Narrow Results

We report on the design and results of the GammeV search for axion-like particles and for chameleon particles. We also discuss plans for an improved experiment to search for chameleon particles, one which is sensitive to both cosmological and power-law chameleon models. Plans for an improved axion-like particle search using coupled resonant cavities are also presented. This experiment will be more sensitive to axion-like particles than stellar astrophysical models or current helioscope experiments.

We briefly review the problems and prospects of the standard lore of dark energy. We have shown that scalar fields, in principle, cannot address the cosmological constant problem. Indeed, a fundamental scalar field is faced with a similar problem dubbed naturalness. In order to keep the discussion pedagogical, aimed at a wider audience, we have avoided technical complications in several places and resorted to heuristic arguments based on physical perceptions. We presented underlying ideas of modified theories based upon chameleon mechanism and Vainshtein screening. We have given a lucid illustration of recently investigated ghost-free nonlinear massive gravity. Again, we have sacrificed rigor and confined to the basic ideas that led to the formulation of the theory. The review ends with a brief discussion on the difficulties of the theory applied to cosmology.

One of the most important issues in modern cosmology is understanding the origin of the accelerated expansion of the universe, known as “dark energy”. The scalar–tensor model is one of the most interesting candidates for describing dark energy. In this paper, we investigate the non-canonical scalar model, which is written in the framework of Chameleon models. In this model, the scalar field is coupled to a mass density ${\rho }_m$ under an arbitrary $g(\varphi )$ coupling function. We study the cosmological aspect of this model according to its Chameleon state and we conclude that for high densities, the mass of the Chameleon field becomes larger, so its effect will not be seen in high-density areas. We also examine the dynamical behavior of this model for different coupling functions and potentials. According to the dynamic behavior of the model, we can conclude that using this model can explain the accelerated expansion of the universe. Finally, we investigate the tensor perturbations in this model and show that the mass of the gravitational waves obtained in the framework of this model can be in good agreement with the observations.

The low-lying states of an organic donor-σ-acceptor dyad, i.e. tetrathiafulvalene-σ-tetracyano-p-quinodimethane (TTF-σ-TCNQ), in gas phase and in various solvents have been investigated by means of hybrid DFT calculations in combination with the conductor-like polarizable continuum model to describe solvent effects. It has been shown that the dyad, though preferring a closed-shell singlet ground state with an eclipsed conformation in gas phase, adopts the charge-separated zwitterionic states with an extended conformation (TTF⁺-σ-TCNQ^-), i.e. open-shell singlet biradical ground state immediately followed by triplet biradical state, in polar solvent (CH₃CN and CH₂Cl₂) as a result of the intramolecular electron transfer (ET) stimulated by solvent polarization. The degree of such intramolecular ET is so strongly dependent on the polarity (dielectric constant) of solvent that the zwitterionic biradical states become more stable with respect to the closed-shell singlet state with increasing polarity of the solvent. As such, the dyad should show a higher ratio of biradicals in more polar solvent and/or at higher temperature and, hence, is chameleonic in nature.

In this paper, we explore the homogeneous and isotropic flat Friedmann–Robertson–Walker (FRW) model in Chameleon cosmology. By considering a non-minimal coupling between the scalar field and matter, we present a non-singular bouncing cosmological scenario of the universe. The universe initially exhibits the ekpyrotic phase during the contracting era, undergoes a non-singular bounce, and then in expanding era, it smoothly transits to the decelerating era having matter and radiation-dominated phases. Further, this decelerating era is smoothly connected to the late-time dark energy-dominated era of the present epoch. We use numerical solution techniques to solve non-minimally coupled gravity equations for understanding the evolution of scalar field along with other quantities like effective potential in the model. The model thus unifies an ekpyrotic, non-singular, asymmetric bounce with the dark energy era of the present epoch. We study the evolution of bouncing model and confront the model with observational results on the equation of state parameter by constraining the model parameters.