Anisotropic dark energy from string compactifications

We discuss new models of an “affine” theory of gravity in multidimensional space-times with symmetric connections. We use and develop ideas of Weyl, Eddington, and Einstein, in particular, Einstein’s proposal to specify the space-time geometry by the use of the Hamilton principle. More specifically, the connection coefficients are determined using a “geometric” Lagrangian that is an arbitrary function of the generalized (nonsymmetric) Ricci curvature tensor (and, possibly, of other fundamental tensors) expressed in terms of the connection coefficients regarded as independent variables. Such a theory supplements the standard Einstein gravity with dark energy (the cosmological constant, in the first approximation), a neutral massive (or tachyonic) vector field (vecton), and massive (or tachyonic) scalar fields. These fields couple only to gravity and can generate dark matter and/or inflation. The new field masses (real or imaginary) have a geometric origin and must appear in any concrete model. The concrete choice of the geometric Lagrangian determines further details of the theory, for example, the nature of the vector and scalar fields that can describe massive particles, tachyons, or even “phantoms.” In “natural” geometric theories, which are discussed here, dark energy must also arise. We mainly focus on intricate relations between geometry and dynamics while only very briefly considering approximate cosmological models inspired by the geometric approach.

We analyze a distinctive mechanism for inflation in which particle production slows down a scalar field on a steep potential, and show how it descends from angular moduli in string compactifications. The analysis of density perturbations -- taking into account the integrated effect of the produced particles and their quantum fluctuations -- requires somewhat new techniques that we develop. We then determine the conditions for this effect to produce sixty e-foldings of inflation with the correct amplitude of density perturbations at the Gaussian level, and show that these requirements can be straightforwardly satisfied. Finally, we estimate the amplitude of the non-Gaussianity in the power spectrum and find a significant equilateral contribution.

In this work, we establish a correspondence between the interacting holographic, new agegraphic dark energy and generalized Chaplygin gas model in Bianchi type I universe. Then, we reconstruct the potential of the scalar field which describes the generalized Chaplygin cosmology. Cosmological solutions are obtained when the kinetic energy of the phantom field is of the order of the anisotropy and dominates over the potential energy of the field. We investigate observational constraints on the generalized Chaplygin gas, holographic and new agegraphic dark energy models as the unification of dark matter and dark energy, by using the latest observational data. To do this we focus on observational determinations of the expansion history H(z) . It is shown that the HDE model is better than the NADE and generalized Chaplygin gas models in an anisotropic universe. Then, we calculate the evolution of density perturbations in the linear regime for three models of dark energy and compare with the results of the $ \Lambda$ CDM model. Finally, the analysis shows that the increase in anisotropy leads to more correspondence between the dark energy scalar field model and observational data.

Here we briefly discuss the Galileon gravity theory and modified Friedmann equations. By considering new holographic dark energy (NHDE) in the framework of Galileon gravity, we found the energy density, pressure, equation of state and the deceleration parameter in terms of the scale factor. Subsequently, we study the correspondence between the NHDE in the framework of Galileon gravity with other dark energies like quintessence, k-essence, tachyon, dilaton, hessence and DBI-essence dark energies and construct the scalar field and corresponding scalar potentials which describe the dynamics of the scalar fields graphically. All the dark energy models, the scalar field and the potential decrease due to the evolution of the universe.

Here we briefly discuss the Einstein-Aether gravity theory by modification of Einstein-Hilbert action. We find the modified Friedmann equations and then we find the effective energy density and pressure for Einstein-Aether gravity sector. These can be treated as dark energy provided some restrictions on the free function F(K), where K is proportional to H 2. Subsequently, we study the correspondence between the effective dark energy coming from Einstein-Aether gravity with other dark energies like k-essence, tachyon, dilaton, hessence and DBI-essence dark energy and construct the scalar field and corresponding scalar potentials which describe the dynamics of the scalar fields graphically. So finally, if the Einstein-Aether gravity behaves like other dark energy models, in this situation, the scalar field increases and potential decreases.

Many dark energy models fail to pass the cosmic age test because of the old quasar APM 08279+5255 at redshift $z=3.91$, the $\Lambda$CDM model and holographic dark energy models being no exception. In this paper, we focus on the topic of age problem in the new agegraphic dark energy (NADE) model. We determine the age of the universe in the NADE model by fitting the observational data, including type Ia supernovae (SNIa), baryon acoustic oscillations (BAO) and the cosmic microwave background (CMB). We find that the NADE model also faces the challenge of the age problem caused by the old quasar APM 08279+5255. In order to overcome such a difficulty, we consider the possible interaction between dark energy and dark matter. We show that this quasar can be successfully accommodated in the interacting new agegraphic dark energy (INADE) model at the $2\sigma$ level under the current observational constraints.

The quest of deciphering the true nature of dark energy has proven to be one of the most exciting in recent times in cosmology. Various ideas have been put forward in this regard besides the usual cosmological constant approach, ranging from scalar field based models like Quintessence and Phantom dark energy to various modified gravity approaches as well. A very interesting idea then is to consider scalar field dark energy models in quantum gravitationally corrected cosmologies with the RS-II Braneworld being one of the most well known in this regard. So in this work, we consider RS-II Braneworld based scalar field dark energy models and try to look out for the existence of finite time singularities in these regimes both through a dynamical system perspective, for which we employ the Goriely–Hyde singularity analysis method, and a physical perspective. Our approach is general in the sense that it is not limited to any particular class of potentials or for any constrained parameter region for the brane tension and is valid for both Quintessence and phantom dark energy regimes. We firstly show through Goriely–Hyde procedure that finite time singularities can exist in these models for a limited set of initial conditions and that this result would hold irrespective of any consideration given to the swampland dS conjecture. We then discuss the physical nature of the singularities that can occur in this regime, where we use a well motivated ansatz for the Hubble parameter and show that these models of dark energy can allow for weak singularities like those of Type III and Type IV and can also allow for strong singularities like the Big Rip (Type I).

This paper describes an approach for interactive visualization of mixed scalar and vector fields, in which vector icons are generated from pre-voxelized icon templates and volume-rendered together with the volumetric scalar data. This approach displays simultaneously the global structure of the scalar field and the detailed features of the vector field. Interactive visualization is achieved with incremental image update, by re-rendering only a small portion of the image wherever and whenever a change occurs. This technique supports a set of interactive visualization tools, including change of vector field visualization parameters, real-time animation of vector icons advected within the scalar field, a zooming lens, and a local probe. Subject Terms: data visualisation; interactive systems; graphical user interfaces; rendering (computer graphics); interactive visualization; scalar fields; vector fields; vector icons; pre-voxelized icon templates; volume-rendered; volumetric scalar data; incremental image update; real-time animation; zooming lens; local probe

Recently, two simple criteria were proposed to assess if vacua emerging from an effective scalar field theory are part of the string “landscape” or “swampland”. The former are the vacua that emerge from string compactifications; the latter are not obtained by any such compactification and hence may not survive in a UV completed theory of gravity. So far, these criteria have been applied to inflationary and dark energy models. Here we consider them in the context of solitonic compact objects made up of scalar fields: boson stars. Analysing several models (static, rotating, with and without self-interactions), we find that, in this context, the criteria are not independent. Furthermore, we find the universal behaviour that in the region wherein the boson stars are expected to be perturbatively stable, the compact objects may be part of the landscape. By contrast, in the region where they may be faithful black hole mimickers, in the sense they possess a light ring, the criteria fail (are obeyed) for static (rotating) ultracompact boson stars, which should thus be part of the swampland (landscape). We also consider hairy black holes interpolating between these boson stars and the Kerr solution and establish the part of the domain of existence where the swampland criteria are violated. In interpreting these results one should bear in mind, however, that the swampland criteria are not quantitatively strict.

Dark matter and dark energy are used as two important concepts in cosmology to explain some of the observed phenomena in the universe. Dark matter is one of the most dominant constituents of the Universe, and it influences the structural formation of the Universe through gravity, including the formation and evolution of galaxies, clusters, and the large-scale structure of the Universe. Dark energy is believed to be one of the causes of the accelerated expansion of the Universe, and its presence explains the observed phenomenon of the accelerating rate of expansion of the Universe. Although their existence has not been directly observed, people understand through the study of the structure and evolution of the universe that they play an important role in the universe. This paper first introduces the background knowledge of dark matter and its related properties and explains the reasons why three types of models, namely WIMP, axion, and sterile neutrino, are candidates for dark matter in the light of existing observations. The paper then discusses the relevant properties of dark energy and analyses the mainstream dark energy models. For the cosmological constant mode, the fine-tuning problem and cosmic coincidence problem it faces are analysed in detail. The evolution of the dark energy equation of state from the past >-1 to the present <-1 is then explained, and this is used to introduce the scalar field model involving dynamic, the Chaplygin gas model, the holographic dark energy model, and the interacting dark energy model.

The problem of the cosmic coincidence is a longstanding puzzle. This conundrum may be solved by introducing a coupling between the two dark sectors. In this Letter, we study two cases of the coupled quintessence scenario. (a) Assume that the mass of dark matter particles depends exponentially on the scalar field associated to dark energy and meanwhile the scalar field evolves in an exponential potential; (b) Assume that the mass of dark matter particles depends on a power law function of the scalar field and meanwhile the scalar field evolves in a power law potential. Since the dynamics of this system is dominated by an attractor solution, the mass of dark matter particles is forced to change with time as to ensure that the ratio between the energy densities of dark matter and dark energy becomes a constant at late times, and one thus solve the cosmic coincidence problem naturally. We perform a statefinder diagnostic to both cases of this coupled quintessence scenario. It is shown that the evolving trajectory of this scenario in the s–r diagram is quite different from those of other dark energy models.

The gravitational wave (GW) event GW170817 from a binary neutron star merger together with the electromagnetic counterpart showed that the speed of GWs [Formula: see text] is very close to that of light for the redshift [Formula: see text]. This places tight constraints on dark energy models constructed in the framework of modified gravitational theories. We review models of the late-time cosmic acceleration in scalar–tensor theories with second-order equations of motion (dubbed Horndeski theories) by paying particular attention to the evolution of dark energy equation of state and observables relevant to the cosmic growth history. We provide a gauge-ready formulation of scalar perturbations in full Horndeski theories and estimate observables associated with the evolution of large-scale structures, cosmic microwave background and weak lensing by employing a so-called quasi-static approximation for the modes deep inside the sound horizon. In light of the recent observational bound of [Formula: see text], we also classify surviving dark energy models into four classes depending on different structure-formation patterns and discuss how they can be observationally distinguished from each other. In particular, the nonminimally coupled theories in which the scalar field [Formula: see text] has a coupling with the Ricci scalar [Formula: see text] of the form [Formula: see text], including [Formula: see text] gravity, can be tightly constrained not only from the cosmic expansion and growth histories but also from the variation of screened gravitational couplings. The cross-correlation of integrated Sachs–Wolfe signal with galaxy distributions can be a key observable for placing bounds on the relative ratio of cubic Galileon density to total dark energy density. The dawn of GW astronomy will open up a new window to constrain nonminimally coupled theories further by the modified luminosity distance of tensor perturbations.

In this paper, we have studied the general evolution of spherical over-densities for thawing class of scalar field dark energy (DE) models. We have considered the scalar fields having canonical as well as non-canonical kinetic energy (particularly the Born-Infeld form of kinectic term) with various type of potentials, and also investigated the situation where DE is homogeneous as well as the case where DE virializes together with matter (inhomogeneous). Our study has shown that models with linear potential, in particular, can have significant deviation from the ΛCDM model in terms of density contrast at the time of virialization, and further study of the cluster number counts has shown that the total cluster number counts of different DE models can have substantial deviation from ΛCDM, and this deviation is most significant for all the models we have considered.

We propose an adynamical, background independent approach to quantum gravity and unification whereby the fundamental elements of Nature are graphical units of space, time and sources (in parlance of quantum field theory). The transition amplitude for these elements of “spacetimesource” is computed using a path integral with discrete Gaussian graphical action. The unit of action for a spacetimesource element is constructed from a difference matrix K and source vector J on the graph, as in lattice gauge theory. K is constructed from graphical relations so that it contains a non-trivial null space (whence gauge invariance), and J is then restricted to the column space of K which ensures it is distributed in a divergence-free fashion over the spacetime defined by the element. This rule for the relational construct of K and J is our proposed fundamental axiom of physics and results in a self-consistency relationship between sources, the spacetime metric, and the stress-energy-momentum content of the element, rather than a dynamical law for time-evolved entities. In its most general form, the set of fundamental elements employed by lattice gauge theory contains scalar fields on nodes and links, and vector fields on nodes. To complete the fundamental set (unification in this view), we propose the addition of scalar fields on plaquettes (basis for graviton) and vector fields on links. We use this approach via modified Regge calculus to correct proper distance in the Einstein-deSitter cosmology model yielding a fit of the Union2 Compilation supernova data that matches ɅCDM without having to invoke accelerating expansion or dark energy.

Plane-symmetric dark energy model with a massive scalar field