  The high ionization level and universal metallicity (1% solar) of the
intergalactic gas at redshifts z<5 implies that nonlinear structure had started
to form in the universe at earlier times than we currently probe. In Cold Dark
Matter (CDM) cosmologies, the first generation of baryonic objects emerges at
redshifts z=10-50. Here we examine the observable consequences of the
possibility that an early generation of stars reionized the universe and
resulted in the observed metallicity of the Lyman-alpha forest. Forthcoming
microwave anisotropy experiments will be sensitive to the damping of
anisotropies caused by scattering off free electrons from the reionization
epoch. For a large range of CDM models with a Scalo stellar mass function, we
find that reionization occurs at a redshift z>10 and damps the amplitude of
anisotropies on angular scales <10 degrees by a detectable amount, of order
10-25%. However, reionization is substantially delayed if the initial stellar
mass function transformed most of the baryons into low mass stars. In this
case, the mass fraction of pre-galactic stars could be constrained from the
statistics of microlensing events in galactic halos or along lines of sight to
quasars. Deep infrared imaging with future space telescopes (such as SIRTF or
the Next Generation Space Telescope) will be able to detect bright star
clusters at z>5. The cumulative Bremsstrahlung emission from these star
clusters yields a measurable distortion to the spectrum of the microwave
background.
