We show that hydrodynamic instabilities arise when a radiative flux is present. These instabilities can be extremely important in various astrophysical settings that at first glance appear to be completely unrelated. Radiatively supported atmospheres as well as the early universe during the time of recombination can exhibit unstable acoustic modes that under certain conditions grow on a short enough time scale as to change the gross behavior of the systems. Atmospheric properties such as the effective opacity as well as the Eddington luminosity limit can change, and phenomena such as the formation of small-scale structure can take place in the early universe.
- Radiative transfer