  Pre--big bang models of inflation based on string cosmology produce a
stochastic gravitational wave background whose spectrum grows with decreasing
wavelength, and which may be detectable using interferometers such as LIGO. We
point out that the gravitational wave spectrum is closely tied to the density
perturbation spectrum, and that the condition for producing observable
gravitational waves is very similar to that for producing an observable density
of primordial black holes. Detection of both would provide strong support to
the string cosmology scenario.

General Relativity and Quantum Cosmology;Astrophysics;High Energy Physics - Theory