Nonsingular FRW cosmology and nonlinear electrodynamics
  The possibility to avoid the cosmic initial singularity as a consequence of nonlinear effects on the Maxwell eletromagnetic theory is discussed. For a flat FRW geometry we derive the general nonsingular solution supported by a magnetic field plus a cosmic fluid and a nonvanishing vacuum energy density. The nonsingular behavior of solutions with a time-dependent $\Lambda(t)$-term are also examined. As a general result, it is found that the functional dependence of $\Lambda(t)$ can uniquely be determined only if the magnetic field remains constant. All these models are examples of bouncing universes which may exhibit an inflationary dynamics driven by the nonlinear corrections of the magnetic field. 
C. S. Camara (UFRN), M. R. de Garcia Maia (UFRN), J. C. Carvalho   (UFRN), J. A. S. Lima (IAG-Usp, Ufrn)