  A primordial spectrum of gravitational waves serves as a backlight to the
relativistic degrees of freedom of the cosmological fluid. Any change in the
particle physics content, due to a change of phase or freeze-out of a species,
will leave a characteristic imprint on an otherwise featureless primordial
spectrum of gravitational waves and indicate its early-Universe provenance. We
show that a gravitational wave detector such as the Laser Interferometer Space
Antenna would be sensitive to physics near 100 TeV in the presence of a
sufficiently strong primordial spectrum. Such a detection could complement
searches at newly proposed 100 km circumference accelerators such as the Future
Circular Collider at CERN and the Super Proton-Proton Collider in China,
thereby providing insight into a host of beyond Standard Model issues,
including the hierarchy problem, dark matter, and baryogenesis.
