ScalingStacks

2.6 Basic length scales of the generic region [022G]

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2.6 Basic length scales of the generic region

We mentioned in the beginning that the generalized Calabi ansatz geometrically describes an iterated fibration, which is supposedly the model for the generic region near infinity on the noncompact Calabi-Yau X=X¯∖DX=\bar{X}\setminus D. Figuring out the order of magnitude of various length scales is essentially a matter of dimensional analysis. In the generic region tt is of order ∼1\sim 1, vv is smooth and of order ∼1\sim 1. Our homogeneous ansatz prescribes

u∼O⁡(|x|(n+2)/n),|d​u|∼O⁡(|x|2/n),|D2​u|∼O⁡(|x|(2−n)/n).u\sim O(|x|^{(n+2)/n}),\quad|du|\sim O(|x|^{2/n}),\quad|D^{2}u|\sim O(|x|^{(2-n)/n}).

We have |D2​u|≪1≪|d​u||D^{2}u|\ll 1\ll|du|. From the descriptions in section 2.1, the Hessian term is responsible for the base and torus direction of the metric, while d​udu is responsible for the effective Kähler class, and therefore the size of the fibres diffeomorphic to Y≃D1∩D2Y\simeq D_{1}\cap D_{2}. Thus

diam​(T2)=O⁡(|D2​u|1/2)=O⁡(|x|(2−n)/2​n),diam​(Y)=O⁡(|d​u|1/2)=O⁡(|x|1/n),\text{diam}(T^{2})=O(|D^{2}u|^{1/2})=O(|x|^{(2-n)/2n}),\quad\text{diam}(Y)=O(|du|^{1/2})=O(|x|^{1/n}),

the distance to the origin is of order

O⁡(∫|D2​u|1/2)=O⁡(|x|(2+n)/2​n),O(\int|D^{2}u|^{1/2})=O(|x|^{(2+n)/2n}),

and the volume within |x|≤r|x|\leq r is O⁡(r2)O(r^{2}) from the two log directions of the base. In terms of geodesic distance to the origin, the volume grows with power 4​n/(n+2)4n/(n+2).

For |x|≫1|x|\gg 1, this reaffirms the intuition that the T2T^{2} length scale is far smaller than Y≃D1∩D2Y\simeq D_{1}\cap D_{2}, which is far smaller than the real 2-dimensional base.

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