The Uncertain Astrophysical Origins of Gravitational Waves

The groundbreaking discoveries of the LIGO experiment have opened a new window on the Universe.  Direct observation of gravitational waves (GWs) has revealed new, previously unknown populations of compact objects and enabled unprecedented tests of general relativity.  In the future, joint ("multimessenger") GW and electromagnetic observations of the same source will enable standard siren cosmology, permitting measurements of cosmological parameters independent of standard sources of systematic error.  Despite the incredible accomplishments and future potential of GW astronomy, a basic, zeroth-order question remains unanswered: what is the astrophysical process that generates the observed sources of GW radiation?  This question is particularly pressing for the black hole binaries (BHBs) that make up the large majority of LIGO/Virgo signals seen to date, as theorists have proposed at least half a dozen distinct formation scenarios that may dominate the overall event rate.  I will survey the complex landscape of ways in which the Universe may assemble BHB systems, emphasizing the empirical tests that could differentiate different formation channels from one another.  Looking towards the future, when space-based laser interferometers such as LISA will find low-frequency GW signals, similar questions emerge concerning "extreme mass ratio inspirals" (EMRIs), the GWs produced when stellar-mass black holes inspiral into supermassive ones.  In the last part of my talk, I will discuss the different formation pathways for these EMRI GWs, which serve as a primary science target for LISA due to the enormous signal-to-noise ratios they accumulate.  For EMRIs, the uncertain contributions of different astrophysical channels is even more consequential, as accurate waveform modeling of EMRIs involves unsolved problems in gravitational self-force, which may be of greater or lesser difficulty depending on which formation scenario predominates.