Raymond Laflamme

Baby universes and the non-renormalizability of gravity

Abstract Little closed universes can branch off from our nearly flat region of spacetime. This leads to an extra degree of uncertainty, over and above that normally associate with quantum field theory. One can describe this extra uncertainty as loss of quantum coherence, or, equivalently, as uncertainty about the initial quantum state. The branching of little closed universes leadsto an infinite number of effective interactions. The strengths of these effective interactions cannot be predicted from the theory, but have to be fixed by measuring them. This implies that quantum gravity is effectively non-renormalizable, even if it is based on some finite underlying theory, such as superstrings.

The euclidean vacuum: Justification from quantum cosmology

Abstract We evaluate the wave function of the universe for a de Sitter minisuperspace with inhomogeneous matter perturbations from a massive scalar field. From the Wheeler-DeWitt equation, we derive Schrodinger equations for the matter modes. We show that the matter part of the Hartle-Hawking wave function is the euclidean vacuum state of quantum field theory in curved spacetime.

Entropy of a Rindler wedge

Abstract The entropy of a Rindler wedge is calculated from the action of its complexified section using the method of Gibbons and Hawking. It is equal to one quarter the area of the event horizon in fundamental units.

Conservation laws in the quantum mechanics of closed systems.

We investigate conservation laws in the quantum mechanics of closed systems and begin by reviewing an argument that exact decoherence implies the exact conservation of quantities that commute with the Hamiltonian. However, we also show that decoherence limits the alternatives that can be included in sets of histories that assess the conservation of these quantities. In the case of charge and energy, these limitations would be severe were these quantities not coupled to a gauge field. However, for the realistic cases of electric charge coupled to the electromagnetic field and mass coupled to spacetime curvature, we show that when alternative values of charge and mass decohere they always decohere exactly and are exactly conserved. Further, while decohering histories that describe possible changes in time of the total charge and mass are also subject to the limitations mentioned above, we show that these do not, in fact, restrict {ital physical} alternatives and are therefore not really limitations at all.

Classical Perturbations From Decoherence of Quantum Fluctuations in the Inflationary Universe

The evolution of a scalar field interacting with an environment in the de Sitter phase of an inflationary Universe is studied. The environment is taken to be a second scalar field. It is shown that the coherence length of the quantum fluctuations rapidly decreases after the wavelength of the perturbation crosses the Hubble radius. Hence, the fluctuations can be interpreted as classical. This lends support to the usual derivation of the spectrum of density perturbations in inflationary Universe models.

Classicality of density perturbations in the early universe

Quantum fluctuations in the de Sitter phase of an inflationary Universe may produce the classical primordial density perturbations required to seed galaxies. Implicit in this theory is a prescription for extracting classical quantities from a quantum theory. We briefly review the original prescriptions for obtaining classical density perturbations. We then solve a simple toy model and show that the original prescriptions can be justified by applying decoherence arguments similar to those used in quantum cosmology.