Novel matter coupling in Einstein gravity

TL;DR

This paper introduces a new framework for matter coupling in Einstein gravity, proving equivalence to GR in vacuum and exploring conditions for consistent matter coupling with potential cosmological implications.

Contribution

It presents a general class of theories with Hamiltonian constraints based on arbitrary functions of GR's Hamiltonian, defining a novel Jordan frame and analyzing matter coupling conditions.

Findings

Theories with Hamiltonian $f(H_g)$ are equivalent to GR in vacuum.

Two types of consistency conditions for matter coupling are identified.

Cosmological implications of matter coupling are discussed.

Abstract

A general framework of the novel matter coupling in the Einstein gravity is introduced. We firstly prove that a class of theories whose Hamiltonian constraint is given by an arbitrary function $f(H_g)$, where $H_g$ is the Hamiltonian constraint of general relativity (GR), is equivalent to GR in the vaccum. A novel Jordan frame is defined when GR is rewritte in terms of one of its equivalents in this class. The transformation between the novel Jordan frame and the Eintein frame is a redifinition of lapse. We discuss two types of consistency condition for matter to couple to Einstein gravity in the novel Jordan frame. The Type I consistency condition is found by demanding all constraints to be first class; the type II consistency condition is found by demanding the algebra is closed when matter minimally couples to gravity in the novel Jordan frame, which an additional gauge condition is required. We discuss the cosmological implications from these two types of matter coupling.