  Modified expansion rates in the early Universe prior to big bang
nucleosynthesis are common in modified gravity theories, and can have a
significant impact on the generation of dark matter, matter-antimatter
asymmetry, primordial black holes and the primordial gravitational wave (PGW)
spectrum. Here we study the PGW spectrum in modified gravity theories, in early
Universe cosmology. In particular, we consider scalar-tensor and
extradimensional scenarios, investigating the detection prospects in current
and future GW observatories. For the scalar-tensor case, PGW could be
potentially observed by laser interferometers operating in the high-frequency
range, while for the extradimensional case they could be detected even at low
frequencies with pulsar timing arrays. We find that data from the planned
network of several GW detectors operating across various frequency ranges could
be able to distinguish between various modified gravity scenarios.
