14. December 2018

**BREMEN-OLDENBURG RELATIVITY SEMINARDouble Feature**

Location: |
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When: | 14. December 2018 14:00 c.t. | |

Speaker: | Hovhannes Demirchyan (Byurakan Astrophysical Observatory, Armenia) | |

Title: | Astrophysical shock waves as impulsive signals (Abstract below) |

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When: | 14. December 2018 15:00 c.t. |

Speaker: | Omar Jesus Franca (Santiago National Autonomous University Mexico) |

Title: | Theories with θ-couplings and some concrete application (Abstract below) |

**First Abstract:** Impulsive signals exist in lightlike hypersurfaces where Riemann curvature tensor contains a Dirac delta function. Such hypersurfaces describe astrophysical shock waves arising from supernovae, collision of neutron stars and other cataclysmic events. These shock waves are a burst of matter traveling with the speed of light and gravitational radiation which together form the impulsive signal. We will discuss approaches for constructing singular hypersurfaces in general space-time. As a particularly interesting example we will discuss the construction of singular hypersurfaces in Minkowski space.

**Second Abstract: **Through the last years the coupling of topological terms to certain field theories has obtained a lot of interest in several branches of physics from both experimental and theoretical point of view. In this talk we focus on the Pontryagin term coupled by a scalar function theta to the Maxwell action, known as theta-Electrodynamics, which models the effective behavior of Topological Insulators (TIs). We also study how to add this theta-term to Einstein-Hilbert action, resulting in theta-Gravity. Here we analyze two applications. First, we apply theta-Electrodynamics to the radiation of a point-like particle with constant velocity, whose radiation gives forward and reverse Čerenkov radiation (RČR), which is very surprising because RČR has been observed only in metamaterials. And finally, in the context of theta-Gravity we will expose a black hole-like solution, which provides a possibility to study the gravitational analog of TIs.