  Observations of the Ly-alpha forest at z~3 reveal an average metallicity
Z~0.01 Z_solar. The high-redshift supernovae that polluted the IGM also
accelerated relativistic electrons. Since the energy density of the CMB scales
as (1+z)^4, at high redshift these electrons cool via inverse Compton
scattering. Thus, the first star clusters emit X-rays. Unlike stellar UV
ionizing photons, these X-rays can escape easily from their host galaxies. This
has a number of important physical consequences: (i) Due to their large mean
free path, these X-rays can quickly establish a universal ionizing background
and partially reionize the universe in a gradual, homogeneous fashion. If
X-rays formed the dominant ionizing background, the universe would have more
closely resembled a single-phase medium, rather than a two-phase medium. (ii)
X-rays can reheat the universe to higher temperatures than possible with UV
radiation. (iii) X-rays counter the tendency of UV radiation to
photo-dissociate H2, an important coolant in the early universe, by promoting
gas phase H2 formation. The X-ray production efficiency is calibrated to local
observations of starburst galaxies, which imply that ~10% of the supernova
energy is converted to X-rays. While direct detection of sources in X-ray
emission is difficult, the presence of relativistic electrons at high redshift
and thus a minimal level of X-ray emission may be inferred by synchrotron
emission observations with the Square Kilometer Array. These sources may
constitute a significant fraction of the unresolved hard X-ray background, and
can account for both the shape and amplitude of the gamma-ray background. This
paper discusses the existence and observability of high-redshift X-ray sources,
while a companion paper models the detailed reionization physics and chemistry.
