Fingerprints of gravity on the cosmic microwave background

Physics Today : According to inflationary cosmologies, the anisotropies in the cosmic microwave background (CMB) manifest tiny quantum fluctuations in the primordial universe that are blown up by the rapidly expanding cosmos. Gravity, too, is presumed to be subject to quantum fluctuations and so the subtle distortions of spacetime imposed early on by gravitons—the particles responsible for gravity—should also be visible in the CMB. Yesterday, at a press conference held at the Harvard–Smithsonian Center for Astrophysics, members of the BICEP2 collaboration announced that they found gravity’s imprint. The research team’s telescope, located at the South Pole, peered through clear, cold, dry air and measured the polarization of CMB photons. Primordial gravitational waves revealed themselves as a tiny component of the polarization field that has a “handedness” (technically, the collaboration observed so-called B modes in the polarization field). The BICEP2 team took its measurements at angular scales of about 1–5 degrees; theoretically, at those scales, the B modes arising from gravitational waves are most readily distinguished from B-mode features generated by the relatively mundane gravitational lensing of CMB light traveling to observers on Earth. Explaining the details of the B-mode field observed by BICEP2 provides a new challenge to specific models of inflation.