The Superhorizon Test of Future B-mode Experiments
  Inflation predicts B-mode polarization with correlations that span
superhorizon scales at recombination. In contrast, the correlations set up by
causal sources, such as phase transitions or defects, necessarily vanish on
superhorizon scales. Motivated by BICEP2's B-mode detection, we consider the
prospects for measuring the inflationary superhorizon signature in future
observations. We explain that the finite resolution of an experiment and the
filtering of the raw data induces a transfer of spurious subhorizon power to
superhorizon scales, and describe ways to correct for it. We also provide a
detailed treatment of possible sources of noise in the measurement. Finally, we
present forecasts for the detectability of the signal with future CMB
polarization experiments.
