In 1998, two groups of astrophysics discovered that the expansion of our
Universe is accelerating, in direct contradiction with our expectations
from General Relativity. This strange behaviour of our Universe could
either be due to a new form of energy, called dark energy, or to a
modification of the laws of gravity at large, cosmological scales. In this
talk I will discuss how cosmological observations can be used to test the
validity of General Relativity. I will present novel observables, that can be
used to search for deviations from the standard cosmological model.