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 ) steplike primordial ( initial ) spectrum of adiabatic perturbations produced in an exactly solvable inflationary model where the inflaton potential has a rapid change of its first derivative at some point .
In contrast to inflationary models with a scale-free initial spectrum , these models may have a GW power spectrum whose amplitude ( though not its shape ) is arbitrary for fixed amplitude and shape of the adiabatic perturbations 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 CMB temperature anisotropy observed in the COBE experiment is due to primordial GW .
Confronting them with existing observational data on CMB angular temperature fluctuations , galaxy clustering and peculiar velocities of galaxies , we find that for the best parameter values \Omega _ { \Lambda } \approx 0.7 and h \approx 0.7 , the GW contribution to the CMB anisotropy can be as large as that of the scalar fluctuations .