In the theory of general relativity, linearized gravity is the application of perturbation theory to the metric tensor that describes the geometry of spacetime. As a consequence, linearized gravity is an effective method for modeling the effects of gravity when the gravitational field is weak. The usage of linearized gravity is integral to the study of gravitational waves and weak-field gravitational lensing.
Weak-field approximation The Einstein field equations (EFE) describing the geometry of spacetime using the MTW sign convention, including the metric signature (−+++), is
R μ ν − 1 2 R g μ ν = κ T μ ν {\displaystyle R_{\mu \nu }-{\frac {1}{2}}Rg_{\mu \nu }=\kappa T_{\mu \nu }}
where R μ ν {\displaystyle R_{\mu \nu }} is the Ricci tensor, R {\displaystyle R} is the Ricci scalar, T μ ν {\displaystyle T_{\mu \nu }} is the energy–momentum tensor, κ {\displaystyle \kappa } is the Einstein gravitational constant, and g μ ν {\displaystyle g_{\mu \nu }} is the spacetime metric tensor that represents the solutions of the equation. Although succinct when written out using Einstein notation, hidden within the Ricci tensor and Ricci scalar are exceptionally nonlinear dependencies on the metric tensor that render the prospect of finding exact solutions impractical in most systems. However, when describing systems for which the curvature of spacetime is small (meaning that terms in the EFE that are quadratic in g μ ν {\displaystyle g_{\mu \nu }} do not significantly contribute to the equations of motion), one can model the solution of the field equations as being the Minkowski metric η μ ν {\displaystyle \eta _{\mu \nu }} plus a small perturbation term h μ ν {\displaystyle h_{\mu \nu }} . In other words:
g μ ν = η μ ν + h μ ν , | h μ ν | ≪ 1. {\displaystyle g_{\mu \nu }=\eta _{\mu \nu }+h_{\mu \nu },\qquad |h_{\mu \nu }|\ll 1.}
In this regime, substituting the general metric g μ ν {\displaystyle g_{\mu \nu }} for this perturbative approximation results in a simplified expression for the Ricci tensor:
R μ ν = 1 2 ( ∂ σ ∂ μ h ν σ + ∂ σ ∂ ν h μ σ − ∂ μ ∂ ν h − ◻ h μ ν ) , {\displaystyle R_{\mu \nu }={\frac {1}{2}}(\partial _{\sigma }\partial _{\mu }h_{\nu }^{\sigma }+\partial _{\sigma }\partial _{\nu }h_{\mu }^{\sigma }-\partial _{\mu }\partial _{\nu }h-\square h_{\mu \nu }),}
where h = η μ ν h μ ν {\displaystyle h=\eta ^{\mu \nu }h_{\mu \nu }} is the trace of the perturbation, ∂ μ {\displaystyle \partial _{\mu }} denotes the partial derivative with respect to the x μ {\displaystyle x^{\mu }} coordinate of spacetime, and ◻ = η μ ν ∂ μ ∂ ν {\displaystyle \square =\eta ^{\mu \nu }\partial _{\mu }\partial _{\nu }} is the d'Alembert operator. Together with the Ricci scalar,
R = η μ ν R μ ν = ∂ μ ∂ ν h μ ν − ◻ h , {\displaystyle R=\eta _{\mu \nu }R^{\mu \nu }=\partial _{\mu }\partial _{\nu }h^{\mu \nu }-\square h,}
the left side of the field equation reduces to
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