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

We investigate cosmological signatures of uncorrelated isocurvature perturbations whose power spectrum is blue-tilted with spectral index 2 less than or similar to p less than or similar to 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 similar or equal to 7 - 8) but that are required to reionize the universe at z similar to 10. We mainly focus on how the formation of dark matter halos can be modified. Based on the Delta chi(2) analysis with other cosmological parameters being fixed, we explore the connection between the spectralshape of CMB anisotropies and the reionization optical depth as a powerful probe of a highly blue-tilted isocurvature primordial power spect rum. 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.