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Measurements of the post-reionization 21-cm bispectrum $B_{{\rmHI}}(\mathbf{k_1},\mathbf{k_2},\mathbf{k_3})$ using various upcoming intensitymapping experiments hold the potential for determining the cosmologicalparameters at a high level of precision. In this paper we have estimated the21-cm bispectrum in the $z$ range $1 \le z \le 6$ using semi-numericalsimulations of the neutral hydrogen (${\rm HI}$) distribution. We determine the$k$ and $z$ range where the 21-cm bispectrum can be adequately modelled usingthe predictions of second order perturbation theory, and we use this to predictthe 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 nearlylinearly with decreasing $z$, and are in good agreement with earlierpredictions 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 providepolynomial fitting formulas for $b_1$ and $b_2$ as functions of $z$. Themodelling presented here is expected to be useful in future efforts todetermine cosmological parameters and constrain primordial non-Gaussianityusing the 21-cm bispectrum.

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