Applications of the Hawking Energy in Inhomogeneous Cosmology

TL;DR

This paper explores the properties and applications of the Hawking energy in inhomogeneous cosmology, deriving bounds on cosmic parameters and analyzing energy behavior along lightcones.

Contribution

It provides explicit cosmological applications of the Hawking energy, including bounds on cosmic density and new insights into energy monotonicity along lightcones.

Findings

Shear-free matter maximizes Hawking energy at high densities.

Constructed a Robertson-Walker reference slice for each lightcone.

Derived bounds on cosmic fluid density and equation-of-state parameter.

Abstract

The past lightcone of an observer in a cosmological spacetime is the unique geometric structure directly linked to observations. After general properties of the Hawking energy along slices of the past lightcone have previously been studied, the present work continues along this path by providing explicit cosmological applications of the Hawking energy associated with a lightcone. Firstly, it is shown that amongst all two-dimensional non-trapped spheres with equal area and average matter density, a shear-free matter distribution maximizes the Hawking energy for sufficiently high densities. Secondly, a Robertson-Walker reference slice is constructed for every lightcone slice based on area and energy. Thirdly, after a few pedagogical examples in concrete FLRW spacetimes, the implications of monotonicity of the energy down the lightcone are explored, arriving at two new bounds on the cosmic fluid's density and equation-of-state parameter.