  The measurement of B-mode polarization of the cosmic microwave background at
large angular scales by the BICEP experiment suggests a stochastic
gravitational wave background from early-universe inflation with a surprisingly
large amplitude. The power spectrum of these tensor perturbations can be probed
both with further measurements of the microwave background polarization at
smaller scales, and also directly via interferometry in space. We show that
sufficiently sensitive high-resolution B-mode measurements will ultimately have
the ability to test the inflationary consistency relation between the amplitude
and spectrum of the tensor perturbations, confirming their inflationary origin.
Additionally, a precise B-mode measurement of the tensor spectrum will predict
the tensor amplitude on solar system scales to 20% accuracy for an exact power
law tensor spectrum, so a direct detection will then measure the running of the
tensor spectral index to high precision.
