Inflation in Flatland
  We investigate the symmetry structure of inflation in 2+1 dimensions. In
particular, we show that the asymptotic symmetries of three-dimensional de
Sitter space are in one-to-one correspondence with cosmological adiabatic modes
for the curvature perturbation. In 2+1 dimensions, the asymptotic symmetry
algebra is infinite-dimensional, given by two copies of the Virasoro algebra,
and can be traced to the conformal symmetries of the two-dimensional spatial
slices of de Sitter. We study the consequences of this infinite-dimensional
symmetry for inflationary correlation functions, finding new soft theorems that
hold only in 2+1 dimensions. Expanding the correlation functions as a power
series in the soft momentum $q$, these relations constrain the traceless part
of the tensorial coefficient at each order in $q$ in terms of a lower-point
function. As a check, we verify that the ${\cal O}(q^2)$ identity is satisfied
by inflationary correlation functions in the limit of small sound speed.
