Abstract. The flat Friedman–Robertson–Walker (FRW) model coupled to the massless scalar field according to the improved, background scale-independent version of Ashtekar, Pawłowski and Singh [1] is considered. The core of the theory is addressed directly: the APS construction of the quantum Hamiltonian is analyzed under the assumption that the cosmological constant Λ ≤ 0. We prove the essential self-adjointness of the operator whose square-root defines in [1] the quantum Hamiltonian operator and therefore provide the explicit definition. If Λ < 0, then the spectrum is discrete. In the Λ = 0 case, the essential and absolutely continuous spectra of the operator are derived. The latter operator is related in the unitary way to the absolutely continuous part of the quantum mechanics operator (a, b > 0 being some constants) plus a trace class operator.

PACS numbers: 98.80.Hw, 04.90.+e

Print publication: Issue 3 (7 February 2008)
Received 26 August 2007, in final form 5 December 2007
Published 14 January 2008