: Cosmic reionization corresponds to the milestone of the Universe where the
intergalactic medium transitions from primarily neutral, dark, and cold into mainly ionized, more
luminous, and warmer. During this transition, the intergalactic medium is violently heated and
ripples from the additional injected energy can be seen millions of years after this milestone.
However, estimates of these ripples tend to focus on large scales (a choice that was partially
motivated by computational resources). In this talk, I will describe the thermodynamic and
hydrodynamic response of the often forgotten intergalactic medium small-scale structure to the
reionization process. Furthermore, I will showcase how this response leads to observational
fossils” that can be observed in both current Lyman-alpha forest surveys (e.g. DESI) and nearfuture
21 cm line intensity mapping experiments (e.g. SKA/PUMA). Likewise, I will highlight the
physical gains of this effect as a window into the physics that govern cosmic reionization and the
Cosmic dawn, and ultimately as a competitive probe of the nature of dark matter.