Inflation from extra dimensions

The radial mode of n extra compact dimensions (the radion b) can cause inflation in theories where the fundamental gravity scale M is smaller than the Planck scale M-P. For radion potentials V(b) with a simple polynomial form, to get the observed density perturbations, the energy scale of V(b) must greatly exceed M similar to 1 TeV: V(b)(1/4)=M(V)similar to 10(-4) M-P. This gives a large radion mass and reheat temperature similar to 10(9) GeV, thus avoiding the moduli problem. Such a value of M-V can be consistent with the classical treatment if the new dimensions started sufficiently small. A new possibility is that b approaches its stable value from above during inflation. The same conclusions about M-V may hold even if inflation is driven by bulk matter fields rather than by the radion.