  We present the first end-to-end nonperturbative analysis of the gravitational
wave power spectrum from a thermal first-order electroweak phase transition
(EWPT), using the framework of dimensionally reduced effective field theory and
pre-existing nonperturbative simulation results. We are able to show that a
first-order EWPT in any beyond the Standard Model (BSM) scenario that can be
described by a Standard Model-like effective theory at long distances will
produce gravitational wave signatures too weak to be observed at existing and
planned detectors. This implies that colliders are likely to provide the best
chance of exploring the phase structure of such theories, while transitions
strong enough to be detected at gravitational wave experiments require either
previously neglected higher-dimension operators or light BSM fields to be
included in the dimensionally reduced effective theory and therefore
necessitate dedicated nonperturbative studies. As a concrete application, we
analyze the real singlet-extended Standard Model and identify regions of
parameter space with single-step first-order transitions, comparing our
findings to those obtained using a fully perturbative method. We discuss the
prospects for exploring the electroweak phase diagram in this model at collider
and gravitational wave experiments in light of our nonperturbative results.

High Energy Physics - Phenomenology;Cosmology and Nongalactic Astrophysics;High Energy Physics - Lattice;High Energy Physics - Theory