curve represents the fiducial value with $f_{\rm X} = 0.6$. Red curve shows the thermal history of IGM in the presence of DM particle annihilation with $m_{\chi} = 10 ~\rm{MeV}$ and $\langle \sigma v \rangle = 10^{-29}~ \rm{cm}^{3}s^{-1}$. Green curve shows the thermal history of IGM in the presence of DM particle decay with $m_{\chi} = 10 ~\rm{MeV}$ and $\tau = 10^{28}~ \rm{s}$. Blue curve shows the thermal history of IGM in the presence of PBH with $M_{\rm PBH} = 10^{16} ~\rm{g}$ and $f_{\rm PBH} = 10^{-4}$.
• Figure 2: 1D power spectrum under different scenarios. The measurement errors are shown by the gray regions. Black curves show the fiducial model of the 1D power spectrum heated by only astrophysical X-ray sources with $f_{\rm X} = 0.6$ (upper panel) and $f_{\rm X} = 0.02$ (lower panel). The red curves correspond to models including DM particle annihilation with $m_{\chi} = 10 ~\rm{GeV}$, and $\langle \sigma v \rangle = 10^{-26}~ \rm{cm}^{3}s^{-1}$ (upper panel) and $\langle \sigma v \rangle = 10^{-28}~ \rm{cm}^{3}s^{-1}$ (lower panel). The green curves correspond to models including DM particle decay with $m_{\chi} = 10 ~\rm{GeV}$ and $\tau = 10^{26}~ \rm{s}$ (upper panel) and $\tau = 10^{29}~ \rm{s}$ (lower panel). The blue curves correspond to models including PBH with $M_{\rm PBH} = 10^{16} ~\rm{g}$ and $f_{\rm PBH} = 10^{-6}$ (upper panel) and $f_{\rm PBH} = 10^{-7}$ (lower panel).
• Figure 3: Constraints on $f_{\rm X}$ and DM parameters using 21 cm forest 1D power spectrum. Left, middle, and right panels show the results for DM annihilation, decay, and PBH Hawking radiation. The fiducial value of $f_{\rm X}$ is 0.6, while DM parameter's fiducial values are set to 0. The dark and light regions represent the $1 \sigma$ and $2 \sigma$ confidence contours, respectively.
• Figure 4: Constraints on $f_{\rm X}$ and DM parameters using 21 cm forest 1D power spectrum. Left, middle, and right panels show the results for DM annihilation, decay, and PBH Hawking radiation. The fiducial value of $f_{\rm X}$ is 0.02, while DM parameter's fiducial values are set to 0. The dark and light regions represent the $1 \sigma$ and $2 \sigma$ confidence contours, respectively.
• Figure 5: $2\sigma$ limits on the DM annihilation cross section from the SKA 21 cm forest 1D power spectrum. The left, middle, and right panels show the constraints for the electron-positron pairs, photon pairs, and bottom-anti-bottom quark pairs annihilation channels, respectively. Our results are shown by the red curves. Also shown for comparison are existing $2\sigma$ upper limits from observations of CMB distortion Zhang:2023usm, gamma-ray observations Cirelli:2020bpcHESS:2018komHESS:2014zqaVERITAS:2017tifMAGIC:2017avyAleksic:2013xeaFermi-LAT:2015att, and electron-positron pairs Cohen:2016uygBoudaud:2018oya, along with constraints from the 21 cm global signals zhao:2024jad and the 21 cm power spectrum zhao:2025ddy. The shaded regions correspond to the excluded parameter space.