Emergence of approximate translation invariance in finite intervals as a speed selection mechanism for propagating fronts

We introduce a velocity selection criterion for fronts propagating into unstable and metastable states. We restrict these fronts to large finite intervals in the comoving frame of reference and require that their centers be insensitive to the locations of the ends of the finite intervals, thus exhibiting effectively an approximate translation invariance. Only one monotonic front has this behavior, and its velocity is the one that is physically selected. We present analytic results in the case of piecewise parabolic potentials and numerical results in other cases.