The Cauchy problem of scalar-tensor theories of gravity

TL;DR

This paper develops a well-posed 3+1 formulation of scalar-tensor theories of gravity in the Jordan frame, enabling future hyperbolicity analysis and applications to astrophysical and cosmological spacetimes.

Contribution

It provides the first first-order in time and space formulation of scalar-tensor theories, challenging previous beliefs and setting the stage for hyperbolicity and well-posedness studies.

Findings

The Cauchy problem for scalar-tensor theories is well formulated.

A generalized harmonic gauge ensures well-posedness.

Applications to astrophysical and cosmological spacetimes are discussed.

Abstract

The 3+1 formulation of scalar-tensor theories of gravity (STT) is obtained in the physical (Jordan) frame departing from the 4+0 covariant field equations. Contrary to the common belief (folklore), the new system of ADM-like equations shows that the Cauchy problem of STT is well formulated (in the sense that the whole system of evolution equations is of first order in the time-derivative). This is the first step towards a full first order (in time and space) formulation from which a subsequent hyperbolicity analysis (a well-posedness determination) can be performed. Several gauge (lapse and shift) conditions are considered and implemented for STT. In particular, a generalization of the harmonic gauge for STT allows us to prove the well posedness of the STT using a second order analysis which is very similar to the one used in general relativity. Some spacetimes of astrophysical and cosmological interest are considered as specific applications. Several appendices complement the ideas of the main part of the paper.