Scalar Field Cosmology Research Articles

Model for Modified Holographic Ricci Dark Energy in Gravitation Theory of Branc Dicke

In this cosmological model, we have studied the spatially homogeneous and anisotropic Bianchi type and axially symmetric model filled with dark matter and dark energy in Brans-Dicke’s [1] theory of gravitation. Here, we consider the modified holographic Ricci dark energy defined by Chen and Jing [21] as the suitable condition of dark energy. To obtain a solution we assumed the scale factor used Mishra et al. [43]. We have solved field equations of Brans-Dicke theory of gravitation with the help of an axially symmetric anisotropic Bianchi-type space-time. We have determined the cosmological parameters, namely, EoS parameter, MHRDE density, matter density, skewness parameter, and BD scalar field. Here the various phenomena like the expanding universe, and shift from anisotropy to isotropy are observed in this model. A detailed physical discussion of these dynamical parameters are presented graphically. Some physical and geometrical behaviours of the models are also discussed and found to be in good agreement with the recent observations (OHD+JLA) datasets.

МОДИФІКАЦІЇ СТАНДАРТНОЇ КОСМОЛОГІЧНОЇ МОДЕЛІ ТА "НОВА ФІЗИКА"

An overview of the problems associated with the possible going beyond the basic cosmological model is given. The focus is mainly on issues related to astronomical observations and the modifications of general relativity, which use a cosmological scalar field or nonlinear in curvature terms in gravitational Lagrangian. The main points are as follows: the problems of flatness and horison, discussion of the “Hubble Tension” and the simplest known models with one scalar field and the modified gravity. The general opinion is that that there is a number of ways to solve problems with the Hubble constant, but none of them explains all the completeness of the observational data. They are either phenomenological in nature or contain arbitrary functions that must be determined from experiment or from some fundamental principles. Therefore, it is natural to first turn to those models that either do not introduce “new entities”, or include a minimum number of new physical elements and a minimum number of additional parameters.

Gauss\u2013Bonnet term corrections in scalar field cosmology

The influence of non-minimal coupling of a scalar field and the Gauss–Bonnet term on the inflationary stage of evolution of the universe is investigated in this paper. The main cosmological effects of such a coupling were considered. The deviations between Einstein–Gauss–Bonnet inflation and standard one based on Einstein gravity were determined. The corrections of a weak GB coupling preserving the type of the scalar field potential to standard inflationary models is considered as well.

Generalized scalar field cosmologies: a global dynamical systems formulation

Local and global phase-space descriptions and averaging methods are used to find qualitative features of solutions for the FLRW and the Bianchi I metrics in the context of scalar field cosmologies with arbitrary potentials and arbitrary couplings to matter. The stability of the equilibrium points in a phase-space as well as the dynamics in the regime where the scalar field diverges are studied. Equilibrium points that represent some solutions of cosmological interest such as: several types of scaling solutions, a kinetic dominated solution representing a stiff fluid, a solution dominated by an effective energy density of geometric origin, a quintessence scalar field dominated solution, the vacuum de Sitter solution associated to the minimum of the potential, and a non-interacting matter dominated solution are obtained. All of which reveal a very rich cosmological phenomenology.

Generalized scalar field cosmologies: theorems on asymptotic behavior

Phase-space descriptions are used to find qualitative features of the solutions of generalized scalar field cosmologies with arbitrary potentials and arbitrary couplings to matter. Previous results are summarized and new ones are presented as theorems, which include the previous ones as corollaries. Examples of these results are presented as well as counterexamples when the hypotheses of the theorems are not fulfilled. Potentials with small cosine-like corrections motivated by inflationary loop-quantum cosmology are discussed. Finally, the Hubble‐normalized formulation for the FRW metric and for the Bianchi I metric is applied to a scalar field cosmology with a generalized harmonic potential, non-minimally coupled to matter, and the stability of the solutions is discussed.

Comment on “Brans-Dicke scalar field cosmological model in Lyra’s geometry”

In the present study, we comment on Brans-Dicke scalar field cosmological model in Lyra's geometry [Maurya \& Zia, Phys. Rev. D \textbf{100}, 023503 (2019)]. In this comment, we investigate that there is no acceleration in the model proposed by the authors of Phys. Rev. D \textbf{100}, 023503 (2019). Therefore, despite the claims to the contrary the Brans-Dicke Scalar Field Cosmological Model in Lyra's Geometry with high Brans-Dicke (BD) coupling parameter $\omega$ and constant $\beta$ can not produce late time acceleration in the universe.

Dynamical systems in perturbative scalar field cosmology

We derive a new regular dynamical system on a three-dimensional compact state space describing linear scalar perturbations of spatially flat Robertson–Walker geometries for relativistic models with a minimally coupled scalar field with an exponential potential. This enables us to construct the global solution space, illustrated with figures, where known solutions are shown to reside on special invariant sets. We also use our dynamical systems approach to obtain new results about the comoving and uniform density curvature perturbations. Finally we show how to extend our approach to more general scalar field potentials. This leads to state spaces where the state space of the models with an exponential potential appears as invariant boundary sets, thereby illustrating their role as building blocks in a hierarchy of increasingly complex cosmological models.

Larger value for H0 by an evolving gravitational constant

We provide further evidence that a massless cosmological scalar field with a non-minimal coupling to the Ricci curvature of the type $M^2_{\rm pl}(1+\xi \sigma^n/M_{\rm pl}^n) $ alleviates the existing tension between local measurements of the Hubble constant and its inference from CMB anisotropies and baryonic acoustic oscillations data in presence of a cosmological constant. In these models, the expansion history is modified compared to $\Lambda$CDM at early time, mimicking a change in the effective number of relativistic species, and gravity weakens after matter-radiation equality. Compared to $\Lambda$CDM, a quadratic ($n=2$) coupling increases the Hubble constant when {\em Planck} 2018 (alone or in combination with BAO and SH0ES) measurements data are used in the analysis. Negative values of the coupling, for which the scalar field decreases, seem favored and consistency with Solar System can be naturally achieved for a large portion of the parameter space without the need of any screening mechanism. We show that our results are robust to the choice of $n$, also presenting the analysis for $n=4$.

Nonlinear Schrödinger-type formulation of scalar field cosmology: Two barotropic fluids and exact solutions

Time-independent nonlinear Schrödinger-type (NLS) formulation of FRW cosmology with canonical scalar field is considered in the case of two barotropic fluids. We derived Friedmann formulation variables in terms of NLS variables. Seven exact solutions found by D’Ambroise [Ph.D. thesis, arXiv:1005.1410 ] and one new found solution are explored and tested in cosmology. The result suggests that time-independent NLS formulation of cosmology case should be upgraded to the time-dependent case.

Dynamical System Perspective of Cosmological Models Minimally Coupled with Scalar Field

The stability criteria for the dynamical system of a homogeneous and isotropic cosmological model are investigated with the interaction of a scalar field in the presence of a perfect fluid. In this paper, we depict the dynamical system perspective to study qualitatively the scalar field cosmology under two special cases, with and without potential. In the absence of potential, we get a two-dimensional dynamical system, and we study the analytical as well as geometrical behavior. For the dynamical system with potential, we analyze different potential forms: simple exponential potential form (Vϕ=Voe−λϕ), double exponential potential form Vϕ=Voexp−Aexp2αϕ, and inverse power law potential form (Vϕ=Voϕ−α). We generate an autonomous system of ordinary differential equations (ASODE) for each case by introducing new dimensionless variables and obtain respective fixed points. We also analyze the type, nature, and stability of the fixed points and how their behavior reflects towards the cosmological scenarios. Throughout the whole work, the investigation of this model has shown us the deep connection between these theories and cosmic acceleration phenomena. The phase plots of the system at different conditions and different values of γ have been analyzed in detail, and their geometrical interpretations have been studied. The perturbation plots of the dynamical system have been analyzed with emphasis on our analytical findings. We have evaluated the total energy density (Ωϕ) at the fixed points and also found out the suitable range of γ and λ for a stable model.

Dynamical Role of Scalar Fields in K = 0 Universe

It is an accepted and well examined practice in cosmology to invoke a suitable form of scalar field with potential V(φ) when the observed evolution of the universe cannot be reconciled with theoretical believes. In this article we investigate the role a class of scalar fields in cosmology under minimal and maximal coupling with background matter. We further analyze the dynamical behavior of scalar fields through its scaling solutions by considering their constant equation of state parameter.

Analytic solutions in Einstein-aether scalar field cosmology

In the context of Einstein-aether scalar field cosmology we solve the field equations and determine exact and analytic solutions. In particular, we consider a model proposed by Kanno and Soda where the aether and the scalar fields interact through the aether coefficient parameters, which are promoted to be functions of the scalar field. For this model, we write the field equations by using the minisuperspace approach and we determine the scalar field potentials which leads to Liouville-integrable systems. We solve the field equations for five families of scalar field potentials and, whether it is feasible, we write down the analytic solutions by using closed-form functions.

Cosmological solutions in Hořava-Lifshitz scalar field theory

Abstract We perform a detailed study of the integrability of the Hořava-Lifshitz scalar field cosmology in a Friedmann-Lemaître-Robertson-Walker background space-time. The approach we follow to determine the integrability is that of singularity analysis. More specifically, we test whether the gravitational field equations possess the Painlevé property. For the exponential potential of the scalar field, we are able to perform an analytic explicit integration of the field equations and write the solution in terms of a Laurent expansion and more specifically write the solution in terms of right Painlevé series.

Dynamics of scalar fields in an expanding/contracting cosmos at finite temperature * * Supported in parts by the NSF China (11775110, 11690034), the European Union’s Horizon 2020 research and innovation programme (RISE) under the Marie Sk′ lodowska-Curie grant agreement (644121), and the Priority Academic Program Development for Jiangsu Higher Education Institutions (PAPD)

This study extends the investigation of quantum dissipative effects of a cosmological scalar field by taking into account cosmic expansion and contraction. Cheung, Drewes, Kang, and Kim calculated the effective action and quantum dissipative effects of a cosmological scalar field in a recent work, where analytical expressions for the effective potential and damping coefficient were presented using a simple scalar model with quartic interactions, and the work was conducted using Minkowski-space propagators in loop diagrams. In this work, we incorporate the Hubble expansion and contraction of the cosmic background and focus on the thermal dynamics of a scalar field in a regime where the effective potential changes slowly. Given that the Hubble parameter, H, attains a small but non-zero value, we carry out calculations to the first order in H. If we set H = 0, all results match those in flat spacetime. Interestingly, we must integrate over the resonances, which in turn leads to an amplification of the effects of a non-zero H. This is an intriguing phenomenon, which cannot be uncovered in flat spacetime. The implications on particle creations in the early universe will be studied in a forthcoming study.

Noise-Induced Origin of the Fundamental Scalar Field

The physical nature of the fundamental scalar field generation and hence the origination of the Universe is a matter of the discussions for many years. We propose to use the statistical approach to the description of the steady states of the quasi stationary systems with the elements of the quantum field theory methods as a basis to explain the appearance of the cosmological scalar field. Particularly, we apply two fundamental principles, i.e., the H-theorem and least-energy principle to show principal possibility of the scalar field origination. Along with the basic statement that in the presence of the fundamental scalar field, the energy of the vacuum ground state is lower than the ground state energy of the vacuum with no scalar field (primary vacuum), and with regard to the nonlinear interaction of fluctuating physical fields with the scalar field, these principles are employed to reveal probable phase transitions that may be associated with origin and further evolution of the Universe. Thus, we propose the possible physical justification of the spontaneous cosmological scalar field generation.

Analytical treatment of small scales matter power spectrum in coupled scalar field (CSF) cosmology

In this paper, we reconstruct matter power spectrum of a cosmological model in which coupled scalar field acts as dark energy. The coupled scalar field (CSF) dark energy we propose here is a generalized model of quintessence, k-essence or phantom coupled to dark matter (non-baryonic matter) via a coupling constant. The existence of coupling between dark energy and matter allows energy to transfer between them, which may give rise to different observational signatures, especially at perturbation level, including matter power spectrum. In this work, through analytical exploration we studied that possible signature in matter power spectrum that maybe induced by this mode.

Matter perturbation in coupled scalar field cosmology

A universe containing a coupled scalar field would have different dynamical evolution compared to uncoupled cases. Significant differences are expected to emerge during matter dominated era, where scalar field density mimics how matter density evolves, by becoming subdominant. Analysis of the dynamics was investigated both analytically and numerically through phase plane method, and we obtained two attractor solutions which are compatible to a late time cosmic acceleration as the ending of matter dominated era. Ordinary scalar field triggers an acceleration with stable attractor solution which converges to ωϕ = ωeff ≈ −1. Whereas, phantom scalar field allows negative values for its own energy (ωeff ˂ −1 and Ωϕ ˂ 0); however this is not stable, therefore it will be caught by attractor solution which is stable to ωϕ = ωeff ≈ −1. In addition, we found cosmological parameters generated from both analytical and numerical calculations, i.e. ωϕ, Ωϕ and ωeff, by assuming a flat universe. The existence of a coupling constant Q between scalar field and matter induces different structure growth compared to uncoupled cases and standard cold dark matter (CDM) model. During matter dominated era, ordinary scalar field induces growth of structures so that it becomes faster than the standard model does, whereas phantom scalar field slows down the growth of structures. During scalar field dominated era (far in the future), we obtained that formed structures or density contrasts will decay in a similar way as in uncoupled cases, whereas following phantom scenario, the decay of formed structures occurs faster and more significant. This result is associated to background dynamics which shows that big rip will be the relevant fate of our universe.

Variable G and $$\Lambda $$ gravity theory and analytical cosmological solutions using Noether symmetry approach

The present work deals with scalar field cosmology in the framework of a quantum gravity modified Einstein-Hilbert Lagrangian with variable $G$ and $\Lambda$. Using Renormalization group, variable $G$ behaves as a minimally coupled filed (not the scalar-tensor theory) and variable $\Lambda$ can be interpreted as a potential function. The point Lagrangian for this model in the background of homogeneous and isotropic flat FLRW space-time model experiences point-like Noether symmetry and equivalent potential function $\Lambda(G)$ is determined. Using a point transformation in the $3D$ augmented space is found that one of the variable become cyclic and as a consequence there is considerable simplification to the physical system. Lastly, the constants of motion can be written in compact form and it is possible to have analytic cosmological solutions in the present context.

Exact solutions in Chiral cosmology

In multi-scalar field cosmologies new dynamical degrees of freedom are introduced which can explain the observational phenomena. Unlike the usual scalar field theory where a single scalar field is considered, the multi-scalar field cosmologies allow more than one scalar field and exhibits interetsing consequences, such as quintom, hybrid inflation etc. The current work studies the existence of exact solutions and integrable dynamical systems in multi-scalar field cosmology. We present the exact analytic solutions for a system of $N$-scalar fields. In particular, we consider a multi scalar field cosmological scenario comprised of $N$-scalar fields that are minimally coupled to the Einstein gravity. The geometry of the universe is described by the spatially flat homogeneous and isotropic line element and the scalar fields may interact in their kinetic or/and potential terms. Within this set up, we show that for a specific geometry in the kinetic part of the scalar fields and specific potential form, the gravitational field equations for the class of $N$-scalar field models can be exactly solved. More specifically, we show that the Einstein field equations in $N$-scalar field cosmology can be reduced to that of a $\left( N+1\right) $ linear system.

δ Gravity, δ Matter and the Accelerated Expansion of the Universe

Introducing an additional symmetry (δ symmetry) in the Einstein-Hilbert action, we obtain a gravitational field model (δ gravity) based on two symmetric tensors, gμv and gμv where an effective metric, given by gμv = gμv + gμv must be defined to describe the trajectories of massless particles. In previous papers, a truncated versions of δ gravity applied to cosmology was presented, where the δ symmetry was fixed in order to simplify the analysis of the model. The model predicts an accelerated expansion of the Universe without a cosmological constant or additional scalar fields, ending in a Big Rip. We present the full-fledged δ gravity, introducing a new kind of matter fields (δ matter) in order to preserve the δ symmetry. In this approach the prediction of the expansion of the Universe is preserved, but the value of the cosmological parameters improve greatly by the inclusion of δ matter. Additionally, δ matter gives us an excellent candidate for Dark Matter.