The Epoch of Reionization is a period in the history of the Universe during which the intergalactic hydrogen gas went from neutral to ionized. This watershed event was powered by the emergence of the first luminous objects when the Universe was a few hundred million years to a billion years old. I will report in this talk our recent progress in advancing our capability to model such an intrinsically nonlinear process more efficiently and more robustly, using numerical 3D radiative transfer simulations, in two aspects. (1) We present a new algorithm for the 3D transport of ionizing radiation, called F2-Ray (Fast Fourier Ray-tracing method), which solves for the radiation transport via a newly discovered formal solution of the radiative transfer equation in Fourier space. This new algorithm overcomes the limitations of the standard ray-tracing methods in the scenario involving X-rays, and, therefore, can be an order of magnitude computationally more efficient than the latter. Our method can be applied to simulate the thermal distribution of the intergalactic medium as well as reionization, when the intergalactic gas was heated by X-rays from first stars and quasars. (2) It is customary to define a subgrid clumping factor, in computing the hydrogen recombination rate, that corrects for unresolved density fluctuations below the coarse-grained resolution in cosmological simulations of reionization. We quantify the correlation between the locally-averaged clumping factor and matter density using high resolution N-body simulations, and investigate in detail the impact of inhomogeneous subgrid clumping factor on cosmic reionization and its observables.

Bio: Dr. Yi MAO obtained his Ph.D. in physics at the Massachusetts Institute of Technology in 2008, supervised
by Professors Max Tegmark and Alan Guth. He was a postdoc at the University of Texas at Austin in 2008 -2012, and now a Lagrange postdoctoral Fellow at the Paris Institute of Astrophysics (IAP) since 2012. Yi is
a theoretical astrophysicist and cosmologist. His research focuses on modeling the epoch of reionization with
both analytical approaches and numerical simulations, and predicting the signatures of reionization on observations
such as the cosmic 21 cm background.