I am trying to figure out what happens in the following situation: You have a fluid that consists of many types of particles that can turn into each other via chemical or nuclear reactions such that if the fluid it left to settle, it will reach a secular equilibrium. If I perturb the system, is there any limit in which entropy production of iso-curvature perturbations is negligible?
The reason I ask is that I was reading lecture notes on cosmology and that claimed it was legitimate to treat cosmological iso-curvature as essentially entropy preserving, but I am having trouble imagining how this can be.
