Dirac-Bergmann analysis and Degrees of Freedom of Coincident $f(Q)$-gravity

TL;DR

This paper performs a detailed Dirac-Bergmann analysis of coincident $f(Q)$-gravity, revealing it has six propagating degrees of freedom and no gauge freedom, due to the violation of diffeomorphism invariance.

Contribution

It provides the first comprehensive Hamiltonian analysis of $f(Q)$-gravity, clarifying its degrees of freedom and constraint structure in the coincident gauge.

Findings

$f(Q)$-gravity has six propagating degrees of freedom.

All constraints are second-class, indicating no gauge freedom.

Violation of diffeomorphism invariance allows multiple sectors.

Abstract

We investigate the propagating degrees of freedom of $f(Q)$-gravity in a $4$-dimensional space-time under the imposition of the coincident gauge by performing the Dirac-Bergmann analysis. In this work, we start with a top-down reconstruction of the metric-affine gauge theory of gravity based only on the concept of a vector bundle. Then, the so-called geometrical trinity of gravity is introduced and the role of the coincident GR is clarified. After that, we reveal relationships between the boundary terms in the variational principle and the symplectic structure of the theory in order to confirm the validity of the analysis for our studied theories. Then, as examples, we revisit the analysis of GR and its $f(\lc{R})$-extensions. Finally, after reviewing the Dirac-Bergmann analysis of the coincident GR and that of $f(T)$-gravity, we perform the analysis of coincident $f(Q)$-gravity. Under the imposition of appropriate spatial boundary conditions, we find that, as a generic case, the theory has five primary, three secondary, and two tertiary constraint densities and all these constraint densities are classified into second-class constraint density; the number six is the propagating degrees of freedom of the theory and there are no longer any remaining gauge degrees of freedom. We also discuss the condition of providing seven pDoF as a generic case. The violation of diffeomorphism invariance of coincident $f(Q)$-gravity make it possible to emerge such several sectors.