Parametrizing fluids in canonical quantum gravity
  The problem of time is an unsolved issue of canonical General Relativity. A possible solution is the Brown-Kuchar mechanism which couples matter to the gravitational field and recovers a physical, i.e. non vanishing, observable Hamiltonian functional by manipulating the set of constraints. Two cases are analyzed. A generalized scalar fluid model provides an evolutionary picture, but only in a singular case. The Schutz' model provides an interesting singularity free result: the entropy per baryon enters the definition of the physical Hamiltonian. Moreover in the co-moving frame one is able to identify the time variable tau with the logarithm of entropy. 
Giovanni Montani (1, 2 and 3), Simone Zonetti (1) ((1) Dipartimento di   Fisica, Universit\`a degli Studi di Roma "La Sapien...