Modul: MAT675 PDE and Mathematical Physics

## [Video] The scalar wave equation on general asymptotically flat spacetimes: Stability and instability results

Prof. Dr. Georgios Moschidis talk

**Date:** 29.11.18 **Time:** 18.10 - 19.00 **Room:** Y27H35/36

Abstract: In this talk, we will examine how certain geometric conditions on general asymptotically flat spacetimes $(\mathcal{M},g)$ are related to stability or instability properties of solutions to the scalar wave equation $\square_{g}\psi=0$ on $\mathcal{M}$. First, in the case when $(\mathcal{M},g)$ possesses an event horizon with positive surface gravity and an ergoregion which is sufficiently small in terms of the near-horizon geometry, we will prove a logarithmic decay result for solutions to $\square_{g}\psi=0$, provided a uniform energy boundedness estimate holds on $(\mathcal{M},g)$. This result, applicable also in the absence of a horizon and an ergoregion, generalises a result of Burq for the wave equation on the complement of an arbitrary compact obstacle in flat space. We will then proceed to enlarge our scope of asymptotically flat backgrounds by relaxing even further our assumptions on the properties of the ergoregion. In this case, we will present a rigorous proof of Friedman's ergosphere instability for scalar waves in the case when $(\mathcal{M},g)$ possesses an ergoregion and no event horizon.