What are Black Holes?

General Relativity conclusively establishes that when a sufficiently large amount of matter is confined within a fixed volume, 
this matter will undergo gravitational collapse and form a singularity i.e. a point where General Relativity is broken. 
What happens to this collapsed object? It is conventionally assumed that the Schwarzschild solution that is derived from General 
Relativity describes this space-time, even though General Relativity cannot be used to describe the evolution of this collapsed 
object once a singularity has formed. This assumption should be questioned since the Schwarzschild geometry leads to the black 
hole information problem. Are there other possible final states? In this talk, I will present spherically symmetric solutions 
to Einstein’s equations which are equivalent to canonical Schwarzschild black holes on the exterior but with singular 
(i.e. Planck density) shells just outside their horizon, leading to a macroscopic singularity. These solutions are inspired by 
the macroscopic singularities that need to exist at the inner horizons of Reissner-Nordstrom and Kerr black holes to solve the 
Cauchy horizon problems of those geometries. While we can show the existence of such solutions and analyze their stability, 
we cannot rigorously address their formation - instead, we suggest plausibility arguments for how “normal” black hole states may 
evolve into these solutions. We argue that the singular nature of these solutions is critical for their formation from generic 
initial conditions, distinguishing this class of solutions from black hole mimickers such as gravastars and fuzzballs. 
We also comment on possible experimental tests in gravitational wave and electromagnetic observatories. 

Meeting Link:

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