Expansion-Free Cavity Evolution: Some exact Analytical Models
  We consider spherically symmetric distributions of anisotropic fluids with a central vacuum cavity, evolving under the condition of vanishing expansion scalar. Some analytical solutions are found satisfying Darmois junction conditions on both delimiting boundary surfaces, while some others require the presence of thin shells on either (or both) boundary surfaces. The solutions here obtained model the evolution of the vacuum cavity and the surrounding fluid distribution, emerging after a central explosion. This study complements a previously published work where modeling of the evolution of such kind of systems was achieved through a different kinematical condition. 
A. Di Prisco, L. Herrera, J. Ospino, N. O. Santos and V. M.   Vi\~na-Cervantes
gr-qc astro-ph.CO