# Quintessence, Branes and Higher Dimensions

## Summary

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.

### Role within the Transregional Collaborative Research Centre

There are strong connections between the theory groups in Bonn, Heidelberg and Munich, as can be
seen by the exchange of people (Dr. Lüdeling from Heidelberg to Bonn, Dr. Vaudrevange from Bonn
to Munich, Dr. Groot Nibbelink from Heidelberg to Munich) and by a number of joint publications.
Within the TR33, this will lead to
collaboration with the projects A2 and A3 which investigate inflationary and more formal aspects,
respectively. Furthermore, the search for indirect evidence about Dark Energy will certainly also
connect to Dark Matter, in particular A4 and C4, and the astrophysical observations of the B
projects. Finally, models of Dark Energy will possibly lead to modifications of gravity due to some
higher-dimensional effects (A6) and might help to answer the "Why Now" problem, project C6.

to top