How Dark Matter Reionized The Universe

Although empirical evidence indicates that that the universe’s gas had become ionized by redshift z ~ 6, the mechanism by which this transition occurred remains unclear. In this article, we explore the possibility that dark matter annihilations may have played the dominant role in this process. Energetic electrons produced in these annihilations can scatter with the cosmic microwave background to generate relatively low energy gamma rays, which ionize and heat gas far more efficiently than higher energy prompt photons. In contrast to previous studies, we find that viable dark matter candidates with electroweak scale masses can naturally provide the dominant contribution to the reionization of the universe. Intriguingly, we find that dark matter candidates capable of producing the recent cosmic ray excesses observed by PAMELA and/or ATIC are also predicted to lead to the full reionization of the universe by z ~ 6.

💡 Research Summary

The authors address the long‑standing problem of how the intergalactic medium became fully ionized by redshift z ≈ 6. While conventional models invoke ultraviolet photons from the first stars and quasars, the required photon budget remains uncertain. This paper revisits the alternative hypothesis that dark‑matter (DM) annihilations supplied the dominant ionizing power. The key insight is that energetic electrons produced in DM annihilations rapidly lose energy by inverse‑Compton scattering off the cosmic microwave background (CMB), generating a copious flux of low‑energy gamma‑rays (∼10⁻³ GeV). These photons have Klein‑Nishina cross sections close to the Thomson value, making them roughly two orders of magnitude more efficient at ionizing hydrogen than the higher‑energy prompt photons that are usually considered.

The authors compute the annihilation rate per comoving volume, R(z), by integrating over the halo mass function dn/dM, adopting an NFW density profile with concentrations from Bulcock et al. Two sets of cosmological parameters (σ₈ = 0.812, nₛ = 0.96 and σ₈ = 0.864, nₛ = 0.986) are used to explore uncertainties at high redshift. The spectrum of photons per annihilation, dN_γ/dE_γ, is propagated forward in redshift, accounting for absorption via an exponential factor A_b(z,z′,E′γ). The ionization probability of a photon, P(E_γ,z), is expressed through the Klein‑Nishina cross section σ{γe} and the baryon density. Energy‑to‑ionization conversion is approximated by N_ion(E_γ) ≈ 2.4 × 10⁷ (E_γ/1 GeV).

Electrons, however, dominate the energy budget. Their inverse‑Compton loss rate, dE_e/dt ≈ 2.3 × 10⁻¹⁷ GeV s⁻¹ (1+z)⁴ (E_e/1 GeV)², ensures that a 1 GeV electron at z = 6 deposits 99 % of its energy into the CMB within ≈5 × 10⁷ yr, producing photons with typical energy E_IC ≈ 3.2 × 10⁻⁴ GeV (1+z)(E_e/10 GeV)². Because these inverse‑Compton photons lie in the regime where σ_{γe}≈σ_T, they ionize the gas far more efficiently than the prompt GeV photons, a point illustrated in Figure 2.

The net ionization rate I(z) (Eq. 16) combines the photon spectrum, absorption, and ionization probability, while the recombination rate R_c(z) (Eq. 18) uses standard temperature‑dependent coefficients for H and He. Gas heating is assumed to receive one‑third of the deposited energy, raising the temperature above the adiabatic cooling law T(z) ≈ 5.3 × 10⁻³