On Unitary Evolution of a Massless Scalar Field In A Schwarzschild   Background: Hawking Radiation and the Information Paradox
  We develop a Hamiltonian formalism which can be used to discuss the physics of a massless scalar field in a gravitational background of a Schwarzschild black hole. Using this formalism we show that the time evolution of the system is unitary and yet all known results such as the existence of Hawking radiation can be readily understood. We then point out that the Hamiltonian formalism leads to interesting observations about black hole entropy and the information paradox. 
Kirill Melnikov and Marvin Weinstein