# Testing the scale-dependent hemispherical asymmetry with the 21-cm power spectrum from the epoch of reionization

## TL;DR

This paper proposes a new method to test scale-dependent hemispherical asymmetry using the 21-cm power spectrum from the epoch of reionization, potentially constraining asymmetry models beyond CMB observations.

## Contribution

It introduces a novel parameterization of scale-dependent asymmetry inspired by multi-speed inflation and demonstrates how future 21-cm observations can constrain this asymmetry.

## Key findings

- SKA Phase 2 can constrain asymmetry amplitude to ΔA ≈ 0.2
- Cosmic variance limited experiments can improve constraints to ΔA ≈ 0.01
- The method provides a new way to test early universe asymmetry models

## Abstract

Hemispherical power asymmetry has emerged as a new challenge to cosmology in early universe. While the cosmic microwave background (CMB) measurements indicated the asymmetry amplitude $A \simeq 0.07$ at the CMB scale $k_{\rm CMB}\simeq 0.0045\,{\rm Mpc}^{-1}$, the high-redshift quasar observations found no significant deviation from statistical isotropy. This conflict can be reconciled in some scale-dependent asymmetry models. We put forward a new parameterization of scale-dependent asymmetric power spectrum, inspired by a multi-speed inflation model. The 21-cm power spectrum from the epoch of reionization can be used to constrain the scale-dependent hemispherical asymmetry. We demonstrate that an optimum, multi-frequency observation by the Square Kilometre Array (SKA) Phase 2 can impose a constraint on the amplitude of the power asymmetry anomaly at the level of $\Delta A \simeq 0.2$ at $0.056 \lesssim k_{\rm 21cm} \lesssim 0.15 \,{\rm Mpc}^{-1}$. This limit may be further improved by an order of magnitude as $\Delta A \simeq 0.01$ with a cosmic variance limited experiment such as the Omniscope.