  Upcoming searches for the stochastic background of inflationary gravitational
waves (GWs) offer the exciting possibility to probe the evolution of our
Universe prior to Big Bang nucleosynthesis. In this spirit, we explore the
sensitivity of future GW observations to a broad class of
beyond-the-Standard-Model scenarios that lead to a nonstandard expansion
history. We consider a new scalar field whose coherent oscillations dominate
the energy density of the Universe at very early times, resulting in a scalar
era prior to the standard radiation-dominated era. The imprint of this scalar
era on the primordial GW spectrum provides a means to probe well-motivated yet
elusive models of particle physics. Our work highlights the complementarity of
future GW observatories across the entire range of accessible frequencies.
