Geometry and Symmetry Structures in 2T Gravity
  2T-gravity in d+2 dimensions predicts 1T General Relativity (GR) in d dimensions, augmented with a local scale symmetry known as the Weyl symmetry in 1T field theory. The emerging GR comes with a number of constraints, particularly on scalar fields and their interactions in 1T field theory. These constraints, detailed in this paper, are footprints of 2T-gravity and could be a basis for testing 2T-physics. Some of the conceptually interesting consequences of the "accidental" Weyl symmetry include that the gravitational constant emerges from vacuum values of the dilaton and other Higgs-type scalars and that it changes after every cosmic phase transition (inflation, grand unification, electroweak phase transition, etc.). We show that this consequential Weyl symmetry in d dimensions originates from coordinate reparametrization, not from scale transformations, in the d+2 spacetime of 2T-gravity. To recognize this structure we develop in detail the geometrical structures, curvatures, symmetries, etc. of the d+2 spacetime which is restricted by a homothety condition derived from the action of 2T-gravity. Observers that live in d dimensions perceive GR and all degrees of freedom as shadows of their counterparts in d+2 dimensions. Kaluza-Klein (KK) type modes are removed by gauge symmetries and constraints that follow from the 2T-gravity action. However some analogs to KK modes, which we call "prolongations" of the shadows into the higher dimensions, remain but they are completely determined, up to gauge freedom, by the shadows in d dimensions. 
Itzhak Bars and Shih-Hung Chen