But now that the observation of the acceleration of
the expansion of the Universe favors a form of dark
energy with the properties of vacuum energy (e.g. an
equation of state parameter w ∼ −1), one has to tackle
the formidable task of providing an explanation to the
vacuum energy problem: the vacuum energy density ρΛ
scales like a mass to the fourth power and should thus
be of order m4
P in the context of quantum gravity: