I've been trying to understand the whole BICEP story. Here's what I think I understand so far: Inflation produces primordial gravitational waves. During recombination, photons decouple from matter and the CMB is formed. Propagating gravity waves stretch space in one direction and squeeze it in an orthogonal direction. The presence of gravity waves during CMB formation would therefore redshift the wavelength of the photons in one direction and blueshift it in an orthogonal direction. Thus, gravity waves cause quadrupole anisotropies in the CMB temperature. Quadrupole temperature anisotropies lead to differing intensities of photons incident on any given electron from orthogonal directions, which in turn leads to the Thomson-scattered outgoing radiation having a linear polarization. This is one of several mechanisms to produce polarization in the CMB, but only this mechanism produces polarization patterns with nonzero curl ("B-mode" polarization). Please feel free to correct, clarify, or expand any of these points, but my primary question is about the very first one. Why do we expect that inflation should generate primordial gravitational waves?