  Null infinity in asymptotically flat spacetimes posses a rich mathematical
structure; including the BMS group and the Bondi news tensor that allow one to
study gravitational radiation rigorously. However, FLRW spacetimes are not
asymptotically flat because their stress-energy tensor does not decay
sufficiently fast and in fact diverges at null infinity. This class includes
matter- and radiation-dominated FLRW spacetimes. We define a class of
spacetimes whose structure at null infinity is similar to FLRW spacetimes: the
stress-energy tensor is allowed to diverge and the conformal factor is not
smooth at null infinity. Interestingly, for this larger class of spacetimes,
the asymptotic symmetry algebra is similar to the BMS algebra but not
isomorphic to it. In particular, the symmetry algebra is the semi-direct sum of
supertranslations and the Lorentz algebra, but it does not have any preferred
translation subalgebra. Future applications include studying gravitational
radiation in FLRW the full nonlinear theory, including the cosmological memory
effect, and also asymptotic charges in this framework.
