Violation of statistical isotropy and homogeneity in the 21-cm power spectrum

TL;DR

This paper proposes using the 21-cm power spectrum during the dark ages to test for statistical isotropy violations, potentially surpassing CMB constraints by orders of magnitude.

Contribution

It introduces a novel method to detect primordial power asymmetries using 21-cm fluctuations, extending tests to smaller scales and higher redshifts.

Findings

21-cm power spectrum can probe small-scale anisotropies

Forecasted sensitivity exceeds current CMB constraints by 1-3 orders of magnitude

Method applicable to SKA and future radio arrays

Abstract

Most inflationary models predict primordial perturbations to be statistically isotropic and homogeneous. Cosmic microwave background (CMB) observations, however, indicate a possible departure from statistical isotropy in the form of a dipolar power modulation at large angular scales. Alternative models of inflation, beyond the simplest single-field slow-roll models, can generate a small power asymmetry, consistent with these observations. Observations of clustering of quasars show, however, agreement with statistical isotropy at much smaller angular scales. Here, we propose to use off-diagonal components of the angular power spectrum of the 21-cm fluctuations during the dark ages to test this power asymmetry. We forecast results for the planned SKA radio array, a future radio array, and the cosmic-variance-limited case as a theoretical proof of principle. Our results show that the 21-cm line power spectrum will enable access to information at very small scales and at different redshift slices, thus improving upon the current CMB constraints by $\sim 2$ orders of magnitude for a dipolar asymmetry and by $\sim 1-3$ orders of magnitude for a quadrupolar asymmetry case.