Did the early universe go through an inflationary phase?

Establish or refute an inflationary epoch by a signature that alternatives cannot reproduce, such as a nearly scale-invariant primordial tensor spectrum with tilt $n_t = -r/8$ (single-field consistency relation); refutation requires a demonstration that a non-inflationary model reproduces all CMB and large-scale-structure data including any detected tensor modes. For r below $\sim 0.01$ the consistency relation cannot be measured, and a yes answer then requires a detected tensor spectrum with amplitude and scale dependence that non-inflationary models are shown unable to produce.

Detection of primordial B-mode polarization with measured amplitude and tilt consistent with inflation, or an alternative model fitting the same data.