Can gravitational collapse sustain singularity-free trapped surfaces?

Abstract

In singularity generating spacetimes both the out-going and in-going expansions of null geodesic congruences \(\theta ^{+}\) and \(\theta ^{-}\) should become increasingly negative without bound, inside the horizon. This behavior leads to geodetic incompleteness which in turn predicts the existence of a singularity. In this work we inquire on whether, in gravitational collapse, spacetime can sustain singularity-free trapped surfaces, in the sense that such a spacetime remains geodetically complete. As a test case, we consider a well known solution of the Einstien Field Equations which is Schwarzschild-like at large distances and consists of a fluid with a \(p=-\rho \) equation of state near \(r=0\). By following both the expansion parameters \(\theta ^{+}\) and \(\theta ^{-}\) across the horizon and into the black hole we find that both \(\theta ^{+}\) and \(\theta ^{+}\theta ^{-}\) have turning points inside the trapped region. Further, we find that deep inside the black hole there is a region \(0\leq r<r_{0}\) (that includes the black hole center) which is not trapped. Thus the trapped region is bounded both from outside and inside. The spacetime is geodetically complete, a result which violates a condition for singularity formation. It is inferred that in general if gravitational collapse were to proceed with a \(p=-\rho \) fluid formation, the resulting black hole may be singularity-free.