Large primordial gravitational wave background in a class of BSL ACDM models

We consider the primordial gravitational wave (GW) background in a class of spatially flat inflationary cosmological models with cold dark matter (CDM ), a cosmological constant, and a broken scale-invariant (BSI) step-like pr imordial (initial) spectrum of adiabatic perturbations produced in an exact ly solvable inflationary model where the inflaton potential has a rapid cha nge of its first derivative at some point. In contrast to inflationary mode ls with a scale-free initial spectrum, these models may have a GW power spe ctrum the amplitude (although not the shape) of which is arbitrary for fixe d amplitude and shape of the adiabatic perturbation power spectrum. In the presence of a positive cosmological constant, the models investigated here possess the striking property that a significant part of the large-angle co smic microwave background (CMB) temperature anisotropy observed in the COBE experiment is a result of primordial GW. Confronting the models with exist ing observational data on CMB angular temperature fluctuations, galaxy clus tering and peculiar velocities of galaxies, we find that, for the best para meter values Omega(Lambda) approximate to 0.7 and h approximate to 0.7, the GW contribution to the CMB anisotropy can be as large as that of the scala r fluctuations.