## Seminars

### Summer term 2017

### Gravitationswellen und ihr direkter Nachweis (Bachelor colloquium)

Abstract:
"Ladies and gentleman, we have detected gravitational waves. We did it!"

Mit diesen Worten trat am 11. Februar 2016 David Reitze, der Direktor von LIGO vor
die Presse.

Durch diese bahnbrechende Entdeckung hat das Thema der Gravitationswellen neue
Bedeutung gewonnen. Als Albert Einstein ihre Existenz postulierte, zweifelte er selbst
noch an seiner Behauptung. Doch jetzt, fast einhundert Jahre später war LIGO in der
Lage, Gravitationswellen nachzuweisen.

Das Ziel dieser Bachelorarbeit ist es, einen Einblick in die Theorie der Gravitationswellen
zu verschaffen und die von LIGO veröffentlichten Ergebnis zu untersuchen. Dabei werde
ich zunächst auf die mathematischen und physikalischen Grundlagen eingehen, um anschlie
ÿend mithilfe der von LIGO bereitgestellten Daten die Chirp Masse zu bestimmen.
Mit der Chirp Masse kann man auf weitere Parameter des vermessenen Doppelsternsystems
schließen, welches die Gravitationswellen verursachte.

### Bouncing cosmologies and accelerated expansion from group field theory condensates

Abstract: I will discuss the cosmological implications of the Group Field Theory (GFT) approach to Quantum Gravity. The work I will present is part of a programme which aims at understanding Early Universe Cosmology by studying the dynamics of the emergent continuum spacetime, as obtained from a fundamentally discrete microscopic theory of the gravitational field. In particular, I will show how it is possible to achieve a bounce and an early epoch of accelerated expansion in this approach.

### Einstein gravity and functional Schrödinger equation from the Weyl-Einstein canonical quantum gravity

Abstract: In this talk I will discuss the semiclassical approximation to the Weyl-Einstein-Wheeler-DeWitt (WEWDW) equation, an equation analogous to the Weheler-DeWitt (WDW) equation appearing in quantum geometrodynamics of Einstein-Hilbert (QEH) action. This equation is the dynamical equation for the wave functional resulting from the canonical quantization of the Weyl-squared action extended by the EH term (and matter fields; in our particular case it will be non-minimally coupled massless scalar field). I will present issues and possible work-arounds of the Born-Oppenheimer type ansatz and WKB approximation related to the role of the additional coupling constant (of the Weyl-squared term). An approach which gives pure classical GR and modified functional Schrödinger equation will be presented.

### Gauge Theories of Gravitation: An external gauge group implies the geometry of spacetime

Abstract: The advent of general relativity in 1915/1916 induced a paradigm shift: since then, the theory of gravity had to be seen in the context of the geometry of spacetime. An outgrowth of this new way of looking at gravity is the gauge principle of Weyl (1929) and Yang–Mills–Utiyama (1954/1956). It became manifest around the 1960s (Sciama–Kibble) that gravity is closely related to the Poincaré group acting in Minkowski space. The gauging of this external group induces a Riemann–Cartan geometry on spacetime. If one generalizes the gauge group of gravity, one discovers still more involved spacetime geometries. If one specializes it to the translation group, one finds a specific Riemann–Cartan geometry with teleparallelism (Weitzenböck geometry). In: D.Lehmkuhl, G.Schiemann, E. Scholz et al. (eds.), Towards a Theory of Spacetime Theories, Einstein Studies Volume 13, pp 145-169, Birkhauser-Springer (2017), arxiv:1204.3672v2

### Decoherence: From Schrödinger's Cat to the Classical World

Abstract: How does the classical behaviour emerge in a world that is fundamentally described by quantum theory? The key to the answer is given by a process that was described for the first time 47 years ago - decoherence. Decoherence is the irreversible emergence of classical properties of a quantum system through the unavoidable interaction with its environment. In my talk, I shall give a general introduction into decoherence and present its most important theoretical and experimental applications. These include, in particular, the localization of objects. I shall then discuss the relevance of decoherence for the interpretation of quantum theory and end with an outlook on quantum cosmology and the origin of the direction of time.

### The unexpected resurgence of Weyl geometry in late 20th century physics

Abstract: Weyl's original scale geometry of 1918 (?purely infinitesimal geometry?) was withdrawn by its author from physical theorizing in the early 1920s. It had a comeback in the last third of the 20th century in different contexts: scalar tensor theories of gravity, foundations of gravity, foundations of quantum mechanics, elementary particle physics, and cosmology. The talk will present the basic ideas of Weyl geometry and its application to gravity. I will argue that even in its most simple form of an integrable scale connections it may lead to at least methodologically interesting modifications of Einstein gravity.

### Effective metrics and a fully covariant description of constitutive tensors in electrodynamics

Abstract: The idea behind analogue space-times is to gain additional testing grounds for space-time physics. This, ideally, helps to get around many of the experimental difficulties encountered in general relativity. Here, we shall demonstrate how to approach macroscopic electrodynamics with this goal in mind. In order to find media that lend themselves to an analogue of vacuum electrodynamics on a curved space-time, we shall develop a fully covariant, four-dimensional formulation of macroscopic electrodynamics. Using this reformulation we shall regain and extend previous results. I shall also contrast the perspective of analogue space-times to the goals encountered in pre-metric electrodynamics.

### The Born-Oppenheimer Approach and the Semiclassical Limit of Canonical Quantum Gravity

Abstract: This thesis outlines and further develops the semiclassical approximation to quantum geometrodynamics. Within a Born-Oppenheimer ansatz, the quantum gravitational corrections to quantum field theory in a fixed curved background spacetime are derived within the functional Schrödinger picture. It is demonstrated that the very choice of a product like / Born-Oppenheimer ansatz leads to arbitrary gauge-like degrees of freedom which turn out to be important for the final result, in particular with respect to unitarity violation. For the choice of a vacuum background, a closed expression containing all quantum gravitational effects for the semiclassical matter Hamiltonian is derived. The formalism is then adjusted such that the semiclassical limit has unitary time evolution and the lowest order correction to the new Hamiltonian is calculated.

## Past seminars

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