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Title: Probing the Cosmic Dawn using the redshifted 21-cm bispectrum
Speaker: Mohd Kamran
Affiliation: Uppsala University
Time: Thursday 24 November 2022, 1400 to 1500
Location: 10134Å (Polhemsalen)
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 into 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 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. The power spectrum cannot probe this intrinsic and time-evolving non-Gaussianity. The bispectrum, being a potential probe of this non-Gaussianity, provides an opening for a comprehensive and correct interpretation of this signal. We for the first time explore the extent to which the bispectrum, probing the non-Gaussianity, can probe the IGM physics during CD-EoR. Compared to the well-studied 21-cm power spectrum, which only captures the evolution of the amplitude of the signal fluctuations, the 21-cm bispectrum captures the evolution of the fluctuations in the signal and the associated IGM physics through the evolution of its magnitude and sign. This work is the first of its kind to probe the IGM physics during CD-EoR by estimating the bispectra for all possible unique k-triangles with the help of a suite of radiative transfer simulations for the signal. We observe that the bispectrum sign and magnitude strongly depend on the competition between the three main processes i.e. Lyman-alpha coupling, X-ray heating, and the ionization of the gas in the IGM led by the first sources. The bipsectrum, therefore, can put a tighter constraint on the CD-EoR parameters compared to the power spectrum.