Implications of a positive cosmological constant for general relativity

Most of the literature on general relativity over the last century assumes that the cosmological constant Lambda is zero. However, by now independent observations have led to a consensus that the dynamics of the universe is best described by Einstein's equations with a small but positive Lambda. Interestingly, this requires a drastic revision of conceptual frameworks commonly used in general relativity, no matter how small Lambda is. We first explain why, and then summarize the current status of generalizations of these frameworks to include a positive Lambda, focusing on gravitational waves.