  The cosmological constant $\Lambda$ used to be a freedom in Einstein's theory
of general relativity, where one had a proclivity to set it to zero purely for
convenience. The signs of $\Lambda$ or $\Lambda$ being zero would describe
universes with different properties. For instance, the conformal structure of
spacetime directly depends on $\Lambda$: null infinity $\mathcal{I}$ is a
spacelike, null, or timelike hypersurface, if $\Lambda>0$, $\Lambda=0$, or
$\Lambda<0$, respectively. Recent observations of distant supernovae have
taught us that our universe expands at an accelerated rate, and this can be
accounted for by choosing $\Lambda>0$ in Einstein's theory of general
relativity. A quantity that depends on the conformal structure of spacetime,
especially on the nature of $\mathcal{I}$, is the Bondi mass which in turn
dictates the mass loss of an isolated gravitating system due to energy carried
away by gravitational waves. This problem of extending the Bondi mass to a
universe with $\Lambda>0$ has spawned intense research activity over the past
several years. Some aspects include a closer inspection on the conformal
properties, working with linearisation, attempts using a Hamiltonian
formulation based on "linearised" asymptotic symmetries, as well as obtaining
the general asymptotic solutions of de Sitter-like spacetimes. We consolidate
on the progress thus far from the various approaches that have been undertaken,
as well as discuss the current open problems and possible directions in this
area.
