  Cosmic strings are generic cosmological predictions of many extensions of the
Standard Model of particle physics, such as a $U(1)^\prime$ symmetry breaking
phase transition in the early universe or remnants of superstring theory.
Unlike other topological defects, cosmic strings can reach a scaling regime
that maintains a small fixed fraction of the total energy density of the
universe from a very early epoch until today. If present, they will oscillate
and generate gravitational waves with a frequency spectrum that imprints the
dominant sources of total cosmic energy density throughout the history of the
universe. We demonstrate that current and future gravitational wave detectors,
such as LIGO and LISA, could be capable of measuring the frequency spectrum of
gravitational waves from cosmic strings and discerning the energy composition
of the universe at times well before primordial nucleosynthesis and the cosmic
microwave background where standard cosmology has yet to be tested. This work
establishes a benchmark case that gravitational waves may provide an
unprecedented, powerful tool for probing the evolutionary history of the very
early universe.
