## 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.### 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.### Static, spherically symmetric wormholes in EvGB theory

Abstract: Approaches to generalize GR feature additional scalar fields, vector fields and even tensor fields in four and more dimensions as well as higher order curvature terms whose inclusion is inspired by the low energy limit of heterotic string theory. These terms are candidates to complete Einstein’s general relativity and may modify its predictions in the strong gravity regime. In specific, a well studied generalized theory in four dimensions is the Einstein-scalar-Gauss-Bonnet theory, in whose action the Ricci scalar is supplemented by a scalar field and the quadratic Gauss-Bonnet curvature invariant, non-minimally coupled to the scalar. Analogously we extend GR by including a real, massless vector field to form the Einstein-vector-Gauss-Bonnet (EvGB) theory. Here we numerically solve the field equations in the static, spherically symmetric case and investigate the two-dimensional phase space of solutions. Next to black holes, we find solutions whose geometry features a "throat". We construct symmetric wormholes to circumvent a singular behaviour beyond the throat and restore geodesic completeness. Also, we find asymmetric wormholes with a pseudo-flat second asymptotic region.### Matter coupled to two-dimensional Causal Dynamical Triangulations

Abstract: Causal Dynamical Triangulations (CDT) is a viable candidate to be a nonperturbative theory of quantum gravity. Although we hope our universe can be described by CDT in four dimensions, the two-dimensional case serves as an important playground to study some of the model's properties and behavior. In this work, we couple matter in the form of Ising spins to 2d CDT, motivated by the fact our own universe is not 'empty' either. We study the model using lattice Monte Carlo methods. Although a similar model was investigated in the 1990s, we expand upon previously known results by investigating curvature properties of the system. For this we apply a newly developed prescription for determining curvature in nonclassical geometries, called the quantum Ricci curvature. We find the matter-coupled system to exhibit curvature properties hinting at an average positive curvature, as opposed to the 'quantum flat' behavior of the pure gravity case.### The Weird and the Not-So-Weird Reverse-Engineered Space-Times

Abstract: Reverse-engineered space-times are those solutions of Einstein's equations found by solving for the matter source when given a metric instead of solving for the metric when given a source. Technically, this is the difference between integration and differentiation. The harder, and physically more relevant question in this case is then to decide whether a given metric as input is physically plausible. While this approach goes back at least to Gödel's investigations of how general relativity does not fully satisfy Mach's principle, modern examples usually go through staples of science fiction: Wormholes, warp drives, tractor beams. For a long time, various energy conditions have been used for claims of their physical limitations. In this talk, we will discuss why most of these space-times should be called into question for different reasons, why energy conditions are trickier than usually assumed, and how this all might lead to a better understanding of what general relativity may or may not allow.### Introduction to BMS Symmetry and Black Hole Information Paradox

Abstract: Recent work on BMS Symmetry suggests a possible resolution to the black hole information paradox by challenging the assumption of no hair theorem and a unique vacuum. This seminar serves as a pedagogical introduction/crash course on the extended BMS symmetry, its relation to conformal field theory and the black hole information paradox. We first introduce the ideas needed to understand BMS symmetry by reviewing the relation between observers who differ by a Poincaré transformation and how they view the observable universe. We then generalize these findings to BMS symmetry and understand what are the physical significance of supertranslations and superrotations. Lastly, a brief introduction to the black hole information paradox and how BMS symmetry might provide a solution to it is presented.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) |

Nov 2 | 12:00 | Claus Kiefer (Universität zu Köln) |
On a Quantum Weyl Curvature Hypothesis | 0.03 new building |

Nov 9 | No seminar | |||

Nov 16 | No seminar | |||

Nov 23 | 12:00 | Ayesha Khan (Universität Bonn) |
A relational approach to Quantum Gravity | Zoom (with password) |

Nov 30 | 12:00 | Simon Barton (Cologne and Oldenburg) |
Static, spherically symmetric wormholes in EvGB theory | Zoom (with password) |

Dec 7 | 12:15 | Willem van der Feltz (Universität zu Köln; Master colloquium) |
Matter coupled to two-dimensional Causal Dynamical Triangulations | 0.03 new building |

Dec 14 | 12:00 | Mauricio Ortiz Torres (Universität zu Köln) |
Computer algebra from two torsion cosmological solutions on quadratic Poincaré gauge theory | 0.03 new building |

Dec 21 | 12:00 | Sebastian Schuster (Charles University Prague) |
The Weird and the Not-So-Weird Reverse-Engineered Space-Times | Zoom (with password) |

Jan 11 | 12:00 | Blanca Hergueta (Universität zu Köln) |
Phantom fields as the source of dark energy | Zoom (with password) |

Jan 18 | 12:00 | Mario Montero (Universität zu Köln) |
Semiclassical gravity and the |
0.03 new building |

Jan 25 | 12:00 | Enes Aktas (Universität zu Köln) |
TBA | 0.03 new building |

Feb 1 | 12:00 | Mark Goh (Universität zu Köln) |
Introduction to BMS Symmetry and Black Hole Information Paradox | 0.03 new building |

Feb 8 | 12:00 | Bhuvan Agrawal (Köln/Leipzig) |
Gauge-invariant observables in Perturbative Quantum Gravity | Zoom (with password) |

## Past seminars

Summer term 2021

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