On the Gauge Invariance of Cosmological Gravitational Waves

The issue of the gauge invariance of gravitational waves arises if they are produced in the early universe at second-order in perturbation theory. We address it by dividing the discussion about the gauge invariance in three parts: the production of gravitational waves, their propagation in the real universe, and their measurement.
Comment: 26 pages, 3 figures, version accepted by JCAP