8. December 2021: RTG Colloquium University Oldenburg

8 December 2021

RTG Colloquium - Online (orig. University Oldenburg)

14:00- 16:30 CET


14:00 - 15:00 Irene Tamborra: Messengers from the Universe

Abstract: tba.

15:00 - 15:30 Coffee break

15:30 - 16:30 Fethi Ramazanoglu: Unwelcome instabilities in theories of gravity

Abstract: Weak field tests already constrain any deviation from general relativity to be small, hence, scenarios in alternative theories where major effects become relevant in the strong gravity regime are particularly interesting. A perfect example of this is the spontaneous scalarization phenomena in scalar-tensor theories where the scalar can naturally grow near compact objects due to a tachyonic instability. The tachyon is welcome, because it is eventually shut off due to nonlinear effects, and leads to stable objects of potential astrophysical relevance. There does not seem to be anything specific to scalars in the instability mechanism at a first sight, hence there have been various efforts to find analogous theories for vectors and other fields. I will summarize these efforts, and then concentrate on our recent finding that such theories are fundamentally different from scalarization. Wheneven an analog of the tachyon in scalarization is introduced in a vector-tensor theory, a ghost (or gradient) instability also appears. Instability time scale of the ghost is faster than that of tachyons, hence it dominates the dynamics. Even more crucially, these ghosts also manifest divergent terms in the field equations, which poses serious questions about the mathematical meaning and physical validity of the theories they appear in. We will discuss these problems, some ideas about potential solutions and factors that hinder the solutions. Reference: arxiv.org/abs/2110.04594

More details will follow.