  We provide a first derivation of the Bekenstein-Hawking entropy of 3d flat
cosmological horizons in terms of the counting of states in a dual field
theory. These horizons appear in the shifted-boost orbifold of R^{1,2}, the
flat limit of non-extremal rotating BTZ black holes. These 3d geometries carry
non-zero charges under the asymptotic symmetry algebra of R^{1,2}, the 3d
Bondi-Metzner-Sachs (BMS3) algebra. The dual theory has the symmetries of the
2d Galilean Conformal Algebra, a contraction of two copies of the Virasoro
algebra, which is isomorphic to BMS3. We study flat holography as a limit of
AdS3/CFT2 to semi-classically compute the density of states in the dual,
exactly reproducing the bulk entropy in the limit of large charges. Our flat
horizons, remnants of the BTZ inner horizons also satisfy a first law of
thermodynamics. We comment on how the dual theory reproduces the bulk first law
and how cosmological bulk excitations are matched with boundary quantum
numbers.
