Seminars
Winter term 2018/19
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 spacetime of an evaporating collapsing shell a derivation of the 2D renormalized energymomentum tensor is presented by using the DaviesFullingUnruh (DFU) formula for a conformal massless scalar field with an exterior generalized Vaidya metric. The flux of particles created  the so called preHawking radiation at future null infinity is obtained for shells following timelike 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 timedependent Hawkinglike flux $\dot{M}(u)\sim 1/M(u)^2$ is that the collapsing shell remains pressureless at the expense of moving at an unphysical superluminal speed. The only physically admissible solution is therefore to have a vanishing preHawking flux at latetimes which makes the formation of an apparent horizon an unavoidable conclusion.
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 LemaîtreTolmanBondi (LTB) model for spherically
symmetric, selfgravitating 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 EinsteinHilbert 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.
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 veryhighredshift surface of last scattering
and the lowredshift largescale 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.
Abstract:
Antide Sitter space (AdS) is a maximally symmetric Loretzian metric characterized by a negative
scalar curvature. It has been shown that a String theory in assymptotic flat AdS bears correspondence to a Quantum field theory invariant under conformal transformations living on the boundary
of the this space such that a strongly coupled CFT implies for the string dual a Classical AdS
gravity theory. But to model systems like heavy ion collisions which involve studying transient,
nonequilibrium solution system, a non stationary study of the dynamics on AdS side is necessary.
Perturbations on Minkowski have been studied to show that it is stable at linear and non linear
level as waves dissipate by dispersion. Recent claim has been that the AdS space on the other
hand is unstable at the non linear level (Dafermos, Anderson 2006). These generic small and finite
perturbations of AdS become large and give rise to formation of small Black Holes. The conjecture
then follows that Black holes are not an exception but the only expected outcome of a perturbed
AdS space. An overview and current status of this conjecture will be presented.
In part II, a brief introduction to Inflation and the attractive reasons that motivate its study is
presented. Inflation, a rapid expansion of the early universe, is well accepted in contemporary cosmology. This phase may be driven be several fields, whose potential is not well known, and often
includes some random component. The discussion will begin with the Old Inflation model leading
on to the Slow Roll Inflation hypothesis and the subsequent landscape of models that came with it.
We discuss how constraints from Planck Data ie, the Universe as we know it today help in narrowing
down the possible realization of inflation from a landscape of models available. We conclude with
the outlook of the future BICEP2, LIGO and PLANCK experiments.
Further reading:
[1] P. Bizon, arXiv:1312.5544
[2] G. Martinon, arXiv:1708.05600
[3] J. Martin, arXiv:1807.11075
[4] S. Tsujikawa et al, arXiv:astroph/0507632
Abstract:
What are observables in Hamiltonian GR with electromagnetism? Do they
change? Hamiltonian observables often have been defined as having 0 Poisson
bracket with each firstclass constraint. A reforming literature has
redefined gauge transformations using not separate firstclass constraints,
but a tuned sum thereof, the gauge generator G. G in GR changes the
4metric by a 4d Lie derivative.
The classical definition of the Lie derivative is like comparing
1am CET and 1am CEST, so demanding a 0 Poisson bracket trivially yields the
absence of change. A more plausible definition of observables would use G
in all cases and allow a 4d Lie derivative (not 0) for the bracket in GR;
then tensor fields like the 4metric are observables.
One can further test this definition using massive photons with
and without artificial gauge freedom (Stueckelberg and Proca) and massive
gravitons with and without clock fields. Unlike earlier definitions, the
new definition yields equivalent observables for these equivalent theories.
But if internal and external gauge symmetries are both present,
do the 0 and Lie derivative conditions mesh? Observables should be
electromagnetically invariant and spatiotemporally covariant. Thus the
electromagnetic field F and spacetime metric g are observables. Hamiltonian
observables are local fields, vary spatiotemporally, and agree with the
Lagrangian.
Abstract: We consider the wave equation, $\Box_g \psi=0$, in fixed flat Friedmann–Lemaître–Robertson–Walker and Kasner spacetimes with topology $\mathbb{R}_+\times\mathbb{T}^3$. We obtain generic blow up results for solutions to the wave equation towards the Big Bang singularity in both backgrounds. In particular, we characterize open sets of initial data prescribed at a spacelike hypersurface close to the singularity, which give rise to solutions that blow up in an open set of the Big Bang hypersurface $\{t=0\}$. The initial data sets are characterized by the condition that the Neumann data should dominate, in an appropriate $L^2$sense, up to two spatial derivatives of the Dirichlet data. For these initial configurations, the $L^2(\mathbb{T}^3)$ norms of the solutions blow up towards the Big Bang hypersurfaces of FLRW and Kasner with inverse polynomial and logarithmic rates respectively. Our method is based on deriving suitably weighted energy estimates in physical space. No symmetries of solutions are assumed.
Abstract: Asymptotic Safety constitutes a mechanism for obtaining a quantum theory of gravity within the framework of quantum field theory. The key ingredient in this scenario is a nontrivial renormalization group fixed point for the gravitational interactions which ensures the consistency and predictive power of the construction. This talk concisely summarizes the current status of the program before discussing properties of the gravitational flows emerging from spacetimes carrying a foliation structure. We also comment on relations to Monte Carlo simulations carried out within the Causal Dynamical Triangulation program and the scaledependence of Lorentzsymmetry violating interactions at low energy.
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
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 20  12:00 
Unnati Akhouri (Oxford)

Discussions on Spacetime  AdS stability and Early Universe Inflation  Konferenzraum 1 (Neubau) 
November 27  12:00 
Brian Pitts (Cambridge)

Changing Observables in Hamiltonian General Relativity and Einstein–Maxwell Theory  Konferenzraum 1 (Neubau) 
December 18  12:00 
Anne Franzen (Lisbon)

Flat Friedmann–Lemaître–Robertson–Walker and Kasner Big Bang Singularities  Konferenzraum 1 (Neubau) 
January 8, 2019  12:00 
Frank Saueressig (Nijmegen)

Quantum gravity on foliated spacetimes: Asymptotically safe and sound  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