  Global cosmic strings are generically predicted in particle physics beyond
the Standard Model, e.g., a post-inflationary global $U(1)$ symmetry breaking
which may associate with axion-like dark matter. We demonstrate that although
subdominant to Goldstone emission, gravitational waves (GWs) radiated from
global strings can be observable with current or future GW detectors. The
frequency spectrum of such GWs is also shown to be a powerful tool to probe the
Hubble expansion rate of the Universe at times prior to the Big Bang
nucleosynthesis where the standard cosmology has yet to be tested.
