Limits on Direct Detection of Gravitational Waves

We compute energy density and strain induced by a primordial spectrum of gravitational waves on terrestrial- and space-based detectors (e.g., LIGO) as constrained by the COBE detection of microwave background anisotropy. For the case where the spectrum is created during inflation, we find new, stricter upper bounds on the induced strain, making detection unlikely. However, detectors might be useful for discovering (or ruling out) exotic, non-inflationary sources.