Total mass and limits of quasi-local mass at future null infinity

Understand

An idealized observer of an astronomical event is situated at future null infinity, where light rays emitted from the source approach.

• Mathematically, null infinity corresponds to the portion of the spacetime boundary defined by equivalence classes of null geodesics.
• But what can we observe at future null infinity? In the note, we start by reviewing the description of null infinity in terms of the Bondi-Sachs coordinate system.
• In particular, we reformulate and extend the invariance of Bondi mass and the mass loss formula in terms of a modified mass aspect 2-form.