Dirac observables and boundary proposals in quantum cosmology
  We study the reduced phase space quantization of a closed Friedmann Universe, where matter content is constituted by two (no-interacting) fluids, namely dust (or cold dark matter) and radiation. It is shown that, for this particular model, specific boundary conditions can be related to the algebra of Dirac observables. 
S. Jalalzadeh and P. V. Moniz
gr-qc