Manifestly Gauge-Invariant General Relativistic Perturbation Theory: II. FRW Background and First Order

TL;DR

This paper develops a gauge-invariant perturbation theory for general relativity on an FRW background, showing it aligns with standard cosmological perturbation theory with negligible late-universe corrections, enabling measurable higher-order analyses.

Contribution

It introduces a manifestly gauge-invariant perturbation framework for GR using dust as a reference, extending previous work and connecting it to standard cosmological perturbation theory.

Findings

Equations match standard cosmological perturbation theory with small corrections.

Corrections decay in the late universe, becoming negligible.

The approach allows for gauge-invariant, measurable quantities at higher perturbation orders.

Abstract

In our companion paper we identified a complete set of manifestly gauge-invariant observables for general relativity. This was possible by coupling the system of gravity and matter to pressureless dust which plays the role of a dynamically coupled observer. The evolution of those observables is governed by a physical Hamiltonian and we derived the corresponding equations of motion. Linear perturbation theory of those equations of motion around a general exact solution in terms of manifestly gauge invariant perturbations was then developed. In this paper we specialise our previous results to an FRW background which is also a solution of our modified equations of motion. We then compare the resulting equations with those derived in standard cosmological perturbation theory (SCPT). We exhibit the precise relation between our manifestly gauge-invariant perturbations and the linearly gauge-invariant variables in SCPT. We find that our equations of motion can be cast into SCPT form plus corrections. These corrections are the trace that the dust leaves on the system in terms of a conserved energy momentum current density. It turns out that these corrections decay, in fact, in the late universe they are negligible whatever the value of the conserved current. We conclude that the addition of dust which serves as a test observer medium, while implying modifications of Einstein's equations without dust, leads to acceptable agreement with known results, while having the advantage that one now talks about manifestly gauge-invariant, that is measurable, quantities, which can be used even in perturbation theory at higher orders.