Topological Quintessence

TL;DR

This paper investigates how topological defects like monopoles could cause accelerated cosmic expansion, modeling inhomogeneous dark energy and fitting observational data similarly to the standard b1CDM model.

Contribution

It introduces a detailed numerical simulation of topological quintessence, demonstrating how monopoles can produce acceleration consistent with observations.

Findings

Monopole cores can induce local acceleration matching b1CDM.

Matter density is anti-correlated with dark energy in monopole regions.

Fitting to Union2 data shows comparable quality to b1CDM.

Abstract

A global monopole (or other topological defect) formed during a recent phase transition with core size comparable to the present Hubble scale, could induce the observed accelerating expansion of the universe. In such a model, topological considerations trap the scalar field close to a local maximum of its potential in a cosmologically large region of space. We perform detailed numerical simulations of such an inhomogeneous dark energy system (topological quintessence) minimally coupled to gravity, in a flat background of initially homogeneous matter. We find that when the energy density of the field in the monopole core starts dominating the background density, the spacetime in the core starts to accelerate its expansion in accordance to a \Lambda CDM model with an effective inhomogeneous spherical dark energy density parameter \Omega_\Lambda(r). The matter density profile is found to respond to the global monopole profile via an anti-correlation (matter underdensity in the monopole core). Away from the monopole core, the spacetime is effectively Einstein-deSitter (\Omega_\Lambda(r_{out}) -> 0) while at the center \Omega_\Lambda(r ~ 0) is maximum. We fit the numerically obtained expansion rate at the monopole core to the Union2 data and show that the quality of fit is almost identical to that of \Lambda CDM. Finally, we discuss potential observational signatures of this class of inhomogeneous dark energy models.