Probing the cosmic dawn using the redshifted 21-cm bispectrum

Abstract

In the evolutionary history of our Universe, the Cosmic Dawn and Epoch of reionization (CD-EoR) is the period when the formation and evolution of the very first luminous sources took place. These first sources transitioned the state of the Universe from cold and neutral to a fully heated and ionized one. This particular phase of the Universe is largely untested via observations and thus at present we have very little insight about it. The 21-cm radiation, produced by the spin-flip transition of the electron-proton system in the 1s ground state of neutral hydrogen atom (Hi), which is abundant in the inter-galactic medium (IGM), is the direct tracer of heating and ionization processes in the IGM during the CD-EoR era. This highly redshifted (cosmological) and extremely feeble signal coming from the CD-EoR is yet to be confirmatively detected via the presently operating radio telescopes. Future radio interferometric surveys with the upcoming Square Kilometre Array (SKA) are expected to be sensitive enough to detect the redshifted 21-cm signal from the CD-EoR both via Fourier statistics and tomographic images. Most of the present generation telescopes, with which imaging is almost impossible, have been trying to detect the fluctuations in this signal mainly using Fourier statistics e.g. the power spectrum. The power spectrum provides a complete statistical description of a field if it is Gaussian random in nature. However, CD-EoR 21-cm fluctuations are expected to be highly non-Gaussian. This non-Gaussianity arises and evolves with time as the heated and ionized regions form, grow, and percolate with each other during this period in the IGM. The power spectrum cannot probe this intrinsic and time-evolving non-Gaussianity.