How Dark Matter Reionized The Universe

TL;DR

This study proposes that annihilating dark matter with electroweak-scale masses can drive cosmic reionization by $z \sim 6$, emphasizing energy transfer from DM-produced electrons to the CMB via inverse Compton scattering, which yields IC photons that ionize gas more efficiently than prompt photons. By modeling the halo mass function, NFW halo profiles, and detailed energy deposition and recombination physics, the authors show that while a canonical $m_X\approx100$ GeV with $\langle\sigma v\rangle\sim3\times10^{-26}$ cm$^3$/s yields only 1–10% ionization by $z\sim6$, non-thermally produced or leptophilic DM (e.g., a $100$ GeV wino with $\langle\sigma v\rangle\sim(3-10)\times10^{-24}$ cm$^3$/s) can reionize the universe by that epoch and match the observed optical depth. The work further connects DM interpretations of PAMELA/ATIC signals to reionization, showing that models explaining these cosmic-ray excesses predict ionization histories compatible with WMAP measurements. Overall, the paper argues that DM properties inferred from cosmic-ray data could have played a major role in shaping the early ionization state of the universe, with significant implications for the interpretation of reionization-era observations.

Abstract

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.

Figures (6)

• Figure 1: The comoving number density of dark matter halos as a function of mass, at redshifts of 80, 60, 40, 20, 10 and 0 (from bottom-to-top). The solid (dashed) lines were calculated using $\sigma_8=0.812$ and $n_s=0.96$ ($\sigma_8=0.864$ and $n_s=0.986$).
• Figure 2: In the left frame, we plot the spectrum of prompt gamma rays (solid) and electrons (dots) from the annihilation of 100 GeV dark matter particles to $W^+ W^-$. We also plot, as dashed lines, the spectrum of inverse Compton photons which results from those electrons scattering with the cosmic microwave background (for redshifts of $z=$0, 10 and 60, from left to right). In the right frame, the Klein-Nishina cross section is shown as a function of photon energy. Due to the rapidly falling cross section, the inverse Compton photons are far more efficient at reionizing the universe than higher energy prompt gamma rays.
• Figure 3: The rate of change of the fraction of ionized baryons (upper left), the temperature of gas (upper right), and the fraction of ionized baryons (lower), as a function of redshift. Here, we have considered a 100 GeV dark matter particle which annihilates to $W^+W^-$ with a cross section of $\langle\sigma v\rangle = 3 \times 10^{-26}$ cm$^3$/s. We show results using two sets of cosmological parameters ($n_s = 0.96, \sigma_8 = 0.812$ and $n_s = 0.986, \sigma_8 = 0.864$). In the upper right frame, the dotted line denotes the evolution of the gas temperture without heating from dark matter annihilations.
• Figure 4: The same as shown in Fig. \ref{['xionrate']}, but for the case of a 100 GeV dark matter particle which annihilates to $W^+W^-$ with a cross section of $4.5\times 10^{-24}$ cm$^3$/s (a wino-like neutralino, for example). Dark matter annihilations in this model lead to nearly total ionization by $z\approx 6$, and constitute the primary source the optical depth as measured by WMAP.
• Figure 5: The contribution to the optical depth of the universe (over $z > 6$) from dark matter annihilations. Here we have considered a 100 GeV dark matter particle which annihilates to $W^+W^-$. The horizontal dotted lines denotes the range of values measured by WMAP, $\delta \tau \approx 0.047 \pm 0.017$. A relatively light (100-200 GeV) wino-like neutralino would naturally lead to an optical depth consistent with this measurement.