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Linearized gravity is an approximation scheme in general relativity in which the nonlinear contributions from the spacetimemetric are ignored, simplifying the study of many problems while still producing useful approximate results.
The Einstein field equations (EFE), being nonlinear in the metric, are difficult to solve exactly and the above perturbation scheme allows linearised Einstein field equations to be obtained. These equations are linear in the metric, and the sum of two solutions of the linearized EFE is also a solution. The idea of 'ignoring the nonlinear part' is thus encapsulated in this linearization procedure.
In a weak-field approximation, the gauge symmetry is associated with diffeomorphisms with small "displacements" (diffeomorphisms with large displacements obviously violate the weak field approximation), which has the exact form (for infinitesimal transformations)
Where is the Lie derivative and we used the fact that η does not transform (by definition). Note that we are raising and lowering the indices with respect to η and not g and taking the covariant derivatives (Levi-Civita connection) with respect to η. This is the standard practice in linearized gravity. The way of thinking in linearized gravity is this: the background metric η is the metric and h is a field propagating over the spacetime with this metric.
In the weak field limit, this gauge transformation simplifies to
The equations are obtained by assuming the spacetime metric is only slightly different from some baseline metric (usually a Minkowski metric). Then the difference in the metrics can be considered as a field on the baseline metric, whose behaviour is approximated by a set of linear equations.
Starting with the metric for a spacetime in the form
where is the Minkowski metric and — sometimes written as — is the deviation of from it. must be negligible compared to : (and similarly for all derivatives of ). Then one ignores all products of (or its derivatives) with or its derivatives (equivalent to ignoring all terms of higher order than 1 in ). It is further assumed in this approximation scheme that all indices of h and its derivatives are raised and lowered with .
The metric h is clearly symmetric, since g and η are. The consistency condition shows that
then the last form above of the linearized Einstein equation simplifies to
To solve it, this can be rewritten as
where ∆ is the Laplacian on a spatial slice. If the stress-energy changes slowly (velocities are low compared to c), then this gives
as a generalization of the Newtonian formula for gravitational potential. This is solved iteratively by first replacing the second time derivative by zero and then inserting the h so obtained repeatedly until convergence.