Abstract:

Soon after the last scattering, electrons and photons began to decouple. The Universe entered a lifeless phase when electrons and charged nucleons had completed the process of recombination. The cosmological space was then filled with neutral hydrogen and helium. It was cold and opaque to optical light and X-rays. This dark age in the cosmic history prevailed until the arrival of the first stars, first galaxies and first AGN. The powerful radiation from these "first objects" was able to carve cavities of ionised gas around neutral gases. When ionised cavities grew, they merged to form bigger cavities. When the neutral gases were sufficiently displaced by the cavities of ionised gas, the Universe became transparent again. At the same time the Universe have continued to grow, settling into the hierarchical orders of voids, filaments, super-clusters, clusters, groups, galaxies and sub-galactic systems. In the talk I will elaborate how ionisation cavities develop (i.e. time-dependent structural evolution) and the difference in the hydrodynamical and thermal structures of the cavities around the "first sources" relevant to cosmological reionisation. I will show the hyperfine 21-cm signals of neutral hydrogen from explicit radiative transfer calculations using our fully covariant C21LRT code developed in house at UCL and demonstrate the effects on the 21-cm line arisen from convolution of local velocity fields, such as turbulence, and global cosmological expansion. I will show how the 21-cm line structures and tomographic power-spectra from our explicit cosmological radiative transfer calculations differ to those obtained by implicit methods, such as the commonly adopted optical-depth approach. I will show what are the conditions that the implicit methods are applicable or inapplicable. I will show some results (yet-to-be-published) obtained from our newly developed computational algoritm based on front propagation formalism, and illustrate how it would properly capture the subtle dynamics of cavity overlapping and merging, and handle inter-cavity and multiple source intra-cavity irradiation. If time allows, I will briefly discuss the effects of local Lyman-alpha pumping on the 21-cm line tomographic signals.