  Light moduli fields in string compactifications can have interesting
implications for particle physics and cosmology. Fifth force bounds impose
stringent constraints on the interactions of such moduli with the visible
sector. To be consistent with the bounds, they need to be part of hidden
sectors which interact with the Standard Model with weaker-than-Planck
suppressed interactions. We consider scenarios in which the visible sector
degrees of freedom are localised in the compactification and light moduli arise
as closed string degrees of freedom associated with hidden sectors which are
geometrically separated (in the extra-dimensions) from the Standard Model.
Kinetic mixings lead to interactions between the moduli and the visible sector
- we compute these using Kaehler potentials of string/M-theory
compactifications. We argue that in general these interactions provide a lower
bound on the strength of the interactions between the moduli and the visible
sector. The interactions scale with inverse powers of the volume of the
compactification, thus fifth force bounds can be translated to lower bounds on
the volume of the extra-dimensions. We find that compactification volumes have
to be large to evade the bounds. This imposes interesting constraints on
quintessence model building in string theory. Our results for the strength of
the interactions can also be used to quantify the fine-tuning necessary for the
stability of the potential of a light modulus against quantum corrections
involving visible sector loops.
