dark energy.

With respect to this problem, there have been two main directions of work:

-   Finding phenomenologically viable vacua (in the Particle Physics, e.g. [@Ibanez:2001nd] and/or cosmological, e.g. [@Kachru:2003sx] sense).

-   Find a vacuum-selection mechanism.

There has been no real progress in the second direction for many years.

The failure to solve the vacuum selection problem through some dynamical mechanism has favored recently a purely statistical approach in which one first has to explore and chart (“classify”) the space of vacua a.k.a. *Landscape*. In this approach, our Universe is the way it is because the probability of this kind of Universe is overwhelming. Of course, this way of thinking can be combined with different forms of the Anthropic Principle.

Charting the superstring landscape is a very difficult problem and some simplifications have been suggested: for instance, one could consider all supersymmetric String Theory vacua, which correspond to different kinds of supergravities [@Susskind:2003kw] or only the vacua with 4-dimensional Poincaré symmetry and a Calabi-Yau internal space, which correspond to $N=1,d=4$ supergravities and give Standard-Model-like theories [@Douglas:2003um]. One could also consider, as proposed by Van Proeyen [@kn:VP], all possible supergravities, even if the stringy origin of many of them is unknown 