Exotic smooth R4 and certain configurations of NS and D branes in string theory
http://arxiv.org/pdf/1101.3169.pdf 20110118

4. Discussion and conclusions
In this paper we tried to give a partial answer to the important question: Is it possible
that string theory deals with 4-dimensional structures directly neither by im-plementing
compactifications nor by phenomenological models-building, and these structures would have a physical meaning?

補足
http://ja.wikipedia.org/wiki/NS5%E3%83%96%E3%83%AC%E3%83%BC%E3%83%B3
NS5ブレーン(NS5-brane)とは、超弦理論に存在する5次元的に広がった物体である。