## Jesús Zavala Franco and Javier Israel Reynoso Córdoba

Math Phys seminar

**Speaker**: Jesús Zavala Franco, Háskóli Íslands and Javier Israel Reynoso
Córdoba, Universidad de Guanajuato

**Title: **The Boltzmann equation for a rarefied fluid in linear perturbation
theory

Room: HB5 (Háskólabíó)

Time: Friday 4^{th} October, 11:40hrs

#### Abstract:

Linear perturbation theory is the basis upon which we understand the initial growth of density perturbations in the early Universe. This theory has been developed and studied extensively in two extreme regimes: i) the fluid regime and ii) the collisionless regime. The former is applicable to the photon-baryon plasma, while the latter is commonly used to describe dark matter. There is however, a relevant class of dark matter models, known generically as self-interacting dark matter, where the Knudsen number is around 1, which lies in between these regimes. Linear perturbation theory in this regime remains essentially unexplored, requiring a full treatment of the Collisional Boltzmann equation. In this talk, we will present an overview of the problem and present our preliminary progress towards describing this regime.