Tiago Mesquita Santos Verissimo: Extension of Symmetries in the Expanding Universe

This master's thesis is a contribution to the rigidity problem for black holes in the case where the cosmological constant \(\Lambda\) is non-zero. We generalise the results of Ionescu and Klainerman in the problem of the extension of Killing vector fields from the assumption that \(\Lambda=0\) to the more general assumption that \(\Lambda \in \mathbb{R}\).