The Setup
Let Greek indices be summed over 0,1,…,d and Latin indices over 1,2,…,d. Consider a vector potential Aμ on Rd,1 defined to gauge transform as Aμ→A′μ=Aμ+∂μθ for some real-valued function θ on Rd,1. The usual claim about Coulomb gauge fixing is that the condition ∂iAi=0 serves to fix the gauge in the sense that ∂iA′i=0 only if θ=0. The usual argument for this (as far as I am aware) is that ∂iA′i=∂iAi+∂i∂iθ, so the Coulomb gauge conditions on Aμ and A′μ give ∂i∂iθ=0, but the only sufficiently smooth, normalizable (Lesbegue-integrable?) solution to this (Laplace's) equation on Rd is θ(t,→x)=0 for all →x∈Rd.
My Question
What, if any, is the physical justification for the smoothness and normalizability constraints on the gauge function θ?
EDIT 01/26/2013 Motivated by some of the comments, I'd like to add the following question: are there physically interesting examples in which the gauge function θ fails to be smooth and/or normalizable? References with more details would be appreciated. Lubos mentioned that perhaps monopoles or solitons could be involved in such cases; I'd like to know more!
Cheers!
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