An inverse approach to Einstein's equations for non-conducting fluids

TL;DR

This paper introduces an inverse method to determine fluid flows in warped spacetimes from the metric, enabling direct phenomenological interpretation of Einstein's equations for anisotropic and viscous fluids without coordinate transformations.

Contribution

It presents an algorithmic approach to derive fluid flows from spacetime metrics and interprets Einstein's equations phenomenologically, applicable to anisotropic and viscous fluids.

Findings

Flow uniquely determined by zero flux condition in certain warped spacetimes

Inverse approach allows direct fluid interpretation from the metric

Implementation as a computer algebra program GRSource

Abstract

We show that a flow (timelike congruence) in any type $B_{1}$ warped product spacetime is uniquely and algorithmically determined by the condition of zero flux. (Though restricted, these spaces include many cases of interest.) The flow is written out explicitly for canonical representations of the spacetimes. With the flow determined, we explore an inverse approach to Einstein's equations where a phenomenological fluid interpretation of a spacetime follows directly from the metric irrespective of the choice of coordinates. This approach is pursued for fluids with anisotropic pressure and shear viscosity. In certain degenerate cases this interpretation is shown to be generically not unique. The framework developed allows the study of exact solutions in any frame without transformations. We provide a number of examples, in various coordinates, including spacetimes with and without unique interpretations. The results and algorithmic procedure developed are implemented as a computer algebra program called GRSource.