It remains also possible that, through some unknown
mechanism, vacuum energy is vanishing and that the acceleration
of the expansion of the Universe is due to
some other cause. It has become customary to envisage
two different classes of solutions, based on which
side of Einstein’s equations (1) one modifies: modifications
of the left-hand side are considered as modifications
of gravity itself; modifications of the right-hand
side amount to introduce a new component (or more
generally new components) to the energy-momentum
tensor, the so-called dark energy. The former modifi-
cations lead to modified Friedmann equations, the latter
to new contributions to the right-side of the standard
Friedmann equation (5).