Abstract
The cosmological constant is the quantity in physics that is most accurately measured to be zero: observations of departures from the Hubble law by distant galaxies place an upper limit of the order of 10 -120 in dimensionless units. On the other hand, the various symmetry breaking mechanisms that we believe are operating in the Universe would give an effective cosmological constant many orders of magnitude larger, unless they are incredibly finely balanced. One answer would be to appeal to the anthropic principle, but a more attractive possibility is that there is a phase transition N = 8 supergravity to a foam-like state which breaks supersymmetry and which appears flat on scales larger than the Planck length.
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