## Seminars

### Particle Creation by Gravitationally Collapsing Dust Shells

Abstract: In this work some aspects of quantum field theory in the gravitational collapse of dust shells are studied. As a model for possible backreaction effects in the background space-time of an evaporating collapsing shell a derivation of the 2D renormalized energy-momentum tensor is presented by using the Davies-Fulling-Unruh (DFU) formula for a conformal massless scalar field with an exterior generalized Vaidya metric. The flux of particles created - the so called pre-Hawking radiation- at future null infinity is obtained for shells following time-like as well as null trajectories in the presence and absence of backreaction. Finally, by using the four dimensional Einstein's equations it is shown that the condition for a compact horizonless object emanating a time-dependent Hawking-like flux $\dot{M}(u)\sim -1/M(u)^2$ is that the collapsing shell remains pressureless at the expense of moving at an un-physical superluminal speed. The only physically admissible solution is therefore to have a vanishing pre-Hawking flux at late-times which makes the formation of an apparent horizon an unavoidable conclusion.

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### Singularity Avoidance of the Quantum LTB Model for Gravitational Collapse

Abstract: In classical General Relativity, singularities signal a breakdown of the theory. One line of inquiry that can be followed to investigate whether quantum gravitational effects could resolve this peculiar behavior is to quantize classical models for gravitational collapse, for example the Lemaître--Tolman--Bondi (LTB) model for spherically symmetric, self-gravitating dust.
Using a symmetry reduced treatment of the marginally bound LTB model based on Quantum Geometrodynamics, we show that close to the classical singularity the dynamics of the system follow a particular effective Hamiltonian. The general form of this Hamiltonian can be recovered in an alternate approach to quantizing the marginally bound LTB, where the dust cloud's general behavior is characterized by its outermost shell.
In this approach, which we develop here, an action for this outermost shell is derived starting from the Einstein--Hilbert action, and the resulting Hamiltonian quantized. Because the dust naturally provides a preferred notion of time, one can construct a Hilbert space as in ordinary quantum mechanics, and impose unitary evolution on its states. We then show that those states avoid the classical singularity. Furthermore, for a wave packet initially approximating the classical trajectory the collapse to a singularity is replaced by a bounce, effectively a transition from black to white hole. Finally we discuss some implications of this bouncing behavior by constructing a quantum corrected spacetime describing dust collapse based on this wave packet: the nature of the horizon, the lifetime of the temporary ‘grey’ hole, and the effective pressure facilitating the bounce.

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### On the various aspects of relativistic inhomogeneous cosmology

Abstract: Modern cosmology has observational access to two very distinct epochs in the history of the Universe: the very-high-redshift surface of last scattering and the low-redshift large-scale structure. One of the aims of relativistic cosmology is to bridge these two epochs with an accurate description of gravitationally induced dynamics that causally lead from one epoch to the other. In my talk I will present some of the constituents of this major endeavour, including:

- methods of modelling gravitational instability evolution and calculating its statistical outcome: the mass function on galaxy cluster scales. These are based on the scalar averaging formalism, with the Zel'dovich approximation serving as a closure condition and the silent universe models (Einstein equations with no rotation and no energy transfer).

- arguments against the Green and Wald formalism which appeared to invalidate the coupling of expansion to structure formation

Some of the presented results will be put in the context of the current and future astronomical sky surveys.

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### A mechanism for quantum correlations in emergent quantum mechanics

Abstract: In this talk, a particular approach to emergent quantum mechanics will be described. In this context, a new way to look to quantum entanglement is discussed. We will highlight differences with the standard description of entanglement in quantum theory

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Date Time Speaker Topic Room
October 16 12:00 Group members Conference reports Konferenzraum 1 (Neubau)
October 23 12:00 Sebastian Arenas
(Master Colloquium)
Particle Creation by Gravitationally Collapsing Dust Shells Konferenzraum 1 (Neubau)
October 30 12:00 Tim Schmitz
(Master Colloquium)
Singularity Avoidance of the Quantum LTB Model for Gravitational Collapse Konferenzraum 1 (Neubau)
November 6 12:00 Jan Ostrowski (ENS de Lyon)
On the various aspects of relativistic inhomogeneous cosmology Konferenzraum 1 (Neubau)
November 27 12:00 Brian Pitts (Cambridge)
TBA Konferenzraum 1 (Neubau)
December 18 12:00 Anne Franzen (Lisbon)
TBA Konferenzraum 1 (Neubau)
January 8, 2019 12:00 Frank Saueressig (Nijmegen)
TBA Konferenzraum 1 (Neubau)
January 22, 2019 12:00 Ricardo Gallego Torromé (Frankfurt)
A mechanism for quantum correlations in emergent quantum mechanics Konferenzraum 1 (Neubau)

## Past seminars

Summer term 2018
Winter term 2017/18
Summer term 2017
Winter term 2016/17
Summer term 2016
Winter term 2015/16
Summer term 2015
Winter term 2014/15
Summer term 2014
Winter term 2013/14
Summer term 2013
Winter term 2012/13
Summer term 2012
Winter term 2011/12
Summer term 2011
Winter term 2010/11
Summer term 2010
Winter term 2009/10
Summer term 2009
Winter term 2008/09
Summer term 2008
Winter term 2007/08
Summer term 2007
Winter term 2006/07
Summer term 2006
Summer term 2005
Winter term 2004/05
Summer term 2004
Winter term 2003/04
Summer term 2003