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# Post-Newtonian expansion

Post-Newtonian expansions in general relativity are used for finding an approximate solution of the Einstein field equations for the metric tensor. The approximations are expanded in small parameters which express orders of deviations from Newton's law of universal gravitation. This allows approximations to Einstein's equations to be made in the case of weak fields. Higher order terms can be added to increase accuracy, but for strong fields sometimes it is preferable to solve the complete equations numerically. This method is a common mark of effective field theories. In the limit, when the small parameters are equal to 0, the post-Newtonian expansion reduces to Newton's law of gravity.

## Expansion in 1/c2

The post-Newtonian approximations are expansions in a small parameter, which is the ratio of the velocity of the matter that creates the gravitational field, to the speed of light, which in this case is more precisely called the speed of gravity.[1] In the limit, when the fundamental speed of gravity becomes infinite, the post-Newtonian expansion reduces to Newton's law of gravity. A systematic study of post-Newtonian approximations was developed by Subrahmanyan Chandrasekhar and co-workers in the 1960s.[2][3][4][5][6]

## Expansion in h

Another approach is to expand the equations of general relativity in a power series in the deviation of the metric from its value in the absence of gravity

${\displaystyle h_{\alpha \beta }=g_{\alpha \beta }-\eta _{\alpha \beta }\,.}$

To this end, one must choose a coordinate system in which the eigenvalues of ${\displaystyle h_{\alpha \beta }\eta ^{\beta \gamma }\,}$ all have absolute values less than 1.

For example, if one goes one step beyond linearized gravity to get the expansion to the second order in h:

${\displaystyle g^{\mu \nu }\approx \eta ^{\mu \nu }-\eta ^{\mu \alpha }h_{\alpha \beta }\eta ^{\beta \nu }+\eta ^{\mu \alpha }h_{\alpha \beta }\eta ^{\beta \gamma }h_{\gamma \delta }\eta ^{\delta \nu }\,.}$
${\displaystyle {\sqrt {-g}}\approx 1+{\tfrac {1}{2}}h_{\alpha \beta }\eta ^{\beta \alpha }+{\tfrac {1}{8}}h_{\alpha \beta }\eta ^{\beta \alpha }h_{\gamma \delta }\eta ^{\delta \gamma }-{\tfrac {1}{4}}h_{\alpha \beta }\eta ^{\beta \gamma }h_{\gamma \delta }\eta ^{\delta \alpha }\,.}$

## Hybrid expansion

Sometimes, as with the parameterized post-Newtonian formalism, a hybrid approach is used in which both the reciprocal of the speed of gravity and masses are assumed to be small.

## Uses

The first use of a PN expansion (to first order) was made by Einstein in calculating the perihelion precession of Mercury's orbit. Today, Einstein's calculation is recognized as a first simple case of the most common use of the PN expansion: Solving the general relativistic two-body problem, which includes the emission of gravitational waves.

## Newtonian gauge

In general, the perturbed metric can be written as[7]

${\displaystyle ds^{2}=a^{2}(\tau )\left[(1+2A)d\tau ^{2}-2B_{i}dx^{i}d\tau -\left(\delta _{ij}+h_{ij}\right)dx^{i}dx^{j}\right]}$

where ${\displaystyle A}$, ${\displaystyle B_{i}}$ and ${\displaystyle h_{ij}}$ are functions of space and time. ${\displaystyle h_{ij}}$ can be decomposed as

${\displaystyle h_{ij}=2C\delta _{ij}+\partial _{i}\partial _{j}E-{\frac {1}{3}}\delta _{ij}\Box ^{2}E+\partial _{i}{\hat {E}}_{j}+\partial _{j}{\hat {E}}_{i}+2{\tilde {E}}_{ij}}$

where ${\displaystyle \Box }$ is the d'Alembert operator, ${\displaystyle E}$ is a scalar, ${\displaystyle {\hat {E}}_{i}}$ is a vector and ${\displaystyle {\tilde {E}}_{ij}}$ is a traceless tensor. Then the Bardeen potentials are defined as

${\displaystyle \Psi \equiv A+H(B-E'),+(B+E')',\quad \Phi \equiv -C-H(B-E')+{\frac {1}{3}}\Box E}$

where ${\displaystyle H}$ is the Hubble constant and a prime represents differentiation with respect to conformal time ${\displaystyle \tau \,}$.

Taking ${\displaystyle B=E=0}$ (i.e. setting ${\displaystyle \Phi \equiv -C}$ and ${\displaystyle \Psi \equiv A}$), the Newtonian gauge is

${\displaystyle ds^{2}=a^{2}(\tau )\left[(1+2\Psi )d\tau ^{2}-(1-2\Phi )\delta _{ij}dx^{i}dx^{j}\right]\,}$.

Note that in the absence of anistropic stress, ${\displaystyle \Phi =\Psi }$.