The Equivalence Principle and The Cosmological Constant Problem

In this essay I point out that, in the context of semi-classical gravity, the equivalence principle can mitigate the cosmological constant problem. On a Minkowski space-time background with the usual ${\mathbb{R}}^4$ topology, the vacuum self-energy is removed by normal ordering; this is allowed because it is not observable; I argue that, in a freely-falling frame of reference, the same must hold true, up to contributions from modes whose wavelength is of the order of the background radius of curvature. Thus, the equivalence principle implies that ultra-violet modes do not contribute to the effective energy-momentum tensor.