Do We Need Stars to Reionize the Universe at High Redshifts? Early Reionization by Decaying Heavy Sterile Neutrinos

TL;DR

WMAP's measurement of a large $\tau_e$ implies an earlier reionization than standard stellar populations alone would naturally provide. The authors propose a two-step history in which a heavy sterile neutrino with $m \approx 200\,\mathrm{MeV}$ and decay time $t \approx 4\times10^{15}\,\mathrm{s}$ injects energetic electrons that upscatter CMB photons to UV/soft X-ray energies, driving initial ionizations, followed by conventional star formation completing reionization by $z\sim6-7$. This mechanism can yield $\tau_e \approx 0.16$ without conflicting with BBN, the diffuse gamma-ray background, or current CMB constraints, and it keeps warm dark matter scenarios viable. The model makes testable predictions for CMB spectral distortions and polarization signatures that future missions, notably Planck, could distinguish from purely stellar histories.

Abstract

A remarkable result of the Wilkinson Microwave Anisotropy Probe (WMAP) observations is that the universe was significantly reionized at large redshifts. The standard explanation is that massive stars formed early and reionized the universe around redshift z=17. Here we explore an alternative possibility, in which the universe was reionized in two steps. An early boost of reionization is provided by a decaying sterile neutrino, whose decay products, relativistic electrons, result in partial ionization of the smooth gas. We demonstrate that a neutrino with a mass of m_nu ~ 200 MeV and a decay time of t ~ 4 * 10^{15} s can account for the electron scattering optical depth tau=0.16 measured by WMAP without violating existing astrophysical limits on the cosmic microwave and gamma ray backgrounds. Reionization is then completed by subsequent star formation at lower redshifts. This scenario alleviates constraints on structure formation models with reduced small-scale power, such as those with a running or tilted scalar index, or warm dark matter models.