2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/149336Models of string cosmology predict a stochastic background of gravitational waves with a spectrum that is strongly tilted towards high frequencies. I give simple approximate expressions for spectral densities of the cosmic background which can be directly compared with sensitivities of gravitational wave detectors.Extrapolation formula and one typo corrected. Latex, 6 pages, 2 eps figuresHigh Energy Physics - TheoryAstrophysicsGeneral Relativity and Quantum Cosmologytext