A fresh look at the gravitational-wave signal from cosmological phase
  transitions
  Many models of physics beyond the Standard Model predict a strong first-order
phase transition (SFOPT) in the early Universe that leads to observable
gravitational waves (GWs). In this paper, we propose a novel method for
presenting and comparing the GW signals that are predicted by different models.
Our approach is based on the observation that the GW signal has an
approximately model-independent spectral shape. This allows us to represent it
solely in terms of a finite number of observables, that is, a set of peak
amplitudes and peak frequencies. As an example, we consider the GW signal in
the real-scalar-singlet extension of the Standard Model (xSM). We construct the
signal region of the xSM in the space of observables and show how it will be
probed by future space-borne interferometers. Our analysis results in
sensitivity plots that are reminiscent of similar plots that are typically
shown for dark-matter direct-detection experiments, but which are novel in the
context of GWs from a SFOPT. These plots set the stage for a systematic model
comparison, the exploration of underlying model-parameter dependencies, and the
construction of distribution functions in the space of observables. In our
plots, the experimental sensitivities of future searches for a stochastic GW
signal are indicated by peak-integrated sensitivity curves. A detailed
discussion of these curves, including fit functions, is contained in a
companion paper [2002.04615]. The data and code that we used in our analysis
can be downloaded from Zenodo [https://doi.org/10.5281/zenodo.3699415].
