  Gravitational waves have been directly detected and astronomical observations
indicate that our Universe has a positive cosmological constant $\Lambda$.
Nevertheless, a theoretical gauge-invariant notion of gravitational waves
arriving at infinity (escaping from the space-time) in the presence of a
positive $\Lambda$ has been elusive. We find the answer to this long-standing
gravitational puzzle and present a geometric, gauge-invariant radiation
condition at infinity.
