## Seminars

### Summer term 2015

(Link to the Physikalisches Kolloquium at the University of Bonn.)### Dice seminar — Hacking the quantum revolution: 1925–1975

The presenter will be determined by the throw of a dice.

Silvan S. Schweber, Eur. Phys. J. H **40** (2015) 1, 53–149

Sections: *Introduction*, *The role of context*, pp. 1–6; *4.5 Leo Kadanoff*, pp. 45–52

### Dice seminar — Identification of a gravitational arrow of time

The presenter will be determined by the throw of a dice.

Julian Barbour, Tim Koslowski, and Flavio Mercati, Phys. Rev. Lett. **113** (2014) 18, 181101, arXiv:1409.0917 [gr-qc]

### Dice seminar — Exotic Statistics for Ordinary Particles in Quantum Gravity

The presenter will be determined by the throw of a dice.

John Swain, Int. J. Mod. Phys. D **17** (2008) 13, 2475, arXiv:0805.2373 [gr-qc]

### Dice seminar — Cosmology and local physics

The presenter will be determined by the throw of a dice.

### Master colloquium — Emergent Gravity and Cosmology: Thermodynamic Perspective

Pranjal Dhole (Köln/Bonn)

This thesis is divided in two parts. First part of this thesis is an attempt to clarify the notion of emergence in Physics. We analyze thermodynamically emergent scenarios of gravity viz. ADS/CFT, Verlinde’s entropic gravity and Padmanabhan’s thermodynamic emergent gravity and clarify how/whether gravity is described as an emergent phenomena in each theory.

The second part of this thesis is dedicated to Padmanabhan’s thermodynamic treatment of gravity. In recent work by T. Padmanabhan et al. [arXiv:1303.1535], a Hamiltonian structure derived from metric density and its corresponding conjugate momentum density was proposed. These variables have thermodynamic analogues in Horizon thermodynamics. We attempt a conformal decomposition of these variables. We start with an alternate description of cosmology that does not originate with gravitational field equations. We postulate that evolution of our universe can be described inside a spacetime box between two de Sitter phases that arises due to breaking of conformal symmetry giving rise to matter degrees of freedom (physical length scales).

We postulate that generation of matter degrees of freedom, in consequence, give rise to holographic discrepancy which drives the evolution of cosmic space along with cosmic time.

### Dice seminar — The Interpretation of Quantum Mechanics: Many Worlds or Many Words?

The presenter will be determined by the throw of a dice.

M. Tegmark, Fortsch. Phys. **46** (1999) 6–8, 855, arXiv:quant-ph/9709032

### Dice seminar — On the Classical Limit of Quantum Mechanics

The presenter will be determined by the throw of a dice.

### Dice seminar — The strange equation of quantum gravity

The presenter will be determined by the throw of a dice.

Carlo Rovelli, Class. Quant. Grav. **32** (2015) 124005, arXiv:1506.00927 [gr-qc]

### Dice seminar — Boundary conditions in quantum cosmology

The presenter will be determined by the throw of a dice.

### Numerical study of a geometric flow for the Bartnik conjecture in axisymmetry

Markus Strehlau (Köln/AEI Potsdam)

The static metric extension conjecture was formulated by Robert Bartnik, motivated by the problem of defining a quasi-local mass. This conjecture can be interpreted as an open boundary problem. The prescribed conditions are the metric on the inner boundary surface of the extension and its mean curvature. To gain further insight into this problem, axisymmetric extensions were used. We define a geometric flow, based on a mean curvature flow, to approach the inner boundary of the extension. This flow was numerically analysed using a pseudo-spectral method.

### Dice seminar — Is Spacetime Countable?

The presenter will be determined by the throw of a dice.

Sean Gryb, It From Bit or Bit From It? The Frontiers Collection 2015, pp 153-168, arXiv:1501.02671 [gr-qc].

### Quasi-normal modes of the BTZ black hole solution of (2+1)-dimensional topological Poincare gauge theory of gravity

Jens Boos (Cologne)

In this thesis, we study the quasi-normal modes of the BTZ black hole with torsion. Generalizing the results already presented in the literature, we derive the scalar, fermionic, and electromagnetic wave equations from a variational principle and solve them analytically for the BTZ background with torsion. In a second step, demanding outgoing perturbations at the horizon and vanishing flux at infinity gives rise to the quasi-normal modes.

These expressions are shown to fulfill the AdS/CFT correspondence. Tensorial wave equations in coframe and the tensorial part of the Lorentz connection—unique to the Mielke–Baekler model—are derived and compared with the Proca–Chern–Simons system. Their formal similarity suggests that also here, the quasi-normal modes can be found analytically.

Derived from the scalar, spinorial, and electromagnetic quasi-normal modes, the semiclassical entropy spectrum of the non-rotating BTZ black hole is evenly spaced.

Moreover, the minimal noise temperature picked up by a quantum amplifier when measuring the quasi-normal mode transitions of the BTZ black hole is of the order of the Hawking temperature. This is an interesting feature of quasi-normal mode quantum gravity” in (2 + 1)D.

### Dice seminar — Phonon creation by gravitational waves

The presenter will be determined by the throw of a dice.

Carlos Sabin, David Edward Bruschi, Mehdi Ahmadi, Ivette Fuentes, New J. Phys. **16** (2014) 085003, arXiv:1402.7009 [quant-ph].

### Soft singularity crossing and transformation of matter properties

Alexander Kamenshchik (Uni Bologna/Landau Inst. Moscow)

We investigate particular cosmological models, based either on tachyon fields or on perfect fluids, for which soft future singularities arise in a natural way. Our main result is the description of a smooth crossing of the soft singularity in models with an anti-Chaplygin gas or with a particular tachyon field in the presence of dust. Such a crossing is made possible by certain transformations of matter properties. Some of these cosmological evolutions involving tachyons are compatible with SNIa data. We compute numerically their dynamics involving a first soft singularity crossing, a turning point and a second soft singulatity crossing during recollapse, ending in a Big Crunch singularity.

## Past seminars

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