Theoretical study of gravitational collapse of topological defects and of their stability under the higher dimensional gravity

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It is a purpose of this research project to get knowledge about the gravitational collapse to form black strings as a peculiar phenomenon under the higher dimensional gravity. And further, using the knowledge obtained, I would study the gravitational collapse of cosmic strings and domain walls whose have infinite extent of energy distribution.Together with my collaborators, I investigated 4-dimensional spacelike hypersurfaces on which a mass distributes homogeneously within an ellipsoidal region. We search for apparent horizons on these spacelike hypersurfaces and confirm the so-called hyperhoop conjecture about the necessary and sufficient condition for the formations of black objects in higher dimensional spacetime.Together with my collaborators, I proposed a scenario of the black string formation by the collisions and mergers of type I cosmic strings and found that even if the tension of the type I cosmic string is almost equal to the energy scale of electroweek phase transition, the black strings will form if such type I cosmic strings exist in out universe.Since a domain wall has a 2-dimantional spatial extent, its spatial codimension is two in the 5-dimensional brane world scenario. Thus a domain wall in the 5-dimensional brane world scenario will behave as a cosmic string in 4-dimensional spacetime. In general it is very difficult to treat a spacetime singularity with spatial codimension 2. Thus I also studied the gravitational collapse with cylindrical symmetry, which form a spacetime singularity with codimension 2. Further, I studied the inflation which occurs within a topological defect, and the cosmological gravitational lensing effects which might observationally reveal the existence of black objects in our universe.