arXiv subjects

Emil Nissimov

Publications and source records attributed to Emil Nissimov.

At least 19 recordsLinked to original sources

arXiv2024

Square-Root Gauge Theory and Modified Gravity -- Gravity-Assisted Confinement/Deconfinement and Emergent Electro-Weak Symmetry Breaking in Cosmology

In the present contribution to the proceedings of MG17, the main aim is to elucidate the physically important effects of a special nonlinear gauge field with a square-root of the standard Maxwell Lagrangian in its action, interacting with a specific non-canonical modified $f(R)= R + R^2$ gravity formulated in terms of metric-independent spacetime volume elements and in addition coupled to the bosonic fields of the standard electroweak particle model. When applied in the context of cosmological evolution, the above theory consistently describes absence (suppression) of charge confinement and electroweak (Higgs) spontaneous breakdown in the ``early'' Universe, whereas in the post-inflationary ``late'' universe primarily the presence of the ``square-root'' nonlinear gauge field dynamically triggers both the appearance of QCD-like confinement and dynamical generation of the Higgs effect, as well as dynamical generation of non-zero cosmological constant.

gr-qc↗

arXiv2020

Non-Canonical Volume-Form Formulation of Modified Gravity Theories and Cosmology

A concise description is presented of the basic features of the formalism of non-canonical spacetime volume-forms and its application in modified gravity theories and cosmology. The well known unimodular gravity theory appears as a very special case. Concerning the hot issues facing cosmology now, we specifically briefly outline the construction of: (a) unified description of dark energy and dark matter as manifestations of a single material entity -- a special scalar field "darkon"; (b) quintessential models of universe evolution with a gravity-"inflaton"-assisted dynamical Higgs mechanism -- dynamical suppression/generation of spontaneous electroweak gauge symmetry breaking in the "early"/"late" universe; (c) unification of dark energy and dark matter with diffusive interaction among them; (d) mechanism for suppression of 5-th force without fine-tuning.

gr-qc↗

arXiv2020

$Λ$CDM as a Noether Symmetry in Cosmology

The standard $Λ$CDM model of cosmology is formulated as a simple modified gravity coupled to a single scalar field ("darkon") possessing a non-trivial hidden nonlinear Noether symmetry. The main ingredient in the construction is the use of the formalism of non-Riemannian spacetime volume-elements. The associated Noether conserved current produces stress-energy tensor consisting of two additive parts -- dynamically generated dark energy and dark matter components non-interacting among themselves. Noether symmetry breaking via an additional scalar "darkon" potential introduces naturally an interaction between dark energy and dark matter. The correspondence between the $Λ$CDM model and the present "darkon" Noether symmetry is exhibited up to linear order w.r.t. gravity-matter perturbations.

astro-ph.CO↗

arXiv2020

Quintessential Inflation with Dynamical Higgs Generation as an Affine Gravity

First, we propose a scale-invariant modified gravity interacting with a neutral scalar "inflaton" and a Higgs-like SU(2)xU(1) iso-doublet scalar field based on the formalism of non-Riemannian (metric-independent) spacetime volume-elements. This model describes in the physical Einstein frame a quintessential inflationary scenario driven by the "inflaton" together with gravity-inflaton assisted dynamical spontaneous SU(2)xU(1) symmetry breaking in the post-inflationary universe, whereas SU(2)xU(1) symmetry remains intact in the inflationary epoch. Next, we find the explicit representation of the latter quintessential inflationary model with a dynamical Higgs effect as an Eddington-type purely affine gravity.

gr-qc↗

arXiv2020

Dynamically Generated Inflationary Lambda-CDM

Our primary objective is to construct a plausible unified model of inflation, dark energy and dark matter from a fundamental Lagrangian action first principle, where all fundamental ingredients are systematically dynamically generated starting from a very simple model of modified gravity interacting with a single scalar field employing the formalism of non-Riemannian spacetime volume-elements. The non-Riemannian volume element in the initial scalar field action leads to a hidden nonlinear Noether symmetry which produces energy-momentum tensor identified as a sum of a dynamically generated cosmological constant and a dust-like dark matter. The non-Riemannian volume-element in the initial Einstein-Hilbert action upon passage to the physical Einstein-frame creates dynamically a second scalar field with a non-trivial inflationary potential and with an additional interaction with the dynamically generated dark matter. The resulting Einstein-frame action describes a fully dynamically generated inflationary model coupled to dark matter. Numerical results for observables such as the scalar power spectral index and the tensor-to-scalar ratio conform to the latest 2018 PLANCK data.

gr-qc↗

arXiv2019

Non-Riemannian Volume Elements Dynamically Generate Inflation

Our primary objective is the formulation of a plausible cosmological inflationary model entirely in terms of a pure modified gravity without any a priori matter couplings within the formalism of non-Riemannian spacetime volume elements. The non-Riemannian volume elements dynamically create in the physical Einstein frame a canonical scalar matter field and produce a non-trivial inflationary scalar potential with a large flat region and a low-lying stable minimum corresponding to the late universe stage. This dynamically generated inflationary potential is a substantial generalization of the classic Starobinsky potential. Our model predicts scalar power spectral index and tensor to scalar ratio in accordance with the available observational data.

gr-qc↗

arXiv2019

Dynamically generated inflationary two-field potential via non-Riemannian volume forms

We consider a simple model of modified gravity interacting with a single scalar field $φ$ with weakly coupled exponential potential within the framework of non-Riemannian spacetime volume-form formalism. The specific form of the action is fixed by the requirement of invariance under global Weyl-scale symmetry. Upon passing to the physical Einstein frame we show how the non-Riemannian volume elements create a second canonical scalar field $u$ and dynamically generate a non-trivial two-scalar-field potential $U_{\rm eff}(u,φ)$ with two remarkable features: (i) it possesses a large flat region for large $u$ describing a slow-roll inflation; (ii) it has a stable low-lying minimum w.r.t. $(u,φ)$ representing the dark energy density in the "late universe". We study the corresponding two-field slow-roll inflation and show that the pertinent slow-roll inflationary curve $φ= φ(u)$ in the two-field space $(u,φ)$ has a very small curvature, i.e., $φ$ changes very little during the inflationary evolution of $u$ on the flat region of $U_{\rm eff}(u,φ)$. Explicit expressions are found for the slow-roll parameters which differ from those in the single-field inflationary counterpart. Numerical solutions for the scalar spectral index and the tensor-to-scalar ratio are derived agreeing with the observational data.

astro-ph.CO↗

arXiv2019

Modified Gravity Theories Based on the Non-Canonical Volume-Form Formalism

We present a concise description of the basic features of gravity-matter models based on the formalism of non-canonical spacetime volume-forms in its two versions: the method of non-Riemannian volume-forms (metric-independent covariant volume elements) and the dynamical spacetime formalism. Among the principal outcomes we briefly discuss: (i) quintessential universe evolution with a gravity-"inflaton"-assisted suppression in the "early" universe and, respectively, dynamical generation in the "late" universe of Higgs spontaneous electroweak gauge symmetry breaking; (ii) unified description of dark energy and dark matter as manifestations of a single material entity - a second scalar field "darkon"; (iii)unification of dark energy and dark matter with diffusive interaction among them; (iv) explicit derivation of a stable "emergent universe" solution, i.e., a creation without Big Bang; (v) mechanism for suppression of 5-th force without fine-tuning.

gr-qc↗

arXiv2019

Dynamically Generated Inflation from Non-Riemannian Volume Forms

We propose a simple modified gravity model without any initial matter fields in terms of several alternative non-Riemannian spacetime volume elements within the metric (second order) formalism. We show how the non-Riemannian volume-elements, when passing to the physical Einstein frame, create a canonical scalar field and produce dynamically a non-trivial inflationary-type potential for the latter with a large flat region and a stable low-lying minimum. We study the evolution of the cosmological solutions from the point of view of theory of dynamical systems. The theory predicts the spectral index $n_s \approx 0.96$ and the tensor-to-scalar ratio $r \approx 0.002$ for 60 e-folds, which is in accordance with the observational data. In the future Euclid and SPHEREx missions or the BICEP3 experiment are expected to provide experimental evidence to test those predictions.

gr-qc↗

arXiv2019

Classical and Quantum Two-Body Problem in General Relativity

The two-body problem in general relativity is reduced to the problem of an effective particle (with an energy-dependent relativistic reduced mass) in an external field. The effective potential is evaluated from the Born diagram of the linearized quantum theory of gravity. It reduces to a Schwarzschild-like potential with two different `Schwarzschild radii'. The results derived in a weak field approximation are expected to be relevant for relativistic velocities.

gr-qc↗

arXiv2018

Gauss-Bonnet Gravity in $D=4$ Without Gauss-Bonnet Coupling to Matter - Cosmological Implications

We propose a new model of $D=4$ Gauss-Bonnet gravity. To avoid the usual property of the integral over the standard $D=4$ Gauss-Bonnet scalar becoming a total derivative term, we employ the formalism of metric-independent non-Riemannian spacetime volume elements which makes the D=4 Gauss-Bonnet action term non-trivial without the need to couple it to matter fields unlike the case of ordinary D=4 Gauss-Bonnet gravity models. The non-Riemannian volume element dynamically triggers the Gauss-Bonnet scalar to be an arbitrary integration constant M on-shell, which in turn has several interesting cosmological implications: (i) It yields specific solutions for the Hubble parameter and the Friedmann scale factor as functions of time, which are completely independent of the matter dynamics, i.e., there is no back reaction by matter on the cosmological metric; (ii) For M>0 it predicts a "coasting"-like evolution immediately after the Big Bang, and it yields a late universe with dynamically produced dark energy density given through M; (iii) For the special value M=0 we obtain exactly a linear "coasting" cosmology; (iv) For M<0 we have in addition to the Big Bang also a Big Crunch with "coasting"-like evolution around both; (v) It allows for an explicit analytic solution of the pertinent Friedmann and scalar field equations of motion, while dynamically fixing uniquely the functional dependence of the scalar potential.

gr-qc↗

arXiv2018

Four-Dimensonal Gauss-Bonnet Gravity Without Gauss-Bonnet Coupling to Matter - Spherically Symmetric Solutions, Domain Walls and Spacetime Singularities

We discuss a new extended gravity model in ordinary $D=4$ spacetime dimensions, where an additional term in the action involving Gauss-Bonnet topological density is included without the need to couple it to matter fields unlike the case of ordinary D=4 Gauss-Bonnet gravity models. Avoiding the Gauss-Bonnet density becoming a total derivative is achieved by employing the formalism of metric-independent non-Riemannian spacetime volume-forms. The non-Riemannian volume element triggers dynamically the Gauss-Bonnet scalar to become an arbitrary integration constant on-shell. We describe in some detail the class of static spherically symmetric solutions of the above modified D=4 Gauss-Bonnet gravity including solutions with deformed (anti)-de Sitter geometries, black holes, domain walls and Kantowski-Sachs-type universes. Some solutions exhibit physical spacetime singular surfaces not hidden behind horizons and bordering whole forbidden regions of space. Singularities can be avoided by pairwise matching of two solutions along appropriate domain walls. For a broad class of solutions the corresponding matter source is shown to be a special form of nonlinear electrodynamics whose Lagrangian L(F^2) is a non-analytic function of F^2 (the square of Maxwell tensor F_mu_nu), i.e., L(F^2) is not of Born-Infeld type.

gr-qc↗

arXiv2018

Modified Gravity and Inflaton Assisted Dynamical Generation of Charge Confinement and Electroweak Symmetry Breaking in Cosmology

We describe a new type of gravity-matter models where modified f(R)=R+R^2 gravity couples non-canonically to a scalar "inflaton", to the bosonic sector of the electroweak particle model and to a special nonlinear gauge field with a square-root of the standard Maxwell/Yang-Mills kinetic term simulating QCD confining dynamics. Our construction is based on the powerful formalism of non-Riemannian space-time volume-forms - alternative metric-independent volume elements defined in terms of auxiliary antisymmetric tensor gauge fields. Our model provides a unified Lagrangian action principle description of: (i) the evolution of both "early" and "late" universe by the "inflaton" scalar field; (ii) gravity-inflaton-assisted dynamical generation of Higgs spontaneous breakdown of electroweak gauge symmetry in the "late" universe, as well as dynamical suppression of electroweak breakdown in the "early" universe; (iii) gravity-inflaton-assisted dynamical generation of QCD-like confinement in the "late" universe and suppression of confinement in the "early" universe due to the special interplay with the dynamics of the QCD-simulating nonlinear gauge field.

hep-th↗

arXiv2018

Confinement/Deconfinement and Gravity-Assisted Emergent Higgs Mechanism in Quintessential Cosmological Model

Motivated by the ideas of Jacob Bekenstein concerning gravity-assisted symmetry breaking, we consider a non-canonical model of f(R)=R+R^2 extended gravity coupled to neutral scalar "inflaton", as well as to SU(2)xU(1) multiplet of fields matching the content of the bosonic sector of the electroweak particle model, however with the following significant difference - the SU(2)xU(1) iso-doublet Higgs-like scalar enters here with a standard positive mass squared and without quartic selfinteraction. Strong interaction dynamics and, in particular, QCD-like confinement effects are also considered by introducing an additional coupling to a strongly nonlinear gauge field whose Lagrangian contains a square-root of the standard Maxwell/Yang-Mills kinetic term. The latter is known to produce charge confinement in flat spacetime. The principal new ingredient in the present approach is employing the formalism of non-Riemannian spacetime volume-forms - alternative generally covariant volume elements independent of the spacetime metric, constructed in terms of auxiliary antisymmetric tensor gauge fields of maximal rank. Although being almost pure-gauge, i.e. not introducing any additional propagating degrees of freedom, their dynamics triggers a series of physically important features when passing to the Einstein frame: (i) Appearance of two infinitely large flat regions of the effective "inflaton" scalar potential with vastly different energy scales corresponding to the "early" and "late" epochs of the Universe; (ii) Dynamical generation of Higgs-like spontaneous symmetry breaking effective potential for the SU(2)xU(1) iso-doublet scalar in the "late" Universe, and vanishing of the symmetry breaking in the "early" Universe; (iii) Dynamical appearance of charge confinement via the "square-root" nonlinear gauge field in the "late" Universe and deconfinement in the "early" Universe.

gr-qc↗

arXiv2018

Quintessence, Unified Dark Energy and Dark Matter, and Confinement/Deconfinement Mechanism

We describe a new type of generalized gravity-matter models where $f(R)=R+R^2$ gravity couples in a non-conventional way to a scalar "inflaton" field, to a second scalar "darkon" field responsible for dark energy/dark matter unification, as well as to a non-standard nonlinear gauge field system, which is responsible for a charge confining/deconfinfing mechanism. The essential non-conventional feature of our models is employing the formalism of non-Riemannian volume forms, i.e. metric-independent non-Riemannian volume elements on the spacetime manifold, defined in terms of auxiliary antisymmetric tensor gauge fields. Although being (almost) pure-gauge degrees of freedom, the non-Riemannian volume-forms trigger a series of important features unavailable in ordinary gravity-matter models. Upon passing to the physical Einstein frame we obtain an effective matter-gauge-field Lagrangian of quadratic "k-essence" type both w.r.t. the "inflaton" and the "darkon", with the following properties: (i) Remarkable effective "inflaton" potential possessing two infinitely large flat regions with vastly different heights ("vacuum" energy densities) describing the "early" and "late" Universe; (ii) Nontrivial effective gauge coupling constants running with the "inflaton", in particular, effective "inflaton"-running coupling constant, which determines the strength of the charge confienement; (iii) The confinement-strength gauge coupling constant is non-zero in the "late" Universe, i.e., charge confinement is operating, whereas it vanishes in the "early" Universe, i.e., confinement-free epoch; (iv) The unification of dark energy and dark matter is explicitly seen within the FLRW reduction, where they appear as dynamically generated effective vacuum energy density and dynamically induced dust-like matter, correspondingly.

gr-qc↗

arXiv2017

Wheeler-DeWitt Quantization of Gravity Models of Unified Dark Energy and Dark Matter

First, we describe the construction of a new type of gravity-matter models based on the formalism of non-Riemannian space-time volume forms - alternative generally covariant integration measure densities (volume elements) defined in terms of auxiliary antisymmetric tensor gauge fields. Here gravity couples in a non-conventional way to two distinct scalar fields providing a unified Lagrangian action principle description of: (i) the evolution of both "early" and "late" Universe - by the "inflaton" scalar field; (ii) dark energy and dark matter as a unified manifestation of a single material entity - the "darkon" scalar field. A physically very interesting phenomenon occurs when including in addition interactions with the electro-weak model bosonic sector - we obtain a gravity-assisted dynamical generation of electro-weak spontaneous gauge symmetry breaking in the post-inflationary "late" Universe, while the Higgs-like scalar remains massless in the "early" Universe. Next, we proceed to the Wheeler-DeWitt minisuperspace quantization of the above models. The "darkon" field plays here the role of cosmological "time". In particular, we show the absence of cosmological space-time singularities.

gr-qc↗

arXiv2017

Quintessence in Multi-Measure Generalized Gravity Stabilized by Gauss-Bonnet/Inflaton Coupling

We consider a non-standard generalized model of gravity coupled to a neutral scalar "inflaton" as well as to the fields of the electroweak bosonic sector. The essential new ingredient is employing two alternative non-Riemannian space-time volume-forms (non-Riemannian volume elements, or covariant integration measure densitities) independent of the space-time metric. The latter are defined in terms of auxiliary antisymmentric tensor gauge fields, which although not introducing any additional propagating degrees of freedom, trigger a series of important features such as: (i) appearance of two infinitely large flat regions of the effective "inflaton" potential in the corresponding Einstein frame with vastly different scales corresponding to the "early" and "late" epochs of Universe's evolution; (ii) dynamical generation of Higgs-like spontaneous symmetry breaking effective potential for the SU(2)\times U(1) iso-doublet electroweak scalar in the late universe, whereas it remains massless in the "early" universe. Next, to stabilize the quintessential dynamics, we introduce in addition a coupling of the "inflaton" to Gauss-Bonnet gravitational term. The latter leads to the following radical change of the form of the total effective "inflaton" potential: its flat regions are now converted into a local maximum corresponding to a "hill-top" inflation in the "early" universe with no spontaneous breakdown of electroweak gauge symmetry and, correspondigly, into a local minimum corresponding to the "late" universe evolution with a very small value of the dark energy and with operating Higgs mechanism.

gr-qc↗

arXiv2016

Einstein-Rosen "Bridge" Revisited and Lightlike Thin-Shell Wormholes

We study in some detail the properties of the mathematically correct formulation of the classical Einstein-Rosen "bridge" as proposed in the original 1935 paper, which was shown in a series of previous papers of ours to represent the simplest example of a static spherically symmetric traversable lightlike thin-shell wormhole. Thus, the original Einstein-Rosen "bridge" is not equivalent to the concept of the dynamical and non-traversable Schwarzschild wormhole, also called "Einstein-Rosen bridge" in modern textbooks on general relativity. The original Einstein-Rosen "bridge" requires the presence of a special kind of "exotic" matter source located on its throat which was shown to be the simplest member of the previously introduced by us class of lightlike membranes. We introduce and exploit the Kruskal-Penrose description of the original Einstein-Rosen "bridge". In particular, we explicitly construct closed timelike geodesics on the pertinent Kruskal-Penrose manifold.

gr-qc↗