We consider a ($1+1$) dimensional scalar field theory that supports oscillons, which are localized, oscillatory, stable solutions to nonlinear equations of motion. We study this theory in an expanding background and show that oscillons now lose energy, but at a rate that is exponentially small when the expansion rate is slow. We also show numerically that a universe that starts with (almost) thermal initial conditions will cool to a final state where a significant fraction of the energy of the universe—on the order of 50%—is stored in oscillons. If this phenomenon persists in realistic models, oscillons may have cosmological consequences.

• Received 8 January 2008

DOI:https://doi.org/10.1103/PhysRevD.77.085019