The Future of Primordial Features with 21 cm Tomography

TL;DR

This paper investigates how 21 cm tomography at high redshift can significantly improve constraints on primordial features in the power spectrum, offering insights into early Universe physics beyond current CMB limits.

Contribution

It provides a forecast of the potential of high-redshift 21 cm observations to detect or constrain primordial features, exploring different models and experimental configurations.

Findings

21 cm tomography can improve bounds on primordial features by several orders of magnitude.

High-redshift 21 cm observations can access small-scale features beyond current CMB limits.

Detection sensitivity depends on feature amplitude, frequency, scale, and experimental resolution.

Abstract

Detecting a deviation from a featureless primordial power spectrum of fluctuations would give profound insight into the physics of the primordial Universe. Depending on their nature, primordial features can either provide direct evidence for the inflation scenario or pin down details of the inflation model. Thus far, using the cosmic microwave background (CMB) we have only been able to put stringent constraints on the amplitude of features, but no significant evidence has been found for such signals. Here we explore the limit of the experimental reach in constraining such features using 21 cm tomography at high redshift. A measurement of the 21 cm power spectrum from the Dark Ages is generally considered as the ideal experiment for early Universe physics, with potentially access to a large number of modes. We consider three different categories of theoretically motivated models: the sharp feature models, resonance models, and standard clock models. We study the improvements on bounds on features as a function of the total number of observed modes and identify parameter degeneracies. The detectability depends critically on the amplitude, frequency and scale-location of the features, as well as the angular and redshift resolution of the experiment. We quantify these effects by considering different fiducial models. Our forecast shows that a cosmic variance limited 21 cm experiment measuring fluctuations in the redshift range $30\leq z \leq 100$ with a 0.01-MHz bandwidth and sub-arcminute angular resolution could potentially improve bounds by several orders of magnitude for most features compared to current Planck bounds. At the same time, 21 cm tomography also opens up a unique window into features that are located on very small scales.