Carl-Johan Haster, MIT, Kavli Institute for Astrophysics and Space Research

"When a black hole might not be a black hole"

Abstract: From the large population of black holes observed through the gravitational waves emitted in their mergers with other compact objects, we can start to test how well this population can collectively be described by General Relativity alone. By exploiting the properties of parameters that are universal across the entire population of observed binaries I will show that this universality present both statistically robust and physically interesting results.

I will here present results from two studies presenting different approaches in probing observations of the coalescing compact-object binary population.

Firstly, I will present such a universal variable that appears at several orders in the post-Newtonian expressions governing a binary with an orbit dominated by gravitational wave emission. This variable can, by allowing it to vary freely, therefore be used as a Null test of General Relativity if found to recover the value predicted from General Relativity when applied to a population of observations. I will present the results from such a study, and also show the robustness of this test against both physical and non-physical gravitational wave signals not described by General Relativity.

Secondly, even though the population of binary black hole observations can individually be described well as just that, a population of black hole binaries, many of the tests of the specific nature of these objects are either not general enough or are penalised by the low signal-to-noise ratios of individual observations. I will present an approach that covers both of these potential pitfalls. Through the use of an effective Equation of State, able to describe both black holes and several classes of mimickers, and by parametrising this Equation of State in terms of a Universal parameter I can put very strong limits on the specific nature of the population of compact objects and how much they can be deformed.