Parameterized Post-Newtonian Cosmology

TL;DR

This paper extends the parameterized post-Newtonian (PPN) formalism to cosmology, creating a unified framework with four time-dependent functions to test gravity theories across scales from solar system to the universe.

Contribution

It introduces a novel PPNC framework that links weak-field and cosmological observations using four functions of time, enabling comprehensive tests of gravity theories.

Findings

Derived equations linking PPN parameters to cosmological models

Unified description of gravity theories across scales

Framework compatible with various dark energy and modified gravity models

Abstract

Einstein's theory of gravity has been extensively tested on solar system scales, and for isolated astrophysical systems, using the perturbative framework known as the parameterized post-Newtonian (PPN) formalism. This framework is designed for use in the weak-field and slow-motion limit of gravity, and can be used to constrain a large class of metric theories of gravity with data collected from the aforementioned systems. Given the potential of future surveys to probe cosmological scales to high precision, it is a topic of much contemporary interest to construct a similar framework to link Einstein's theory of gravity and its alternatives to observations on cosmological scales. Our approach to this problem is to adapt and extend the existing PPN formalism for use in cosmology. We derive a set of equations that use the same parameters to consistently model both weak fields and cosmology. This allows us to parameterize a large class of modified theories of gravity and dark energy models on cosmological scales, using just four functions of time. These four functions can be directly linked to the background expansion of the universe, first-order cosmological perturbations, and the weak-field limit of the theory. They also reduce to the standard PPN parameters on solar system scales. We illustrate how dark energy models and scalar-tensor and vector-tensor theories of gravity fit into this framework, which we refer to as "parameterized post-Newtonian cosmology" (PPNC).