When attempting to find the spectrum of the theory after spontaneous symmetry breaking, of a gauge symmetry or otherwise, should we expand the potential in a Taylor series about the VEV or just substitute the shifted fields into the potential?
I am asking this question because I have seen both methods in various textbooks (eg Aitchison/Hey, Quigg).
For example, when attempting to showcase the existence of massless particles in the spectrum when we break a continuous symmetry, most books I have seen will give a potential, eg. $U(\phi) = \mu^2 |\phi|^2 + \lambda (|\phi|^2 )^2$, and expand in a Taylor series about the VEV.
Whereas in derivations for the Higgs mechanism with a Higgs potential $$\lambda \left[H^{\dagger} H - \frac{v^2}{2}\right] $$ In the unitary gauge, are left with a single real scalar field:
$$ H(x) \rightarrow \begin{pmatrix} 0 \\ \bar{h}(x) \end{pmatrix}$$
we shift the Higgs field by its VEV: $$ \bar{h}(x) = h(x) + v $$ and then substitute this directly into the potential. Why do we not simply undertake a Taylor expansion about the VEV for the Higgs field? Please excuse me if I am missing something obvious.
I feel I am mixing up concepts for Goldstone bosons emerging from spontaneous symmetry breaking of a global continuous symmetry and massive gauge bosons arising from the spontaneous symmetry breaking of a gauge theory.