I will describe recent results in gravitational wave observations of binary black hole mergers. By tracking a feature in the binary black hole mass spectrum across cosmic time it will be possible for Advanced LIGO and Virgo to measure the expansion history of the universe to few-percent accuracy at redshifts z ~ 0.7. Measurements at these redshifts are particularly interesting because they correspond to the transition from a matter-dominated to dark-energy-dominated universe; in concert with other percent-level cosmographical measurements, binary black hole observations could constrain the dark energy equation of state parameter to better than 10%. Because binary black hole mergers are standard(izable) sirens, these measurements are independent of any of the other distance ladders or standard rulers employed for cosmography. Binary black hole mergers also enable precision tests of general relativity as a theory of gravity. One such test is black hole spectroscopy---measurement of the normal modes of the spacetime near a black hole horizon through their gravitational wave emission---which is analogous to the use of atomic spectral lines to test quantum mechanics. I will explain the first-ever measurement of multiple modes of oscillation from a black hole spacetime (the remnant black hole from the first binary black hole merger observed by LIGO, GW150914) and discuss the future of such measurements and the constraining power they have over general relativity. In both black hole cosmography and spectroscopy, advanced-era gravitational wave detectors are delivering precision and power at a level not anticipated until the next generation ("3G") of gravitational wave detectors.
Will Farr earned his BS in Physics from Caltech in 2003, a PhD in Physics from MIT in 2010, was a postdoc and then CIERA fellow at Northwestern from 2010 to 2013, faculty (Lecturer, then Senior Lecturer) at U. Birmingham in the UK from 2013 to 2018, and is currently an Associate Professor at Stony Brook University and lead the Gravitational Wave Astronomy group at the CCA. Most of his work these days centers around the population of merging binary black holes detected and detectable in LIGO/Virgo; Will studies the population to understand the astrophysics of massive stellar binary evolution, to use as tracers of cosmic expansion for cosmography, and also as laboratories to detect potential deviations from the behavior predicted by general relativity. Will has a deep and abiding interest in (astro)statistics, particularly as applied and computed; and has dabbled in exoplanet dynamics and populations, asteroseismology, and astronomical time-series analysis.