  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, \emph{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.
