Friday, 28 March 2008
The Cosmological Constant Problem
Christopher Smeenk, ‘03, University of Western Ontario
3:30 pm, 817R Cathedral of Learning

Abstract: There is widespread agreement within the physics community that the cosmological constant problem is a crisis in theoretical physics: the predicted value of the cosmological constant disagrees with observational constraints by 120 orders of magnitude! There has been a proliferation of attempted solutions to the problem within the physics literature, and my purpose is not to survey these approaches or propose a new approach. My aim is instead to reconsider the nature of the problem. The disastrous prediction follows from treating the vacuum energy density of quantum fields as a source of the gravitational field. Two aspects of this inference are troubling. First, the connection between vacuum energy density and the successful predictions of quantum field theory is tenuous. Physicists typically invoke the Casimir effect as evidence that the zero-point energies of quantum fields contribute to the vacuum energy density. This invocation overlooks the availability of a derivation of the Casimir effect that does not depend upon zero point energies. Second, there is a further claim that the vacuum energy density enters into Einstein’s field equations for general relativity as an effective cosmological constant term. But it is not clear that this claim is even well-formulated in spacetimes that lack the structure needed to single out a unique vacuum state. I will further argue that the cosmological problem is an instance of a general methodological problem that arises in attempts to combine theories: the need to identify surplus structure that can be eliminated in the formulation of a new theory.