Chameleon Models Research Articles

THE GammeV SUITE OF EXPERIMENTAL SEARCHES FOR AXION-LIKE PARTICLES

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.

Strong Coupling Regime of Chameleon Models

In chameleon theories where the field couples to matter much more strongly than gravity does, the fifth force between two bodies with thin-shell is independent of their coupling to the field. As a consequence present days bounds on these models can be exponentially relaxed.

Slow-roll suppression of adiabatic instabilities in coupled scalar field-dark matter models

We study the evolution of linear density perturbations in the context of interacting scalar field-dark matter cosmologies, where the presence of the coupling acts as a stabilization mechanism for the runaway behavior of the scalar self-interaction potential as in the case of the chameleon model. We show that, in the ``adiabatic'' background regime of the system, the rise of unstable growing modes of the perturbations is suppressed by the slow-roll dynamics of the field. Furthermore, the coupled system behaves as an inhomogeneous adiabatic fluid. In contrast, instabilities may develop for large values of the coupling constant, or along nonadiabatic solutions, characterized by a period of high-frequency dumped oscillations of the scalar field. In the latter case, the dynamical instabilities of the field fluctuations, which are typical of oscillatory scalar field regimes, are amplified and transmitted by the coupling to dark matter perturbations.

Detecting chameleons through Casimir force measurements

The best laboratory constraints on strongly coupled chameleon fields come not from tests of gravity per se but from precision measurements of the Casimir force. The chameleonic force between two nearby bodies is more akin to a Casimir-like force than a gravitational one: The chameleon force behaves as an inverse power of the distance of separation between the surfaces of two bodies, just as the Casimir force does. Additionally, experimental tests of gravity often employ a thin metallic sheet to shield electrostatic forces; however, this sheet masks any detectable signal due to the presence of a strongly coupled chameleon field. As a result of this shielding, experiments that are designed to specifically test the behavior of gravity are often unable to place any constraint on chameleon fields with a strong coupling to matter. Casimir force measurements do not employ a physical electrostatic shield and as such are able to put tighter constraints on the properties of chameleons fields with a strong matter coupling than tests of gravity. Motivated by this, we perform a full investigation on the possibility of testing chameleon models with both present and future Casimir experiments. We find that present-day measurements are not able to detect the chameleon. However, future experiments have a strong possibility of detecting or rule out a whole class of chameleon models.

K-chameleon and the coincidence problem

In this paper we present a hybrid model of $k$-essence and chameleon, named as $k$-chameleon. In this model, due to the chameleon mechanism, the directly strong coupling between the $k$-chameleon field and matters (cold dark matters and baryons) is allowed. In the radiation-dominated epoch, the interaction between the $k$-chameleon field and background matters can be neglected; the behavior of the $k$-chameleon therefore is the same as that of the ordinary $k$-essence. After the onset of matter domination, the strong coupling between the $k$-chameleon and matters dramatically changes the result of the ordinary $k$-essence. We find that during the matter-dominated epoch, only two kinds of attractors may exist: one is the familiar $\mathbf{K}$ attractor and the other is a completely new, dubbed $\mathbf{C}$ attractor. Once the Universe is attracted into the $\mathbf{C}$ attractor, the fraction energy densities of the $k$-chameleon ${\ensuremath{\Omega}}_{\ensuremath{\phi}}$ and dust matter ${\ensuremath{\Omega}}_{m}$ are fixed and comparable, and the Universe will undergo a power-law accelerated expansion. One can adjust the model so that the $\mathbf{K}$ attractor does not appear. Thus, the $k$-chameleon model provides a natural solution to the cosmological coincidence problem.