Figure of the month

In General Relativity, the {\it singularity theorems} predict that a spacetime singularity forms when a sufficiently large mass is concentrated in a small region of an asymptotically flat spacetime, as occurs in the complete gravitational collapse of a star, for instance. An open question is weather or not this singularity in naked (light rays emanating from it can be detected by arbitrarily distant observers) or hidden (no visible light rays) inside a black hole. Professor Olivier Sarbach , researcher of the IFM, and his PhD student, Néstor Ortiz , have analyzed the gravitational collapse of a spherically symmetric star with zero pressure. They have developed a method that generates a conformal diagram inside the collapsing dust cloud from given data for the initial density and velocity distributions. Their method provides a valuable tool for understanding the causal structure of the spacetime. In particular, it enables one to determine in a systematic way whether or not a given initial data set results in a singularity that is hidden inside a black hole. The method is based on a combination of analytic and numerical techniques. By generating the conformal diagrams for different initial data sets, Sarbach and Ortiz found that it is possible to obtain spacetimes with naked singularities which are globally visible without fine-tuning, indicating that these are generic within the class of spherically symmetric dust collapse. The figure shows a complete conformal diagram for a spacetime describing a collapsing, spherically symmetric dust cloud. In this case, there exist light rays emanating from the singularity which arrive at the surface of the cloud earlier than the apparent horizon (AH). Therefore, a portion of the singularity is visible to distant observers outside the black hole region. The results are published in Classical and Quantum gravity, 28 235001, 2011 (IoP Select), see also arXiv:1106.2504 .