I am interested in looking at the dynamics of a hypothetical static (ie space not expanding or contracting) and closed "universe" filled only with radiation (only photons, no matter). (similar to Penrose's "Hawking's Box" - a closed universe in a box with perfectly reflecting walls so that no energy or fields escape the box). Would such a universe exhibit gravitational clumping (ie would the photons attract each other gravitationally)? If yes, what are the equations governing the dynamics, and where can I get more information on this? I would expect the gravitational clumping would depend on the photon energy density (eV/m3) and the mean photon frequency (energy per photon, eV). For sufficiently low energy density, or sufficiently high energy per photon, I would expect the "universe" to remain homogeneous and not exhibit gravitational clumping - is this correct? Is such a universe stable, or would it evolve over time to a more stable configuration? If there is gravitational clumping, could Black Holes be formed, and if yes how would the mean Black Hole density be related to the background photon energy density and energy per photon at equilibrium? Thanks for your help!