## Seminars

### Summer term 2014

Date | Time | Speaker | Topic | Room |
---|---|---|---|---|

April 8 | 12:00 | Antonin Coutant
(AEI Potsdam) |
Unitary and non-unitary transitions around a cosmological bounce | R 215 |

April 15 | 12:00 | Arezu Dehghanfar
(Köln) |
The cosmological constant and Dark Energy | R 215 |

May 6 | 12:00 | Branislav Nikolić
(Köln/Bonn) |
Shape dynamics as a timeless theory | R 215 |

May 27 | 12:00 | Marcel Zimmer
(Köln) |
Bachelorkolloquium: „Fermionen und das Spin-Statistik-Theorem“ | R 215 |

12:30 | Anne Franzen
(Utrecht) |
Boundedness of a massless scalar wave on Reissner–Nordström interior backgrounds | R 215 | |

June 3 | 12:00 | Patrick Dürr
(Tübingen) |
Aspects of Alternative Theories of Gravity | R 215 |

June 17 | 12:00 | Jens Boos
(Köln) |
Poincaré Gauge Theory of Gravity | R 215 |

June 24 | 12:00 | Jens Boos
(Köln) |
Exterior calculus and Einstein–Cartan theory | R 215 |

July 1 | 12:00 | Branislav Nikolić
(Köln/Bonn) |
Quantization of higher-derivative gravity | R 215 |

July 8 | 12:00 | Heinrich Päs
(Dortmund) |
Sterile neutrino altered dispersion relations in particle physics, astrophysics and cosmology | R 215 |

July 15 | 12:00 | Ali Akpinar
(Köln) |
Bachelorkolloquium | R 215 |

12:30 | Pranjal Dhole
(Köln/Bonn) |
Cosmological constant from emergent gravity | R 215 | |

Sept. 11 | 10:00 | Nassim Tanha
(Köln) |
Master colloquium: “First Excited State: Quantum Gravitational Corrections to the Power Spectrum of Density Fluctuations from Canonical Quantum Cosmology” | R 215 |

## Past seminars

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

Antonin Coutant (AEI Potsdam)

In this presentation, we will discuss the notion of time and unitarity in the vicinity of a bounce in quantum cosmology, that is, a turning point for the scale factor. This starts from the Vilenkin approach to the interpretation of the solutions of the Wheeler-DeWitt equation. In this approach, unitarity is defined through the conserved current and is by nature an approximate concept. In minisuperspace it amounts to using the scale factor as a time variable. A unitary evolution is recovered when the latter becomes semi-classical enough.

Unfortunately, WKB methods drastically fail near a turning point and the scale factor cannot play the role of time in scenarios with a bounce or a recollapsing phase for the universe. In this work, we extend the results of Vilenkin Massar and Parentani to momentum representation. For this, we investigate the dynamics of matter transitions when using its conjugate momentum as a time. In a first part, we describe the precise conditions so as to recover unitarity, and hence, a consistent notion of probability. In a second part, we discuss a concrete example in the vicinity of a bounce.

Close

**Unitary and non-unitary transitions around a cosmological bounce**In this presentation, we will discuss the notion of time and unitarity in the vicinity of a bounce in quantum cosmology, that is, a turning point for the scale factor. This starts from the Vilenkin approach to the interpretation of the solutions of the Wheeler-DeWitt equation. In this approach, unitarity is defined through the conserved current and is by nature an approximate concept. In minisuperspace it amounts to using the scale factor as a time variable. A unitary evolution is recovered when the latter becomes semi-classical enough.

Unfortunately, WKB methods drastically fail near a turning point and the scale factor cannot play the role of time in scenarios with a bounce or a recollapsing phase for the universe. In this work, we extend the results of Vilenkin Massar and Parentani to momentum representation. For this, we investigate the dynamics of matter transitions when using its conjugate momentum as a time. In a first part, we describe the precise conditions so as to recover unitarity, and hence, a consistent notion of probability. In a second part, we discuss a concrete example in the vicinity of a bounce.

Close

Patrick Dürr (Tübingen)

Modifying standard theories of gravity is looking back upon a long history. In this talk, we shall examine the contemporary motivation behind and some exciting results and aspects of one of the most natural extensions of GR, Brans–Dicke-like theories, in which gravity is also mediated by a scalar (thus, rendering the gravitational coupling “constant” a dynamical variable).

After a brief review of the foundations and a few generic viability criteria, mainly concerning stability issues and constraints imposed by cosmological data, our discussion will focus upon weak field astrophysics, which turns out to have some surprises in store for us: How do such modifications fare with the Weak Equivalence Principle and classical tests, viz. in our solar system? Might they contribute to the solution of the Dark Matter mystery? What do they imply for gravitational wave phenomenology, perhaps even of observational relevance?

Close

**Aspects of Alternative Theories of Gravity**Modifying standard theories of gravity is looking back upon a long history. In this talk, we shall examine the contemporary motivation behind and some exciting results and aspects of one of the most natural extensions of GR, Brans–Dicke-like theories, in which gravity is also mediated by a scalar (thus, rendering the gravitational coupling “constant” a dynamical variable).

After a brief review of the foundations and a few generic viability criteria, mainly concerning stability issues and constraints imposed by cosmological data, our discussion will focus upon weak field astrophysics, which turns out to have some surprises in store for us: How do such modifications fare with the Weak Equivalence Principle and classical tests, viz. in our solar system? Might they contribute to the solution of the Dark Matter mystery? What do they imply for gravitational wave phenomenology, perhaps even of observational relevance?

Close

Anne Franzen (Utrecht)

We consider solutions of the scalar wave equation ☐

The proof depends on novel weighted energy estimates in the black hole interior which, in combination with commutation by angular momentum operators and Sobolev embedding, yield uniform pointwise estimates.

Close

**Boundedness of a massless scalar wave on Reissner–Nordström interior backgrounds**We consider solutions of the scalar wave equation ☐

_{g}*φ*= 0, without symmetry, on fixed subextremal Reissner–Nordström backgrounds (*M*,*g*). Previously, it has been shown that for*φ*arising from sufficiently regular data on a two ended Cauchy hypersurface, the solution and its derivatives decay suitably fast on the event horizon*H*^{+}. Using this, we show here that*φ*is in fact uniformly bounded, |*φ*| ≤*C*, in the black hole interior up to and including the bifurcate Cauchy horizon*CH*^{+}.The proof depends on novel weighted energy estimates in the black hole interior which, in combination with commutation by angular momentum operators and Sobolev embedding, yield uniform pointwise estimates.

Close

Heinrich Päs (Dortmund)

The search for sterile neutrinos is motivated by the LSND and MiniBooNE, reactor and Gallium anomalies. The fact that this evidence is partly conflicting can be a consequence of either experimental systematics or of non-standard neutrino properties such as altered dispersion relations. Altered dispersion relations can arise from various effects such as Lorentz violation, shortcuts in extra dimensions and standard or non-standard matter effects. We analyze the effects of sterile neutrino altered dispersion relations on neutrino oscillations the flavor ratios of astrophysical neutrinos and on big bang nucleosynthesis (BBN).

Close

**Sterile neutrino altered dispersion relations in particle physics, astrophysics and cosmology**The search for sterile neutrinos is motivated by the LSND and MiniBooNE, reactor and Gallium anomalies. The fact that this evidence is partly conflicting can be a consequence of either experimental systematics or of non-standard neutrino properties such as altered dispersion relations. Altered dispersion relations can arise from various effects such as Lorentz violation, shortcuts in extra dimensions and standard or non-standard matter effects. We analyze the effects of sterile neutrino altered dispersion relations on neutrino oscillations the flavor ratios of astrophysical neutrinos and on big bang nucleosynthesis (BBN).

Close