Searching for blue in the dark
Jessie de Kruijf, Eleonora Vanzan, Kimberly K. Boddy, Alvise, Raccanelli, Nicola Bartolo

TL;DR
This paper explores how 21-cm intensity mapping during the dark ages can detect small-scale enhancements in the primordial power spectrum, offering new insights into early universe physics and inflation models.
Contribution
It presents forecasts for detecting blue-tilted primordial spectra using Earth- and Moon-based 21-cm observations, highlighting the potential to probe previously inaccessible small scales.
Findings
Earth-based instruments can detect strong small-scale enhancements.
Lunar-based instruments can probe shallower slopes with higher precision.
Scales up to k ~ 8 Mpc^{-1} (Earth) and 250 Mpc^{-1} (Moon) are accessible.
Abstract
The primordial power spectrum of curvature perturbations has been well-measured on large scales but remains fairly unconstrained at smaller scales, where significant deviations from CDM may occur. Measurements of 21-cm intensity mapping in the dark ages promise to access very small scales that have yet to be probed, extending beyond the reach of cosmic microwave background and galaxy surveys. In this paper, we investigate how small-scale power-law enhancements -- or blue tilts -- of the primordial power spectrum affect the 21-cm power spectrum. We consider generic enhancements due to curvature modes, isocurvature modes, and runnings of the spectral tilt. We present forecasts for Earth- and lunar-based instruments to detect a blue-tilted primordial spectrum. We find that an Earth-based instrument capable of reaching the dark ages could detect any enhancements of power on nearly…
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