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E. I. Guendelman

Publications and source records attributed to E. I. Guendelman.

At least 37 records · Page 2Linked to original sources
arXiv2012

Extra Dimensional Curvature Supression of the Effective Four Dimensional Vacuum Energy Density

Considering a very large number of extra dimensions, $N\rightarrow \infty$, we show that in the effective four dimensional picture, to leading order in $N$, both the cosmological constant in $N+4$ dimensions and the curvature of the extra dimensions (curved as spheres) give the same type of contributions. Furthermore in this limit, the extra dimensional curvature naturally supress the effect of a positive Cosmological Constant, so that the resulting effective potential governing the vacuum energy in the effective $4-D$ picture has a leading 1/N dependence (i.e. vanishing in the large $N$ limit). We can understand qualitatively this effect in a heuristic picture, by thinking that all dimensions, both visible and extra have an equal sharing of the curvature caused by $Λ$, in this case when increasing the overall number of dimensions by adding $N$ extra dimensions, then if $N$ is large, the visible dimensions do not have to curve too much, hence a small four dimensional vacuum energy follows. In the large $N$ picture the potential can be also stabilized by a small (i.e. vanishing at large $N$) expectaction value of a four index field strength.

gr-qc↗
arXiv2011

Non Singular Origin of the Universe and the Cosmological Constant Problem (CCP)

We consider a non singular origin for the Universe starting from an Einstein static Universe in the framework of a theory which uses two volume elements $\sqrt{-{g}}d^{4}x$ and $Φd^{4}x$, where $Φ$ is a metric independent density, also curvature, curvature square terms, first order formalism and for scale invariance a dilaton field $ϕ$ are considered in the action. In the Einstein frame we also add a cosmological term that parametrizes the zero point fluctuations. The resulting effective potential for the dilaton contains two flat regions, for $ϕ\rightarrow \infty$ relevant for the non singular origin of the Universe and $ϕ\rightarrow -\infty$, describing our present Universe. Surprisingly, avoidance of singularities and stability as $ϕ\rightarrow \infty$ imply a positive but small vacuum energy as $ϕ\rightarrow -\infty$. Zero vacuum energy density for the present universe is the "threshold" for universe creation.

gr-qc↗
arXiv2011

Non Singular Origin of the Universe and its Present Vacuum Energy Density

We consider a non singular origin for the Universe starting from an Einstein static Universe, the so called "emergent universe" scenario, in the framework of a theory which uses two volume elements $\sqrt{-{g}}d^{4}x$ and $Φd^{4}x$, where $Φ$ is a metric independent density, used as an additional measure of integration. Also curvature, curvature square terms and for scale invariance a dilaton field $ϕ$ are considered in the action. The first order formalism is applied. The integration of the equations of motion associated with the new measure gives rise to the spontaneous symmetry breaking (S.S.B) of scale invariance (S.I.). After S.S.B. of S.I., it is found that a non trivial potential for the dilaton is generated. In the Einstein frame we also add a cosmological term that parametrizes the zero point fluctuations. The resulting effective potential for the dilaton contains two flat regions, for $ϕ\rightarrow \infty$ relevant for the non singular origin of the Universe, followed by an inflationary phase and $ϕ\rightarrow -\infty$, describing our present Universe. The dynamics of the scalar field becomes non linear and these non linearities are instrumental in the stability of some of the emergent universe solutions, which exists for a parameter range of values of the vacuum energy in $ϕ\rightarrow -\infty$, which must be positive but not very big, avoiding the extreme fine tuning required to keep the vacuum energy density of the present universe small. Zero vacuum energy density for the present universe defines the threshold for the creation of the universe.

gr-qc↗
arXiv2010

Bags and Confinement Governed by S.S.B. of Scale Invariance

A general coordinate invariant theory is constructed where confinement of gauge fields and gauge dynamics in general is governed by the spontaneous symmetry breaking (s.s.b.) of scale invariance. The model uses two measures of integration in the action, the standard $\sqrt{-g}$ where $g$ is the determinant of the metric and another measure $Φ$ independent of the metric. To implement scale invariance (S.I.), a dilaton field is introduced. Using the first order formalism, curvature ( $ΦR$ and $\sqrt{-g}R^{2}$ ) terms , gauge field term and dilaton kinetic terms are introduced in a conformally invariant way. Exponential potentials for the dilaton break down (softly) the conformal invariance down to global scale invariance, which also suffers s.s.b. after integrating the equations of motion. The model has a well defined flat space limit. As a result of the s.s.b. of scale invariance phases with different vacuum energy density appear. Inside the bags, that is in the regions of larger vacuum energy density, the gauge dynamics is normal, that is non confining, while for the region of smaller vacuum energy density, the gauge field dynamics is confining. Likewise, the dynamics of scalars, like would be Goldstone bosons, is suppressed inside the bags.

hep-th↗
arXiv2010

Wormholes and Child Universes

Evidence to the case that classical gravitation provides the clue to make sense out of quantum gravity is presented. The key observation is the existence in classical gravitation of child universe solutions or "almost" solutions, "almost" because of some singularity problems. The difficulties of these child universe solutions due to their generic singularity problems will be very likely be cured by quantum effects, just like for example "almost" instanton solutions are made relevant in gauge theories with breaking of conformal invariance. Some well motivated modifcations of General Relativity where these singularity problems are absent even at the classical level are discussed. High energy density excitations, responsible for UV divergences in quantum field theories, including quantum gravity, are likely to be the source of child universes which carry them out of the original space time. This decoupling could prevent these high UV excitations from having any influence on physical amplitudes. Child universe production could therefore be responsible for UV regularization in quantum field theories which take into account semiclassically gravitational effects. Child universe production in the last stages of black hole evaporation, the prediction of absence of tranplanckian primordial perturbations, connection to the minimum length hypothesis and in particular the connection to the maximal curvature hypothesis are discussed. Some discussion of superexcited states in the case these states are Kaluza Klein excitations is carried out. Finally, the posibility of obtaining "string like" effects from the wormholes associated with the child universes is discussed.

gr-qc↗
arXiv2009

Gravitational Theory with a Dynamical Time

A gravitational theory involving a vector field $χ^μ$, whose zero component has the properties of a dynamical time, is studied. The variation of the action with respect to $χ^μ$ gives the covariant conservation of an energy momentum tensor $ T^{μν}_{(χ)}$. Studying the theory in a background which has killing vectors and killing tensors we find appropriate shift symmetries of the field $χ^μ$ which lead to conservation laws. The energy momentum that is the source of gravity $ T^{μν}_{(G)}$ is different but related to $ T^{μν}_{(χ)}$ and the covariant conservation of $ T^{μν}_{(G)}$ determines in general the vector field $χ^μ$. When $ T^{μν}_{(χ)}$ is chosen to be proportional to the metric, the theory coincides with the Two Measures Theory, which has been studied before in relation to the Cosmological Constant Problem. When the matter model consists of point particles, or strings, the form of $ T^{μν}_{(G)}$, solutions for $χ^μ$ are found. For the case of a string gas cosmology, we find that the Milne Universe can be a solution, where the gas of strings does not curve the spacetime since although $ T^{μν}_{(χ)} \neq 0$, $ T^{μν}_{(G)}= 0$, as a model for the early universe, this solution is also free of the horizon problem. There may be also an application to the "time problem" of quantum cosmology.

gr-qc↗
arXiv2009

String Gas Shells, their Dual Radiation and Hedgehog Signature Control

We search for spherically symmetric, stationary solutions with a string gas shell as a source. The requirement of a uniform newtonian potential, or constancy of the 00 component of the metric, implies the existence of a "dual" radiation, which we argue can be interpreted as representing the virtual quantum fluctuations that stabilize the shell. A string hedgehog can be introduced also into the solution. For zero or small hedgehog strength the string gas shell is of a regular nature, while the dual radiation is of a spacelike nature. For higher hedgehog strengths however the radiation "materializes" and becomes timelike while the string gas shell becomes space like. The significance of these solutions for the quantum theory is discussed.

gr-qc↗
arXiv2008

Physical Consequences of a Theory with Dynamical Volume Element

We survey motivation, basic ideas and physical consequences of a theory where the underlying action involves terms both with the usual volume element $\sqrt{-g}d^{4}x$ and with the new one $Φd^{4}x={4!}dφ_{1}\wedge dφ_{2}\wedge dφ_{3}\wedge dφ_{4}$. The latter may be interpreted as the 4-form determined on the 4-D space-time manifold (not necessary Riemannian). Regarding the scalar fields $φ_{a} (a=1,...4)$ as new dynamical variables and proceeding in the first order formalism we realize the so-called Two Measures Theory which possesses a number of attractive features. We discuss new physical effects which arise from this theory and in particular strong gravity effect in high energy physics experiments.

gr-qc↗
arXiv2008

Transition to Zero Cosmological Constant and Phantom Dark Energy as Solutions Involving Change of Orientation of Space-Time Manifold

Solutions with degenerate metric ($det(g_{μν})=0$ or $g_{μν}=0$) in the first order formalism (FOF) are physically acceptable: they may describe topology changes (Horowitz) and reduction of "metrical dimension" (Tseytlin) of space-time. The latter implies disappearance of the volume element $\sqrt{-g}d^4x$ of 4-D space-time. We pay attention that besides $\sqrt{-g}$, the 4-D space-time differentiable manifold possesses also a "manifold volume measure" (MVM)described by a 4-form which is sign indefinite and generically independent of the metric. The FOF proceeds with originally independent connection and metric structures of the space-time manifold. We bring up the question whether the FOF should be supplemented with degrees of freedom of MVM. Adding such manifold degrees of freedom to the action principle in the FOF we realize very interesting dynamics. Such Two Measures Theory enables radically new approaches to resolution of the cosmological constant problem. We show that fine tuning free solutions describing a transition to $Λ=0$ state involve oscillations of $g_{μν}$ and MVM around zero. The latter can be treated as a dynamics involving changes of orientation of the space-time manifold. As we have shown earlier, in realistic scale invariant models (SIM), solutions formulated in the Einstein frame satisfy all existing tests of General Relativity (GR). Here we reveal surprisingly that in SIM, all ground state solutions with $Λ\neq 0$ appear to be degenerate either in $g_{00}$ or in MVM. Sign indefiniteness of MVM in a natural way yields a dynamical realization of a phantom cosmology ($w<-1$). For all solutions, the metric tensor rewritten in the Einstein frame has regularity properties exactly as in GR. A possibility of a strong gravity effect in LHC experiments is discussed.

gr-qc↗
arXiv2008

Photon and Axion Splitting in an Inhomogeneous Magnetic Field

The axion photon system in an external magnetic field, when the direction of propagation of axions and photons is orthogonal to the direction of the external magnetic field, displays a continuous axion-photon duality symmetry in the limit the axion mass is neglected. The conservation law that follow in this effective 2+1 dimensional theory from this symmetry is obtained. The magnetic field interaction is seen to be equivalent to first order to the interaction of a complex charged field with an external electric potential, where this ficticious "electric potential" is proportional to the external magnetic field. This allows one to solve for the scattering amplitudes using already known scalar QED results. From the scalar QED analog the axion and the photon are symmetric and antisymmetric combinations of particle and antiparticle. If one considers therefore scattering experiments in which the two spatial dimensions of the effective theory are involved non trivially, one observes that both particle and antiparticle components of photons and axions are preferentially scattered in different directions, thus producing the splitting or decomposition of the photon and axion into their particle and antiparticle components in an inhomogeneous magnetic field. This observable in principle effect is of first order in the axion photon coupling, unlike the "light shining through a wall phenomena ", which is second order.

hep-th↗
arXiv2008

Localized Axion Photon States in a Strong Magnetic Field

We consider the axion field and electromagnetic waves with rapid time dependence, coupled to a strong time independent, asymptotically approaching a constant at infinity "mean" magnetic field, which takes into account the back reaction from the axion field and electromagnetic waves with rapid time dependence in a time averaged way. The direction of the self consistent mean field is orthogonal to the common direction of propagation of the axion and electromagnetic waves with rapid time dependence and parallel to the polarization of these electromagnetic waves. Then, there is an effective U(1) symmetry mixing axions and photons. Using the natural complex variables that this U(1) symmetry suggests we find localized planar soliton solutions. These solutions appear to be stable since they produce a different magnetic flux than the state with only a constant magnetic field, which we take as our "ground state". The solitons also have non trivial U(1) charge defined before, different from the uncharged vacuum.

hep-th↗
arXiv2007

Axion Photon Oscillations From a "Particle-Antiparticle" View Point

We observe that it is very usefull to introduce a complex field for the axion photon system in an external magnetic field, when for example considered with the geometry of the experiments exploring axion photon mixing, where the real part is the axion and the imaginary part is the photon polarization that couples to the axion when the magnetic field is present. In the absence of the external magnetic field, the theory displays charge conjugation symmetry. In this formulation the axion and photon are the symmetric and antisymmetric combinations of particle and antiparticle (as defined from the complex field) respectively and they do not mix if the external magnetic field is set to zero. The magnetic field interaction is seen to be equivalent to first order to the interaction of the complex charged field with an external electric potential, where this ficticious "electric potential" is proportional to the external magnetic field. This interaction breaks the charge conjugation symmetry and therefore symmetric and antysymmetric combinations are not mantained in time. As a result one obtains axion photon mixing in the presence of an external magnetic field, a well known result understood in a different way.

hep-ph↗
arXiv2007

Continuous Axion Photon Duality and its Consequences

The axion photon system in an external magnetic field, when for example considered with the geometry of the experiments exploring axion photon mixing, displays a continuous axion-photon duality symmetry in the limit the axion mass is neglected. The conservation law that follows from this symmetry is obtained. The magnetic field interaction is seen to be equivalent to first order to the interaction of a complex charged field with an external electric potential, where this ficticious "electric potential" is proportional to the external magnetic field. This allows to solve for the scattering amplitudes using already known scalar QED results. It is argued that in more generic conditions (not just related to these experiments) axion-photon condensation could be obtained for high magnetic fields. Finally an exact constraint originating fromthe current conservation on the amplitudes of reflected and transmited waves is obtained.

hep-th↗
arXiv2007

Symmetry Supporting a Transition to Zero Cosmological Constant State

In a number of previous publications we demonstrated that the Two Measures Field Theory (TMT) enables to resolve the old cosmological constant (CC) problem avoiding the Weinberg's no-go CC theorem and together with this TMT agrees with all tests of the Einstein's general relativity and allows inflationary scenarios. Analysis performed in the present paper shows that there exists an intrinsic symmetry of TMT which emerges in the $Λ=0$ ground state. This symmetry contains a subgroup of reflections of the metric $g_{μν}\to -g_{μν}$ studied recently by a number of authors as the symmetry imposing zero CC. We show that realization of this idea in TMT is free of fine tuning and has no problems typical to other approaches.

hep-th↗
arXiv2007

Absence of the Fifth Force Problem in a Model with Spontaneously Broken Dilatation Symmetry

A scale invariant model containing dilaton $ϕ$ and dust (as a model of matter) is studied where the shift symmetry $ϕ\toϕ+const.$ is spontaneously broken at the classical level due to intrinsic features of the model. The dilaton to matter coupling "constant" $f$ appears to be dependent of the matter density. In normal conditions, i.e. when the matter energy density is many orders of magnitude larger than the dilaton contribution to the dark energy density, $f$ becomes less than the ratio of the "mass of the vacuum" in the volume occupied by the matter to the Planck mass. The model yields this kind of "Archimedes law" without any especial (intended for this) choice of the underlying action and without fine tuning of the parameters. The model not only explains why all attempts to discover a scalar force correction to Newtonian gravity were unsuccessful so far but also predicts that in the near future there is no chance to detect such corrections in the astronomical measurements as well as in the specially designed fifth force experiments on intermediate, short (like millimeter) and even ultrashort (a few nanometer) ranges. This prediction is alternative to predictions of other known models.

gr-qc↗
arXiv2007

Strong Interaction Dynamics from Spontaneous Symmetry Breaking of Scale Invariance

Using the mechanism of spontaneous symmetry breaking of scale invariance obtained from the dynamics of maximal rank field strengths, it is possible to spontaneously generate confining behavior. Introducing a dilaton field, the study of non trivial confining and de-confining transitions appears possible. This is manifest in two ways at least: One can consider bags which contain an unconfined phase in the internal region and a confined phase outside and also one obtains a simple model for deconfinement at high Temperature from the finite Temperature dynamics of the dilaton field.

hep-th↗
arXiv2007

Fine Tuning Free Paradigm of Two Measures Theory: K-Essence, Absence of Initial Singularity of the Curvature and Inflation with Graceful Exit to Zero Cosmological Constant State

The dilaton-gravity sector of the Two Measures Field Theory (TMT)is explored in detail in the context of cosmology. The model possesses scale invariance which is spontaneously broken due to the intrinsic features of the TMT dynamics. The effective model represents an explicit example of the effective k-essence resulting from first principles without any exotic term in the fundamental action. Depending of the choice of regions in the parameter space, TMT exhibits different possible outputs for cosmological dynamics: a) Absence of initial singularity of the curvature while its time derivative is singular. This is a sort of "sudden" singularities studied by Barrow on purely kinematic grounds. b) Power law inflation in the subsequent stage of evolution. Depending on the region in the parameter space (but without fine tuning) the inflation ends with a graceful exit either into the state with zero cosmological constant (CC) or into the state driven by both a small CC and the field phi with a quintessence-like potential. c) Possibility of resolution of the old CC problem. From the point of view of TMT, it becomes clear why the old CC problem cannot be solved (without fine tuning) in conventional field theories. d) TMT enables two ways for achieving small CC without fine tuning of dimensionfull parameters: either by a seesaw type mechanism or due to a correspondence principle between TMT and conventional field theories (i.e theories with only the measure of integration sqrt{-g} in the action. e) There is a wide range of the parameters such that in the late time universe: the equation-of-state w=p/ρ<-1; w asymptotically (as t\to\infty) approaches -1 from below; ρapproaches a constant, the smallness of which does not require fine tuning of dimensionfull parameters.

gr-qc↗