  Motivated by aLIGO's recent discovery of gravitational waves we discuss
signatures of new physics that could be seen at ground and space-based
interferometers. We show that a first order phase transition in a dark sector
would lead to a detectable gravitational wave signal at future experiments, if
the phase transition has occurred at temperatures few orders of magnitude
higher than the electroweak scale. The source of gravitational waves in this
case is associated with the dynamics of expanding and colliding bubbles in the
early universe. At the same time we point out that topological defects, such as
dark sector domain walls, may generate a detectable signal already at aLIGO.
Both -- bubble and domain wall -- scenarios are sourced by semi-classical
configurations of a dark new physics sector. In the first case the
gravitational wave signal originates from bubble wall collisions and subsequent
turbulence in hot plasma in the early universe, while the second case
corresponds to domain walls passing through the interferometer at present and
is not related to gravitational waves. We find that aLIGO at its current
sensitivity can detect smoking-gun signatures from domain wall interactions,
while future proposed experiments including the fifth phase of aLIGO at design
sensitivity can probe dark sector phase transitions.
