Extra dimensions present a new flatness problem

There is no known fundamental reason to demand as a cosmological initial condition that the bulk possess an $\mathrm{SO}(3,1)$ isometry. On the contrary, one expects bulk curvature terms that violate the $\mathrm{SO}(3,1)$ isometry at early epochs, leading to a violation of Lorentz invariance on our brane. Demanding that the Lorentz noninvariant terms are small leads to a new ``flatness'' problem, not solved by the usual formulation of inflation for a class of braneworld solutions. Furthermore, unlike in four dimensions, the Lorentz violations induced from the bulk curvature cannot always be removed as the infrared cutoff is taken arbitrarily large. This means that the equivalence principle in higher dimensions does not guarantee the equivalence principle in dimensionally reduced theories. Near-future experiments are expected to severely constrain these Lorentz-violating ``signatures'' of extra dimensions.