Narrow Results

The (2+1)-dimensional gauged O(3) nonlinear sigma model with Chern–Simons term is canonically quantized. Furthermore, we study a nonminimal coupling in this model implemented by means of a Pauli-type term. It is shown that the set of constraints of the model is modified by the introduction of the Pauli coupling. Moreover, we found that the quantum commutator relations in the nominimal case is independent of the Chern–Simons coefficient, in contrast to the minimal one.

We show that in the realm of general relativity, a nonminimal coupling between the electromagnetic and the gravitational fields may produce an era of accelerated expansion.

We investigate the cosmological effects of an alternative theory of gravity on the four-dimensional Randall–Sundrum braneworld of type II with a higher-order string curvature term added to the action. We discuss the possibility of a varying speed of light, which has recently attracted considerable attention, in the presence a Maxwell field and of a dynamically evolving bulk scalar field nonminimally coupled to scalar curvature in a quadratic form, together with a dark matter–dark energy interaction term. After deriving the modified Friedmann equation on the brane, as well as the scalar field equations, we then analyze the dynamical equations obtained so far. Special attention is paid to scaling solutions which could be important building blocks in constructing the models of dark energy. The constructed model exhibits several features of cosmological and astrophysical interest for both the early and the late universe, consistent with recent observations, in particular the ones concerned with celerity of light, four and five gravitational constants, black hole masses and entropies.

The purpose of this paper is to study braneworld cosmologies in the presence of stringy corrections coupled to a canonical scalar field. Two independent models are explored which in their own right provide cosmologies exhibiting the present cosmic acceleration of the universe. The evolution of the cosmological scale factor is studied in four and ten-dimensional space–times. The four-dimensional model is preformed in the context of the nonminimal Maxwell–Gauss–Bonnet gravity while the ten-dimensional model is constructed from the implications of Kaluza–Klein cosmology together with the Gauss–Bonnet Lagrangian in the action with matter fields nonminimally coupled to gravity. Both the low energy and high energy limits are discussed for the two models and many interesting features are described in some detail.

Motivated by the recent work of Zhang and Chen,¹ we generalize their work to the nonminimally coupled case. We consider a quintom model of dark energy with a single scalar field T given by a Lagrangian inspired by a tachyonic Lagrangian in string theory. We consider nonminimal coupling of the tachyon field to the scalar curvature, and then we reconstruct this model in the light of three forms of parametrization for dynamical dark energy.

From a variational action with nonminimal coupling with a scalar field and classical scalar and fermionic interaction, cosmological field equations can be obtained. Imposing a Friedmann–Lemaître–Robertson–Walker (FLRW) metric, the equations lead directly to a cosmological model consisting of two interacting fluids, where the scalar field fluid is interpreted as dark energy and the fermionic field fluid is interpreted as dark matter. Several cases were studied analytically and numerically. An important feature of the non-minimal coupling is that it allows crossing the barrier from a quintessence to phantom behavior. The insensitivity of the solutions to one of the parameters of the model permits it to find an almost analytical solution for the cosmological constant type of universe.

We perform a phase space analysis of a nonminimally coupled modified gravity theory with the Lagrangian density of the form , where f₁(R) and f₂(R) are arbitrary functions of the curvature scalar R and is the matter Lagrangian density. We apply the dynamical system approach to this scenario in two particular models. In the first model we assume f₁(R) = 2R with a general form for f₂(R) and set favorable values for effective equation of state parameter which is related to the several epochs of the cosmic evolution and study the critical points and their stability in each cosmic eras. In the second case, we allow the f₁(R) to be an arbitrary function of R and set f₂(R) = 2R. We find the late-time attractor solution for the model and show that this model has a late-time accelerating epoch and an acceptable matter era.

We generalize the scalar-curvature coupling model ξΦ²R of Higgs inflation to ξΦ^aR^b to study inflation. We compute the amplitude and spectral index of curvature perturbations generated during inflation and fix the parameters of the model by comparing these with the Planck + WP data. We find that if the scalar self-coupling λ is in the range 10^-5–0.1, parameter a in the range 2.3–3.6 and b in the range 0.77–0.22 at the Planck scale, one can have a viable inflation model even for ξ ≃ 1. The tensor to scalar ratio r in this model is small and our model with scalar-curvature couplings is not ruled out by observational limits on r unlike the pure theory. By requiring the curvature coupling parameter to be of order unity, we have evaded the problem of unitarity violation in scalar-graviton scatterings which plague the ξΦ²R Higgs inflation models. We conclude that the Higgs field may still be a good candidate for being the inflaton in the early universe if one considers higher-dimensional curvature coupling.

In light of BICEP2, we re-examine single field inflationary models in which the inflation is a composite state stemming from various four-dimensional strongly coupled theories. We study in the Einstein frame a set of cosmological parameters, the primordial spectral index n_s and tensor-to-scalar ratio r, predicted by such models. We confront the predicted results with the joint Planck data, and with the recent BICEP2 data. We constrain the number of e-foldings for composite models of inflation in order to obtain a successful inflation. We find that the minimal composite inflationary model is fully consistent with the Planck data. However it is in tension with the recent BICEP2 data. The observables predicted by the glueball inflationary model can be consistent with both Planck and BICEP2 contours if a suitable number of e-foldings are chosen. Surprisingly, the super Yang–Mills inflationary prediction is significantly consistent with the Planck and BICEP2 observations.

The nonminimal coupling of the inflaton is known to alleviate the smallness of the quartic coupling λ in the chaotic inflation with ϕ⁴ potential. A large ξ is required to obtain the cosmic microwave background (CMB) power spectrum while a small value ~ 1/6 seems to be preferred from spectral index. There are issues related to conformal transformations, choice of frame and natural value(s) of ξ for a given potential. We revisit some of these issues and invoke field theoretic arguments (which exist in different context and have not been employed previously in the context of inflation) in order to address the same. A rather strong and general conclusion reached, based on the requirements of renormalizability and finiteness of specific matrix elements in a quantum theory, is that it is generically not possible to eliminate the nonminimal coupling by going from the Jordan to the Einstein frame via conformal transformations. We also comment on Higgs inflation.

Although equivalent to general relativity, teleparallel gravity (TG) is conceptually speaking a completely different theory. In this theory, the gravitational field is described by torsion, not by curvature. By working in this context, a new model is proposed in which the four-derivative of a canonical scalar field representing dark energy is nonminimally coupled to the “vector torsion”. This type of coupling is motivated by the fact that a scalar field couples to torsion through its four-derivative, which is consistent with local spacetime kinematics regulated by the de Sitter group $\mathrm{\text{SO}}(1,4)$. It is found that the current state of accelerated expansion of the universe corresponds to a late-time attractor that can be (i) a dark energy-dominated de Sitter solution (${\omega }_{\varphi }=-1$), (ii) a quintessence-type solution with ${\omega }_{\varphi }\ge -1$, or (iii) a phantom-type ${\omega }_{\varphi }<-1$ dark energy.

We study a nonminimal inflation which is driven by a superpotential. By adopting the Arnowitt–Deser–Misner formalism, we explore the primordial perturbations and its non-Gaussianity in this framework. By expanding the action up to the second-order in perturbations, we seek the scalar spectral index, its running and the tensor-to-scalar ratio. In this regard, we find the ranges of the nonminimal coupling and superpotential parameters which lead to the observationally viable perturbations parameters. The non-Gaussian feature in both the equilateral and orthogonal configurations in this setup, is studied by exploring the third-order action. We show that in some ranges of the nonminimal and superpotential parameters, the model predicts large non-Gaussianity. By comparing the numerical results with Planck2015 data, we test the viability of the model and find some constraints on the model’s parameters space.

Here, we concisely review the nonminimal coupling dynamics of a single scalar field in the context of purely affine gravity and extend the study to multifield dynamics. The coupling is performed via an affine connection and its associated curvature without referring to any metric tensor. The latter arises a posteriori and it may gain an emergent character like the scale of gravity. What is remarkable in affine gravity is the transition from nonminimal to minimal couplings which is realized by only field redefinition of the scalar fields. Consequently, the inflationary models gain a unique description in this context where the observed parameters, like the scalar tilt and the tensor-to-scalar ratio, are invariant under field reparametrization. Overall, gravity in its affine approach is expected to reveal interesting and rich phenomenology in cosmology and astroparticle physics.

We investigate polarizations of gravitational waves in generalized theories of gravity whose action contains a nonminimal coupling (NMC) between an exponential function of $f(R)$ gravity and the matter field. Specifically, we consider two classes of such models. In the first case, we assume an explicit NMC between the matter Lagrangian density and curvature while in the second case, we consider the exponential $f(R)$ gravity coupled to the trace of the stress-energy tensor of a scalar field. Using the Newman–Penrose quantities, we show that these models can be considered as suitable choices in order to produce new polarization modes.

The case of a self-gravitating scalar field under-going the first-order phase transition is studied. Particularly the effects of self gravity on the nucleation of vacuum bubbles is investigated. We also investigate what modifications are induced by the introduction of nonminimal coupling of the scalar field. The possibility of nucleation of false vacuum bubbles within the true vacuum background in the case of a nonminimally coupled scalar field is discussed.

We discuss constraints to some nonminimally (NMC) coupled curvature-matter models of gravity by means of Solar System experiments.

First we discuss a NMC gravity model which constitutes a natural extension of 1/Rⁿ gravity to the nonminimally coupled case. Such a NMC gravity model is able to predict the observed accelerated expansion of the Universe. Differently from the f (R) = 1/Rⁿ gravity case, which is not compatible with Solar System observations, it turns out that this NMC model is a viable theory of gravity.

Then we consider a further NMC gravity model which admits Minkowski spacetime as a background, and we derive the 1/c expansion of the metric. The nonrelativistic limit of the model is not Newtonian, but contains a Yukawa correction. We look for trajectories around a static, spherically symmetric body. Since in NMC gravity the energy-momentum tensor of matter is not conserved, then the trajectories deviate from geodesics. We use the NMC gravity model to compute the perihelion precession of planets and we constrain the parameters of the model from radar observations of Mercury.