  Gravitational waves are a unique probe of the early Universe, as the Universe
is transparent to gravitational radiation right back to the beginning. In this
article, we summarise detection prospects and the wide scope of primordial
events that could lead to a detectable stochastic gravitational wave
background. Any such background would shed light on what (if anything) lies
beyond the Standard Model, sometimes at remarkably high scales. We overview the
range of strategies for detecting a stochastic gravitational wave background
before delving deep into three major primordial events that can source such a
background. Finally, we summarize the landscape of other sources of primordial
backgrounds.
