Seminars

Winter term 2021/22


 

Bouncing Black Holes From Canonical Quantum Gravity

Abstract: It is commonly believed that the ubiquitous singularities of general relativity will be cured in a theory of quantum gravity. One particular scenario for this is bouncing gravitational collapse: in it, quantum gravitational effects prevent the matter from fully collapsing to a singularity, and instead cause it to re-expand. We investigate this scenario by constructing a quantum Oppenheimer-Snyder model, in which both the comoving observer and an observer exterior to the collapsing matter are included. For both observers a bounce emerges. However, for the exterior observer the minimal radius of the bounce is so large that no horizon forms. Further, we investigate what exterior geometries can be matched classically to a bouncing dust cloud. In particular, we show that static exteriors necessarily have a more involved causal structure, and we discuss a specific dynamic exterior in which the horizon retracts into the collapsing body at the moment of the bounce.

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Decoherence in collapsing null shell solutions

Abstract: In one of the earliest solutions to the Einstein equations, the Schwarzschild solution, the gravitational field of a spherically symmetric mass distribution was derived. It predicts the existence of a singularity at its center. Thus, it predicts the breakdown of general relativity, which is among the most successfully confirmed theories. Certain endeavors to resolve the issue of singularities involve considering collapsing models and the implementation of quantum mechanics, thus including approaches to quantum gravity to resolve the issue. A common result across many models has been that the in-falling matter does not form a singularity, but rather expands outward again, possibly even past the formed event horizon, if the horizon forms in the first place. Such a ‘bounce’ would imply that black holes are not the final state of gravitational collapse. It can be understood that the infalling and outgoing matter form a superposition state and the singularity is eradicated by destructive interference. We will investigate the influence of an external scalar field on the collapse of null dust shells, specifically to analyse the influence of decoherence between the components of the superposition of in- and outgoing shell due to interaction with the scalar field. Starting by formulating a Lagrangian, incorporating the collapsing shell and the scalar field, and quantizing it with the Dirac method, we make use of the Born-Oppenheimer approximation scheme to formulate a superposition between the ingoing and outgoing shell. The decoherence parameter cannot be computed directly without describing the states explicitly. Thus, we propose an expansion in moments, regarding the overlap of collapsing and expanding states. Where we consider a time evolution operator to describe the interaction. We are able to show that the Born-Oppenheimer approximation scheme is not capable of resolving situations that involve null shells as considered here. Furthermore, we derive an analytical expression for the amount of decoherence and show that it does not depend on the shell.

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Date Time Speaker Topic Room
Oct 12 12:00 Group discussion 0.03 new building
Oct 19 12:00 Tim Schmitz
(Universität zu Köln; disputation)
Bouncing Black Holes From Canonical Quantum Gravity Zoom (with password)
Oct 26 12:00 Alexander Hermanns
(Universität zu Köln; Master colloquium)
Decoherence in collapsing null shell solutions Zoom (with password)
Dec 21 12:00 Sebastian Schuster
(Charles University Prague)
0.03 new building

 


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