Description
The spectrum of gravitational-wave sources is rapidly broadening. At the higher-frequency end, the LIGO-Virgo-KAGRA collaboration detects black-hole mergers at a rate of roughly one every other day when the detectors are operating. At the lower-frequency end, pulsar timing arrays have found strong evidence for a stochastic gravitational-wave background. The intermediate frequencies will be measured by the LISA detector in the 2030s. These observatories are measuring the properties of curved spacetime from the most strongly gravitating and dynamically evolving astrophysical sources that have been discovered. General relativity predicts that the gravitational waves from these sources will have distinctive features that arise from nonlinear interactions of gravitational waves themselves as they propagate away from an isolated source. After reviewing the latest status of gravitational-wave observations, I will focus on one of the more distinctive nonlinear phenomena: the gravitational-wave memory effect. Its observational feature is a lasting change in the gravitational-wave strain before and after a burst of gravitational waves pass through a detector. I will discuss the features of the memory effect that gravitational-wave detectors can measure and discuss the measurement prospects for the effect, which has yet to be detected. I will briefly discuss the connection between the memory effects, soft theorems, Bondi-Metzner-Sachs symmetry, and the conserved charges. Finally, I will also describe a hierarchy of higher memory effects that are more challenging to measure, but which could be related to aspects of celestial holography.
