  Many motivated extensions of the Standard Model predict the existence of
cosmic strings. Gravitational waves originating from the dynamics of the
resulting cosmic string network have the ability to probe many otherwise
inaccessible properties of the early universe. In this study we show how the
spectrum of gravitational waves from a cosmic string network can be used to
test the equation of state of the early universe prior to Big Bang
Nucleosynthesis (BBN). We also demonstrate that current and planned
gravitational wave detectors such as LIGO, LISA, DECIGO/BBO, and ET/CE have the
potential to detect signals of a non-standard pre-BBN equation of state and
evolution of the early universe (e.g., early non-standard matter domination or
kination domination) or new degrees of freedom active in the early universe
beyond the sensitivity of terrestrial collider experiments and cosmic microwave
background measurements.
