A general formalism for coupling scalar fields to the Einstein equations without a variational principle

TL;DR

This paper introduces a new formalism for coupling scalar fields to Einstein's equations without using a variational principle, enabling more flexible models and analysis of cosmological solutions.

Contribution

It proposes a generic coupling method for scalar fields to gravity that does not rely on a Lagrangian, broadening the scope of scalar field models in General Relativity.

Findings

Reproduces minimally and $k$-essence scalar couplings with a potential

Allows free fields as constitutive freedoms in the formalism

Analyzes stability and asymptotic behavior of Bianchi I solutions near singularities

Abstract

The purpose of this work is to discuss how matter fields are coupled to gravity within the framework of General Relativity. Our particular focus here is on the coupling of scalar field models. In a first step, we suggest a new method for coupling scalar fields to the Einstein equations \emph{without} the use of a variational principle or Lagrangian. We show that, under the appropriate assumptions, this new method (for coupling scalar fields to gravity) reproduces the minimally and $k$-essence scalar field couplings with a non-zero potential. We therefore interpret this formalism as describing a \emph{generic} method for coupling scalar fields to gravity. The approach described here allows for a number of free fields which we interpret as constitutive freedoms. In a second step, we choose these free fields in such a way that the resulting system is somehow ``near minimal''. In this setting we investigate Bianchi I type solutions. We establish conditions under which the solutions are asymptotically Kasner, near the initial singularity, and investigate their stability properties.