  Gravitational waves provide a novel and powerful way to test astrophysical
models of compact objects, early universe processes, beyond the Standard Model
particle physics, dark matter candidates, Einstein's theory of General
Relativity and extended gravity models, and even quantum gravity candidate
theories. A short introduction to the gravitational-wave background and the
method we are using to detect it will be presented. Constraints on various
astrophysical/cosmological models from the non-detectability of the
gravitational-wave background will be discussed. Gravitational waves from
transients will be highlighted and their physical implications will be
summarised.
