On a repelling force between particles in a bath of thermal radiation

In an optically thin medium two particles in a bath of thermal radiation repel each other with a force directly proportional to the product of their cross sections and inversely proportional to the square of the distance between them. This force, which is mediated by the scattered photons, was in the past much larger than the gravitational attraction between scattering particles, and could have played a role in the formation of the first gravitationally bound systems.