Current seminars

In addition to the Vienna relativity seminars, the calendars above sometimes contain other events of interest to members of the relativity group. The seminars of the Vienna relativity group are listed below.

Currently (unless indicated otherwise) all seminars take place on Thursday at 15:15 in Seminarraum A, Währinger Strasse 17, 2nd Floor.

The Mathematical Physics Seminars take place on Tuesdays at 13.45.

The Particle Physics Seminars take place on Tuesdays at 16.15.

  • WEDNESDAY, December 7th, 14:15
    Pedro Cunha (University of Aveiro, Portugal): The fate of the light ring instability

Abstract: The ringdown and shadow of the astrophysically significant Kerr Black Hole (BH) are both intimately connected to a special set of bound null orbits known as Light Rings (LRs). Similarly, ultracompact objects with LRs but without an horizon could mimic BHs in some of their strong gravity phenomenology. However, are such ultracompact objects dynamically viable? Stationary and axisymmetric ultracompact objects that can form from smooth, quasi-Minkowski initial data must have at least one stable LR, which has been argued to trigger a spacetime instability. Until recently its development and fate had been unknown. We confirm that the LRs indeed triggered an instability in fully non-linear numerical evolutions of ultracompact bosonic stars free of any other known instabilities. The results shows that the LR instability can be effective in destroying horizonless ultracompact objects that could be plausible BH imitators.

  • Thursday, December 15th, 15:15, via zoom
    Miguel Zumalacárregui (Albert Einstein Institute, Potsdam): On the propagation of gravitational waves: diffraction, dispersion & birefringence

Abstract: Just like light, gravitational waves (GWs) are deflected and magnified by the large-scale structure of the Universe, a phenomenon known as gravitational lensing. Their low frequency, phase coherence and capacity to propagate with no absorption makes GWs an ideal signal in which to observe wave-propagation phenomena. I will describe how GWs deflected by cosmic structures produce diffractive, wave-optics phenomena, whose measurement will allow us to infer the properties of galactic and dark matter halos. For GWs in strong gravitational fields, such as the vicinity of a massive black hole, their propagation depends on the frecuency (dispersion) and polarization (birefringence) through the gravitational spin-hall effect. I will describe how observations of sources near central black holes of galaxies may enable the observation of dispersive GWs. While birefringence might be too suppressed to observe in Einstein’s general relativity, alternative theories predict that the two GW polarizations travel at different speeds near massive objects. Searches for this effect provide one of the most stringent tests of gravity so far.