Cosmic reionization by primordial cosmic rays
  After the so-called cosmic recombination, the expanding universe entered into
a period of darkness since most of the matter was in a neutral state. About a
billion years later, however, the intergalactic space was once again ionized.
The process, known as the cosmic reionization, required the operation of
mechanisms that are not well understood. Among other ionizing sources,
Population III stars, mini-quasars, and X-ray emitting microquasars have been
invoked.
  In this article we propose that primordial cosmic rays, accelerated at the
termination points of the jets of the first microquasars, may have contributed
to the reionization of the intergalactic space as well. For this we quantify
the ionization power of cosmic rays (electrons and protons) in the primordial
intergalactic medium using extensive particle cascade simulations.
  We establish that, depending on the fraction of electrons to protons
accelerated in the microquasar jets, cosmic rays should have contributed to the
reionization of the primordial intergalactic medium as much as X-rays from
microquasar accretion disks. If the primordial magnetic field was of the order
of $10^{-17}$ G, as some models suggest, cosmic rays had an important role in
ionizing the neutral material far beyond the birth places of the first stars.
