Inflationary Tensor Perturbations after BICEP2

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.

Figure 1

Likelihood countours in the $r\text{−}{n}_T$ plane for the model polarization experiments in Table 1. The fiducial model is $r=0.2$ and $n_T=-0.025$, indicated by $*$. The dotted line indicates the inflation consistency relation. The vertical line indicates $n_T=0$ for reference. (Left) Full lensing contribution to the cosmic variance error, (center) residual lensing contribution after delensing (see Table 1 for residual lensing noise levels), and (right) no lensing contribution to the cosmic variance, for comparison.