# What are Black Holes?

#### What are Black Holes?

- Event time: 2:00pm
- Event date: 28th October 2020
- Speaker: Prof Surjeet Rajendran (John Hopkins University)
- Location: Online - link below

### Event details

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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https://ed-ac-uk.zoom.us/j/89108840817

Meeting ID: 891 0884 0817

Passcode: 8rJhS^gh

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https://ed-ac-uk.zoom.us/skype/89108840817

#### Event resources

### About Particle Physics Theory seminars

The Particle Physics Theory seminar is a weekly series of talks reflecting the diverse interests of the group. Topics include analytic and numerical calculations based on the Standard Model of elementary particle physics, theories exploring new physics, as well as more formal developments in gauge theories and gravity..