Abstract. Hydrogen is the most abundant visible element in the universe and observing its spatial distribution at different cosmic times can be the most direct probe of the evolution of underlying structures. Radio interferometric observations of the redshifted 21-cm line originating from the spin-flip transition in the neutral hydrogen (HI) are our best bet to observe the HI distribution over a wide redshift range.

After the matter-radiation decoupling, the epoch at which most of the HI goes through its first major phase change is the Cosmic Dawn and the Epoch of Reionization. This is the phase when the first sources of lights are formed, gradually heating and ionizing the HI in the IGM. A large number of present and upcoming radio interferometers are in a race to observe the HI from this phase using the redshifted 21-cm signal. Most of this ongoing effort focuses on the statistical detection of the signal through its power spectrum. However, the HI distribution during this phase is expected to be highly non-Gaussian due to its evolving morphology. The power spectrum, therefore, cannot give us a comprehensive picture of the signal.

In this talk I will discuss a higher order Fourier statistics, the bispectrum, that is capable of capturing the evolving non-Gaussianity inherent in the CD-EoR 21-cm signal. With the end of the Epoch of Reionization, that is, when most of the HI in the universe is ionized, some HI remains in high-density regions of the IGM that can be self-shielded from the ionization background. This HI distribution is also expected to be non-Gaussian in nature. I will discuss how the bispectrum estimates from the intensity-mapping observations of the post-EoR 21-cm signal can contribute to our understanding of the underlying matter distribution.