Cosmological signatures of tilted isocurvature perturbations: reionization and 21cm fluctuations

We investigate cosmological signatures of uncorrelated isocurvature perturba- tions 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 mod- ified, 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.