Gravitating magnetic monopole via the spontaneous symmetry breaking of pure $R^2$ gravity

TL;DR

This paper explores how spontaneous symmetry breaking in pure $R^2$ gravity with matter fields leads to gravitating magnetic monopole solutions in various cosmological backgrounds, revealing new insights into gravity-matter interactions.

Contribution

It demonstrates that in a restricted Weyl invariant $R^2$ gravity framework, spontaneous symmetry breaking yields Einstein-Yang-Mills-Higgs dynamics with gravitating monopoles in Minkowski, AdS, and dS spaces.

Findings

Minkowski space can be a viable gravitating background after symmetry breaking.

Numerical solutions for magnetic monopoles are obtained in different backgrounds.

The theory links $R^2$ gravity symmetry breaking to Einstein-Yang-Mills-Higgs phenomena.

Abstract

The pure $R^2$ gravity is equivalent to Einstein gravity with cosmological constant and a massless scalar field and it further possesses the so-called restricted Weyl symmetry which is a symmetry larger than scale symmetry. To incorporate matter, we consider a restricted Weyl invariant action composed of pure $R^2$ gravity, SU(2) Yang-Mills fields and a non-minimally coupled massless Higgs field (a triplet of scalars). When the restricted Weyl symmetry is spontaneously broken, it is equivalent to an Einstein-Yang-Mills-Higgs (EYMH) action with a cosmological constant and a massive Higgs non-minimally coupled to gravity i.e. via a term $\tilde{\xi} R |\Phi|^2$. When the restricted Weyl symmetry is not spontaneously broken, linearization about Minkowski space-time does not yield gravitons in the original $R^2$ gravity and hence it does not gravitate. However, we show that in the broken gauge sector of our theory, where the Higgs field acquires a non-zero vacuum expectation value, Minkowski space-time is a viable gravitating background solution. We then obtain numerically gravitating magnetic monopole solutions for non-zero coupling constant $\tilde{\xi}=1/6$ in three different backgrounds: Minkowski, anti-de Sitter (AdS) and de Sitter (dS), all of which are realized in our restricted Weyl invariant theory.