Tensors, BICEP2, prior dependence, and dust
  We investigate the prior dependence on the inferred spectrum of primordial
tensor perturbations, in light of recent results from BICEP2 and taking into
account a possible dust contribution to polarized anisotropies. We highlight an
optimized parameterization of the tensor power spectrum, and adoption of a
logarithmic prior on its amplitude $A_T$, leading to results that transform
more evenly under change of pivot scale. In the absence of foregrounds the
tension between the results of BICEP2 and Planck drives the tensor spectral
index $n_T$ to be blue-tilted in a joint analysis, which would be in
contradiction to the standard inflation prediction ($n_T<0$). When foregrounds
are accounted for, the BICEP2 results no longer require non-standard
inflationary parameter regions. We present limits on primordial $A_T$ and
$n_T$, adopting foreground scenarios put forward by Mortonson & Seljak and
motivated by Planck 353 GHz observations, and assess what dust contribution
leaves a detectable cosmological signal. We find that if there is sufficient
dust for the signal to be compatible with standard inflation, then the
primordial signal is too weak to be robustly detected by BICEP2 if Planck+WMAP
upper limits from temperature and E-mode polarization are correct.
