Probing BSM with gravitational waves over 20 decades of frequency

Due to their extremely weak interactions, gravitational waves are a very promising, but also challenging, probe of the early Universe. In this talk I will highlight some promising sources of strong stochastic gravitational wave backgrounds, and illustrate the lessons for the underlying BSM models that can be learned from a detection or null-detection of such signals. I will in particular focus on metastable cosmic strings, which can be probed by pulsar timing arrays and current laser interferometers, as well as on inflation models, which will be probed by the next generation of laser interferometers. I will also discuss challenges and opportunities related to the measurement of stochastic gravitational wave backgrounds more broadly, including the measurement of chirality and new ideas to search for gravitational waves at MHz and GHz frequencies.