BICEP's acceleration
  The recent BICEP2 detection of, what is claimed to be primordial $B$-modes,
opens up the possibility of constraining not only the energy scale of inflation
but also the detailed acceleration history that occurred during inflation. In
turn this can be used to determine the shape of the inflaton potential
$V(\phi)$ for the first time - if a single, scalar inflaton is assumed to be
driving the acceleration. We carry out a Monte Carlo exploration of
inflationary trajectories given the current data. Using this method we obtain a
posterior distribution of possible acceleration profiles $\epsilon(N)$ as a
function of $e$-fold $N$ and derived posterior distributions of the primordial
power spectrum $P(k)$ and potential $V(\phi)$. We find that the BICEP2 result,
in combination with Planck measurements of total intensity Cosmic Microwave
Background (CMB) anisotropies, induces a significant feature in the scalar
primordial spectrum at scales $k\sim 10^{-3}$ Mpc$^{-1}$. This is in agreement
with a previous detection of a suppression in the scalar power.
