Reconstruction Between Generalized Hybrid Metric--Palatini Gravity and $\Phi(R,\phi,X)$ Theories

TL;DR

This paper presents a method to reconstruct equivalent scalar-tensor theories in Einstein frame from generalized hybrid metric-Palatini gravity models, highlighting the non-uniqueness and providing explicit examples.

Contribution

It develops a practical reconstruction framework linking $ ext{Phi}(R, extphi,X)$ theories with hybrid metric-Palatini gravity in vacuum, including explicit examples and analysis of non-uniqueness.

Findings

The reconstruction equation has a Clairaut-type structure.

Inverse reconstruction yields a family of theories parametrized by kinetic coupling.

Explicit examples demonstrate the method and parameter translation.

Abstract

We develop a local reconstruction framework between $\Phi(R,\phi,X)$ theories with linear dependence on $X$ and generalized hybrid metric--Palatini gravity. The construction is formulated in vacuum in the Einstein frame, where both formulations can be written as two-scalar theories with the same field-space geometry. The framework provides a practical method for finding $\Phi(R,\phi,X)$ and $f(R,\mathcal R)$ functions that describe the same regular Einstein-frame two-scalar sector. Starting from a given $\Phi(R,\phi,X)$ model, we derive the equation that determines the compatible hybrid functions $f(R,\mathcal R)$ and show that it has a Clairaut-type structure. We also show that the inverse reconstruction is not unique: a regular hybrid Einstein-frame potential determines a family of compatible $\Phi(R,\phi,X)$ theories, parametrized by the kinetic coupling. Explicit examples illustrate the reconstruction procedure, its domain of validity, and the translation of model parameters between the $\Phi(R,\phi,X)$ and $f(R,\mathcal R)$ formulations.