almost independently of initial conditions. One interesting way to exit the string landscape is through MSSM inflation. Note that, when MSSM inflation starts, the “bare” cosmological constant (that not associated with the MSSM inflaton) might still be considerably larger than the present value. This means that further instanton decays should take place to reduce the bare cosmological constant, and these decays should occur during MSSM inflation in order to percolate efficiently. Fortu- nately, MSSM inflation naturally includes a self-reproduction (eternal inflation) regime prior to slow-roll [87–89], see Sec. V E 3. There are other interesting paradigms, such as string gas cosmology and bouncing cosmol- ogy, for a review see [939]. The basic setup relies on stringy ingredients, such as statistical properties of a gas of strings and branes, and some aspects of string field theory. Their pri- mary aim is to explain the seed perturbations for CMB without invoking cosmic inflation, see also [137, 138, 940]. However, all these scenarios suffer from the same symptoms, they require the universe to be exponentially large from the very beginning [941]. In [942], Brandenberger and Vafa (BV) proposed a seemingly very natural initial condition for cosmology in string theory. In BV cosmology, all nine spatial dimensions are compact (and toroidal in the simplest case) and initially at the string radius. The matter content of the universe is provided by a Hagedorn temperature gas of strings. In addition to proposing a very interesting initial condition and analyzing the thermodynamics of string at that point, however, BV argued that string theory in such a background provides a natural mechanism for decompactifying up to three spatial dimensions (that is, allowing three spatial dimensions to become macroscopic). The BV mechanism works because winding strings provide a negative pressure, which causes contraction of the scale factor, as was shown explicitly in [943, 944]. BV then gives a classical argument that long winding strings can only cross each other in three or fewer large spatial dimensions. Therefore, since winding strings freeze out quickly in four or more large spatial dimensions, the winding strings would cause re-collapse 1. String gas cosmology E. Other stringy paradigms 236