Measurement of Cosmic Microwave Background Polarization Power Spectra From Two Years of Bicep Data

Background Imaging of Cosmic Extragalactic Polarization (B icep ) is a bolometric polarimeter designed to measure the inflationary B -mode polarization of the cosmic microwave background (CMB) at degree angular scales. During three seasons of observing at the South Pole (2006 through 2008), Bicep mapped ∼2% of the sky chosen to be uniquely clean of polarized foreground emission. Here, we present initial results derived from a subset of the data acquired during the first two years. We present maps of temperature, Stokes Q and U , E and B modes, and associated angular power spectra. We demonstrate that the polarization data are self-consistent by performing a series of jackknife tests. We study potential systematic errors in detail and show that they are sub-dominant to the statistical errors. We measure the E -mode angular power spectrum with high precision at 21 ⩽ ℓ ⩽ 335, detecting for the first time the peak expected at ℓ ∼ 140. The measured E -mode spectrum is consistent with expectations from a ΛCDM model, and the B -mode spectrum is consistent with zero. The tensor-to-scalar ratio derived from the B -mode spectrum is r = 0.02 +0.31 −0.26 , or r < 0.72 at 95% confidence, the first meaningful constraint on the inflationary gravitational wave background to come directly from CMB B -mode polarization.