  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.
