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The existence of cosmological compact objects with very strong gravity is aprediction of General Relativity and an exact solution of the Einsteinequations. These objects are called black holes and recently we had the firstobservations of them. However, the theory of black hole formation has somedisadvantages. In order to avoid these, some scientists suggest the existenceof gravastars (gravitation vacuum stars), an alternative stellar model whichseems to solve the problems of the black hole theory. In this work we compareblack holes and gravastars using a wide range of the literature and weemphasize the properties of gravastars, which are consistent with the currentcosmological observations. Also, we propose gravastars as the solution of the"pair-instability" effect and a possible explanation for the observed masses ofthe compact objects, before the collapse, from the gravitational signalGW190521, since in the formation of a gravastar there aren't mass restrictions.

Black hole or Gravastar? The GW190521 case