  We study the observational signatures of a potential population of
low-luminosity quasars at high redshifts in a LambdaCDM cosmology. We derive
the evolution of the quasar luminosity function at fainter luminosities and
higher redshifts than currently detected, based on three assumptions: (1) the
formation of dark-matter halos follows the Press-Schechter theory, (2) the
ratio of central black hole mass to halo mass is the same for all halos, and
(3) the light-curve of quasars, in Eddington units, is universal. We show that
a universal light-curve provides an excellent fit to the observed quasar
luminosity function at redshifts 2.6 < z < 4.5. By extrapolating the evolution
of this luminosity function to higher redshifts (4.5 < z < 20), we find that
the associated early population of low-luminosity quasars reionizes the
universe around redshift z=12. The reprocessing of the UV light of these
quasars by dust from early type II supernovae, distorts the microwave
background spectrum by a Compton y-parameter, y = 1e-5, comparable to the upper
limit set by COBE. The Next Generation Space Telescope could detect tens of
quasars from redshifts z > 10 per square arcminute, with its proposed 1nJy
sensitivity at 1-3.5 microns. Absorption spectra of several such quasars would
reveal the reionization history of the universe.
