Impact of the primordial fluctuation power spectrum on the reionization history

Understand

We argue that observations of the reionization history can be used as a probe of primordial density fluctuations, particularly on small scales.

• Although the primordial curvature perturbations are well constrained from measurements of cosmic microwave background (CMB) anisotropies and large-scale structure, these observational data probe the curvature perturbations only on large scales, and hence its information on smaller scales will give us further insight on primordial fluctuations.
• Since the formation of early galaxies is sensitive to the amplitude of small-scale perturbations, and then, in turn, gives an impact on the reionization history, one can probe the primordial power spectrum on small scales through observations of reionization.
• In this work, we focus on the running spectral indices of the primordial power spectrum to characterize the small-scale perturbations, and investigate their impact on the reionization history using the numerical code \texttt{21cmFAST}, which adopts a simple but commonly used reionization model.