It is possible that there may be differences in the fundamental physical parameters from one side of the observed universe to the other. I show that the cosmological constant is likely to be the most sensitive of the physical parameters to possible spatial (or temporal) variation, because a small variation in any of the other parameters produces a huge variation of the cosmological constant. It therefore provides a very powerful indirect evidence against spatial gradients or temporal variation in the other fundamental physical parameters, at least 40 orders of magnitude more powerful than direct experimental constraints. Moreover, a gradient may potentially appear in theories where the variability of the cosmological constant is connected to an anthropic selection mechanism, invoked to explain the smallness of this parameter. Some suggestions are made about how one may observe such a spatial gradient. In the Hubble damping mechanism, I calculate the possible gradient. While this mechanism demonstrates the existence of this effect, it is too small to be seen experimentally, except possibly if inflation happens around the Planck scale.
JOURNAL OF HIGH ENERGY PHYSICS