As far as I am aware there are two pictures of branes:
- for gSN≫1: branes as solitonic solutions to the field equations of supergravity (i.e. black branes)
- for gSN≪1: branes as fundamental objects, on which we can build σ-models (e.g. the fundamental string).
where gS is the string coupling and N is the units of flux or number of branes in a stack.
My understanding is that these two are seen as complementary pictures of the same objects, e.g. a condensate of fundamental p-branes turns into a black brane. However, I am not sure whether the black brane picture is valid at strong string coupling too. Some seemingly incompatible excerpts from the literature:
- In Maldacena's 1997 paper, it is mentioned that the black brane limit gSN≫1 is understood to be at small gS. Indeed, in the classical review hep-th/9905111, the limit is written as N>gSN≫1 to emphasise this.
- From Clifford Johnson's D-brane primer: gSN≫1 "corresponds to either having N small and gS large, or vice versa".
So I am not sure what to believe: are black branes (as solutions to supergravity) at small gS, or at large gSN with gS free to take any value (as long as gSN≫1)? I see reasons for both:
- Since we are talking about (10-d) supergravity solutions, we ought to be at small gS (in addition to small α′), since SUGRA is the low-energy limit of string theory (which is perturbative in gS). However, I guess as we increase gS we would simply go to 11-d SUGRA, so perhaps gS need not be small?
- In the black brane picture, the branes backreact so much that they create the geometry. This makes sense if gS is large. Also, the large gS argument seems to me to be at the heart of entropy calculations of black holes.
Or is the whole point that (some) branes are BPS objects? And so even if originally the SUGRA solutions were at small gS (since it is SUGRA we're talking about), they are protected objects and we may follow them at strong coupling too, and so effectively the black brane picture is valid at any gS?
Many thanks.