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Exact Solutions and Scalar Fields in Gravity - Instituto Avanzado de ...

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Self–gravitat<strong>in</strong>g stationary axisymmetric perfect fluids 9<br />

subspace, a 1-form which reduces <strong>in</strong> vacuum to ( be<strong>in</strong>g<br />

the Weyl canonical coord<strong>in</strong>ate), <strong>and</strong> the <strong>de</strong>f<strong>in</strong>ition<br />

(exten<strong>de</strong>d by l<strong>in</strong>earity to any 1-form), the full set of equations to be<br />

consi<strong>de</strong>red is the follow<strong>in</strong>g:<br />

This equation corresponds to (2.62) of reference [9]. Please note that<br />

there is a typographical error <strong>in</strong> the last sign of that equation.<br />

Equations (19)-(22) are the first structural equations of Cartan, (23)-<br />

(26) their <strong>in</strong>tegrability conditions (first Bianchi i<strong>de</strong>ntities), (27)-(32) the<br />

field equations (<strong>in</strong> particular, (28) is the Raychaudhuri equation), <strong>and</strong><br />

(33) the contracted second Bianchi i<strong>de</strong>ntity (Euler equation for the fluid).<br />

While <strong>and</strong> are fixed, there is a rema<strong>in</strong><strong>in</strong>g gauge freedom of spatial<br />

rotations <strong>in</strong> the subspace. Un<strong>de</strong>r such rotations, however,<br />

<strong>and</strong> rema<strong>in</strong> <strong>in</strong>variant. The operation is not <strong>in</strong>tr<strong>in</strong>sically <strong>de</strong>f<strong>in</strong>ed,<br />

<strong>and</strong> gauge rotations change it, but the equations rema<strong>in</strong> <strong>in</strong>variant <strong>in</strong><br />

form as a set. Fluids with a barotropic equation of state are obviously<br />

characterized by the condition by us<strong>in</strong>g (23) <strong>and</strong> the exterior<br />

<strong>de</strong>rivative of (33), it is easy to see that (disregard<strong>in</strong>g the <strong>de</strong>generate case<br />

) is closed, <strong>and</strong> <strong>and</strong> are coll<strong>in</strong>ear:

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