Following Dirac's brane variation prescription, the brane must not be deformed during the variation process, or else the linearity of the variation may be lost. Alternatively, the variation of the brane is done, in a special Dirac frame, by varying the bulk coordinate system itself. Imposing appropriate Dirac style boundary conditions on the constrained 'sandwiched' gravitational action, we show how Israel junction conditions get relaxed, but remarkably, all solutions of the original Israel equations are still respected. The Israel junction conditions are traded, in the Z2-symmetric case, for a generalized Regge-Teitelboim type equation (plus a local conservation law), and in the generic Z2-asymmetric case, for a pair of coupled Regge-Teitelboim equations. The Randall-Sundrum model and its derivatives, such as the Dvali-Gabadadze-Porrati and the Collins-Holdom models, get generalized accordingly. Furthermore, Randall-Sundrum and Regge-Teitelboim brane theories appear now to be two different faces of the one and the same unified brane theory. Within the framework of unified brane cosmology, we examine the dark matter/energy interpretation of the effective energy/momentum deviations from General Relativity.
I. INTRODUCTIONIn an almost forgotten paper [1] entitled "An extensible model of the electron", Dirac has made an attempt to picture a classical spinless electron as a breathing bubble in the electromagnetic field 'with no constraints fixing its size and shape'. Some positive surface tension has been invoked in order 'to prevent the electron from flying apart under the Coulomb repulsion of it surface charge'. On the practical side, this naive model has not made any impact on particle physics, so in this paper, we make no use of the model itself. On the field theoretical side, however, although gravity was switched off in this paper, it was nonetheless the first brane model. The equations of motion are derivable from an action principle, and in particular, the model offers a detailed prescription how to consistently perform brane variation. In the present paper, we switch on gravity, and apply the Dirac brane variation prescription to modern brane theories.Our main result is that the Israel[2] junction conditions (IJC), which are known to play a central role in all modern brane theories, get in fact relaxed. While every IJC solution is still strictly respected, it represents now a whole (continuous) family of new solutions. This opens the door for brane unification. To be more specific, as schematically illustrated in Fig.1 The Randall-Sundrum theory is very well known, and has rightly attracted lots of attention from General Relativity [7], cosmology [8] in particular, and also from string theory[9] points of view. See Ref.[10] for some brane world reviews, and Ref.[11] for some earlier brane models. The much older Regge-Teitelboim theory, on the other hand, a generalized Nambu-Goto type brane theory [3] for quantum gravity, has remained quite unfamiliar. This is partially due to the fact that the theory was o...