Einstein-Yang-Mills Solitons: The Role of Gravity

The canonical Bartnik-McKinnon solitons are regular solutions of the coupled Einstein-Yang-Mills system in which gravity may balance the repulsive nature of the Yang-Mills field. We examine the role played by gravity in balancing the system and determine its strength. In particular, we obtain an analytic lower bound on the fundamental mass-to-radius ratio, max{2m®/r}>2/3, which is a necessary condition for the existence of globally regular Einstein-Yang-Mills solitons. Our analytical results are in accord with numerical calculations.