We investigate cosmological signatures of uncorrelated isocurvature
perturbations whose power spectrum is blue-tilted with spectral index 2<p<4.
Such an isocurvature power spectrum can promote early formation of small-scale
structure, notably dark matter halos and galaxies, and may thereby resolve the
shortage of ionizing photons suggested by observations of galaxies at high
redshifts (z~7-8) but that are required to reionize the universe at z~10. We
mainly focus on how the formation of dark matter halos can be modified, and
explore the connection between the spectral shape of CMB anisotropies and the
reionization optical depth as a powerful probe of a highly blue-tilted
isocurvature primordial power spectrum. We also study the consequences for 21cm
line fluctuations due to neutral hydrogens in minihalos. Combination of
measurements of the reionization optical depth and 21cm line fluctuations will
provide complementary probes of a highly blue-tilted isocurvature power
spectrum.Comment: 18 pages, 9 figures, 1 tabl
