Non-minimal Kinetic Coupling Research Articles

Cosmological constraints on the background dynamics of a two-field nonsingular bounce model

In this study, we consider a nonsingular two-field bounce scenario with non-minimal kinetic coupling between two scalar fields. We derive constraints on the model parameters from the finiteness of the physical quantities at the classical level and from the relation between the late-time accelerated expansion and particle production up to the bounce phase. We then determine the allowed parameter space for the model.

Gravitational baryogenesis in non-minimal kinetic coupling model

In this work, we consider the gravitational baryogenesis in the framework of non-minimal derivative coupling model. A mechanism to generate the baryon asymmetry based on the coupling between the derivative of the Ricci scalar curvature and the baryon current in context of non-minimal derivative coupling model is investigated. We show that, in this model, the temperature increases during the reheating periods to the end of reheating period or beginning of radiation dominated era. Therefore the reheating temperature is larger then decoupling temperature. It can be demonstrated that, the evaluation of baryon asymmetry is not depends on coupling constant. In this model we can generate baryon asymmetry at low and high reheating temperature, by considering the high friction constraint.

Inflation in Scalar-Tensor Theory with Nonminimal Kinetic Coupling

Inflation in Scalar-Tensor Theory with Nonminimal Kinetic Coupling

Generation of primordial black holes from an inflation model with modified dispersion relation

A primordial black hole (PBH) is interesting to people for its ability of explaining dark matter as well as supermassive astrophysical objects. In the normal inflation scenario, the generation of PBHs usually requires an enhanced power spectrum of scalar perturbation at the end of inflation era, which is expected when the dispersion relation of the inflaton field gets modified. In this work, we study a kind of inflation model called the Dirac-Born-Infeld-inspired nonminimal kinetic coupling (DINKIC) model, where the dispersion relation is modified by a square root existing in the field Lagrangian. We discuss the enhancement of scalar power spectrum due to the modified dispersion relation, as well as the abundance of PBHs produced by the Press-Schechter collapse mechanism. We also discuss the formation of scalar-induced gravitational waves by linear scalar perturbations.

Anisotropic inflation in non-minimal kinetic coupling model

We study anisotropic inflation in non-minimal derivative coupling model where the scalar field non-minimally coupled to the U(1) gaugefields and derivative of the scalar field non-minimally coupled to the Einstein tensor.Within the framework we find power-law anisotropic solutions in this model when both the inflaton potential and the gauge kinetic function are power-law type in the high friction regime.We show the ratio of anisotropy to the expansion rate is nearly constant, small and proportional to the slow-roll parameters of the theory.As a demonstration, we consider numerically calculation of the model to show that the behavior of anisotropy by changing the parameters of the model for quadratic inflationary potential.There is anisotropic attractor solution for a wide range of values of the model parameters.We show both numerically and analytically that there are two phases of inflation, similar to those an anisotropic inflation in minimal coupling model, isotropic and anisotropic phase.We can change the number of e-folds corresponding to each phase of slow roll inflation by changing the gauge coupling constant or non-minimal derivative coupling constant.There are the best agreement with the numerically solutions and analytically solutions in this investigation.

Remnants and thermal corrections in Horndeski black holes with non-minimal kinetic coupling

Remnants and thermal corrections in Horndeski black holes with non-minimal kinetic coupling

Generalized Horndeski-like Einstein Gauss-Bonnet inflation with massless primordial gravitons

In this work we shall introduce a theoretical framework comprised by a non-minimal coupled canonical scalar field, a non-minimal coupling to the Gauss-Bonnet invariant and a non-minimal kinetic coupling. This theoretical framework is basically a non-minimally coupled Einstein-Gauss-Bonnet theory with extra corrections of the form of a non-minimal kinetic coupling. In order to comply with the GW170817 event, we shall impose a constraint on the propagation speed of the primordial tensor perturbations that it is equal to that of light's in vacuum, and this constraint basically specifies the way that the scalar potential and the non-minimal couplings of the theory can be chosen. The whole theoretical framework, which belongs to the larger class of Horndeski theories, cannot yield viable results, due to the fact that the primordial gravitational wave speed is not equal to that of light's. Thus we study this theory by also imposing the constraint of having gravity wave speed equal to the light speed. We directly examine the inflationary phenomenology of our theoretical framework and by assuming the slow-roll conditions, we derived the equations of motion in such a way so that analytical results may be extracted. By using several well motivated models we demonstrate the framework leads to a viable phenomenology.

On Viability of Inflation in Nonminimal Kinetic Coupling Theory

We consider initial conditions for inflation in non-minimal kinetic coupling theory. If inflation is driven solely by kinetic term with no potential, the resulting number of e-folds depends only upon initial velocity of the scalar field $\dot \phi$. We write down the expression for the number of e-folds explicitly, and show that for physically reasonable values of the coupling constant, we can get 60 e-folds only for exponentially big initial $\dot \phi$. When the scalar field potential is taken into account, on the other hand, we need not very large $\dot \phi$ to start with for 60 e-folds, on the other hand, some initial condition lead to physically inadmissible eternal inflation. We show numerically that in the measure used in the present paper only smaller part of initial conditions lead to eternal inflation for reasonable values of the coupling constant.

Hairy Taub-NUT/bolt-AdS solutions in Horndeski theory

We present different Taub-NUT/Bolt-anti de Sitter (AdS) solutions in a shift-symmetric sector of Horndeski theory of gravity possessing nonminimal kinetic coupling of scalar fields to the Einstein tensor. In four dimensions, we find locally and asymptotically locally AdS solutions possessing nontrivial scalar field. In higher dimensions, analytical Taub-NUT/Bolt-AdS p-branes and solitons are obtained, supported by the existence of p Horndeski scalar fields with axionic profile. The thermodynamical properties are studied through Euclidean methods and it is found that the first law of thermodynamics is satisfied. Moreover, constraints on the parameter space of Horndeski gravity and NUT charge are obtained by demanding positivity of the mass, entropy, and specific heat of the p-branes and soliton. We briefly comment on future applications in holography.

Inflation with non-minimal kinetic and Gauss\u2013Bonnet couplings

The Mukhanov–Sasaki equation is deduced from linear perturbations for a general scalar-tensor model with non-minimal coupling to curvature, to the Gauss–Bonnet invariant and non-minimal kinetic coupling to curvature. The general formulas for the power spectra of the primordial scalar and tensor fluctuations are obtained for arbitrary coupling functions. The results have been applied to models with power-law, exponential, natural and double-well potentials. It was found that the presence of these non-minimal couplings affect the inflationary observables leading to values favored by the latest observations, while some interesting results like sub-planckian symmetry breaking scale in natural inflation and sub-planckian v.e.v. of the scalar filed in the double-well potential were obtained. The consistency with the reheating process was discussed and some numerical cases were shown. The equivalence of the model to a sector of generalized Galileons was shown and the functions that establish the correspondence were found.

Reviving non-minimal Horndeski-like theories after GW170817: kinetic coupling corrected Einstein–Gauss–Bonnet inflation

After the recent GW170817 event of the two neutron stars merging, many string corrected cosmological theories confronted the non-viability peril. This was due to the fact that most of these theories produce massive gravitons primordially. Among these theories were the ones containing a non-minimal kinetic coupling correction term in the Lagrangian. In this work we demonstrate how these theories may be revived and we show how these theories can produce primordial gravitational waves with speed in natural units, thus complying with the GW170817 event. As we show, if the gravitational action of an Einstein–Gauss–Bonnet theory also contains a kinetic coupling of the form ∼ξ(ϕ)G μν ∂ μ ϕ∂ ν ϕ, the condition of having primordial massless gravitons, or equivalently primordial gravitational waves with speed in natural units, results to certain conditions on the scalar field dependent coupling function of the Gauss–Bonnet term, which is also the non-minimal coupling of the kinetic coupling. We extensively study the phenomenological implications of such a theory focussing on the inflationary era, by only assuming slow-roll dynamics for the scalar field. Accordingly, we briefly study the case that the scalar field evolves in a constant-roll way. By using some illustrative examples, we demonstrate that the viability of the theoretical framework at hand may easily be achieved. Also, theories containing terms of the form ∼ξ(ϕ) □ ϕg μν ∂ μ ϕ∂ ν ϕ and are also lead to the same gravitational wave speed as the theory we shall study in this paper, so this covers a larger class of Horndeski theories.

General constraints on Horndeski wormhole throats

In this work, we consider the full Horndeski Lagrangian applied to wormhole geometries and present the full gravitational field equations. We analyse the general constraints imposed by the flaring-out conditions at the wormhole throat and consider a plethora of specific subclasses of the Horndeski Lagrangian, namely, quintessence/phantom fields, $k$-essence, scalar-tensor theories, covariant galileons, nonminimal kinetic coupling, kinetic gravity braiding, and the scalar-tensor representation of Gauss-Bonnet couplings, amongst others. The generic constraints analysed in this work serve as a consistency check of the main solutions obtained in the literature and draws out new avenues of research in considering applications of specific subclasses of the Horndeski theory to wormhole physics.

A study of DINKIC inflationary dynamics with non-interacting imperfect fluid

This paper is keen to study the generalized DBI-inspired non-minimal kinetic coupling inflationary model with non-interacting imperfect fluid in the framework of FRW universe. This is a newly proposed model with an interesting feature of having a correction term [Formula: see text] in the perturbed equation of motion due to the nonlinearity of the kinetic term. The power spectrum remains scale invariant both in large-[Formula: see text] and small-[Formula: see text] limits. In order to discuss the inflationary dynamics, the non-interacting inflaton and imperfect fluid including bulk viscosity are considered to be the cosmic matter contents. We consider large field potential and derive the exact solution of inflaton and perturbed parameters. Further, we investigate the nature of the perturbed parameters in detail, specifically on the potential-driven case, and compare the consequences to the current PLANCK/BICEP observational data. We also analyze the corresponding tensor spectrum, which will be tested by the future observations on primordial gravitational waves.

Cosmology in a model with Lagrange multiplier, Gauss–Bonnet and nonminimal kinetic couplings

A scalar-tensor model with Gauss–Bonnet and nonminimal kinetic couplings is considered, in which ghost modes are eliminated via a Lagrange multiplier constraint. A reconstruction procedure is devised for the scalar potential and Lagrange multiplier, valid for any given cosmological scenario. In particular, inflationary and dark energy cosmologies of different types (power-law, Little-Rip, de Sitter, quasi-de Sitter) are reconstructed in such models. It is shown that, for various choices of the kinetic coupling terms, it is possible to obtain a viable inflationary phenomenology compatible with the most accurate values of the observational indices.

Slow-roll inflation with exponential potential in scalar-tensor models

A study of the slow-roll inflation for an exponential potential in the frame of the scalar-tensor theory is performed, where non-minimal kinetic coupling to curvature and non-minimal coupling of the scalar field to the Gauss-Bonnet invariant are considered. Different models were considered with couplings given by exponential functions of the scalar field, that lead to graceful exit from inflation and give values of the scalar spectral index and the tensor-to-scalar ratio in the region bounded by the current observational data. Special cases were found, where the coupling functions are inverse of the potential, that lead to inflation with constant slow-roll parameters, and it was possible to reconstruct the model parameters for given ns and r. In first-order approximation the standard consistency relation maintains its validity in the model with non-minimal coupling, but it modifies in presence of Gauss–Bonnet coupling. The obtained Hubble parameter during inflation, Hsim 10^{-5} M_p and the energy scale of inflation V^{1/4}sim 10^{-3} M_p, are consistent with the upper bounds set by latest observations.

Slow-roll inflation in scalar-tensor models

The linear and quadratic perturbations for a scalar-tensor model with non-minimal coupling to curvature, coupling to the Gauss-Bonnet invariant and non-minimal kinetic coupling to the Einstein tensor are developed. The quadratic action for the scalar and tensor perturbations is constructed and the power spectra for the primordial scalar and tensor fluctuations are given. A consistency relation that is useful to discriminate the model from the standard inflation with canonical scalar field was found. For some power-law potentials it is shown that the Introduction of additional interactions, given by non-minimal, kinetic and Gauss-Bonnet couplings, can lower the tensor-to-scalar ratio to values that are consistent with latest observational constraints, and the problem of large fields in chaotic inflation can be avoided.

Ultralight massive photons in Stueckelberg QED cosmology with self- and curvature non-minimal couplings

In this communication, we study non-minimal massive Stueckelberg QED for the evolution of the universe in the presence of non-minimal kinetic coupling to itself and to the curvature and where massive photons in the theory are of the order of the Hubble parameter or ultralight mass. By adopting the idea that the gauge vector field depends on the Hubble parameter, we observe that the universe is undergoing accelerated expansion with time and is dominated by quintessence, a decaying scalar field, and ultra-light massive photons. The existence of massive photons or massive particles of light is therefore able to explain the current acceleration of the quintessential universe in the absence of a cosmological constant.

Homogeneous black strings in Einstein\u2013Gauss\u2013Bonnet with Horndeski hair and beyond

In this paper we construct new exact solutions in Einstein–Gauss–Bonnet and Lovelock gravity, describing asymptotically flat black strings. The solutions exist also under the inclusion of a cosmological term in the action, and are supported by scalar fields with finite energy density, which are linear along the extended direction and have kinetic terms constructed out from Lovelock tensors. The divergenceless nature of the Lovelock tensors in the kinetic terms ensures that the whole theory is second order. For spherically, hyperbolic and planar symmetric spacetimes on the string, we obtain an effective Wheeler’s polynomial which determines the lapse function up to an algebraic equation. For the sake of concreteness, we explicitly show the existence of a family of asymptotically flat black strings in six dimensions, as well as asymptotically hbox {AdS}_{5}times mathbb {R} black string solutions and compute the temperature, mass density and entropy density. We compute the latter by Wald’s formula and show that it receives a contribution from the non-minimal kinetic coupling of the matter part, shifting the one-quarter factor coming from the Einstein term, on top of the usual non areal contribution arising from the quadratic Gauss–Bonnet term. Finally, for a special value of the couplings of the theory in six dimensions, we construct strings that contain asymptotically AdS wormholes as well as rotating solutions on the transverse section. By including more scalars the strings can be extended to p-branes, in arbitrary dimensions.

Hairy black hole solutions in a three-dimensional Galileon model

We investigate stationary rotationally symmetric solutions of a particular truncation of Horndeski theory in three dimensions, including a non-minimal scalar kinetic coupling to the curvature. After discussing the special case of a vanishing scalar charge, which includes most of the previously known solutions, we reduce the general case to an effective mechanical model in a three-dimensional target space. We analyze the possible near-horizon behaviors, and conclude that black hole solutions with degenerate horizons and constant curvature asymptotics may exist if the minimal and non-minimal scalar coupling constants have the same sign. In a special case, we find a new analytic rotating black hole solution with scalar hair and degenerate horizon. This is geodesically and causally complete, and asymptotic to the extreme BTZ metric. We also briefly discuss soliton solutions in another special case.

Phantom divide crossing with non-minimal kinetic and gauss-bonnet couplings

Phantom divide crossing with non-minimal kinetic and gauss-bonnet couplings