Unification of extremal length geometry on Teichmuller space via intersection number

In this paper, we give a framework for the study of the extremal length geometry of Teichm"uller space after S. Kerckhoff, F. Gardiner and H. Masur. There is a natural compactification using extremal length geometry introduced by Gardiner and Masur. The compactification is realized in a certain projective space. We develop the extremal length geometry in the cone which is defined as the inverse image of the compactification via the quotient mapping. The compactification is identified with a subset of the cone by taking an appropriate lift. The cone contains canonically the space of measured foliations in the boundary. We first extend the geometric intersection number on the space of measured foliations to the cone, and observe that the restriction of the intersection number to Teichm"uller space is represented explicitly by the formula in terms of the Gromov product with respect to the Teichm"uller distance. From this observation, we deduce that the Gromov product extends continuously to the compactification. As an application, we obtain an alternative approach to Earle-Ivanov-Kra-Markovic-Royden’s characterization of isometries. Namely, with some few exceptions, the isometry group of Teichm"uller space with respect to the Teichm"uller distance is canonically isomorphic to the extended mapping class group. We also obtain a new realization of Teichm"uller space, a hyperboloid model of Teichm"uller space with respect to the Teichm"uller distance.