Note on the gravitational and electromagnetic radiation for the Einstein-Maxwell equations with cosmological constant

TL;DR

This paper revisits Bondi's boundary conditions for Einstein-Maxwell equations, revealing that the original conditions are incompatible with a nonzero cosmological constant, and proposes a new boundary condition to restore physical gravitational wave solutions.

Contribution

It introduces a new Bondi-type boundary condition for Einstein-Maxwell equations with cosmological constant, ensuring consistent gravitational wave solutions.

Findings

Original boundary condition leads to unphysical results with cosmological constant.

New boundary condition restores expected gravitational wave behavior.

Electromagnetic wave boundary conditions are unaffected by the cosmological constant.

Abstract

In the middle of last century, Bondi and his coworkers proposed an out going boundary condition for the Einstein equations. Based on such boundary condition the authors theoretically solved the puzzle of the existence problem of gravitational wave. Currently many works on gravitational wave source modeling use Bondi's result to do the analysis including the gravitational wave calculation in numerical relativity. Recently more and more observations imply that the Einstein equations should be modified with an nonzero cosmological constant. In this work we use Bondi's original method to treat the Einstein equations with cosmological constant for theoretical curiosity. We find that the gravitational wave does not essentially exist if Bondi's original out going boundary condition is imposed. We find this unphysical result is due to the non-consistency between Bondi's original out going boundary condition and the Einstein equations with cosmological constant. Based on theoretical analysis, we propose an new Bondi-type out going boundary condition for the Einstein equations with cosmological constant. With this new boundary condition, the gravitational wave behavior for the Einstein equations with cosmological constant is similar to the Einstein equations without cosmological constant. In particular, when the cosmological constant goes to zero, the behavior of the Einstein equations without cosmological constant is recovered. Together with gravitational wave, electromagnetic wave is also analyzed. The boundary conditions for electromagnetic wave does not depend on the cosmological constant.