The Growth Dynamics of Ionized Bubbles During Cosmic Reionization

Abstract

The Epoch of Reionization (EoR) represents a key transition in the history of the universe, during which the first luminous sources ionized the neutral intergalactic medium (IGM). In this work, the author investigates the growth and morphology of ionized bubbles by combining insights from large-scale simulations and simplified analytic models. Using data from the state-of-the-art THESAN project, bubble sizes are quantified with the Mean Free Path (MFP) method, which captures both their temporal evolution and environmental dependence. The analysis reveals a clear inside-out reionization topology, where overdense regions ionize first and later merge into extended ionized structures. To complement these results, the author implements a semi-analytic model inspired by the Furlanetto–Zaldarriaga–Hernquist (FZH04) framework. This model generates ionization maps from Gaussian density fields and applies photon-counting criteria across multiple scales, reproducing key features of bubble growth, such as the transition from isolated to percolated regions. Comparisons between THESAN and the simplified model demonstrate strong qualitative agreement, highlighting the robustness of the excursion set formalism. Overall, this study emphasizes the complementary roles of numerical simulations and semi-analytic approaches in understanding the dynamics and topology of cosmic reionization.