Primordial Gravitational Wave Detectability with Deep Small-Sky CMB
  Experiments
  We use Bayesian estimation on direct T-Q-U CMB polarization maps to forecast
errors on the tensor-to- scalar power ratio r, and hence on primordial
gravitational waves, as a function of sky coverage fsky. This method filters
the quadratic pixel-pixel space into the optimal combinations needed for r
detection for cut skies, providing enhanced information over a first-step
linear separation into a combination of E, B and mixed modes, and ignoring the
latter. With current computational power and for typical resolutions
appropriate for r detection, the large matrix inversions required are accurate
and fast. We explore two classes of experiments. One is motivated by a long
duration balloon experiment like Spider, with pixel noise \propto \sqrt{fsky}
for a specified observing period, but also applies to ground-based array
experiments. We find that, ignoring systematic effects and foregrounds, an
experiment with Spider-like noise over fsky ~0.02-0.2 could place a 2sigma_r
~0.014 (~95% CL) bound, rising to 0.02 with an L-dependent foreground residual.
We contrast this with a Planck-like fixed instrumental noise as fsky varies,
which gives a Galaxy-masked (fsky = 0.75) 2sigma_r ~0.015, rising to ~0.05 with
the foreground residuals. Using for a figure of merit the (marginalized) 1D
Shannon entropy of r, taken relative to the first 2003 WMAP1 CMB-only
constraint, gives -1.7 (-1.9) bits from the 2010 WMAP7+ACT (2011 WMAP7+SPT)
data, forecasts of -6 bits from Spider (plus Planck), of up to -11 bits for
post-Planck satellites and -13 bits for a perfect cosmic variance limited
experiment. We thus confirm the wisdom of the current strategy for r detection
of deeply probed patches covering the fsky minimum-error trough with balloon
and ground experiments.
