SO colloquio: To be black, or not?

Observational tests of strong field gravity are improving rapidly. This allows us to test whether the compact objects observed in the sky are truly black holes as described in general relativity or some other "exotic" objects. These tests, however, require exquisite theoretical modeling of black holes as well as their alternatives. This talk will discuss two such examples: the impact of the astrophysical environment on black holes and the stability of black hole alternatives. In particular, I will discuss an astrophysical effect important in extreme mass ratio inspirals (whereby a stellar mass object spirals into a super massive black hole): tidal resonances. If not modeled carefully, these resonances may incorrectly be identified as beyond GR effects. Next, I will show that a popular class of black hole alternatives with membranes are unstable whenever the membrane's pressure and energy density have the same sign.