Dark energy from strings

A long-standing problem of theoretical physics is the exceptionally small value of the cosmological constant, 10 exp -120, measured in natural Planckian units. Here we derive this tiny number from a toroidal string cosmology based on closed strings. In this picture, the dark energy arises from the correlation between momentum and winding modes that for short distances has an exponential fall-off with increasing values of the momenta. The freeze-out by the expansion of the background universe for these transplanckian modes may be interpreted as a frozen condensate of the closed-string modes in the three non-compactified spatial dimensions. (Author)