  It has recently been pointed out that Gaia is capable of detecting a
stochastic gravitational wave background in the sensitivity band between the
frequency of pulsar timing arrays and LISA. We argue that Gaia and THEIA has
great potential for early universe cosmology, since such a frequency range is
ideal for probing phase transitions in asymmetric dark matter, SIMP and the
cosmological QCD transition. Furthermore, there is the potential for detecting
primordial black holes in the solar mass range produced during such an early
universe transition and distinguish them from those expected from the QCD
epoch. Finally, we discuss the potential for Gaia and THEIA to probe
topological defects and the ability of Gaia to potentially shed light on the
recent NANOGrav results.
