The Stress-Energy Operator

TL;DR

This paper analyzes the properties of the stress-energy operator for a scalar quantum field in curved spacetime, revealing fundamental issues with Hamiltonian self-adjointness, unitary implementability, and unbounded below expectation values in Hadamard states.

Contribution

It demonstrates that, in curved spacetime, the Hamiltonian operators for scalar fields generally lack self-adjointness and unitarity, and their expectation values are unbounded below, highlighting intrinsic local quantum effects.

Findings

Hamiltonians cannot be self-adjoint in general.

Field algebra automorphisms cannot be unitarily implemented.

Expectation values of Hamiltonians are unbounded below.

Abstract

We compute the stress--energy operator for a scalar linear quantum field in curved space-time, modulo c-numbers. For the associated Hamiltonian operators, even those generating evolution along timelike vector fields, we find that in general on locally Fock-like (`Hadamard') representations: (a) The Hamiltonians cannot be self-adjoint operators; (b) The automorphisms of the field algebra generated by the evolution cannot be unitarily implemented; (c) The expectation values of the Hamiltonians are well-defined on a dense family of states; but (d) These expectation values are unbounded below, even for evolution along future-directed timelike vector fields and even on Hadamard states. These are all local, ultraviolet, effects.