Long-time behaviour of sphalerons in $φ^4$ models with a false vacuum

Abstract: Sphalerons in nonlinear Klein-Gordon models are unstable lump-like solutions that arise from a saddle point between true and false vacua in the energy functional. Numerical simulations are presented which show the sphaleron evolving into an accelerating kink-antikink pair whose separation approaches the speed of light asymptotically at large times. Utilizing a nonlinear collective coordinate method, an approximate analytical solution is derived for this evolution. These results indicate that an exact solution is expected to exhibit a gradient blow-up for large times,caused by energy concentrating at the flanks of the kink-antikink pair.