Possible explanations of Dark Energy arise from a space-time in more than four dimensions,
characteristic of a theoretical unification with other fundamental interactions, as found e.g. in
string and M-theory. Such schemes offer new, unconventional and promising ways to address cosmological
questions. They also allow the investigation of a relation between the source of Dark Energy and other
questions, like inflation and Dark Matter of the universe. Light scalar fields related to Dark Energy
may correspond to moduli fields in compactified string theory that play a significant role in the
process of supersymmetry breaking. These in turn might be tested indirectly in particle physics
experiments. The adjustment of the vacuum energy in string theory often requires a specific uplifting
scheme from AdS space in higher dimensions, which leaves characteristic traces in the low energy
effective theories. It is now the time to consider explicit string theory realisations of particle
physics models and explore the wide range of applications towards an understanding of the nature of Dark
Energy and Dark Matter.
Another route explores the relation between quintessence and higher-dimensional dilatation symmetry.
Dilatation symmetry may be realised approximately if the late time behaviour of a cosmological runaway
solution approaches a fixed point. The light scalar field can then be associated to the Goldstone boson
of spontaneously broken dilatation symmetry. This cosmon field acquires a potential and a mass only due
to dilatation anomalies that vanish for asymptotically large time as the fixed point is approached.
Interesting suggestions that higher-dimensional dilatation symmetry forbids an effective
four-dimensional cosmological constant will be pursued. Time varying scalar fields may also lead to a
time variation of fundamental couplings. We will investigate the consequences for observational tests of
such variations.
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