Prospects for direct detection of primordial gravitational waves

We study the primordial gravitational wave background produced in models of single-field inflation. Using the inflationary flow approach, we investigate the amplitude of gravitational wave spectrum, ωgw, in the frequency range 1 mHz--1 Hz pertinent to future space-based laser interferometers. For models that satisfy the current observational constraint on the tensor-to-scalar ratio, r0.36, we derive a strict upper bound of ωgw1.6×10^-15 independent of the form of the inflationary potential. Applying, in addition, the observational constraints on the spectral index nₛ and its running, ωgw is expected to be considerably lower than this bound unless the shape of the potential is finely tuned. We contrast our numerical results with those based on simple power-law extrapolation of the tensor power spectrum from cosmic microwave background (CMB) scales. In addition to single-field inflation, we summarize a number of other possible cosmological sources of primordial gravitational waves and assess what might be learned from direct detection experiments such as LISA, Big Bang Observer and beyond.