# Why holographic renormalization?

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Why is there a need to perform holographic renormalization for the normal $AdS_5\times S^5$/CFT$_4$ correspondence if the brane theory is conformal? Since the flow along the AdS direction $r$ is related to the renormalization scale does this not explicitly introduce an "energy" parameter that breaks conformal invariance in the SYM side of the duality?

This post imported from StackExchange Physics at 2015-02-05 10:16 (UTC), posted by SE-user Marion
retagged Feb 5, 2015

This post imported from StackExchange Physics at 2015-02-05 10:16 (UTC), posted by SE-user Qmechanic
Hi, thanks for the tip. The above question though I did not take it from a question out of a paper or so. It is my luck of understanding of the whole branch maybe.

This post imported from StackExchange Physics at 2015-02-05 10:16 (UTC), posted by SE-user Marion

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The fact that the boundary theory is conformal means that renormalization does not induce running of the coupling. However, there are divergences which have to be regularized and renormalized. The regularization requires the introduction of an arbitrary scale, which is not Weyl invariant and leads to a conformal anomaly (in even dimensions).

Correspondingly, also the bulk theory has to be regularized by introducing a cutoff $\epsilon$ on the radial coordinate. The supergravity fields have to be expanded close to the horizon and local counterterms have to be introduced to subtract the divergences when taking the limit of $\epsilon\to 0$. For the metric, the regularization procedure requires picking a reference metric $g_{(0)}$ from the conformal structure on the boundary. For $d$ (boundary dimension) even, the dependence of the counterterm on the chosen reference metric leads to a renormalized Lagrangian, that is not Weyl invariant. One picks up exactly the expected Weyl anomaly.

This is a very neat example of a connection of boundary UV physics (the cutoff) and bulk IR physics (divergences close to the boundary) which lead to the same Weyl anomaly.

For details, see the paper by Henningson and Skenderis. There are also these very instructive lecture notes on holographic renormalization with the example of renormalization of the action of a massive bulk scalar.

This post imported from StackExchange Physics at 2015-02-05 10:16 (UTC), posted by SE-user physicus
answered Feb 5, 2015 by (105 points)

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