| Date |
Time |
Speaker |
Topic |
Room |
| April 2, 2019 |
12:00 |
Viacheslav Emelyanov
(KIT, Karlsruhe)
|
Black-hole evolution from stellar collapse
|
Konferenzraum 1 (Neubau) |
Black-hole evolution from stellar collapse
Abstract:
We present a local approach towards black-hole evaporation, which
relies on the absence of quantum phase transition across the stellar surface.
We show that this approach augmented with Bekenstein’s black-hole entropy
gives the Hawking effect if the null energy condition is violated in the initial
quantum-field vacuum. If otherwise, an astrophysical black hole may then be
expanding, that corresponds to the anti-Hawking effect, i.e. a positive-energy
radiation flows into the black hole, taking its origin far away from the event
horizon. This quantum process is reverse to the Hawking effect, as the latter
is described by a negative-energy influx nearby the horizon, which goes over
to a positive-energy outflux in the far-horizon region. We also provide examples
of quantum vacua well-known in the literature, which give rise to both quantum
effects from stellar collapse.
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| April 9 |
12:00 |
Adamantia Zampeli
(Charles University, Prague)
|
Canonical quantization of minisuperspace models with variational symmetries
|
Konferenzraum 1 (Neubau) |
Canonical quantization of minisuperspace models with variational symmetries
Abstract:
In this talk I will describe how the symmetries of the minisuperspace
action are used to integrate the system at the classical and quantum
level. For the latter case, we use the canonical quantisation and impose
the symmetries as operators on the wave function together with the
constraints. This leads to a selection rule which prevents their
simultaneous imposition on the wave function and consequently to the
choice of subalgebras. Some of them lead to the classical solution but
there are cases where we obtain quantum corrections. These, in the Bohmian
interpretation we used, are indicated when the quantum potential in the
quantum Hamilton–Jacobi equation does not vanish. I will discuss some
examples of physical interest.
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| April 23 |
12:00 |
Yi-Fan Wang
(Uni Cologne)
|
Dynamically assisted Schwinger effect
|
Konferenzraum 1 (Neubau) |
| April 30 |
12:00 |
Tatevik Vardanyan
(Uni Bonn)
|
Wheeler–DeWitt quantum cosmology of Bianchi II model
|
Konferenzraum 1 (Neubau) |
| May 14 |
12:00 |
David Chay Benisty
(BGU Negev / GU Frankfurt)
|
The Cosmological Constant and the mass of the Local Group
|
Konferenzraum 1 (Neubau) |
The Cosmological Constant and the mass of the Local Group
Abstract:
The two-body problem of M31 and the Milky Way (MW) galaxies with a
cosmological constant background is studied, with emphasis on the
possibility that they experienced past encounters. By implementing the
initial conditions of the big bang and the last measured relative
distance and velocities (i.e. the Timing Argument), it is shown that if
M31 and the MW had more than one encounter then the mass of the Local
Group (LG) would be a few times higher than if there had been no
encounters. Past encounters are possible only for non-zero transverse
velocity, and their viability is subject to observations of the imprints
of such near collisions. While it has been previously shown that the
presence of the cosmological constant requires a higher mass for the LG,
here, using a recent Gaia - based measurement of the transverse velocity
the derived LG mass is (3.36 +1.14 -0.7) · 10^(12) M☉ with no cosmological
constant or (4.54 +1.2 -0.75) · 10^(12)M☉ with a cosmological constant
background. If the LG has had one past encounter, LG mass is (9.70 +2.19
-1.55) · 10^(12) M☉ or (9.99 +2.22 -1.58) · 10^(12) M☉ with a cosmological constant
background. Modified Newtonian Dynamics (MOND) is also studied, as the
accelerations of the Local Group are fully in the deep MOND regime. MOND
yields the order of magnitude for the expected baryonic mass only after
one past encounter, assuming MOND does not include dark matter. While we
only consider the LG as two point masses, our calculations provide a
benchmark for future work with simulations to test the effect of the
finite size of galaxies and tidal fields due to the neighbouring
structures. This model can be also used to test screening mechanisms and
alternative theories of gravity.
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| May 21 |
12:00 |
Victor Berezin
(INR RAS, Moscow)
|
On surface terms and double layers in quadratic gravity
|
Konferenzraum 1 (Neubau) |
On surface terms and double layers in quadratic gravity
Abstract:
The talk consists of three parts.
In the first part it is demonstrated using the simple example of the spherically
symmetric Weyl + Einstein gravity, that the junction conditions in the presence of
the double layer admit the appearance of the arbitrary function determined by
the bulk solutions and, on the other hand, the very stringent requirements are
imposed on the structure of the matching surface, namely, its extrinsic curvature
tensor has to be zero.
In the second part the junction conditions on the singular hypersurface (Israel
equations) are derived from the least action principle.
And, in the third part, the same procedure is applied for the case of the quadratic
gravity. It is shown how the abovementioned “unusual” features can be
implemented quite naturally in the least action principle.
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| June 4 |
12:00 |
Pouria Pedram
(IAU, Tehran)
|
How to construct wave packets with complete classical–quantum correspondence in quantum cosmology
|
Konferenzraum 1 (Neubau) |
How to construct wave packets with complete classical–quantum correspondence in quantum cosmology
Abstract:
I discuss “canonical” wave packets in quantum cosmology which exhibit complete classical-quantum correspondence. I will present a prescription for initial conditions that leads to the classical description. I also study the situation from de-Broglie Bohm interpretation of quantum mechanics and show that the corresponding Bohmian trajectories are in complete agreement with the classical counterparts. As an interesting application, I will apply this method to the Schrödinger equation and obtain wave functions with complete classical-quantum correspondence for a large class of one-dimensional potentials.
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| June 6 |
10:00 (c.t.) |
Dimitrios Gkiatas
(Uni Bonn; Master Colloquium)
|
A covariant treatment of Lorentzian spacetimes in the Einstein–Hilbert truncation
|
Seminarraum 1, BCTP |
A covariant treatment of Lorentzian spacetimes in the Einstein–Hilbert truncation
Abstract:
The perturbative non-renormalizability of pure Quantum
Einstein Gravity at a 2-loop level signifies the need of
a new treatment for the theory. Asymptotically Safe Gravity
is a newly developed field of study where one using non-perturbative
methods tries to prove the Ultraviolet completion of the theory within
the framework of Quantum Field Theory. Such a completion is shown with
the existence of a non-Gaussian fixed point at high energies, during the
study of the renormalization group trajectories. Here, the tools
essential to generate such trajectories for causal (Lorentzian) spacetimes
are established. Furthermore, a careful treatment of the gravitational partition
functional over foliatable spacetimes in a covariant manner takes place. Finally,
employing a specific approximation (Einstein–Hilbert truncation) we bring the
Functional Renormalization Group Equation in a solvable form, which provides
evidence for the existence of the non-Gaussian fixed point.
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| June 25 |
12:00 |
Benjamin Bahr
(Uni Hamburg)
|
Background–independent renormalization in spin foam quantum gravity
|
Konferenzraum 1 (Neubau) |
Affine Coherent State Quantization: A Brief Introduction and
Some Applications
Abstract:
Loop quantum gravity (LQG) is a background-independent
approach to the unification of general relativity and quantum theory.
While LQG has many desirable properties, the independence of a fixed
metric makes it difficult to discuss the renormalisation group, since
there is no way of defining an a priori notion of scale (such as a
maximum energy or lattice length), since that would depend on space-time
geometry, which is fully fluctuating in the theory.
In this talk I will present recent progress in developing a
background-independent notion of RG flow for the path-integral
formulation of LQG. I will show first numerical explorations of the RG
flow, and discuss the relation to standard notions of renormalisation.
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| July 2 |
12:00 |
Tim Schmitz
(Uni Cologne)
|
Affine Coherent State Quantization: A Brief Introduction and
Some Applications
|
Konferenzraum 1 (Neubau) |
Affine Coherent State Quantization: A Brief Introduction and
Some Applications
Abstract:
I will go through the basics of affine coherent state
quantization (ACSQ), and illustrate the advantages and drawbacks of
ACSQ by discussing some examples, including the Lemaître–Tolman–Bondi
model.
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| July 9 |
12:00 |
Nick Kwidzinski
(Uni Cologne)
|
Dynamical symmetries of the Bianchi models
|
Konferenzraum 1 (Neubau) |
Dynamical symmetries of the Bianchi models
Abstract:
In the first part of this talk we will focus on the derivation of the following statement concerning the general relativistic dynamics of the Bianchi models:
The special automorphism group 𝑆Aut(𝔤) is the symmetry group of the equations of motion, satisfied by the metric ℎ𝑖𝑗, in the absence of matter sources.
In the second part we will employ the notion of homogeneity preserving diffeomorphisms in order to shed light onto this statement from a spacetime point of view.
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| August 20 |
16:00 |
Jens Boos
(UAlberta, Edmonton)
|
Non-local "ghost-free" gravity
|
Neubau |
Non-local "ghost-free" gravity
Abstract:
Singularities are a well known problem of Einstein's General Theory
of Relativity. It is believed that any consistent theory of gravity
should resolve them. In this talk, we will explore such an avenue in
the context of classical non-local gravity.
In particular, I will present the model of so-called "ghost-free
gravity" (at the linearized level). This can be thought of as a
generalization of Fierz--Pauli theory with infinitely many
derivatives. These derivatives, unlike in Pauli–Villars
regularization schemes, are combined in such a way that there appear
no unphysical ghost modes in the propagator at tree level. I will
demonstrate that the Newtonian potential is regularized in this
theory, and comment on steps towards exploring ghost-free gravity in
the strong field regime of black holes.
If time permits I will also comment on non-local gravity formulated
via a non-local constitutive law. At the linear level, this
formalism is closely related to ghost-free gravity, even though the
origin of non-locality might be of an entirely different nature.
Close
| September 16 |
14:00 |
Branislav Nikolic
(Uni Cologne; disputation)
|
Quantum Geometrodynamics of Higher Derivative Theories with and without Conformal Symmetry
|
Seminarraum II (II. Physikalische Institut) |
Quantum Geometrodynamics of Higher Derivative Theories with and without Conformal Symmetry
Abstract:
This thesis concerns with a framework of canonical quantization of gravity based on the Einstein–Hilbert action extended by terms quadratic in curvature. The aim is to investigate the semiclassical limit of such a theory and compare it with the semiclassical limit of the canonical quantization of the Einstein–Hilbert action alone, the latter of which is the usual approach in this framework.
General Relativity has passed the tests from the length scales of micrometers up to the cosmological scales. The classical evolution of our Universe seems to be described by the so-called $\Lambda$CDM model, which was recently tested by the Planck satelite with success. The recent discovery of gravitational waves seems to confirm also the linearized, long-range behavior of vacuum General Relativity. However, the behavior of gravity at short scales and relatively high energies, i.e. in the regimes where quantum effects of matter fields and spacetime become relevant, remains so far within the many possible theoretical approaches to its understanding. It is expected that near the initial singularity of our Universe — the Big Bang — the description of gravity drastically deviates from General Relativity and a theory of quantum gravity is necessary. But already near the theoretical limit of the highest observable energy scale (energy per excitation of a quantum field) — the Planck energy scale — it is expected that the effects of quantum field-theoretical description of matter propagating on classical curved spacetimes play a significant role. Because of this, General Relativity changes in two ways. First, the energy-momentum tensor is replaced by the expectation value of the energy-momentum tensor operator. Second, since the latter diverges, the regularization of these divergences has shown that it is necessary to modify General Relativity by adding to the Einstein–Hilbert action, among others, terms quadratic in curvature such as the square of the Ricci scalar and the square of the Weyl tensor. Since these terms generate fourth order derivatives in the modified Einstein equations, the doors were opened for investigating modified classical theories of gravity, in order to provide alternative interpretations of dark matter and the accelerated expansion of the Universe. However, an often neglected fact in these classical approaches is that these terms are suppressed at the present, classical scales. This is also reflected in the fact that the respective coupling constants of these new terms are proportional to the Planck constant and are thus of perturbative nature. Therefore they are only relevant at high energy/strong curvature regimes, typical for the very early universe. At extremely high energy scales, i.e. near and above the Planck energy scale, it is expected that the perturbative description breaks down and that a full quantum theory of gravity — which assumes that the spacetime itself is quantized as well — is necessary.
The main goal of this thesis is to quantize the Einstein–Hilbert action extended by the quadratic curvature terms is within the canonical quantization approach, thus formulating quantum geometrodynamics of the higher derivative theories. The motivation is to provide an alternative to the standard canonical quantization based on the Einstein–Hilbert action alone, because the latter does not generate the quadratic curvature terms in the semiclassical limit. A particular formulation of a semiclassical approximation scheme is employed which ensures that the effects of the quadratic curvature terms become perturbative in the semiclassical limit. This leaves the classical General Relativity intact, while naturally giving rise to its first semiclassical corrections.
Another topic of interest is a classical theory where the quadratic Ricci scalar and the Einstein–Hilbert term are absent from the action, which then enjoys the symmetry with respect to the conformal transformation of fields (local Weyl rescaling). We pay a special attention to this case, because near and beyond Planck scales it is expected that conformal symmetry plays a
very important role, since it provides a natural setting for the absence of the notion of a physical length scale. Certain useful model-independent tools are also constructed in this thesis. Firstly, it is shown that if coordinates are treated as dimensionless and if a set of variables based on the unimodular decomposition of the metric is introduced, the only conformally variant degree of freedom becomes apparent. This makes the geometrical origin of the physical length scale apparent as well, which is especially important in the interpretations of conformally invariant quantum theories of gravity. With such an approach several earlier results become much more transparent. Secondly — which naturally follows from the application of the set of these new variables — a model-independent generator of conformal field transformations is constructed in terms of which a reformulation of the definition of conformal invariance is given. Thirdly, it is argued that a canonical quantization scheme makes more sense to be based on the quantization of generators of relevant transformations, than on the first class constraints.
The thesis thus attempts to combine several minor but important aspects of a theoretical approach and use them to pursue the main goal.
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