Institute for Advanced Study / Princeton University Joint Astrophysics Colloquium

Recent breakthroughs in gravitational wave and electromagnetic observations have given better and better insights into black holes and extreme gravity. While one usually thinks of black holes as the unyielding final state of gravitational collapse, there are several features of black holes, and ultracompact objects in general, that bring them precipitously close to instability, with the black hole's horizon playing a crucial stabilizing role. I will talk about how the absence of a horizon in an ultracompact object generically gives rise to new instabilities, and some recent insights into the fate of these. The stability of spinning black holes itself depends on whether or not certain ultralight fields exist, which means that black holes can act as probes for dark particles. I will discuss how a good understanding of the nonlinear development of such instabilities allows one to harness strong gravity observations to test the black hole paradigm and look for new particles.