Emergent modified gravity: Polarized Gowdy model on a torus

TL;DR

This paper extends emergent modified gravity theories to polarized Gowdy models on a torus, demonstrating models with local degrees of freedom, stability, and compatibility with gravitational waves, offering new insights into quantum gravity effects.

Contribution

The paper introduces new covariant emergent modified gravity models in polarized Gowdy systems, expanding beyond spherical symmetry and analyzing their stability and physical implications.

Findings

Models are free of higher derivative instabilities.

Some models support gravitational waves and matter fields traveling at the same speed.

Different modifications lead to varied singularity outcomes in Bianchi I dynamics.

Abstract

New covariant theories of emergent modified gravity exist not only in spherically symmetric models, as previously found, but also in polarized Gowdy systems that have a local propagating degree of freedom. Several explicit versions are derived here, depending on various modification functions. These models do not have instabilities from higher time derivatives, and a large subset is compatible with gravitational waves and minimally coupled massless matter fields travelling at the same speed. Interpreted as models of loop quantum gravity, covariant Hamiltonian constraints derived from the covariance conditions found in polarized Gowdy systems are more restricted than those in spherical symmetry, requiring new forms of holonomy modifications with an anisotropy dependence that has not been considered before. Assuming homogeneous space, the models provide access to the full anisotropy parameters of modified Bianchi I dynamics, in which case different fates of the classical singularity are realized depending on the specific class of modifications.