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We construct an inflationary model in 6D supergravity that is based onexplicit time-dependent solutions to the full higher-dimensional fieldequations, back-reacting to the presence of a 4D inflaton rolling on aspace-filling codimension-2 source brane. Fluxes in the bulk stabilize allmoduli except the `breathing' modulus (that is generically present inhigher-dimensional supergravities). Back-reaction to the inflaton roll causesthe 4D Einstein-frame on-brane geometry to expand, a(t) ~ t^p, as well asexciting the breathing mode and causing the two off-brane dimensions to expand,r(t) ~ t^q. The model evades the general no-go theorems precluding 4D de Sittersolutions, since adjustments to the brane-localized inflaton potential allowthe power p to be dialed to be arbitrarily large, with the 4D geometry becomingde Sitter in the limit p -> infinity (in which case q = 0). Slow-roll solutionsgive accelerated expansion with p large but finite, and q = 1/2. Because theextra dimensions expand during inflation, the present-day 6D gravity scale canbe much smaller than it was when primordial fluctuations were generated -potentially allowing TeV gravity now to be consistent with the much highergravity scale required at horizon-exit for observable primordial gravity waves.Because p >> q, the 4 on-brane dimensions expand more quickly than the 2off-brane ones, providing a framework for understanding why the observed fourdimensions are presently so much larger than the internal two. If uplifted to a10D framework with 4 dimensions stabilized, the 6D evolution described herecould describe how two of the six extra dimensions evolve to become much largerthan the others, as a consequence of the enormous expansion of the 4 largedimensions we can see.

Sculpting the Extra Dimensions: Inflation from Codimension-2 Brane Back-reaction