We study field theory models in the context of a gravitational theory without the cosmological constant problem (CCP). The theory is based on the requirement that the measure of integration in the action is not necessarily √ −g but it is determined dynamically through additional degrees of freedom, like four scalar fields ϕa. We study three possibilities for the general structure of the theory: (A) The total action has the form S = ΦLd 4 x where the measure Φ is built from the scalars ϕa in such a way that the transformation L → L + const does not effect equations of motion. Then an infinite dimensional shifts group of the measure fields (SGMF) ϕa by arbitrary functions of the Lagrangian density L, ϕa → ϕa + fa(L), is recognized as the symmetry group of the action up to an integral of a total divergence. (B) The total action has the form S = S1 + S2, S1 = ΦL1d 4 x, S2 = √ −gL2d 4 x which is the only model different from (A) and invariant under SGMF (but now with fa = fa(L1)). Similarly, now only S1 satisfies the requirement that the transformation L1 → L1 + const does not effect equations of motion. Both in the case (A) and in the case (B) it is assumed that L, L1, L2 do not depend on ϕa. (C) The action includes a term which breaks the SGMF symmetry. It is shown that in the first order formalism, a constraint appears which allows us to solve the scalar field related to the dynamical measure degrees of freedom, in terms of matter fields. The remarkable feature of models discussed in this paper is that for all cases ((A), (B) and (C)), after the change of variables to the conformal Einstein frame, the classical field equations take exactly the form of General Relativity (GR). Therefore the models are free from the well known problem of the usual scalar-tensor theories in what is concerned with the classical GR tests. The only difference of the field equations in the Einstein frame from the canonical equations of the selfconsistent system of Einstein's gravity and matter fields, is the appearance of the effective scalar field potential which vanishes in a true vacuum state (TVS) without fine tuning in cases (A) and (B). To illustrate how the theory works, we present a few explicit field theory models where it is possible to combine the solution of the cosmological constant problem (CCP) with: 1) possibility for inflationary scenario; 2) spontaneously broken gauge unified theories (including fermions). In the case (C), the breaking of the SGMF symmetry induces a nonzero energy density for the TVS. When considering only a linear potential for a scalar field φ in S1, the continuous symmetry φ → φ + const is respected. Surprisingly, in this case SSB takes place while no massless ("Goldstone") boson appears. We discuss the role of the SGMF symmetry for quantization and the possible connection of this theory with theories of extended objects.

We develop the principle of nongravitating vacuum energy, which is implemented by changing the measure of integration in the action from ͱϪgd D x to an integration in an internal space of D scalar fields a . As a consequence of such a choice of the measure, the matter Lagrangian L m can be changed by adding a constant while no cosmological term is induced. Here we develop this idea to build a new theory which is formulated through the first order formalism, for example, when using vielbein e a and spin connection ab (a,bϭ1,2, . . . ,D) as independent variables. The equations obtained from the variation of e a and the fields a imply the existence of a nontrivial constraint. This approach can be made consistent with invariance under arbitrary diffeomorphisms in the internal space of scalar fields a ͑as well as in ordinary space-time͒, provided that the matter model is chosen so as to satisfy the above-mentioned constraint. If the matter model is not chosen so as to satisfy automatically this constraint, the diffeomorphism invariance in the internal space is broken. In this case the constraint is dynamically implemented by the degrees of freedom that become physical because of the breaking of the internal diffeomorphism invariance. However, this constraint always dictates the vanishing of the cosmological constant term and the gravitational equations in the vacuum coincide with vacuum Einstein's equations with zero cosmological constant. The requirement that the internal diffeomorphisms be a symmetry of the theory points towards the unification of forces in nature such as in the KaluzaKlein scheme. ͓S0556-2821͑97͒02010-9͔

We consider a self-consistent Einstein-Maxwell-Kalb-Ramond system in the bulk D=4 space-time interacting with a variable-tension electrically charged lightlike brane. The latter serves both as a material and charge source for gravity and electromagnetism, as well as it dynamically generates a bulk space varying cosmological constant. We find an asymmetric wormhole solution describing two "universes" with different spherically symmetric black-hole-type geometries connected through a "throat" occupied by the lightlike brane. The electrically neutral "left universe" comprises the exterior region of Schwarzschild-de-Sitter (or pure Schwarzschild) space-time above the inner (Schwarzschild-type) horizon, whereas the electrically charged "right universe" consists of the exterior Reissner-Nordstroem (or Reissner-Nordstroem-de-Sitter) black hole region beyond the outer Reissner-Nordstroem horizon. All physical parameters of the wormhole are uniquely determined by two free parameters - the electric charge and Kalb-Ramond coupling of the lightlike brane.Comment: 17 pages, to appear in the proceedings "Lie Theory and Its Applications in Physics 08" (Varna, June 2009), eds. V. Dobrev and H. Doebner (Heron Press, Sofia, 2010

Abstract:We discuss properties of a new class of -brane models, describing intrinsically lightlike branes for any world-volume dimension, in various gravitational backgrounds of interest in the context of black hole physics. One of the characteristic features of these lightlike -branes is that the brane tension appears as an additional nontrivial dynamical world-volume degree of freedom. Codimension one lightlike brane dynamics requires that bulk space with a bulk metric of spherically symmetric type must possess an event horizon which is automatically occupied by the lightlike brane while its tension evolves exponentially with time. The latter phenomenon is an analog of the well known "mass inflation" effect in black holes.PACS (

We study a scale invariant two measures theory where a dilaton field φ has no explicit potentials. The scale transformations include a translation of a dilaton φ → φ + const. The theory demonstrates a new mechanism for generation of the exponential potential: in the conformal Einstein frame (CEF), after SSB of scale invariance, the theory develops the exponential potential and, in general, non-linear kinetic term is generated as well. The scale symmetry does not allow the appearance of terms breaking the exponential shape of the potential that solves the problem of the flatness of the scalar field potential in the context of quintessential scenarios. As examples, two different possibilities for the choice of the dimensionless parameters are presented where the theory permits to get interesting cosmological results. For the first choice , the theory has standard scaling solutions for φ usually used in the context of the quintessential scenario. For the second choice, the theory allows three different solutions one of which is a scaling solution with equation of state p φ = wρ φ where w is predicted to be restricted by −1 < w < −0.82. The regime where the fermionic matter dominates (as compared to the dilatonic contribution) is analyzed. There it is found that starting from a single fermionic field we * guendel@bgumail.bgu.ac.il † alexk@bgumail.bgu.ac.il obtain exactly three different types of spin 1/2 particles in CEF that appears to suggest a new approach to the family problem of particle physics. It is automatically achieved that for two of them, fermion masses are constants, the energy-momentum tensor is canonical and the "fifth force" is absent. For the third type of particles, a fermionic self-interaction appears as a result of SSB of scale invariance.