Anisotropic inflation and the origin of four large dimensions

In the context of (4 + d)-dimensional general relativity, we propose an inflationary scenario wherein three spatial dimensions grow large, while d extra dimensions remain small. Our model requires that a self-interacting d-form acquires a vacuum expectation value along the extra dimensions. This causes three spatial dimensions to inflate, whilst keeping the size of the extra dimensions nearly constant. We do not require an additional stabilization mechanism for the radion, as stable solutions exist for flat, and for negatively curved compact extra dimensions. From a four-dimensional perspective, the radion does not couple to the inflaton; and, the small amplitude of the CMB temperature anisotropies arises from an exponential suppression of fluctuations, due to the higher-dimensional origin of the inflaton. The mechanism triggering the end of inflation is responsible both for heating the universe and for avoiding violations of the equivalence principle due to coupling between the radion and matter.