[Wientje]

If I recall correctly, the dimensions of the early universe in these kinds of models depends on counting the degrees of freedom of the universe. At high energies, everything becomes an internal degree of freedom, hence the universe behaves two dimensional (space + time). At lower energies, more dof shift from internal to external and the universes behaves multidimensional.

It's a similar pov to 11D supergravity for example. Since 7dimensions are so small, the look like internal dof. The huge added advantage is that gravity simplifies below 4 dimensions and the incompatibilities between GR and quantummechanics disappear.

I'll try to use this bad analogy. Suppose you see a high speed train passing (high E), the people in train would seem to move at high speed aswell from left to right. If a slow train passes, you'll see they'll also move from the far side of the aisle to your side and perhaps from the upper level to the lower level. So you 1D high E train becomes 3D low E.