# The massless Einstein-Boltzmann system with a conformal-gauge singularity in an FLRW background

## TL;DR

This paper proves finite-time existence of solutions to the massless Einstein-Boltzmann system in an FLRW universe with initial singularity, and long-time existence for small data when a positive cosmological constant is present.

## Contribution

It establishes the existence of solutions to the Einstein-Boltzmann system with soft cross-sections in a cosmological setting, including initial singularity and long-time behavior.

## Key findings

- Finite-time existence for the massless Boltzmann equation in FLRW background.
- Long-time existence for small data with positive cosmological constant.
- Applicability to a range of soft cross-sections.

## Abstract

We obtain finite-time existence for the massless Boltzmann equation, with a range of soft cross-sections, in an FLRW background with data given at the initial singularity. In the case of positive cosmological constant we obtain long-time existence in proper-time for small data as a corollary.