# 21-cm power spectrum and ionization bias as a probe of long-mode modulated non Gaussian sky

## TL;DR

This paper explores how long-wavelength mode modulation and primordial non-Gaussianity affect the 21-cm brightness temperature power spectrum, proposing that upcoming surveys like SKA could detect hemispherical power asymmetry.

## Contribution

It introduces a model linking long-mode modulation and non-Gaussianity to the 21-cm power spectrum, highlighting the potential for observational detection.

## Key findings

- Primordial non-Gaussianity can amplify the 21-cm power spectrum by up to four orders of magnitude.
- Long-mode modulation may cause detectable hemispherical power asymmetry in upcoming 21-cm surveys.
- The bias parameter of ionized fraction is formulated to relate neutral hydrogen distribution to matter fluctuations.

## Abstract

The observed hemispherical power asymmetry in cosmic microwave background radiation can be explained by long wavelength mode (long-mode) modulation. In this work we study the prospect of the detection of this effect in the angular power spectrum of 21-cm brightness temperature. For this task, we study the effect of the neutral Hydrogen distribution on the angular power spectrum. This is done by formulating the bias parameter of ionized fraction to the underlying matter distribution. We also discuss the possibility that the long mode modulation is companied with a primordial non-Gaussianity of local type. In this case, we obtain the angular power spectrum with two effects of primordial non-Gaussianity and long mode modulation. Finally, we show that the primordial non-Gaussianity enhances the long mode modulated power of 21-cm signal via the non-Gaussian scale-dependent bias up to four orders of magnitude. {Accordingly, the observation of 21-cm signal with upcoming surveys such as the Square Kilometer Array (SKA) is probably capable of detecting hemispherical power asymmetry in the context of the long mode modulation.