  The color flux tube connecting a static quark-anti-quark pair in Yang-Mills
theory supports massless transverse fluctuations, which are the Goldstone
bosons of spontaneously broken translation invariance. Just as in chiral
perturbation theory, the dynamics of these Goldstone bosons is described by a
systematic low-energy effective field theory. We use the effective theory to
calculate the width of the fluctuating string at the 2-loop level, using both
cylindrical and toroidal boundary conditions. At zero temperature, the string
width diverges logarithmically with the quark-anti-quark distance r. On the
other hand, at low but non-zero temperature T = 1/\beta, for r >> \beta, the
string width diverges linearly.
