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) |
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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October 30 |
12:00 |
Tim Schmitz
(Master Colloquium)
|
Singularity avoidance of the quantum LTB model for gravitational collapse
|
Konferenzraum 1 (Neubau) |
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.
Close
November 6 |
12:00 |
Jan Ostrowski (ENS de Lyon)
|
On the various aspects of relativistic inhomogeneous cosmology
|
Konferenzraum 1 (Neubau) |
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.
Close
November 20 |
12:00 |
Unnati Akhouri (Oxford)
|
Discussions on spacetime - AdS stability and early universe inflation
|
Konferenzraum 1 (Neubau) |
Abstract:
Anti-de 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,
non-equilibrium 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:astro-ph/0507632
Close
November 27 |
12:00 |
Brian Pitts (Cambridge)
|
Changing observables in Hamiltonian general
relativity and Einstein–Maxwell theory
|
Konferenzraum 1 (Neubau) |
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 first-class constraint. A reforming literature has
redefined gauge transformations using not separate first-class constraints,
but a tuned sum thereof, the gauge generator G. G in GR changes the
4-metric by a 4-d 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 4-d Lie derivative (not 0) for the bracket in GR;
then tensor fields like the 4-metric 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 spatio-temporally covariant. Thus the
electromagnetic field F and space-time metric g are observables. Hamiltonian
observables are local fields, vary spatio-temporally, and agree with the
Lagrangian.
Close
December 18 |
12:00 |
Anne Franzen (Lisbon)
|
Flat Friedmann–Lemaître–Robertson–Walker and Kasner big bang singularities
|
Konferenzraum 1 (Neubau) |
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.
Close
January 8, 2019 |
12:00 |
Frank Saueressig (Nijmegen)
|
Quantum gravity on foliated spacetimes: Asymptotically safe and sound
|
Konferenzraum 1 (Neubau) |
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 non-trivial 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
scale-dependence of Lorentz-symmetry violating interactions at low energy.
Close
January 22 |
12:00 |
Ricardo Gallego Torromé (Frankfurt)
|
A mechanism for quantum correlations in emergent quantum mechanics (cancelled)
|
Konferenzraum 1 (Neubau) |
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.
Close
January 22 |
12:00 |
Leonardo Chataignier
|
Gauge fixing and the semiclassical interpretation of quantum cosmology
|
Konferenzraum 1 (Neubau) |
Gauge fixing and the semiclassical interpretation of quantum cosmology
Abstract:
We make a critical review of the semiclassical interpretation of quantum cosmology and emphasise that it is not necessary to consider that time emerges only when the gravitational field is (semi)classical. We show that the usual results of the semiclassical interpretation can be obtained by gauge fixing, both at the classical and quantum levels. By ‘gauge fixing’ we mean a particular choice of the time coordinate, which determines the arbitrary Lagrange multiplier that appears in Hamilton’s equations. In the quantum theory, we adopt a tentative definition of the (Klein–Gordon) inner product, which is positive definite for solutions of the quantum constraint equation found via an iterative procedure that corresponds to a weak coupling expansion in powers of the inverse Planck mass. We conclude that the wave function should be interpreted as a state vector for both gravitational and matter degrees of freedom, the dynamics of which is unitary with respect to the chosen inner product and time variable.
Close
January 29 |
12:00 |
Dimitrios Gkiatas
|
Functional renormalization group equation for Lorentzian spacetimes
|
Konferenzraum 1 (Neubau) |
Functional renormalization group equation for Lorentzian spacetimes
Abstract:
In the covariant approach to quantum gravity, a perturbative
treatment of the Fierz–Pauli Lagrangian for the massless spin-2 graviton
results in a theory which is power-counting non-renormalizable i.e. new
divergences are expected to arise at each order of perturbation theory.
Explicit calculations verified that cancellation of the divergent terms
occur to one-loop in the absence of matter rendering the theory one-loop
renormalizable, but at two-loops the existence of a divergent pole, called
the Garoff–Sagnotti term, proves two-loop perturbative non-renormalizability.
Asymptotic safety, provides a method to determine a well-defined theory of
gravity, both in the IR and UV, using methods developed in Quantum Field
Theory, following an initial non-perturbative treatment in the implementation
of the background field. In this scenario, the flow of the coupling constants
(Newton's constant and Cosmological constant) in the UV is studied via the
Functional Renormalization Group Equation (FRGE). The existence of a
non-Gaussian fixed point in their flow provides the UV completion of the
theory implying a well-defined theory at all energy scales. In this talk, we
will develop the tools essential for the construction of the FRGE for
Lorentzian spacetimes. Then, we shall establish the asymptotic behavior
obtained and compare it with the results found using a Euclidean signature.
Finally, a covariant way of considering only foliated spacetimes will be
introduced.
Close
February 19 |
12:00 |
Ana Alonso Serrano (AEI Potsdam)
|
Black hole evaporation: Entropy analysis and GUP corrections
|
Konferenzraum 1 (Neubau) |
Black hole evaporation: Entropy analysis and GUP corrections
Abstract:
We have studied the entropy budget per particle emitted in
blackbody radiation and determined explicit coarse-grainning models for
classical and quantum entropies. As the process is unitary, the entropy
is exactly compensated by the “hidden information” in the correlations
that we choose not to consider within the specific selected
coarse-graining. Our goal is to extend these ideas to a black hole
evaporation process. In order to carry out this calculation we adopted a
variant of the “average subsystem” approach, but consider a multipartite
pure system that includes the influence of the rest of the universe. In
addition, the entropy budget should be corrected at the last stages of
evaporation, due to quantum gravity effects. We have been shown recently
how these effects (expressed in terms of the generalized uncertainty
principle) modify the Hawking flux when we approach the Planck size.
Close