Visibility Of Singularity Research Articles

Visibility of a spacetime singularity

We investigate here the causal structure of spacetime in the vicinity of a spacetime singularity. The particle and energy emission from such ultradense regions forming in gravitational collapse of a massive matter cloud is governed by the nature of nonspacelike paths near the same. These trajectories are examined to show that if a null geodesic comes out from the singularity, then there exist families of future-directed nonspacelike curves which also necessarily escape from the same. The existence of such families is crucial to the physical visibility of the singularity. We do not assume any underlying symmetries for the spacetime, and earlier considerations on the nature of causal trajectories emerging from a naked singularity are generalized and clarified.

Gravitational collapse in asymptotically anti–de Sitter or de Sitter backgrounds

We study here the gravitational collapse of a matter cloud with a nonvanishing tangential pressure in the presence of a nonzero cosmological term $\ensuremath{\Lambda}$. It is investigated how $\ensuremath{\Lambda}$ modifies the dynamics of the collapsing cloud and whether it affects the cosmic censorship. Conditions for bounce and singularity formation are derived. It is seen that when the tangential pressure vanishes, the bounce and singularity conditions reduce to the dust case studied earlier. The collapsing interior is matched to an exterior which is asymptotically de Sitter or anti--de Sitter, depending on the sign of the cosmological constant. The junction conditions for matching the cloud to the exterior are specified. The effect of $\ensuremath{\Lambda}$ on apparent horizons is studied in some detail and the nature of central singularity is analyzed. The visibility of singularity and implications for the cosmic censorship conjecture are discussed. It is shown that for a nonvanishing cosmological constant, both black hole and naked singularities do form as collapse end states in spacetimes which are asymptotically de Sitter or anti--de Sitter.