Reionization Topology as a Probe of Self-Interacting Dark Matter

The topology of cosmic reionization, the sizes, shapes, and connectivity of ionized bubbles is a primary observable of next-generation 21,cm experiments. We show that this topology is sensitive to the microphysics of dark matter. Self-interacting dark matter (SIDM), with cross-sections $\sigma \mathrm{/}m\sim 1$–$10\mathrm{c}{\mathrm{m}}^2\mathrm{/}\mathrm{g}$ motivated by small-scale structure anomalies, reduces halo gas binding energies and increases the duty cycle of ionizing-photon escape. At fixed global neutral fraction ${\bar{x}}_{\mathrm{H}\mathrm{I}}$, this reshapes the source population from rare, very bright emitters to more numerous, moderate emitters, producing qualitatively different ionization morphology. We decompose the effect into two scale-dependent levers: a $2$–$3\mathrm{\%}$ emissivity-weighted bias shift at $k\lesssim 0.1h\mathrm{/}\mathrm{M}\mathrm{p}\mathrm{c}$, and a factor $2$–$4$ shot-noise suppression at $k\sim 0.1$–$1h\mathrm{/}\mathrm{M}\mathrm{p}\mathrm{c}$. A halo-by-halo semi-numerical simulation at ${128}^3$ resolution confirms a $\sim 60$–$70\mathrm{\%}$ increase in the Euler characteristic of the ionization field for $\sigma \mathrm{/}m\gtrsim 2\mathrm{c}{\mathrm{m}}^2\mathrm{/}\mathrm{g}$, detected at $3.8\sigma$ across ten independent realizations. A blowout model connecting the binding-energy reduction to the duty cycle through the ISM column density distribution yields a detection threshold at $\sigma \mathrm{/}m\sim 1$–$2\mathrm{c}{\mathrm{m}}^2\mathrm{/}\mathrm{g}$. The signal exceeds the CDM baryonic uncertainty band and is robust to the functional form of the emissivity parametrization. The signal persists even if gravitational heating offsets $50$–$75\mathrm{\%}$ of the blowout enhancement, and is not diluted by unresolved low-mass sources. Velocity-dependent SIDM produces a qualitatively distinct opposite-sign bias shift. These predictions are testable with SKA1-Low, establishing reionization as a new arena for probing dark matter models complementary to dwarf galaxies and galaxy clusters.