A hydrodynamical description of gravitational waves

TL;DR

This paper demonstrates that near null infinity, a fluid model can accurately represent gravitational waves, satisfying both physical acceptability and relativistic hydrodynamics, supporting the fluid/gravity equivalence concept.

Contribution

It provides a concrete example where the fluid/gravity equivalence holds for gravitational wave spacetimes near null infinity.

Findings

Fluid near null infinity obeys relativistic hydrodynamics.

The fluid has physically acceptable properties.

The model supports the fluid/gravity equivalence hypothesis.

Abstract

It is easy to reason that gravity might be the effect of a fluid in disguise, as it will naturally arise in emergent gravity models where gravity is due to the effect of some fundamental particles, with the latter expected to behave collectively like a fluid at the macroscopic scale. We call this the fluid/gravity equivalence. The key difficulty with the fluid/gravity equivalence is to find the correct metric-fluid relation (the relation between the emergent metric and the fluid properties) so that the fluid not only has physically acceptable properties but also obeys the usual hydrodynamic equations, while at the same time the emergent metric also obeys the Einstein equations. Faced with the problem, we have previously made a tentative proposal of the metric-fluid relation, focusing only on obtaining physically acceptable predictions on the fluid properties. In this paper, however, we find that for the general gravitational wave spacetime near the null infinity, the underlying fluid not only has physically acceptable properties, but also satisfies the expected relativistic hydrodynamic equations in the Minkowski background, thus providing a concrete example satisfying both of the major requirements expected for the fluid/gravity equivalence.