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We study gravitational radiation for a positive value of the cosmological
constant $\Lambda$. We rely on two battle-tested procedures: (i) We start from
the same null coordinate system used by Bondi and Sachs for $\Lambda = 0$, but,
introduce boundary conditions adapted to allow radiation when $\Lambda>0$. (ii)
We determine the asymptotic symmetries by studying, \`a la Regge-Teitelboim,
the surface integrals generated in the action by these boundary conditions. A
crucial difference with the $\Lambda=0$ case is that the wave field does not
vanish at large distances, but is of the same order as de Sitter space. This
novel property causes no difficulty; on the contrary, it makes quantities
finite at every step, without any regularization. The asymptotic symmetry
algebra consists only of time translations and space rotations. Thus, it is not
only finite-dimensional, but smaller than de Sitter algebra. We exhibit
formulas for the energy and angular momentum and their fluxes. In the limit of
$\Lambda$ tending to zero, these formulas go over continuously into those of
Bondi, but the symmetry jumps to that of Bondi, Metzner and Sachs. The
expressions are applied to exact solutions, with and without radiation present,
and also to the linearized theory. All quantities are finite at every step, no
regularization is needed.
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