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 MP . 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 ∼ 1 TeV: V (b) ≡ MV ∼ 10−4MP . This gives a large radion mass and reheat temperature ∼ 10 GeV, thus avoiding the moduli problem. Such a value of MV 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 MV may hold even if inflation is driven by matter fields rather than by the radion.