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The central idea of brane cosmology is that the visible, three-dimensional universe is restricted to a brane inside a higher-dimensional space, called the "bulk". What is "bulk"? Is this a higher-dimensional brane?

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The bulk is the higher-dimensional space in which our brane lives. It need not be a brane, although it could be.

As an example, in the Randall-Sundrum model, our observable Universe is a brane living at one end of a warped-throat geometry. The word "bulk" describes the full spacetime.

It's also useful to think of an analogy to condensed matter physics. Let's say you have some chunk of material, with paint on one surface. The painted surface is like the brane, and the "chunk" or "bulk" of the material is, well, like the bulk.

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