Multi-mode black hole spectroscopy: detection and parameter estimation

As current gravitational-wave detectors undergo technological improvements, we will soon reach an era when it would be possible to identify black hole merger remnants by measuring their quasinormal mode frequencies. This idea, often called “black hole spectroscopy”, is similar to identifying atomic elements through their spectral lines. I will address the question of event rates and detectability of quasinormal modes with current and planned detectors. I will show that while second-generation detectors are incapable of observing subdominant modes, space-based detector - like LISA - will see all harmonics currently available from numerical simulations. I will also explain how the detection of multiple ringdown modes can help in parameter estimation by breaking various degeneracies. This degeneracy breaking will be very important when detectors see only the ringdown (i.e., for intermediate-mass black hole mergers observed by ground-based detectors and for the heavier supermassive black hole mergers observed by LISA).