link between QCD and a higher dimensional theory in a curved space-time.

D-branes and $AdS$ Space
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The most important property of D-branes is that they contain gauge theories on their world volume. In particular, the massless spectrum of open strings living on a Dp-brane contains a (maximally supersymmetric) U(1) gauge theory in p + 1 dimensions. Moreover, it appears that if we consider the stack of N coincident D-branes, then there are $N^2$ different species of open strings which can begin and end on any of the D-branes, allowing us to have (maximally supersymmetric) U(N) gauge theory on the world-volume of these D-branes. Now, if N is sufficiently large, then this stack of D-branes is a heavy object embedded into a theory of closed strings that contains gravity. This heavy object curves the space which can then be described by some classical metric and other background fields.

Thus, we have two absolutely different descriptions of the stack of coincident Dp-branes. One description is in terms of the U(N) supersymmetric gauge theory on the world volume of the Dp-branes, and the other is in terms of the classical theory in some gravitational background. It is this idea that lies at 