# Modelling the post-reionization neutral hydrogen (${\rm HI}$) 21-cm bispectrum

## TL;DR

This paper models the post-reionization 21-cm bispectrum using semi-numerical simulations to estimate bias parameters across redshifts, aiding future cosmological parameter and non-Gaussianity constraints.

## Contribution

It provides a semi-numerical modeling framework for the 21-cm bispectrum and estimates the evolution of HI bias parameters from redshift 1 to 6.

## Key findings

- b_1 decreases nearly linearly with decreasing z
- b_2 drops sharply, becomes zero at z~3, and stabilizes around -0.36 at z<2
- polynomial formulas for bias parameters as functions of z

## Abstract

Measurements of the post-reionization 21-cm bispectrum $B_{{\rm HI}}(\mathbf{k_1},\mathbf{k_2},\mathbf{k_3})$ using various upcoming intensity mapping experiments hold the potential for determining the cosmological parameters at a high level of precision. In this paper we have estimated the 21-cm bispectrum in the $z$ range $1 \le z \le 6$ using semi-numerical simulations of the neutral hydrogen (${\rm HI}$) distribution. We determine the $k$ and $z$ range where the 21-cm bispectrum can be adequately modelled using the predictions of second order perturbation theory, and we use this to predict the redshift evolution of the linear and quadratic ${\rm HI}$ bias parameters $b_1$ and $b_2$ respectively. The $b_1$ values are found to decreases nearly linearly with decreasing $z$, and are in good agreement with earlier predictions obtained by modelling the 21-cm power spectrum $P_{{\rm HI}}(k)$. The $b_2$ values fall sharply with decreasing $z$, becomes zero at $z \sim 3$ and attains a nearly constant value $b_2 \approx - 0.36$ at $z<2$. We provide polynomial fitting formulas for $b_1$ and $b_2$ as functions of $z$. The modelling presented here is expected to be useful in future efforts to determine cosmological parameters and constrain primordial non-Gaussianity using the 21-cm bispectrum.