Does the BICEP2 Observation of Cosmological Tensor Modes Imply an Era of
  Nearly Planckian Energy Densities?
  BICEP2 observations, interpreted most simply, suggest an era of inflation
with energy densities of order ($10^{16}\, {\rm GeV})^4$, not far below the
Planck density. However, models of TeV gravity with large extra dimensions
might allow a very different interpretation involving much more modest energy
scales. We discuss the viability of inflation in such models, and conclude that
existing scenarios do not provide attractive alternatives to single field
inflation in four dimensions. Because the detection of tensor modes strengthens
our confidence that inflation occurred, it disfavors models of large extra
dimensions, at least for the moment.
