  In a plasma at temperature close to the fundamental scale a small fraction of
particles will experience transplanckian collisions that may result in
microscopic black holes (BHs). We study the dynamics of a system (a black hole
gas) defined by radiation at a given temperature coupled to a distribution of
BHs of different mass. Our analysis includes the production of BHs in
photon-photon collisions, BH evaporation, the absorption of radiation,
collisions of two BHs to give a larger one, and the effects of the expansion.
We find that the system may follow two different generic paths depending on the
initial temperature of the plasma.
