Abstract: There is very good evidence that the initial spectrum of cosmological perturbations was laid down in the very early universe, either by inflation or one of its competitors, such as ekpyrosis. Present constraints on inflationary mechanisms (and alternatives) are insensitive to the subsequent evolution of the universe, but I will show that Planck and other next-generation astrophysical datasets effectively constrain the overall expansion history of the universe, rather than the inflationary phase on its own. Consequently, over the next decade(s) we will explore the "primordial dark ages" that lie between any inflationary phase and nucleosynthesis. While the universe must become radiation dominated before nucleosynthesis, the expansion history during the primordial dark ages is unknown, and depends on particle physics beyond the TeV scale. I will highlight processes that can cause this expansion history to differ from the "default case" of radiation domination, along with other probes of the primordial dark ages, including mechanisms that generate stochastic backgrounds of high-frequency gravitational radiation, which can provide an entirely new window into the very early universe.