Wednesday, September 18, 2019

general relativity - Breaking of diffeomorphism invariance after fixing a background metric


The Lagrangian for the gravitational field in absence of matter is the following L=1/kdx4gR,

where k=G, g is the determinant of the metric and R the Ricci scalar. It's possible to fix a background metric like ηuv and then study the perturbations huv around it by guv=ηuv+khuv
The Lagrangian becomes L=L0+kL1+k2L2+.......
which can be interpreted as an effective field theory of self-interacting particles called gravitons. Now, given the transformation law of huv, how is it possible to say the entire Lagrangian is invariant, order by order, under local diffeomorphisms? Of course the symmetry is still there, but I was wondering if there is some kind of Spontaneus Symmetry Breaking associated with the perturbation field huv and the diffeomorphisms group. The procedure resemble the SSB for the Higgs Boson, where the Lagrangian is L=uϕuϕm2ϕ2+λϕ4
This Lagrangian is invariant under parity in ϕ, but after the redefinition around the vacuum v, the minimum of the potential, you deal with ϕ=v+δϕ and the Lagrangian in δϕ is no more parity invariant. Does this happen in the previous example after fixing a background?




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