Fifth force in string theory I can't seem to get a clear description of what the "fifth force" in string theory is. What is the fifth force in string theory? What does it do? What mediates it? 
 A: This is a good question. At a simplified level, string theory introduces a large number of new scalar fields associated with the shape and size of extra dimensions. These so-called moduli fields can be thought of as force-carrying bosons in certain situations.
Most of the time when you compactify string theory you want to control the moduli fields, so that they're stable and don't do much. In a sense that's because we only observe four fundamental forces. But you can build models in which these moduli fields act as further forces, for example to explain inflation.
Nowadays people usually include branes in their models. These give rise to yet more fields which could lead to new forces. A key problem is to create a model where you just reproduce the known interactions at low energies.
To conclude, string theory has a variety of candidates for "fifth forces". You can build models using compactifications and branes which give new force fields, to explain inflation for instance. It's likely that cosmology will give us the best evidence for the veracity of such theories, since in principle you can observe the relics of extremely high energy events (like the Big Bang).
