The early universe was less like a calm pond and more like a boiling cauldron. Matter flowed, collided, and swirled against the backdrop of rapid spatial expansion. To understand how stars and galaxies emerged from this hot soup, physicists build digital simulations.

This new work is the third part in a series on lattice-based cosmological modeling. The first two focused on fields; this one deals with fluids. The authors walk through, step by step, how to translate the laws of fluid flow onto a computational lattice when the stage itself, the expanding universe, is not static. They consider four scenarios: an ideal frictionless fluid, a viscous fluid, a fluid mixed with other fields, and a fluid exchanging energy with scalar fields (fields that have just a single number at each point in space).

The main engineering triumph is algorithms that do not lose accuracy as space expands. They preserve the symmetries of the theory to machine precision, which is critical when calculating, for instance, the generation of gravitational waves. Such waves, ripples in the fabric of spacetime, should have arisen from violent processes in the early universe, and now they can be computed honestly, including all fluid effects.

Thus, the digital cauldron becomes increasingly realistic. Scientists will be able to peer into epochs when the universe was less than a second old and see how the first flows of matter left imprints that reach our detectors today.